GEOLOGICAL SURVEY CIRCULAR 898
Summary of Workshops Concerning Regional Seismic Source Zones of Parts of the Conterminous United States, Convened by the U.S. Geological Survey 1979-1980, Golden, Colorado Summary of Workshops Concerning Regional Seismic Source Zones of Parts of the Conterminous United States Convened by the U.S. Geological Survey 1979-1980, Golden, Colorado
Edited by Paul C. Thenhaus
GEOLOGICAL SURVEY CIRCULAR 898
J983 United States Department of the Interior WILLIAM P. CLARK, Secretary
Geological Survey Dallas L. Peck, Director
Free on application to the Branch of Distribution, Eastern Region, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22034 CONTENTS
Page Abstract ...... 1 Introduction, by P. C. Thenhaus and F. A. McKeown ...... 1 Acknowledgments ...... 4 Great Basin seismic source zones Summary of workshop convened October 10-11, 1979, by R. C. Bucknam and P. C. Thenhaus ...... 4 Northern Rocky Mountains seismic source zones Summary of workshop convened December 6-7, 1979, by F. A. McKeown, D. C. Ross, and P. C. Thenhaus ... 9 Southern Rocky Mountains seismic source zones Summary of workshop convened January 23-24, 1980, by R. E. Anderson, W. P. Irwin, and P. C. Thenhaus ...... 14 Central Interior seismic source zones Summary of workshop convened June 10-11, 1980, by F. A. McKeown, D. P. Russ, and P. C. Thenhaus ...... 19 Northeastern United States seismic source zones Summary of workshop convened Sep tember 10-11, 1980, by W. H. Diment, F. A. McKeown, and P. C. Thenhaus . 24 Summary, by P. C. Thenhaus ...... 31 References cited ...... 32
ILLUSTRATIONS
FIGURES 1-11. Maps: 1. Five regions of the conterminous United States discussed at seismic source zone workshops . . 2 2. Index of the Great Basin region ...... 5 3. Seismic source zones of the Great Basin ...... 7 4. Index of Northern Rocky Mountain region ...... 10 5. Seismic source zones of the Northern Rocky Mountains ...... 13 6. Index of the Southern Rocky Mountains region ...... 16 7. Seismic source zones of the Southern Rocky Mountains ...... 17 8. Index of the Central Interior ...... 21 9. Seismic source zones of the Central Interior ...... 22 10. Index of the Northeastern United States ...... 26 11. Seismic source zones of the Northeastern United States ...... 27
TABLES
Page TABLE 1. Estimated maximum magnitudes and recurrence rates from geologic data for earthquakes M8>7 in the Great Basin seismic source zones ...... 8 2. Estimated maximum magnitudes (M8) and estimated relative recurrence of maximum magnitude events for the source zones of the Northern Rocky Mountains region ...... 14 3. Estimated maximum magnitudes (Ms) and estimated relative recurrence of maximum magnitude events for the source zones of the Southern Rocky Mountains region ...... 18 4. Estimated maximum magnitudes (Mb) and estimated recurrence rates for maximum magnitude events for the seismic source zones of the Central Interior region ...... 23 5. Estimated maximum magnitudes for seismic source zones of the northeastern United States and adjacent Canada ...... 28
III Summary of workshops concerning regional seismic source zones of parts of the conterminous United States, convened by the U.S. Geological Survey, 1979-1980, Golden, Colorado
Edited by Paul C. Thenhaus
ABSTRACT interval of earthquakes in each of the zones, and (3) calculation of cumulative probability distribu Workshops were convened by the U.S. Geological Survey to obtain the latest information and concepts relative to defin tions of expected acceleration exceedences for ing seismic source zones for five regions of the United States. points in the region. A major part of the first task The zones, with some modifications, have been used in prepara in the preparation of the new maps is to incorpo tion of new national probabilistic ground motion hazard maps rate the most recent information and ideas related by the U.S. Geological Survey. to the seismicity, geology, and geophysics of vari The five regions addressed are the Great Basin, the North ern Rocky Mountains, the Southern Rocky Mountains, the ous parts of the country. This report is primarily Central Interior, and the northeastern United States. Discus a description of the first task the delineation of sions at the workshops focussed on possible temporal and spa seismic source zones. tial variations of seismicity within the regions, latest ages of In an effort to utilize the most recent, pertinent surface-fault displacements, most recent uplift or subsidence, geologic structural provinces as they relate to seismicity, and information from a variety of disciplines and for speculation on earthquake causes. a variety of areas across the country, informal Within the Great Basin region, the zones conform to areas workshops were convened by the U.S. Geological characterized by a predominance of faults that have certain Survey in Golden, Colo., in late 1979 and early ages of latest surface displacements. In the Northern and 1980. The number of workshops convened was lim Southern Rocky Mountain regions, zones primarily conform to distinctive structural terrane. In the Central Interior, primary ited by budget considerations. A difficult decision, emphasis was placed on an interpretation of the areal distribu therefore, was to determine which regions of the tion of historic seismicity, although geophysical studies in the United States would benefit greatest from these Reelfoot rift area provided data for defining zones in the New workshops. Seismic source zoning of the east and Madrid earthquake area. An interpretation of the historic seis west coasts had been addressed in studies com micity also provided the basis for drawing the zones of the pleted just prior to the planning of the 1979-1980 New England region. Estimates of earthquake maximum magnitudes and of re seismic source zone workshops (Perkins and currence times for these earthquakes are given for most of others, 1979; Perkins and others, 1980; Thenhaus the zones and are based on either geologic data or opinion. and others, 1980). A predecessor to the 1979-1980 workshops was convened in September 1978 at the INTRODUCTION U.S. Geological Survey's office in Woods Hole,
By P. C. Thenhaus and F. A. McKeown Mass., to gather recent information that might have a bearing on seismic source zones for the Under the Earthquake Hazards Reduction act eastern United States. Source zones defined as a of 1977 (Public Law 95-124, Executive Office of result of information acquired at that meeting are the President, 1978) the U.S. Geological Survey discussed by Perkins and others (1979). Because has been given the responsibility for producing source zones for the east and west coasts incorpo earthquake hazard and seismic risk maps on both rated the most recent information available as of a regional and national scale. The maps are in 1978-1979, they were excluded from consideration tended to aid in the mitigation of the short-term in planning the 1979^1980 workshops. Throughout earthquake hazard to buildings of standard con the remaining part of the United States, the gen struction. The preparation of these maps requires eral governing principle was to choose those re completion of three major tasks: (1) delineation of gions where geological, geophysical, and seis- seismic source zones, (2) analysis of the recurrence mological research of the past 5 years contributed -4.^ NORTHEAST I/ NORTHERN ^ UNITE
NTAINS | /
*U
FIGURE 1. Five regions of the conterminous United States discussed at seismic source zone workshops convened by the U.S. Greological Survey, 1979-1980. Numbered zones are shown on figure indicated in parentheses. significantly to the understanding of the seis- length, continuity, mode of displacement, or in lo motectonics of the region. The regions chosen cation with respect to range fronts or within ba were the (1) Great Basin, (2) Northern Rocky sins, and so forth? If differences can be recog Mountains, (3) Southern Rocky Mountains, (4) nized, is there any relationship to seismicity pat Central Interior, and (5) northeastern United terns or size of earthquakes? States (fig. 1). 7. Is there strong evidence that various structural, The workshops provided a forum for (1) present tectonic, or geophysical features, including aver ing and discussing current research, some of which age elevation, heatflow, gravity or magnetic gra is in a formative stage, (2) speculating on the na dients, or volcanic centers, are correlative with ture of the earthquake-generating process operat seismicity and, therefore, indicate constraints on ing on a regional scale, (3) voicing concerns and the distribution of seismicity? recommendations for various seismic source zones, Zones resulting from these considerations are and (4) suggesting various treatments of these shown in figures 3, 5, 7, 9, and 11. The zones zones in application to probabilistic hazard maps. are numbered consecutively (1 through 75) in ac Prior to each workshop, participants were asked cord with the chronological order of the workshops to ponder a number of considerations in relating for easy reference to the source zone descriptions. geological, geophysical, and seismological informa Some source zones are duplicated among work tion to seismic source zones. These considerations shop summaries and tables 1, 2 and 3 because of are enumerated here as they were the focus of the overlap of the three western United States the discussions at the workshops. regions (see fig. 1). Information requested from the workshops, other than outlines of source 1. A clear distinction should be drawn (or at least zones, were estimates of maximum magnitude and attempted) between estimating ground motion in recurrence of large events for the zones. This in the short term (say 50 years) and the long term formation is summarized in tables 1 through 5. (say 10,000 years). To what extent can our present Geologic data that support these estimates are understanding of regional seismotectonics be ap available for some zones; for other zones for which plied to this problem? little or no pertinent data are available, estimates 2. Will the spatial distribution of earthquake activ of the magnitude and recurrence are made relative ity in the region remain stationary or change in among the zones. Outside the Basin and Range the next 50 years, or in the next 10,000 years? and the New Madrid regions, these estimates, al If there are changes, what will be the directions though intuitive, still are informative when made and rates? Is the Neogene tectonic history of use by a group of knowledgeable researchers. Inter to predict changes in the distribution of earth estingly, an opinion as to the reasonableness of quake activity? areas belonging to the same (or different) zones 3. With regard to the previous question, will large commonly is based on an intuitive distinction of shocks recur within that zone in the next 10-500 both maximum magnitude and earthquake recur years or will other areas become active? What rence among different areas. other areas might become active? We must point out that the zones derived from 4. To what extent is it now possible to estimate the workshops are not necessarily the zones used frequency of occurrence of earthquakes in a region in the national seismic hazard maps (Algermissen by means other than the magnitude distribution and others, 1982). A brief description of those of earthquakes (for example, by using data on av zones accompanies the maps. erage rates of slip on faults)? The configurations of the source zones in this 5. Can a relationship be drawn between the age report represent current thinking in seismic re- of mapped faulting in various parts of a region gionalization and can be expected to change in the and (1) current seismicity, (2) seismicity in the future as research pertinent to the subject next 50-500 years, and (3) seismicity in the next evolves. Therefore, just as important as the exact 10,000 years? Is there a difference in the preced zone configurations outlined here, is the need to ing relationships for (1) earthquakes of magnitude indicate the current trends in thinking. Discus M<6.5, and (2) earthquakes M>6.5? sions at the workshops covered a wide scope, 6. Can faults in different parts of the region be ranging from detailed accounts of geologic data characterized by differences in style, such as, in about earthquake occurrences on a particular fault to relatively unconstrained speculation on regional GREAT BASIN SEISMIC SOURCE ZONES- tectonics and underlying earthquake causes. In SUMMARY OF WORKSHOP deed, participants were encouraged to range as CONVENED OCTOBER 10-11, 1979 far as they could to construct a reasonable hypothesis. The detailed, better founded hypoth By R. C. Bucknam and P. C. Thenhaus eses are represented in the source zone maps and PARTICIPANTS are documented in the accompanying source zone descriptions. They are, as would be expected, the R. C. Bucknam ...... U.S. Geological Survey (USGS) (Moderator and Recorder) information that the groups were comfortable S. T. Algermissen ...... USGS using as a basis for zoning. Many other ideas that R. E. Anderson ...... USGS related to uncertain geologic associations with W. J. Arabasz ...... University of Utah seismicity, or ideas that may have involved J. W. Bell ...... Nevada Bureau of Mines and Geology speculative causal mechanisms are not rep L. S. Cluff ...... Woodward-Clyde Consultants D. M. Perkins ...... USGS resented in the source zone maps. However, each A. S. Ryall ...... University of Nevada idea has some degree of merit, and, limited as it P. C. Thenhaus ...... USGS might be at present in scope or application, with R. E. Wallace ...... USGS further study may result in a useful zoning princi C. M. Wentworth ...... USGS ple. Accordingly, the "Workshop Summary" sec M. L. Zoback ...... USGS tions in this report reflect the nature of these more speculative ideas and discussions. WORKSHOP SUMMARY The source zones described herein represent A substantial part of the workshop was spent solely the views of the various committees. They discussing the significance of spatial and temporal do not represent the authors' viewpoints nor are variation of seismicity along both the Nevada Seis they to be interpreted as an official position of mic Zone (NSZ) and the Wasatch fault (see fig. the U.S. Geological Survey. 2 for location). Ryall (1977) argued two points that were reiterated at the workshop: (1) that, based ACKNOWLEDGMENTS on the distribution of late Quaternary faulting in Nevada, the seismic activity in the western Great Thanks are due to all the participants of the Basin migrates with time, and (2) that the sites workshops for their contribution in this effort, of large historic events are probably the least particularly for their willingness to supply or to likely areas to have large events in thejuture discuss unpublished data and ideas. Their unself based on the distribution and recurrence charac ishness in this respect has led to the use of the teristics of recent seismicity in the vicinity of the most up-to-date information for delineation of seis past large events. A clear division of opinion mic source zones. The following workshop sum existed as to whether the present state of knowl maries and zone descriptions have been excerpted, edge of the seismotectonics in the western Great either in part or in whole, from workshop sum Basin allows delineation of areas where seismic ac maries prepared by the Recorder of each work tivity could be expected to increase in the future shop. The Recorders' extra efforts are greatly ap as opposed to areas where seismicity could be ex preciated as are the efforts of Frank McKeown pected to decrease. Seismologic data indicate a de for organizing the workshops. creasing rate of low-magnitude seismicity along faults that have had historic rupture, indicating A special thanks is extended to Frank that perhaps the microseismicity is part of a wan McKeown, Porter Irwin, Otto Nuttli, John Bell, ing aftershock pattern. Also, clusters of seismicity Michael Stickney, Yngvar Isachsen, Gabriel Le- are prominent at the ends of certain faults that blank, William Diment, Anthony Qamar, Allan have late Quaternary but not historic displace Sanford, S. T. Algermissen, and David Perkins ment. Other late Quaternary faults, however, have for helpful comments during the preparation of moderate levels of seismicity along their length, this text. perhaps indicating stress accumulation on these 110*
CALIFORNIA NEVDA
41°
SEISMIC ZONE
STILLWATER R ANGE
_____I
FIGURE 2. Index map of the geographic region addressed at the Great Basin seismic source zone workshop showing selected topographic, geologic, and geographic features that are referenced in the text. Two-degree topographic quadrangles shown are those in which Quater nary faults have been mapped. Dashed line indicates geographical extent of the region considered in the workshop. particular faults and possibly impending fault rup reasonably can be inferred to represent M>7 ture. Although the concept generally was accepted events, yet in historic times no earthquake as having merit, the unclear nature of the seis- larger than M=5.5 has occurred on the fault motectonic cycle in the western Great Basin pre (Arabasz and others, 1980). Second, two areas cluded its use in a form of detailed zoning of indi along the fault show anomalously low instrumen- vidual faults. The high incidence of historic fault tally detected seismic activity (Arabasz and ing in the NSZ is apparently without precedent others, 1980; Smith and Sbar, 1974; Smith, 1972) in Holocene time, indicating perhaps that rather but these same areas have geologic evidence of broad zones in the western Great Basin are "un repeated Holocene movement (Swan and others, iformly" activated but only for geologically short 1980). The implication is that seismicity is periodic periods of time. The concept of zones of reactiva along the fault. Pavlis and Smith (1980) have tion that contain a number of faults involves still suggested that the Wasatch fault is segmented other considerations for determining temporal and and bounds independent crustal blocks along its spatial variation of seismicity. A suggestion was length. Possibly, block adjustments could control made that perhaps the NSZ has run the course the spatial character of the seismicity along the of a seismic cycle in the western Great Basin and, Wasatch fault. therefore, that large earthquakes should not be Arabasz and others (1980) provided an excellent expected there in the future. An objection was discussion of considerations involving seismic that large events could not be precluded on range- "gaps" along the Wasatch fault. They distilled the bounding faults next to those faults that have rup arguments down to the fact that if seismicity is tured in historic time. Also, how likely are areas periodic along the Wasatch fault, the seismic cycle outside the NSZ but still within the area of Holo has yet to be defined with any confidence. Avail cene faulting to be reactivated in the near future? able geologic data are not adequate to determine Discussions at the workshop did not resolve any whether seismic potential is increasing or decreas of the myriad questions raised through considera ing along individual segments of the fault. tions of spatial and temporal variations of seismic The primary seismic zonation of the Basin and ity in the western Great Basin. The source zones Range region has been on the basis of broad re (fig. 3) do not represent any special consideration gions that are relatively homogeneous with re along the Nevada Seismic Zone. spect to the age of most recent surface faulting Along the Sierra Nevada-Great Basin boundary within certain broad age categories (Holocene, no zone (SNGBZ), west of the Nevada Seismic Zone, Holocene/late Pleistocene, no late Quaternary). Ryall and Van Wormer (1980) noted that existing The zones can be inferred to represent distinctive seismic hazard maps do not show as high a hazard regional variations in the rate of occurrence of as fault-scarp studies, rate of uplift, and instru large (M>7) earthquakes as averaged over mental seismicity indicate (see Slemmons and thousands of years (Bucknam and others, 1980). others, 1979; Bell and Slemmons, 1979; Sanders The approach used does not preclude the presence and Slemmons, 1979). This area is of considerable of a younger fault within a region of predomin significance as it includes the Reno-Carson City antly older faults. Individual faults can be defined area which is the most heavily populated area in as source zones where the details of their late western Nevada. Ryall and Van Wormer (1980) Quaternary history are known well enough to as have demonstrated reasonably the low estimation sign a specific magnitude and recurrence interval of ground acceleration hazard along the SNGBZ to them. The recurrence estimates in table 1 are in existing hazard maps (Algermissen and Perkins, minimum estimates. The recurrence estimates of 1976; Applied Technology Conference, 1978) and events assumed to be M>7 are made on what is therefore have called for a reconsideration of these thought to be a nearly complete record of these hazard maps for that region. events to the extent that the events are preserved The Wasatch fault at the east margin of the in the geologic record as fault scarps. Basin and Range province in Utah poses two prob lems that were discussed at the workshop. First, DESCRIPTION OF INDIVIDUAL SOURCE ZONES the entire fault zone has evidence of late Quater nary displacement, and at two sites shows evi For documentation of the ages of faulting in dence of repeated Holocene movement (Swan and western Utah and northeastern Nevada, the others, 1980). These Holocene displacements reader is referred to figure 2 on which is outlined areas of I°x2° topographic quadrangle maps in commun., 1980; T. P. Barnhard and R. C. which morphometric scarp studies have been made Bucknam, written commun., 1980; Bell, 1981). The (Bucknam and Anderson, 1979b; Anderson and seismic source zones as defined at the workshop Bucknam, 1979; R. L. Dodge and others, written are shown in figure 3. Table 1 lists estimated 120° 115° 110°
100 200KILOMETERS
FIGURE 3. Seismic source zones of the Great Basin. Numbers refer to descriptions of seismic source zones given in text. Unconnected lines indicate that boundary continues beyond the area considered at the Great Basin workshop. Dashed line indicates geographical extent of the region considered in the workshop. TABLE 1. Estimated maximum magnitudes and recurrence the structural style in this area.) The northern rates from geologic data for earthquakes Ms>7 in the part of the boundary extends to the west side of Great Basin seismic source zones the White Mountains and then joins the northern [Ms, surface wave magnitude; leaders ( ) indicate that no value was given at end of Fish Lake Valley. the workshop] Zone 7. This zone includes the Nevada Seismic Zone (NSZ) which, except for the historic activity, does not appear to have been more active in Holo- Zone No. Estimated Estimated geologic (from maximum rate for earthquakes cene or late Quaternary time than the rest of the fig. 3) magnitude M >7.0 given as region. The zone boundary represents a region (Mg ) number of events per 10 years per 10 within which Holocene faulting is widely distrib square kilometer uted. Note, though, that determination of faulting 1 6.5 as Holocene outside the region studied by Wallace 2 (1979; see fig. 1) is largely subjective and is not 3 7.0 4 well documented. The rate of occurrence of mag 5 7.5 nitude 7-7.5 events within Wallace's study area 6 8.0 7 7.5 "l~T, 2 3.4 is 3.4/104 yr/104 km2. Wallace believed that the 8 __ density of Holocene age scarps in his study area 9 7.5 10.7 is at least as great, or perhaps greater, than that 10 11 7.5 *2.4 of the entire region. The rate of faulting, there 12 fore, may represent an upper bound. The small 13 7.75 14 branch of zone 7 extending from western Nevada
For western Utah zones from Bucknam and others (1980). into northeastern California includes the Walker o For Wallace (1977) study area shown on figure 1. Lane in zone 7. Holocene age faulting is recog nized along the Walker Lane (J. W. Bell, written maximum magnitudes and geologic recurrence communs., 1981, 1982). rates for events M>7.0 in the zones. The significance of the Nevada Seismic Zone re Zone 1. A zone of late Quaternary faulting lo ceived considerable discussion but was not re cated along the western flank of the Sierra solved definitively. Ryall argued that the occur Nevada. rence of major events in the NSZ has lowered the Zone 2. A zone of no late Quaternary faulting. likelihood of large earthquakes in the rupture Zone 3. A zone of late Quaternary faulting zones of these events (Ryall, 1977; Ryall and Van north of the Sierra Nevada. Wormer, 1980). In general, the group believed Zone 4. Central Sierra Nevada zone. Late Ce- that the concept was of merit but some would pre nozoic deformation in this zone is restricted to fer to show some indication of the high rate of broad uplift and westward tilting in the northern historic seismicity. One participant believed that part with more intense warping and tilting in the large events cannot be precluded on other major southern part (Christiensen, 1966). faults adjacent to the zone of large historic events, Zone 5. This zone trends northerly along the such as the west side of the Stillwater Range. The Sierran front along a series of en-echelon warps point clearly was not resolved, although there was and faults to the vicinity of Reno, Nev. A basis not strong pressure to show the Nevada Seismic for the western boundary was poorly defined; in Zone as a separate zone. The map was left with a general way, the zone encloses the western ex much of western Nevada as a single zone. tent of the en-echelon segments. The northwest ern and southeastern boundaries of the zone were Zone 8. A zone of no late Quaternary faulting. supplied by J. W. Bell (written commun., 1981). Zone 9. This zone is defined on the basis of Zone 6. This zone includes Owens Valley, widespread faults of late Quaternary age. Holo Panamint Valley, and Death Valley. The boundary cene faulting is not known but may be present. within the Sierran block has been drawn along the Faults here tend to be short; Wallace believed that "frontal fault" to the vicinity of Bishop, Calif. the maximum magnitude expected for this region From Bishop north, the Sierran front diverges shoald be less than that of zone 6 to the west. from a linear north-northwest trend and steps "Zone 10. A zone of no late Quaternary fault westerly on a series of warps and en-echelon ing. (Zones 10 through 14 are from Bucknam and faults. (See Bateman, 1965, for a good account of others, 1980.)
8 Zone 11. A zone of Holocene faulting. zoning on the spatial extent of different ages of Zone 12. A zone of no Holocene, but late Pleis latest displacements as is true for the Basin and tocene faulting. Range province. Instead, the spatial extent of a Zone 13. Wasatch fault, multiple Holocene particular age of faulting (known from specific movement. faults within a distinctive structural region) is in Zone 14. A zone of Holocene faulting. ferred by assuming that the age of faulting is characteristic of a region of similar structure. Em phasis is placed on the spatial distribution of his NORTHERN ROCKY MOUNTAINS toric activity in drawing only one zone zone 21. SEISMIC SOURCE ZONES SUMMARY The remaining zones generally conform to distinc OF WORKSHOP CONVENED tive structural terranes under the assumption that DECEMBER 6-7, 1979 distinctive structural terranes also have different seismotectonic characteristics that govern pres BY F. A. McKeown, D. C. Ross, and P. C. Thenhaus ent-day seismicity.
PARTICIPANTS Considerable discussion was devoted to the sig nificance of the Lewis and Clark line (fig. 4). Fea F. A. McKeown ...... U.S. Geological Survey (USGS) (Moderator) tures defining the lineament are the St. Marys D. C. Ross ...... USGS fault trend and parallel faults to the south. These (Recorder) faults extend across western Montana and into S. T. Algermissen ...... USGS Idaho (Witkind, 1977). There is evidence of right R. C. Bucknam ...... USGS lateral movement of Pleistocene age on the St. J. M. Dewey ...... USGS M. D. Kleinkopff ...... USGS Marys fault; sand boils and faulted talus cones are L. C. Pakiser ...... USGS evident along its trace. Some strike-slip movement D. M. Perkins ...... USGS of similar age on faults paralleling the St. Marys T. A. Qamar ...... University of Montana trend is suspected. The zone bends to the south M. W. Reynolds ...... USGS near Helena where there has been historic seis W. P. Rogers ...... Colorado Geological Survey R. G. Schmidt ...... USGS micity (Eppley, 1965; Stickney, 1978). The 1935 P. C. Thenhaus ...... USGS earthquake had a strike-slip focal mechanism C. M. Wentworth ...... USGS (Smith and Sbar, 1974) and is consistent with (Observer) geologically inferred type of fault movement. Par- I. J. Witkind ...... USGS dee (1950) stated that the 1935 series of earth quakes (M=6.25, M=6.7) at Helena, Mont., proba WORKSHOP SUMMARY bly were associated with a fault at the south end of Prickly Pear basin as indicated by surface ef Systematic regional mapping of Quaternary fects and locations from instrumental records. He faults in the Northern Rocky Mountains region noted, though, that no surface displacement was has not been done. However, Witkind (1975a, b, found from the earthquakes. Pardee (1926) located 1976) has summarized information from many the 1925 event (M=6.75) 80 km southeast of sources on late Cenozoic faulting. Pardee (1950) Helena. This location coincides with a projection mapped displacements of range-bounding faults in of the St. Marys trend to the southeast. Freidline western Montana of late Tertiary to recent age, but many of these scarps need to be reevaluated and others (1976) defined a northwest-trending in light of the new understanding of fault-scarp zone of seismicity between Helena and Marysville morphology and methods of study (Wallace, 1977; that had both strike-slip and normal faulting indi Bucknam and Anderson, 1979a, b). If a systematic cated by focal mechanisms. They suggested an as search were made for young faults, probably many sociation on a regional scale between seismicity more would be found, and ages of faulting could and northwest-trending zones of weakness in the be revised on a number of known faults. basement. One such zone, the Lake Basin linea Because of this lack of regional faulting informa ment, coincides with a possible extension of the tion, workshop participants adopted a zoning Lewis and Clark line to the southeast and reasona rationale based on information on the age of bly might extend the structure into the northern youngest faulting within regions of similar tectonic parts of Wyoming. Stickney (1978) also noted a setting and structural style. This approach is not northwest trend in earthquakes of magnitude
9 116" 114' 112' 110' 108* 108° 104° 102°
FIGURE 4. Index map of the geographic region addressed at the Northern Rocky Mountains seismic source zone workshop showing selected topographic, geologic, and geographic features that are referenced in the text. Dashed line indicates geographical extent of the region considered in the workshop. greater than 2.5 recorded between July, 1974, and preexisting zones of weakness that bound crustal March, 1977. The trend is about 80 km wide and blocks. extends from Helena, Mont., to Flathead Valley. The committee considered the possibility of the Fault-plane solutions and hypocenter distributions Lewis and Clark line being a separate source zone. in that part of the trend covered by the Helena The lack of late Quaternary faults north of the seismograph array indicate earthquake swarms on line suggests that faults there are less recently northwest-trending normal faults and northeast- active than are those near Helena; although exten- trending oblique-slip faults. He noted that the sional tectonism has affected both areas north and lengths of active, continuous faults are short (less south of the line, it is much more impressive to than 10 km), which suggests faulting along the south. Also, gravity and magnetic signatures
10 differ on either side of the line. An interesting ous east-west strike as compared with the north fact is that the Marysville geothermal area lies west trends of Laramide-age structures. Geophys along the Lewis and Clark line near Helena. Such ical evidence indicates that there is a distinct dif geothermal anomalies may be indicative of a deep ference in crustal thickness north and south of the crustal flaw that provides an avenue for the ascent trend; therefore, the Uinta trend may be a funda of hydrothermal fluids. However, the lineament mental crustal structure. Geophysically, the Uinta itself is not clearly a primary source for historic trend can be traced eastward into Colorado to the earthquakes. Areal rates of seismic activity to the south end of the Park Range, near Steamboat north in western Montana appear to be higher. Springs. There, the Neogene-age structures of the Other topics of significant discussion were the northern extension of the Rio Grande rift swing Snake River Plain, the Uinta Mountains, and the west along the Uinta trend (Tweto, 1979), indica seismic source zones of Wyoming. ting perhaps a fundamental influence on younger In southeast Idaho, formation of the Snake tectonism. Quaternary faulting has been noted on River Plain has progressed from southwest to a north-south oriented uplift extending south from northeast since Miocene time, and perhaps is re the Uinta extension in Colorado (M. West, oral lated to the gross movement of the North Ameri commun., 1980). Although the Uinta trend appar can plate over a mantle thermal anomaly (Suppe ently has influenced the structure of northeastern and others, 1975; Smith, 1978; Christiansen and Utah and northwestern Colorado for a long span McKee, 1978; Armstrong, 1978). The Yellowstone of time, its influence on present-day seismicity is area now forms the advancing front of tectonism unclear. Lack of seismological and other support to the northeast. Strong northwest-oriented grav ing evidence precludes the identification of any ob ity gradients in the older, thicker western part vious relationship. of the plain indicate that perhaps the plain has Other discussions centered around the source been superimposed over older northwest-trending zones of Wyoming. Witkind (1975a) has noted that Basin and Range structures. These structures are ages of latest displacements of east-trending faults evident both north and south of the basalt cover in central Wyoming are Pliocene. Similar east- of the plain. Holocene "pull-apart" structures form trending faults occur in the southern Bighorn an arcuate pattern across the plain so as to follow Mountains and in the Owl Creek Mountains. Zones the grain of the Basin and Range structures to 26 and 27 were drawn on this basis, although there the north and south (Prinz, 1970). These "pull- was considerable argument as to whether a Plio aparts" show no apparent lateral or vertical dis cene age of latest displacements is significant to placement across them (M. A. Kuntz and H. R. present-day seismicity. One point of view holds Covington, oral commun., 1979). The relatively that, regardless of the ages of tectonism, the area warmer, younger, eastern part of the Snake River of most recent tectonism in a region should be con Plain has been aseismic in historic time. The aseis- sidered the most hazardous. Another point of view mieity may be due to the young crust being unable holds that any tectonism older than Quaternary to store enough elastic energy for the generation is, by itself, irrelevant to the relative hazard of earthquakes. The older western part of the among areas. The argument was not resolved plain, however, has a thicker crust (see Hill and clearly, although the zones defined by Pliocene others, 1961; LaFehr and Pakiser, 1962; Hill, 1963; faulting were retained. Hill and Pakiser, 1967). At Shoshone, Idaho, on An alternative zoning procedure suggested by the western part of the plain, (fig. 4) an intensity VII was felt from an event occurring on some committee members was that the large November 11, 1905. Greensfelder (1976) indicated Laramide-age basins be separated from a large that the epicenter probably was located within 24 zone encompassing the central Rocky Mountain re km of Shoshone. Not known is what structure may gion. have been the earthquake source. In the workshop R. B. Smith (written commun., 1979) recom there was a general consensus that the western mended an east-trending zone from Norris Geyser part of the plain has a higher potential for earth Basin in Yellowstone National Park west to quakes than the eastern part because of the differ Hebgen. This zone would include the Centennial ent crustal rheologies. Valley frontal faults and other east-trending tec Discussion of the Uinta trend centered around tonic features along the southern Montana State its eastern margin. The structure has an anomal line.
11 DESCRIPTIONS OF INDIVIDUAL SOURCE ZONES Zone 24. Western Snake River Plain. The western plain has an older, thicker surface than The following seismic source zones are shown the eastern part of the plain. in figure 5. Table 2 lists estimated maximum mag Zone 25. Eastern Snake River Plain. The sur nitudes and estimated relative recurrence times of the maximum magnitudes of earthquakes for face is generally 20,000-50,000 years old (based the individual zones. on studies of undisturbed loess) and has young vol canic features superimposed upon it. Limited Zone 15. This zone is ambiguous as it is de breakage consisting of pull-apart structures with fined primarily by its contrast with the surround no apparent displacement (Prinz, 1970; M. A. ing zones. On the southwest, the zone boundary Kuntz, and H. R. Covington, oral commun., 1979) is drawn along the north edge of the Columbia is evident on this surface. The breaks form an ar Plateaus province. On the west, the boundary sep cuate pattern across the plain (Printz, 1970). The arates this zone from the Cascade Mountains of aseismicity may be attributable to a warm, thin north-central Washington. sialic crust incapable of sustaining earthquake Zone 16. This zone encompasses a northwest- stresses. trending group of faults that had pre-middle Pleis Zone 26. This zone encompasses the Bighorn tocene movement. It coincides with an area of Mountains, Absaroka Range, Wind River Range, Basin and Range type normal faulting that may and the Owl Creek uplift of northwestern Wyo be a continuation of the Rocky Mountain trench ming. The zone excludes the east-trending faults into Montana (Mudge, 1970). that terminate each of these ranges to the south. Zone 17. The disturbed belt of western Mon Zone 27. This zone includes the east-trending tana. The zone is characterized by large imbricate faults of central Wyoming that cut late Pliocene thrust sheets of the Northern Rocky Mountains but not Pleistocene sediments in the Picket Lake (Mudge, 1970). area. Faults in the southern Bighorns and the Owl Zone 18. Idaho batholith exclusive of the Chal- Creek area have much the same pattern. The area lis geothermal area (zone 21). of east-trending faults may be potentially more Zone 19. This zone has a structural style simi hazardous than the rest of Wyoming (see previous lar to that of zone 20 and similarly, has Quater discussion), even though youngest documented nary faults. However, the rate of seismic activity movement is Pliocene. is noticeably lower than in zone 20 to the east, Zone 28. The boundary of this zone is poorly or in zone 16 to the north. constrained, but structural style here is different Zone 20. This zone is characterized by mixed from that of the Snake River Plain to the north. east-, north-, and northwest-trending structure The north-northwest-trending faults in the zone and both normal and strike-slip focal-plane solu apparently had no late Quaternary'movement. tions (Smith and Sbar, 1974; Qamar and Hawley, Zone 29. Faults in this area have the same 1979). Basin and Range character as those north and Zone 21. Challis geothermal area. This region south of the Snake River Plain. Southeast Idaho is characterized by swarm events that typify areas has well-developed extensional tectonic features. of high geothermal anomalies. Faults are present This north-south-trending, normal extensional in Eocene volcanics. The petrology of the volcanic faulting extends south past the west end of the and plutonic rocks is similar to that of the rocks Uinta Mountains in Utah. in the Yellowstone area and may indicate that the Zone SO. No distinction was made in this zone Challis area is an old analog to the Yellowstone between age of faulting or tectonic style. area. Zone 31. Bear Lake area (Utah). Age of fault Zone 22. Trenching studies in this zone ing is Holocene. On the basis of scarp heights of (Malde, 1971) indicated faulting from Pleistocene more than 1 m and on lengths of fault segments, to at least 10,000 years before the present. magnitude 7 + earthquakes can be expected in this Zone 23. This zone is the volcano-tectonic area zone. of Yellowstone National Park. The zone has a high Zone 32. Wasatch fault zone. Multiple Holo heat flow and may have a low maximum mag cene offsets are evident along the fault. nitude. Zone 33. Uinta Mountains. Geophysical data
12 115°
40° -
0 100 200 MILES 11 i x 0 100 200 KILOMETERS
FIGURE 5. Seismic source zones of the Northern Rocky Mountains region. Unconnected lines indicate a continuation of the boundary beyond the area considered at the workshop. Dashed line indicates geographical extent of the region considered in the workshop.
13 TABLE 2. Estimated maximum magnitudes (Mg) and estimated relative recurrence of maximum magnitude events for the source zones of the Northern Rocky Mountains region [Mg, surface wave magnitude; leaders ( ) indicate that no value was given at the workshop]
Zone No. Estimated Estimated Zone No. Estimated Estimated (from maximum relative (from maximum relative fig. 5) magnitude recurrence fig. 5) magnitude recurrence (Mg ) (Mg )
15 6.0 C 28 16 6.5-7.0 B 29 6.5-7.0 B 17 6.0 C 30 6.0 D 18 5.0 D 31 7.0+ A 19 7.0 B 32 7.75 A+ 20 7.5 A 33 7.0 C
21 6.0-6.5 B 34 6.5-7.0 A- 22 7.5 B 35 6.5 C-D 23 6.5 A 36 6.5 C 24 6.5 C 37 6.5 C 25 6.0 C-D 38 7.0-7.5 A-B 26 6.0 C 39 6.5 27 6.0 C
A relative scale agreed upon by committee. A+ is assigned to the Wasatch fault zone as the zone of highest likelihood of seismic events of maximum estimated magnitude during a 50-year exposure time. Decreasing in likelihood are categories A, B, C, and D. The following numerical values were assigned by committee consensus: A=40, B=10, C=3, and D=l. In other words, a zone rated A would have 40 times higher likelihood of a maximum estimated magnitude earthquake than a zone rated D within the return period. indicate different crustal thickness north and Range. Quaternary-age fault displacements are south of an eastward extension of the Uinta Moun evident in the zone. tains. Also, Neogene structures bend westward Zone 37. This zone is the Front Range of the along an extension of the Uinta Mountains to the Northern Rocky Mountains. Quaternary-age fault south end of the Park Range in Colorado. The Rio displacements are evident in the zone. Grande rift structures to the south terminate at this eastward extension. Historic seismicity pro vides no compelling evidence that these features SOUTHERN ROCKY MOUNTAINS control present-day seismicity. SEISMIC SOURCE ZONES SUMMARY OF WORKSHOP CONVENED Zone 34. This zone encompasses a north-south- JANUARY 23-24, 1980 trending group of normal, extensional faults that terminate the Colorado Plateaus to the northwest. By R. E. Anderson, W. P. Irwin, and P. C. Thenhaus The faults extend beyond the west end of the Uinta trend. PARTICIPANTS Zone 35. This zone is defined by the Mullen R. E. Anderson ...... U.S. Geological Survey (USGS) Creek-NashiFork shear zone (Houston and McCal- (Moderator) lum, 1961) that juxtaposes strongly contrasting W. P. Irwin ...... USGS Precambrian rocks. This feature is a fundamental (Recorder) basement trend that may represent an old plate S. T. Algermissen ...... USGS boundary. The zone includes a possible extension E. H. Baltz ...... USGS R. C. Bucknam ...... USGS of the feature to the northeast, the Hartville fault J. W. Dewey ...... USGS (Droullard, 1963). S. M. Dubois ...... Arizona Bureau of Geology and Mining Zone 36. This zone is a subdivision of the Park Technology Range at the north end of the Sangre de Cristo L. H. Jaksha ...... USGS
14 M. N. Machette ...... USGS rift. Earthquakes of this large size probably are F. A. McKeown ...... USGS reported completely from 1869 and therefore W. R. Muehlberger ...... University of Texas at Austin should be free of biased reporting due to non- D. M. Perkins ...... USGS W. P. Rogers ...... Colorado Geological Survey uniform population density. The period 1962-1977 A. R. Sanford ...... New Mexico Institute of Mining and is one of low seismicity compared with the seismic Technology ity of previous years. Sanford and others (1979) G. R. Scott ...... USGS concluded that the seismicity in the rift is episodic, P. C. Thenhaus ...... USGS related perhaps to episodic spreading of the rift. 0. G. Tweto ...... USGS M. West ...... Water and Power Resources Service Sanford and others (1981) noted that on the Col R. L. Wheeler ...... USGS orado Plateau in New Mexico, seismic activity is relatively high along the eastern and northwestern WORKSHOP SUMMARY margins of the San Juan basin. Also, an epicentral trend crosses the southern part of the Colorado As in the Northern Rocky Mountains, systema Plateau and extends through the Rio Grande rift tic morphometric studies of fault scarps have not into the High Plains of northeastern New Mexico. been made throughout the Southern Rocky Moun The trend is coincident with a trend of Pliocene tains. The age-of-faulting information that is avail to Pleistocene volcanic centers that define the able has been assumed to hold throughout distinc Jemez lineament. tive tectonic areas, the boundaries of which in DuBois and Smith (1980) recently have pub cluded consideration of basin and mountain blocks, lished the results of their investigation of the 1887 alinements of volcanic centers, possible buried (estimated M=7.2) Sonoran earthquake. The epi magma chambers, basement configuration, and center was about 60 km south of Douglas, Ariz., physiography. in the State of Sonora, Mexico. Surface rupturing Expression of late Paleogene and Neogene tec extended about 50 km north of the epicentral area. tonic movements in reactiviated older structures A critical task for seismic zoning is to define the is widespread in the Southern Rockies (Tweto, area over which a similar event might reasonably 1979; Kirkham and Rodgers, 1978, 1981). The be expected in the future. Sumner (1976) has most impressive tectonic movements, however, noted a seismicity trend that extends from the are associated with the formation of the Rio area of the Sonoran event northwestward across Grande rift (see fig. 6 for location). The structure the San Francisco volcanic field and into the west of the Rio Grande rift and its associated seismicity ern Grand Canyon region of Arizona (fig. 6). How therefore dominated the discussions of seismic ever, it remains to be established that seismic po source zones in this region. tential is similar along the entire seismicity trend Although significant faulting has occurred with that includes such a diversity of geologic struc in the rift in late Quaternary time, the low level ture. The workshop participants chose a geologic of recent seismicity within the rift does not appear approach to zoning this area as described in the to represent sustained long-term regional tectonic following descriptions of source zones. activity. This observation has prompted Cordell (1978) and Sanford and others (1979) to suggest that the current low level of seismicity is not rep DESCRIPTION OF INDIVIDUAL SOURCE ZONES resentative of the long-term trends. From micro- seismic data collected in recent years, Sanford and The seismic source zones as they were defined others (1979) noted that the current seismicity at the workshop are shown in figure 7. Table 3 level in the rift is comparable to those levels of lists maximum estimated magnitudes and relative the neighboring Colorado Plateaus and High recurrence estimates of the maximum magnitudes for the zones of the Southern Rocky Mountain re Plains provinces. Sanford and others (1979) have gion. related much of the current microseismicity to Zone 38. Sangre de Cristo Range. The zone movement of magma in the crust. However, they contains the Sangre de Cristo fault that bounds noted that the historic record of earthquakes from the San Luis Valley on the northeast. The fault 1849 through 1961 indicates that the Rio Grande may be potentially active (Kirkham and Rogers, rift was by far the most active area in New 1978, 1981). Mexico. Their listing of six intensity VII events Zone 39. Uncompahgre Plateau and San Juan shows that all these events occurred within the volcanic field. The northeastern and southwestern
15 102
SAN FRANCISCO VOLCANIC FIELD
FIGURE 6. Index map of the geographic region addressed at the Southern Rocky Mountains seismic source zone workshop showing selected topographic, geologic, and geographic features that are referenced in the text. Dashed line indicates geographical extent of the region considered in the workshop. flanks of the Uncompahgre Plateau are structur the southwestern flank (Kirkham and Rogers, ally complex. On the northeast flank, only one 1978). fault can be proved to have moved in Quaternary Zvne 40. Golden fault. Kirkham (1977) time (Kirkham and Rogers, 1978) although other documented evidence of recurrent Quaternary dis faults are suspect. Quaternary faults also exist on placement on the fault. If the small size of the
16 110° 105°
40
35'
100 200MILES 30' 0 100 200 KILOMETERS
FIGURE 7. Seismic sources zones of the Southern Rocky Mountains region. Unconnected Unes indicate that boundaries extend beyond the area considered at the workshops. Queried lines indicate uncertainty that the zone is a significant seismic source. Dashed line indicates geographical extent of the region considered in the workshop. zone limits its usefulness to seismic zoning, it Zone 41. Rampart Range fault. Delineation of could be deleted in favor of a larger zone of this zone is based on evidence of Quaternary dis- Quaternary faulting that covers all the Colorado placement along the fault (Kirkham and Rogers, Front Range. 1978, 1981).
17 TABLE 3. Estimated maximum magnitudes (MJ northern Albuquerque basin. Although systematic and estimated relative recurrence of maximum morphometric studies have not been made, wide magnitude events for the source zones of the spread, easily recognized scarps in the Trans- Southern Rocky Mountains region Pecos area (Muehlberger and others, 1978) [Ms, surface wave magnitudes; leaders ( ) indicate that no values suggest late Quaternary faulting there also. Jus were given at the workshop] tification for the placement of the zone boundaries comes, in part, from recognition of the Rio Grande Zone No. Estimated Relative rift as a series of structurally integrated basins (from maximum recurrence that have an overall integrity and continuity im fig. 7) magnitude rate 1 posed across a wide variety of older structures. (Mg ) The boundaries of the Trans-Pecos part of the 38 7.0-7.5 A zone are determined by the distribution of a coher 39 6.5 D ent system of north- and northwest-trending 40 scarps. Linear dry lakes in that region parallel the 41 scarp trends and suggest perhaps a Holocene 42 6 C 43 7.0-7.5 B structural control possibly by faulting. 44 7.0-7.5 A Zone 44. Evidence for Holocene faulting in 45 this zone is mainly from studies by Michael Machette who has found evidence for offset of Hol 46 6.0 C ocene deposits of as much as 6 m on the La Jencia 47 6.0 C 48 fault a major basin-margin structure. The fault 49 7.0-7.5 A shows evidence of young rupture for 35 km but 50 7.0-7.5 A not all the rupture may be related to a single seis 51 7.0-7.5 D mic event (Machette, 1980). A. R. Sanford (oral 52 6.5-7.0 B commun., 1980) noted that seismic activity has not 53 5.5-6.0 D been detected along the La Jencia fault and that 54 6.0 D a geodetic network installed across the fault by J. Savage showed no indication of extension. Relative recurrence is based on a Zone 45. Delineation of this zone is based on dimensionless scale and indicates a a lack of evidence for late Cenozoic faulting in the subjective likelihood of a zone having central part of the Colorado Plateaus province. the maximum magnitude earthquake take place in a 50-year period of interest. Zone 46. This zone is the Jemez lineament west of the Rio Grande rift. The zone is drawn Zone 42. Jemez lineament, east of the Rio on an alinement of Pliocene-Holocene volcanic cen Grande rift. The zone includes the Cimarron and ters that extends diagonally across Arizona and Raton-Clayton-Capulin volcanic fields. In terms of New Mexico. magnitude and frequency of earthquakes, this Zone 47. Delineation of this zone is based on zone is inferred to be similar to zone 48. evidence of minimal faulting in Pleistocene lavas Zone 43. This is an elongate zone that follows in the San Francisco volcanic field (E. Wolfe and the trend of the Rio Grande rift and includes the G. Ulrich, unpub. data). A few faults in the north Trans-Pecos area of west Texas. Evidence for late ern part of the volcanic field may have moved dur Quaternary faulting does not exist everywhere in ing late Quaternary time, but they average only this zone, but in some parts the evidence is wide about 2 km in surface trace and have small dis spread. In the Albuquerque-Belen basin, Bachman placement. In general, there is a conspicuous lack and Machette (1977) have identified a widespread of large-magnitude faulting associated with vol- geomorphic surface that is 5-6 xlO6 years old. This canism in the San Francisco field. Infrequent large and younger surfaces are displaced by perhaps a seismic events probably have an upper-bound hundred faults including scarps along about 50 km magnitude of 5.5-6.0. of fault trace at the eastern margin of the Al Zone 48. Delineation of this zone is based on buquerque basin. Detailed studies by Machette a lack of evidence for late Quaternary faulting (1978) indicated at least four displacement events within a tectonic province that was deformed by during Quaternary time on a single fault in the late Cenozoic faulting.
18 Zone 49. A recently published study by CENTRAL INTERIOR SEISMIC SOURCE DuBois and Smith (1980) examined about 90 per ZONES SUMMARY OF WORKSHOP cent of the historic record of the 1887 Sonoran CONVENED JUNE 10-11, 1980 earthquake (estimated M=7.1). The earthquake produced surface breakage over a distance of By F. A. McKeown, D. P. Russ, and P. C. Thenhaus about 50 km. Studies of young faulting in that area PARTICIPANTS are in progress. Geomorphic evidence suggests the possibility of four events on the 1887 earthquake F. A. McKeown ...... U.S. Geological Survey (USGS) fault, but whether a prehistoric Holocene event (Moderator) occurred is not known. The zone is extended into D. P. Russ ...... USGS Arizona on the basis of the possibility of Holocene (Recorder) S. T. Algermissen ...... USGS surface faulting in southeasternmost Arizona ac R. E. Anderson ...... USGS cording to R. C. Bucknam and S. M. DuBois (oral R. C. Bucknam ...... USGS commun., 1980). T. C. Buschbach ...... Nuclear Regulatory Agency Zone 50. Evidence for Holocene faulting in A. J. Crone ...... USGS this zone is based mainly on unpublished studies D. W. Gordon ...... USGS by L. Gile, who first demonstrated the presence R. M. Hamilton ...... USGS T. G. Hildenbrand ...... USGS of mid-Holocene faulting, and by Machette (1980) A. C. Johnston . Tennessee Earthquake Information Center who has found evidence for 3-4 m of displacement M. F. Kane ...... USGS where the Cox Ranch fault trace crosses Holocene G. R. Keller ...... University of Texas at El Paso fans. The mid-Holocene scarp is locally 9-10 m F. J. Mauk ...... University of Michigan high and the fault shows evidence of breakage for 0. W. Nuttli ...... St. Louis University D. M. Perkins ...... USGS at least 25 km (W. Seager, written commun., K. M. Shedlock ...... USGS 1980). Faults other than the La Jencia and Cox R. G. Stearns ...... Vanderbilt University Ranch in zones 44 and 51 have "reported" but un P. C. Thenhaus ...... USGS documented Holocene displacement. R. L. Wheeler ...... USGS Zone 51. This zone includes an area of the WORKSHOP SUMMARY Trans-Pecos, Texas, for which no fault scarps cut ting Quaternary units have been identified but A consensus was that the central United States which has major late Cenozoic faults cutting rocks zones should be defined on patterns of historical of early Oligocene age and older. It is similar to seismicity except those areas for which geological zone 49. or geophysical knowledge allow delineation of Zone 52. This,zone includes the terrane com zones on the basis of deep structure. Recent analy monly referred to as the western Grand Canyon sis of reflection profiles by USGS investigators has region in the transition zone between the Basin resulted in the identification of several deeply and Range and Colorado Plateaus provinces. The buried faults believed to be active. For the most location of the boundary is not narrowly con part, however, little is known about buried faults strained. Evidence for Quaternary faulting relates in the central United States. Therefore, diverse to (1) Quaternary lava flows that are offset by factors such as gravity, aeromagnetics, and widely spaced normal faults in the western Grand geologic province boundaries also were used to de Canyon region (Hamblin, 1970; Koons, 1945; An- lineate seismic source zones. derson, 1978), (2) offset of Quaternary alluvium A substantial part of the committee's discus in northwestern Arizona (I. Lucchitta, written sions was on the characteristics of source areas commun., 1980), and (3) displacement of Quater and possible causes of earthquakes of the central nary alluvium in Chino Valley (I. Lucchitta, oral United States. Subjects considered include the fol commun., 1976). lowing: Zone 53. Delineation of this zone is based on 1. The relation of seismicity to ancient rifts and lack of evidence for late Cenozoic faults. aulacogens. Zone 54. Delineation of this zone is based on 2. The relation of seismicity to plutons. Several lack of evidence for Quaternary faulting, although investigators have suggested that stress is concen the area was deformed in late Cenozoic time. trated near the contacts of plutons with country
19 rock because of differences in their elastic proper DESCRIPTION OF INDIVIDUAL SOURCE ZONES ties. The seismic source zones are shown in figure 3. The relationship of the Mississippi Embay- 9. Maximum estimated magnitudes and recurrence ment gravity field to thick masses of post-Paleozo estimates of the maximum magnitudes are listed ic sediments indicates perhaps that during late in table 4. Mesozoic time there were widespread intrusions Zone 55. This zone is the area of the largest of magma because of mantle upwarping. As these earthquakes to affect the central United States masses cooled, they contracted and subsided caus (Nuttli, 1973, 1979; Nuttli and Herrmann, 1978). ing downwarping of the embayment. Loading by It is also the area of most frequent moderate seis post-Paleozoic sediments caused further tectonic micity (Stauder, 1982). Although there is little evi perturbations. Questionable, however, is if con dence for surface faulting within the zone (Russ, traction on cooling would be sufficient to produce 1979), subsurface faults, believed to be seis- the net subsidence and if the isostatic phenomenon mogenic, have been found recently (Zoback and could operate in such a short time to produce the others, 1980). The zone has been assigned a observed geology (W. J. .Hinze, oral commun., maximum estimated magnitude (mb) of 7.5 (see 1980). Nuttli, 1973) and a recurrence time of 600-700 years (see Nuttli, 1974; Russ, 1979; Algermissen, 4. The enigma of the lack of moderate-size 1969, 1972). Zone 55 is defined as being generally earthquakes on large fault zones such as the Ste. coincident with the Reelfoot rift as identified by Genevieve (see fig. 8 for location). Some of these Hildenbrand and others (1977) and Hildenbrand fault zones seem to be suitably oriented to the and others (1980) on the basis of aeromagnetic east-west stress field to have movement occur in anomalies. The northwest and southeast bound them. Perhaps the question should not be why the aries of the zone are drawn on differences in the northwest-trending structures are inactive, but pattern and intensity of the magnetic field and are rather, why only the northeast-trending struc believed to be the location of the border faults tures are active. Although both directions are con of the rift. Concealed plutons lie along these jugate to an east-west stress field (and therefore boundaries and may control the occurrence and theoretically would have an equal opportunity to distribution of nearby seismicity (Kane, 1977; be active), the northeast-trending structures may McKeown, 1978). The southwest and northeast have a preferred structural fabric or other phys boundaries of the zone are not identified easily. ical characteristic that make them preferentially Although most of the seismicity associated with active. zone 55 does not extend south of about Marked 5. The suggestion of block uplift and tilting of Tree, Ark. Gat 35°30' N.) (Stauder, 1982), the rift the Ozarks as revealed by terrain relief studies. continues its geophysical expression to at least lat These studies suggest that the Ozarks may behave 34°30' N. At this location, the rift boundaries be as a block bounded by northwest-trending faults. come poorly defined and gravity data show a Uplift and tilting about a northeast-striking axis northwest-trending zone of many areally small but would depress the Mississippi Embayment, gener intense highs that are tentatively interpreted to ating and concentrating stress along preexisting be plutons. The northeast boundary of the gravity structural elements. Thus, modern stresses in the highs is nearly coincident with what traditionally embayment may be related more closely to the has been mapped as the buried Ouachita front. tectonics of the Ozarks than to ambient stresses The change in the character of the gravity is used associated with a drifting continental plate. Inter as the southwestern boundary of seismic source estingly, a projection of the lineament bounding zone 55 because recent studies throw doubt upon the southwest edge of the inferred block is approx the existence of the so-called buried Ouachita front imately coincident with the southwestern terminus (a boundary previously suggested as the south of seismicity within the Reelfoot rift zone. western limit of modern seismicity). The implica tion is that large earthquakes can occur along the 6. A possible relationship between seismicity entire length of the rift. The rift initially continued and the intersection of the rift and the Pascola farther south than lat 34°30' N. but burial or tec- arch. tonism has masked or destroyed the structure
20 f- ^7"
FIGURE 8. Index map of the geographic area addressed at the Central Interior seismic source zone workshop showing topographic, geologic, and geographic features that are referenced in the text. Dashed line indicates geographical extent of the region considered in the workshop. south of this location. The extension of the zone or is the result of spatial variation in seismic activ to the southwest invoked dissenting opinions be ity within the zone (that is, the zone presently cause the southwestern part has been much less is quiescent, but periodically may be active) re active in historic times relative to the northeast mains to be determined. Because of these uncer ern part. An alternative might have been to divide tainties and because the rift structure with which the zone into two parts, both having the same esti many of the earthquakes seem to be associated mated maximum magnitude, but with the south extends at least to the vicinity of Stuttgart, Ark., western part having a recurrence time about dou the committee decided that zone 55 should be re ble the 600- to 700-year estimate for the northeast tained as drawn. The northeastern boundary of ern part. The cause of the cessation of seismicity zone 55 is defined as the southwesternmost of a just south of Marked Tree, Ark., is unknown. series of prominent northwest-trending Whether it marks a change in subsurface structure aeromagnetic anomalies that truncate the rift
21 KENTUCKYy ^-^ VIRGINIA
NORTH TENNESSEE CAROL INA - V
ALA BAMA \ GEORG I A 100 200 MILES
0 100 200 KILOMETERS (\
FIGURE 9. Seismic source zones of the Central Interior. Unconnected lines extend beyond the region considered at the Central Interior workshop. Dashed line indicates geographical extent of the region considered in the workshop. about 15 km southwest of Paducah, Ky. The fault zone. Although Hildenbrand and others southwesternmost anomaly is believed to be along (1980) projected the rift a short distance northeast the subsurface extension of the Ste. Genevieve of the anomalies, the anomalies have been selected
22 TABLE 4. Estimated maximum magnitudes quakes are along the margin of the Fairfield basin and estimated recurrence rates far maximum (a flexural zone) and are not on Wabash Valley magnitude events for the seismic source zones faults. The estimated maximum magnitude (mb) of the Central Interior region for earthquakes in zone 56 is 6.5 and the recur [mb, body wave magnitude; leaders ( ) indicate that no values were given at the workshop] rence time is 1000 yr (see Nuttli and Herrmann, 1978). Zone No. Estimated Estimated Zone 57. This zone encompasses the St. Fran (from maximum recurrence cois Mountains and surrounding regions. Its fig. 9) magnitude (in years) boundary, however, is not based upon physiog (mb ) raphy or structure but rather on the spatial pat 55 7.5 ^00-700 tern of seismicity which takes the form of a ring 56 6.5 2 1000 surrounding the mountains (Nuttli, 1979). Earth 57 6.5 2 1000 quakes in the center of the ring are not as common 58 6.5 22000 59 5.5-6.0 as those along the perimeter. The southern part 60 of the zone has been extended to the south in 61 order to include a number of events in northeast 62 ___ Arkansas with 4.0 63 signed a maximum estimated magnitude (mb) of 6.5 and a recurrence time of 1000 years (Nuttli ^Frorn Russ (1979). 2From Nuttli and Hermann (1978). and Herrmann, 1978). Zone 58. This zone includes the St. Louis, Mo., as the source zone boundary because they repre area and much of south-central Illinois. It is sent a prominent shift in the orientation and char situated on the deepest part of the Illinois basin. acter of deep geologic structure and because the The zone is delineated, however, solely on the areas of intense modern seismicity are southwest basis of seismicity. Several events with of the anomalies. 5.0 23 includes elements of the Kentucky River fault movement) as faults in zone 55. However, faults zone, the Bryant Station-Hickman Creek fault and in zone 55 may not be related genetically to those the Rome trough. The zone has been extended to in the wedge-shaped area. Zone 55 faults are as the south to include the area of the Sharpsburg, sociated with the Reelfoot rift (Zoback and others, Ky., earthquake sequence of July-August, 1980. 1980) whereas faults in the Kentucky fluorspar Zone 61. Delineation of this zone is based en area may have formed during the uplift and subsi tirely on seismicity and includes most of northern dence of a regional dome in Pennsylvanian time Illinois and a small part of southern Wisconsin. (Heyl and Brock, 1961; Krausse and Treworgy, Earthquakes of 3.0 24 tions of W.E.T. Smith (1962, 1966), except that east which is in accord with the structural grain some of the recent epicenters seem to be more to the north of the region but which is discordant closely bunched in areas of high seismicity. The with the northeast structural trends in the vicinity closer bunching can be explained easily by imper of the earthquakes. The suggestion is that the fections in the earlier record. For small magnitude seismicity is controlled by older and deeper struc events, Dewey and Gordon (1980) noted that about tures that have little manifestation at the surface half of the instrumentally recorded earthquakes (Wheeler and Bollinger, 1980). that they have studied are in persistent source Another aspect of the association of geologic zones whereas the remainder are more than 20 conditions with seismicity is that earthquakes ap km from other earthquakes. pear to occur in crystalline rock; that is, in highly Temporal changes in activity have been noted metamorphosed or igneous rocks. This association in some areas. For example, the seismicity in east seems to be so in the northeast, except where sol ern Massachusetts in recent years was low with ution mining of salt in the overlying sediments is respect to that of the historic record. In examining involved (Fletcher and Sykes, 1977) or where the seismicity of southern New England, Shakal there are other manmade perturbations. A and Toksoz (1977) found that the seismicity was rationale (for example, Diment, 1980) is that the higher in the period 1725-1824 than in the follow sediments are too thin, too soft, or too decoupled ing 100 years. Also, there was little activity prior from the basement for sufficient strain to accumu to the Attica, N.Y. earthquake (I0=VIII) of 1929. late within them to produce significant earth In this sense, the border (PQ/ME) earthquake of quakes. Basement is exposed in much of the north 1973 (Wetmiller, 1975) also was something of a east including the Adirondack Mountains and most surprise. of New England and appears to be peppered with Problems with focal depths and focal-plane solu shallow earthquakes, judging from instrumental tions were reviewed. The problems pose a severe determinations and the prevalence of earthquake handicap in relating seismicity to geologic struc sounds which some would attribute to shallowness ture. Measurements of focal depths, and, to a of foci (Sbar and others, 1972; Anderson and somewhat lesser extent, focal-plane solutions are Fletcher, 1976). The fact remains, however, that of questionable reliability except in those areas some earthquakes seem to be of mid-crustal depth covered by dense networks, such as the Ramapo or deeper. Some have suggested (for example, fault area (Aggarwal and Sykes, 1978) (see fig. Sbar and Sykes, 1977; Acharya, 1980) that these 10 for location), Blue Mountain Lake (Sbar and are the regions where large earthquakes are likely others, 1972; Sbar and Sykes, 1977), Attica, some to occur. This suggestion is plausible but one that times (for example, Fletcher and Sykes, 1977; remains to be evaluated more fully. A hypothesis Herrmann, 1978), and La Malbaie (Leblank and was suggested relating seismicity to residual pore Buchbinder, 1977). For a seismotectonic interpre pressure and higher porosity in alkalic intrusives tation of seismicity in New England, the depth compared with rocks of normal alkali content. of the foci must be known whether it is 1 or 10 km or even deeper. Depths of hypocenters make DESCRIPTION OF INDIVIDUAL SOURCE ZONES a difference as to which structures might be causal. Ratcliffe (1971) outlined the intricate his tory of deformation in the area of the Ramapo The seismic source zones for the northeastern fault system, and, at the workshop, stressed the United States are shown in figure 11. Table 5 lists wide range of structures that could be seismogenic the estimated maximum magnitudes assigned to in the 1-10 km depth interval in which earth the zones. No recurrence estimates of these quakes have been observed (Aggarwall and Sykes, maximum magnitudes were made due to a lack 1978). of geologic data to support such estimates. A good illustration of the importance of reliable Zone 64. Offshore zone. The northwestern depth determinations comes from the studies of boundary of this zone corresponds roughly to the Bollinger and his collegues in Giles County, Va. western edge of zone 3 in a report of seismic haz Their detailed network reveals focal depths be ard for the east coast of the United States by Per- tween 5 and 25 km (Bollinger and Wheeler, 1980). kins and others (1980). The boundary roughly coin Moreover, the trend of epicenters is north-north cides with the western edges of deep Jurassic ba- 25 74° CHARLEVOIX SEISMIC ZONE" 46' TREAL g*f ----A _.. n^T i 44? \ < * r* 1DACK I » \ J BLUE MOUNTAIN \ / OLAKE ^RE . \JtLWT LOWWLLE ^ /1\ I ^cm8tj|FAUL- /" BOONVILLE M. r\IS\ _ F1 ^ \ GETTYSBUR x.Q&<;iNx^, FIGURE 10. Index map of the geographic area addressed at the Northeastern seismic source zone workshop showing selected topographic, geologic, and geographic features referenced in the text. Dashed line indicates geographical extent of the region considered in the workshop. sins (Klitgord and Behrendt, 1979) that underlie Grand Banks earthquake of 1929, which occurred the continental shelf and slope. about 700 km to the east of the area covered by Recent earthquakes near the Bermuda Rise the map, was east of the western edge of the Ju (Nishenko and Kafka, 1980) are a reminder that rassic basins. This earthquake was assigned a the oceanic plate is not inactive. Indeed, the magnitude of 7.2. Intensity IV effects were felt 26 75C 70° 65C CANADA 200 MILES 100 200 KILOMETERS \ FIGURE 11. Seismic source zones of the northeastern United States and adjacent Canada. Unconnected lines indicate that boundaries continue beyond the area considered at the workshop. Dashed line indicates geographical extent of the region considered in the workshop. in the United States about 1,000 km from the epi- ter magnitude 7 have occurred in this zone. The center. zone's historical and instrumental seismicity is as Zone 65. Charlevoix zone. Earthquakes esti- high as, or maybe exceeds, that of the New Ma- mated to have equalled or slightly exceeded Rich- drid region of the Mississippi Embayment. The 27 TABLE 5. Estimated maximum to the northeast and southwest of the Charlevoix magnitudes for seismic source zone, that are considerably less seismic than the zones of the northeastern United Charlevoix zone. However, these regions probably States and adjacent Canada are considerably more seismic than the [Leaders ( ) indicate that no value was given at the workshop] background zones to the northwest and southeast. Thus, a zone has been introduced that extends on the northeast from the lower St. Lawrence zone Zone No. Estimated (Basham and others, 1979), (off fig. 12 to the (from maximum northeast) to the southwest through the Char fig. ID magnitude levoix zone to the western Quebec zone. 64 Zone 67. Western Quebec-northern New York 65 7.5 zone. This zone is drawn essentially as outlined 66 6.0 by Basham and others (1979) who were careful 67 7.0 to state that the zone is "simply intended to encir 68 6.0 69 5.5 cle concentrations of seismicity." 70 6.5 Basham and others (1979, fig. 11) defined a zone 71 6.5 of higher seismicity within the western Quebec 72 6.5 zone (as they did within the Charlevoix zone) and 73 6.0 call it the "Gatieau Triangle" (from Forsyth, 1977). 74 6.0 75 5.0 Near the United States border it is about half the width of the western Quebec zone. Magnitude scale not The northwestern edge of the western Quebec specified at the workshop zone could be extended far to the northwest on the basis of the known seismicity and in accor zone is drawn essentially as presented by Basham dance with the highly speculative notions of Di- and others (1979). They noted that if the zone was ment and others (1980) and Muller and others drawn on the basis of distribution of microearth- (1980). These authors suggested that zones of high quakes (Leblank and others, 1973; Leblank and seismicity may occur along, or may be bounded Buchbinder, 1977) the zone would be smaller. The by, certain northwesterly trending lineaments on microearthquakes extend to depths of about 20 km gravity and magnetic maps. and appear to be confined to the Precambrian The western Quebec zone is terminated in crystalline rocks that dip southeasterly under Pa northern New York near the northern edge of the leozoic sediments and metasediments. Microearth- high Adirondack Mountains. quake data obtained during the past 4 years con Some of the earthquakes in the western Quebec firm the earlier observations. Stevens (1980) reas zone are rather deep. The ManisaM earthquake sessed the locations of the larger instrumentally of 1975 was assigned a depth of 17 km (Horner recorded earthquakes (1924-1978) and found them and others, 1978) and the St. Donat earthquake to fall largely in the zone of microseismicity (70 of 1978 a depth of 7 km (Horner and others, 1979). km long) with concentrations at both ends. On the Moreover, a systematic change of focal depth has basis of Dewey and Gordon's (1980) instrumental been suggested across the western Quebec zone relocations, the major shocks tend to occur near from upper-crustal depth north of Montreal to the northeastern end of the zone. mid-crustal depth north of Ottawa (Horner and Basham and others (1979, table 3) and Weichert others, 1979). The focal depths of some northern and Milne (1979, table 2) both used a maximum New York earthquakes are thought to be of mid- magnitude of 8.0 for their probabilistic studies. At crustal depth (Aggarwal and Yang, 1977). the workshop, a maximum magnitude of 7.5 was Zone 68. The Adirondack Mountains and envi suggested, apparently to represent more recent rons. Until recently the seismicity of the Adiron- thinking about the maximum size of intraplate dacks has been portrayed as low, perhaps because earthquakes and to bring the value more in line of the low density of population and because most with those assigned in the central United States. of the seismicity is distributed around the margins Zone 66. St. Lawrence River zone. There are of the Adirondacks. Recent studies indicate an regions along the St. Lawrence River valley, both abundance of small earthquakes in the Adirondack 28 Mountains (Aggarwal and Sykes, 1978) although 12, 1929 is the largest earthquake of the region. the principal anorthosite bodies tend to be aseis- Street and Turcotte (1977) assigned an mb=5.2. mic (Aggarwal and Yang, 1977). Herrmann (1978) found focal depths between 2 and No large earthquake has been attributed to this 3 km for two more recent events (1966 and 1967) region, nor have any been attributed to its near Attica and suggested that the anomalously periphery, except to the north, which falls in the high intensity of the 1929 shock also might be the western Quebec zone. A maximum magnitude of result of such shallow focal depth. 6 seems appropriate. Zone 70. The Clarendon-Linden fault. The Serious attempts have been made to relate seis- north-northeast-trending Clarendon-Linden fault micity to geologic structure in this region (for ex is close to Attica and the 1929 earthquake could ample, Isachsen and McKendree, 1977a-d; have occurred along one of its strands, but this Pomeroy and Fakundiny, 1976). So far the results is not entirely clear. Certainly, the small earth are most interesting, but rather equivocal. The quakes (1972-1975) induced by solution mining of maximum compressive stress, however, has been salt did occur along a strand of the Clarendon-Lin shown to have an east-northeast direction (Sbar den system but they ranged in depth from 0.5 to and Sykes, 1973, 1977) as it seems to have in those 1.1 km (Fletcher and Sykes, 1977, fig. 8). A focal regions west of the Ordovician suture (for exam mechanism with thrust motion on a nodal plane ple, Sykes, 1978), however it might be defined. nearly parallel to the fault was obtained from The southwestern limit of seismicity falls close these earthquakes (Fletcher and Sykes, 1977). The to a northwesterly trending lineament constructed mechanisms obtained from the 1966 and 1967 by Diment and others (1980) and Muller and events also yield a north-northeast-trending nodal others (1980) on the basis of terminations of grav plane, but require approximately equal compo ity and magnetic features in the region. This linea nents of right-lateral and reverse faulting along ment has not been examined in detail as yet. The this nodal plane (Herrmann, 1978). Lowville earthquake of 1853 (Coffman and von The Clarendon-Linden fault falls on the west Hake, 1973) and the Booneville earthquake of 1980 flank of a pronounced gravity and magnetic fea (Kafka and others, 1980) appear to occur along it. ture that extends north-northeast to northeast Moreover, the focal-plane solution for the Boon- across Lake Ontario (Diment and others, 1974; ville earthquake suggests a northwesterly trend Bothner and others 1980). This geophysical anom ing plane with northeasterly directed compressive aly must owe its presence to contrasts primarily stresses, as do most earthquakes in the Adiron- within the Precambrian basement. However, the dacks. Precambrian structures may have influenced the Some earthquakes in the Adirondacks are development of the Paleozoic Clarendon-Linden known to be shallow (<3.5 km) as at Blue Moun fault. tain Lake (for example, Sbar and others, 1972; An- Many earthquakes in the zone clearly are not derson and Fletcher, 1976), but they cannot be related to the Clarendon-Linden fault. An east- related unequivocally with geologic structure west trend has been noted by many, a northwest (Isachsen and Geraghty, 1979). trend by some (Diment and others, 1980), an as Zone 69. Niagara-Attica Zone. Basham and sociation of earthquakes with mafic plutons as re others (1979) defined a zone of diffuse seismicity vealed by gravity and magnetic anomalies (Kane, extending from the Niagara peninsula to Attica 1977), and an association of earthquakes with and a little beyond. Because the zone is delineated edges of blocks as defined by geophysical and mainly on the basis of historical seismicity, its pos stratigraphic studies (Fakundiny, this workshop). sible extent into Lake Ontario and Lake Erie is The Precambrian basement of northwestern New not well known, although results from the LDGO York appears somewhat anomalous with respect net do show epicenters in the western part of to the surrounding region in that local but intense Lake Ontario (Sykes, 1978). Because many of the gravity and magnetic highs are more common (Re- small earthquakes near Hamilton and Toronto and vetta and Diment, 1971). The character of the sig on the Niagara peninsula were the result of shal natures appears to extend northward into Lake low pop-ups, the zone might be restricted to west Ontario. ern New York. Zone 71. Southeastern New York and north The intensity VIII Attica earthquake of August ern New Jersey. This zone was drawn on the basis 29 of historical and recent instrumental seismicity palachian platform, there is a zone in which the (Aggarwal and Sykes, 1978; Sykes, 1978; Chiburis orientation of the stress directions must change. and Ahner, 1980). The seismicity in a part of the The zone of change must intersect the Boston-Ot zone appears to be closely related to the tawa trend, probably in Vermont in the region of northeasterly trending Ramapo fault (TriassicnJu- relative low seismicity. There was little discussion rassic) or to earlier faults that may date back to of these ideas and uncertainties among the partici late Precambrian time. However, there are many pants. However, some participants insisted that historic as well as instrumentally recorded earth the seismicity in southern New Hampshire and quakes in the general area that are well outside northeastern Massachusetts is higher than in adja the Ramapo zone (W.E.T. Smith, 1962, 1966; cent areas along the coast to the north and to the Sykes, 1978; Chiburis and Ahner, 1980). Indeed, south. some epicenters extend a considerable distance Zone 73. Northeastern New England. This offshore. Thus, the zone has been extended large zone could be subdivided if we had more in offshore to the arbitrary offshore boundary of zone formation and greater insight, but at this moment 64 (see previous discussion). Perhaps it extends it would be difficult to subdivide with a systematic farther into the ocean where it might be associated rationale. A number of comments were made with transform faults, although which of the many about the seismicity of the region and its possible faults may be seismogenic is not clear from the relation to geologic structure, but time was short maps of Klitgord and Behrendt (1979). The south and the comments were brief and sometimes con ern termination of the zone corresponds to the flicting. change in structural trends from southwest to 1. Although the historic seismicity near the west-southwest, and to a reduction in seismicity. coast is somewhat high relative to that of the in The northern termination corresponds to the in terior (W.E.T. Smith, 1962, 1966; Coffman and tersection of the northeasterly structural trends Von Hake, 1973; Stover and others, 1977), the with the more northerly trends of the Berkshire contrast may be due to the fact that the interior Hills, and to a reduction in seismicity. There is was settled late (U.S. Geological Survey, 1970) evidence for significant minor seismicity along the and remains sparsely populated. Hudson River valley and the zone might be ex 2. The border (PQ, ME) earthquake of 1973 of tended to the north, perhaps to join with the areas magnitude mb**5 (Wetmiller, 1975) was something of moderate seismicity in, and peripheral to, the of a surprise and is a principal reason for extend southern and central Adirondack Mountains ing the zone so far to the northwest. (Pomeroy and Fakundiny, 1976). 3. Recent focal mechanisms for this zone Zone 72. The Boston-Ottawa trend. Over the (Graham, 1978; Pulli and Toksoz, 1980) are suffi years, several suggestions have been made that ciently different that they do not necessarily sup link a diffuse northwesterly trending zone of seis port the notion of uniform west-northwest-trend micity (W.E.T. Smith, 1962, 1966) that extends ing compressive regime but rather suggest a com from offshore through eastern Massachusetts, plex stress pattern along coastal New England. southeastern New Hampshire and into Canada in Zone 74. Southeastern New England. Seismic the region of Montreal and Ottawa, with an ill-de activity seems to be clustered loosely in the vicin fined zone of Mesozoic alkaline magmatism (Di- ity of the Triassic-Jurassic grabens, but area! dis ment and others, 1972; Sbar and Sykes, 1973; tribution may be higher a little to the east in McHone and others, 1976; McHone,, 1977; Sykes, southern Connecticut, which might raise the ques 1978). Although the notion may have some merit, tion of the relation of seismicity to strands of the it has some significant imperfections: (1) there is Lake Char and Honey Hill fault zones or possibly a gap in seismicity in Vermont, although results to the northeasterly trending lineaments in this from recently established stations indicate that region. this area may not be aseismic (Chiburis and Zone 75. Zone of background seismicity. Ahner, 1980); (2) if the orientation of the principal Large areas have been assigned to this category compressive stress in the coastal zone (west-north (mb=5) of low historic and instrumental seismicity. west) is correct, and, if the orientation of this com An enumeration of ideas expressed at the work pressive stress is east-northeast in the Canadian shop and in the literature may be useful: Shield, the Adirondack Mountains, and the Ap 1. Many believed that earthquakes in the zone 30 are attributed to shallow phenomena and have vidual faults, or area! extent of faulting where the been expunged from the record. However, many faults have geologically young displacements, or quarry blasts remain unculled from the record (an have distinct association with seismicity; (2) zoning important problem inasmuch as they outnumber primarily on regional structural style, particularly natural events by more than a hundred to one in where regional seismicity is associated strongly some regions). Moreover, some earthquakes with distinctive structural terrane; and (3) zoning caused by collapse of subsurface workings, pop- on areal distribution of historic seismicity. Typi ups due to -natural unloading or quarrying cally, some combination of the three approaches (Pomeroy and Fakundiny, 1976), and solution min is used to best define the zones of a region; how ing or reinjection of wastes remain in the record. ever, one approach usually predominates. Although most of these earthquakes are con Comparing the regions discussed in this report, sequences of man's activities, some actually are much more recent faulting information is available indicators of high stress levels near the surface. on the Great Basin region than on either the The latter may be regional in extent and should Northern or Southern Rocky Mountain regions. not be ignored entirely in seismic zoning. This detailed information allows geologic estimates 2. The aseismicity of certain regions may be due of earthquake recurrence for high-magnitude to the presence of shallow sediments that are so events. These estimates have high uncertainty, soft or so decoupled that they cannot accumulate but are informative and useful for comparison with sufficient strain to produce shallow earthquakes statistically derived estimates of recurrence. (for example, Diment, 1980). However, this does not mean that deeper earthquakes might not occur Where recency of faulting information was avail in such regions. able for the Rocky Mountain regions (primarily in 3. Within the zones of background seismicity are the Rio Grande rift and the Intermountain Seismic areas that are more seismic than others, but that Belt), it has been taken into account in defining are not recognized as such because consideration zones. Large areas exist, though, that have not of larger variations obscures them. Sometimes been studied in this respect. The zoning philoso these subtle variations of seismicity within phy, therefore, has been modified for the Rocky "background regions" represent extensions of Mountain regions to include zones defined by simi more obvious trends into regions where seis- lar structural or tectonic setting. The available mogenic structures are present at greater depths data on ages of latest faulting has been assumed (for example, the possible extension of the Attica to hold throughout a distinctive structural region. zone to the southeast). Note that this second approach is different from that used in the Basin and Range where the zone SUMMARY boundaries conform to areas characterized by cer tain ages of latest fault displacements. By P. C. Thenhaus In contrast to the western United States, The procedures used in delineating seismic throughout most of the eastern United States spe source zones are ill defined. No single standard cific seismotectonic structures are unknown. Also, exists by which source zones across the nation can the diversity of geologic and tectonic terrane is be drawn, primarily because of the nonuniform not nearly as marked as in the west. These facts level of pertinent seismological, geological, and pose major handicaps in attempting to define geophysical information available for areas of vast zones primarily on geologic information. Accord ly differing tectonic and geologic settings. The ingly, in the Central Interior and the northeastern equivocal association of seismicity with geologic United States, a third approach for developing structure throughout most of the United States source zones is used that has primary emphasis compounds the problem. The zones described on the spatial distribution of historic seismicity. herein, resulting from a general consensus of each Considerations for defining seismic source zone of the committees, illustrate three useful ap boundaries within regions parallel those for devis proaches in defining regional seismic source zones. ing a zoning technique for the region. Within a Each of the approaches represents the different region, a nonuniform level of pertinent information level of understanding of seismotectonics in any exists among source zones being considered under one of the regions. They are: (1) zoning on indi a single zoning technique. This fact bears on the 31 certainty of the zone boundaries within each re Algermissen, S. T., and Perkins, D. M., 1976, A probabilistic gion. Illustrating this intraregion variation, the estimate of maximum acceleration in rock in the contiguous given zone boundaries within the Basin and Range United States: U.S. Geological Survey Open-File Report 76-416, 45 p., 2 plates. have a higher degree of certainty at the eastern Algermissen, S. T., Perkins, D. M., Thenhaus, P. C., Hanson, and western margins than in the central area. The S. L., Bender, B. B., 1982, Probabilistic estimates of accel reasons for this are obvious: (1) the margins of eration and velocity in the contiguous United States: U.S. the Basin and Range are the most seismically ac Geological Survey Open-File Report 82-1033, 99 p., 6 tive, and (2) the hazard threatens the population plates, scale 1:7,500,000. Anderson, J. G., and Fletcher, J. B., 1976, Source properties which is more densely concentrated at these mar of a Blue Mountain Lake earthquake: Seismological Society gins. Therefore, more research effort has been of America Bulletin, v. 66, p. 677-684. concentrated in these areas. This effort results in Anderson, R. E., 1978, Holocene faulting in the western Grand more and better quality data on ages of latest dis Canyon, Arizona; discussion (followed by reply by Hun- placements. toon): Geological Society of America Bulletin, v. 90, p. In contrast, certainty of zone boundaries defined 221-224. Anderson, R. E., and Bucknan, R. C., 1979, Map of fault under the remaining two techniques of zoning (use scarps on unconsolidated sediments, Richfield 1° by 2° of structural provinces and spatial distribution of quadrangle, Utah: U.S. Geological Survey Open-File Re seismicity) can be assessed only according to the port 79-1236, 15 p. extent that they appear to reasonably organize Arabasz, W. J., Smith, R. B., and Richins, W. D., 1980, Earth historic seismicity. Because these techniques are quake studies along the Wasatch Front, Utah; Network monitoring, seismicity and seismic hazards: Seismological not based on geologic effects of earthquakes (that Society of America Bulletin, v. 70, no. 5, p. 1479-1499. is, number and size of fault scarps), their certainty Armstrong, R. L., 1978, Cenozoic igneous history of the U.S. cannot be assessed in terms of completeness or Cordillera from lat 42° to 49° N., in Smith, R. B., and quality of a particular kind of quantitative data Eaton, G. P., eds., Cenozoic tectonics and regional set. Judgments on both the tectonics of an area geophysics of the Western Cordillera: Geological Society and the historic record of events are involved. It of America Memoir 152, p. 265-282. Bachman, G. 0., and Machette, M. N., 1977, Calcic soils and is because of these judgments that the source calcretes of the southwestern United States: U.S. Geologi zones represent a general but qualified consensus cal Survey Open-File Report 77-794, 163 p. on the part of the workshop participants. There Basham, P. W., Weichert, P. H., and Berry, M. J., 1979, Re usually exists dissenting opinions on the part of gional assessment of seismic risk in eastern Canada: Seis a few as to the reasonableness of some zones. mological Society of America Bulletin, v. 69, p. 1567-1602. Bateman, P. C., 1965, Geology and tungsten mineralization of the Bishop district, California: U.S. Geological Survey Professional Paper 470, 204 p. Bell, J. W., 1981, Quaternary fault map of the Reno 1° by 2° quadrangle: U.S. Geological Survey Open-File Report REFERENCES CITED 81-982, 62 p., 1 plate, scale 1:500,000. Acharya, H. K., 1980, Spatial correlation of large historical Bell, E. J., and Slemmons, D. B., 1979, Recent crustal move earthquakes and moderate shocks greater than 10 km deep ments in the central Sierra Nevada-Walker Lane Region in eastern North America: EOS, Transactions of the of California-Nevada; Part II, the Pyramid Lake right-slip American Geophysical Union, v. 61, p. 1030. fault zone segment of the Walker Lane: Tectonophysics, Aggarwal, Y. P., and Sykes, L. R., 1978, Earthquakes, faults v. 52, p. 571-583. and power plants in southern New York and northern New Bollinger, G. R., and Wheeler, R. L., 1980, The Giles County, Jersey: Science, v. 200, p. 425-429. Virginia, seismic network Monitoring results, 1978-1980 Aggarwal, Y. P., and Yang, J. P., 1977, Seismological investi [abs.]: Seismological Society of America, Eastern Section, gations in the Adirondacks and environs [abs.]: Geological Programs and Abstracts, p. 7. Society of America Abstracts with Programs, v. 9, p. 234. Bothner, W. A., Simpson, R. W., and Diment, W. H., compil Algermissen, S. T., 1969, Seismic risk studies in the United ers, 1980, Bouger gravity map of the northeastern United States: Proceedings of the Fourth World Conference on States and adjacent Canada: U.S. Geological Survey Open- Earthquake Engineering, Chilean Association for Seismol File Report 80-2012, 4 sheets, scale 1:1,000,000. ogy and Earthquake Engineering, Santiago, Chile, 1969. Braile, L. W., Hinze, W. J., Keller, G. R., Lidiak, E. G., 1972, The seismic risk map of the United States De 1982, The north extension of the New Madrid fault zone, velopment, use and plans for future refinement, in Pro in McKeown, F. A., and Pakiser, L. C., eds., Investiga ceedings of conference on seismic risk assessment for tions of the New Madrid, Missouri earthquake region: U.S. building standards, March 16, 1972: Washington, D.C., Geological Survey Professional Paper 1236, 201 p. U.S. Department of Housing and Urban Development and Bucknam, R. C., Algermissen, S. T., and Anderson, R. E., U.S. Department of Commerce, p. 11-16. 1980, Patterns of late Quaternary faulting in western Utah 32 and an application in earthquake hazard evaluation, in Pro intensity patterns in Arizona: State of Arizona Bureau of ceedings of Conference X; earthquake hazards along the Geology and Mineral Technology, the University of Wasatch and Sierra-Nevada frontal fault zones: U.S. Arizona, Tucson, Arizona, Special Paper 3,112 p. Geological Survey Open-Pile Report 80-801, p. 299-314. Eppley, R. A., 1965, Earthquake history of the United States Bucknam, R. C., and Anderson, R. E., 1979a, Estimation of (exclusive of California and western Nevada): Washington, fault-scarp ages from a scarp-height-slope-angle relation D.C., U.S. Government Printing Office, 120 p. ship: Geological Society of America Bulletin, v. 7, p. 11-14. Fletcher, J. B., and Sykes, L. R., 1977, Earthquakes related 1979b, Map of fault scarps on unconsolidated sediments, to hydraulic mining and natural seismic activity in western Delta 1° by 2° quadrangle, Utah: U.S. Geological Survey New York State: Journal of Geophysical Research, v. 82, Open-File Report 79-366,1 plate, scale 1:500,000. no. 26, p. 3767-3780. Chiburis, E. P., and Ahner, R. 0., 1980, Seismicity of the Forsyth, D. A., 1977, Relationships between seismicity, free northeastern United States; Northeastern U.S. seismic air gravity and structure in Arctic and eastern Canada network bulletin no. 17: Available from National Technical [abs.]: Earthquake Notes, v. 48, p. 15. Information Service, Springfield, VA 22161 as report Freidline, R. A., Smith, P. B., and Blackwell, D. D., 1976, NUREG, WES 238-030. Seismicity and contemporary tectonics of the Helena, Mon Christiensen, M. N., 1966, Cenozoic crustal movements in the tana area: Seismological Society of America Bulletin, v. Sierra Nevada: Geological Society of America Bulletin, v. 66, no. 1, p. 81-95. 77, p. 163-182. Graham, T., 1978, Fault plane solutions of three New England Christiansen, R. L., and McKee, E. H., 1978, Late Cenozoic earthquakes [abs.]: Earthquake Notes, v. 49, no. 3, p. 13. volcanic and tectonic evolution of the Great Basin and Co Greensfelder, R. W., 1976, Maximum probable earthquake ac lumbia intermontane regions, in Smith, R. B., and Eaton, celeration on bedrock in the state of Idaho: Idaho Depart G. P., eds., Cenozoic tectonics and regional geophysics of ment of Transportation, Division of Highways Research the Western Cordillera: Geological Society of America Project 79, 69 p. Memoir 152, p. 283-312. Hamblin, W. K., 1970, Structure of the western Grand Canyon Coffman, J. L., and von Hake, C. A., 1973, Earthquake history region, in The western Grand Canyon district: Utah of the United States: Washington, D.C., U.S. Department Geological Society Guidebook to the Geology of Utah, no. of Commerce Publication 41-1, 208 p. 23, p. 3-20. Herrmann, R. B., 1978, A Seismological study of two Attica, Cordell, L., 1978, Regional geophysical setting of the Rio New York earthquakes: Seismological Society of America Grande Rift: Geological Society of America Bulletin, v. 89, Bulletin, v. 68, p. 641-351. p. 1073-1090. Heyl, A. V., Jr., and Brock, M. R., 1961, Structural Dewey, J. W., and Gordon, D. W., 1980, Instrumental seismic- framework of the Illinois-Kentucky mining district and its ity of the eastern United States and adjacent Canada relation to mineral deposits, in Short papers in the [abs.]: Seismological Society of America, Eastern Section, geologic and hydrologic sciences: U.S. Geological Survey Programs and Abstracts, p. 13. Professional Paper 424-D, p. D3-D6. Diment, W. H., 1980, Significance of the aseismic regions of Hildenbrand, T. G., Kane, M. F., and Hendricks, J. D., 1982, the eastern United States [abs.]: Geological Society of Magnetic basement in the Upper Mississippi Embayment America Abstracts with Programs, v. 12, p. 3031. region A preliminary report, in McKeown, F. A., and Diment, W. H., Muller, 0. H., and Lavin, P. M., 1980, Base Patoser, L. C., eds., Investigations of the New Madrid, ment tectonics of New York and Pennsylvania as revealed Missouri, earthquake region: U.S. Geological Survey Pro by gravity and magnetic studies, in Wones, D. R., ed., fessional Paper 1236, 201 p. Proceedings of The Caldonides in the USA: Blacksburg, Hildenbrand, T. G., Kane, M. P., and Stauder, W., 1977, Mag Va., Virginia Polytechnic Institute and State University, netic and gravity anomalies in the northern Mississippi Department of Geological Sciences Memoir 2, p. 221-227. Embayment and their spatial relation to seismicity: U.S. Diment, W. H., Urban, T. C., Mequid, F., and Revetta, F. Geological Survey Miscellaneous Field Studies Map MF- A., 1974, Speculations about the Precambrian basement 914, 2 plates, scale 1:1,000,000. of New York and Pennsylvania from gravity and magnetic Hill, D. P., 1963, Gravity and crustal structure in the Western anomalies [abs.]: Geological Society of America Abstracts Snake River Plain, Idaho: Journal 33 Horner, R. B., Wetmiller, R. J., and Hasegawa, H. S., 1979, LaFehr, T. R., and Pakiser, L. C., 1962, Gravity, volcanism The St. Donat, Quebec, earthquake sequence of February and crustal deformation in the eastern Snake River Plain, 18-23, 1978: Canadian Journal of Earth Sciences, v. 16, Idaho; Article 141 in Geological Survey Research 1962, p.. 1892-1898. Short papers in geology, hydrology, and topography, arti Houston, R. S., and McCallum, M. E., 1961, Mullen Creek- cles 120-179: U.S. Geological Survey Professional Paper Wash Fork shear zone, Medicine Bow Mountains, south 450-D, p. D76-D78. eastern Wyoming [abs.]: Geological Society of America Leblank, G., and Buchbinder, G., 1977, Second microearth- Special Paper 68, p. 91. quake survey of the St. Lawrence Valley near La Malbaie, isachen, Y. W., and McKendree, W. G., 1977a, Preliminary Quebec: Canadian Journal of Earth Sciences, v. 14, p. brittle structures map of New York: New York State 2778-2789. Museum Map and Chart Series 31A through 31D, 4 sheets, Leblank, G., Stevens, A. E., Wetmiller, R. J., Du Berger, scale 1:250,000. R., 1973, A microearthquake survey of the St. Lawrence 1977b, Preliminary brittle structures map of New York: Valley near La Malbaie, Quebec: Canadian Journal of New York State Museum Map and Chart Series 31E, 1 Earth Sciences, v. 10, p. 42-53. sheet, scale 1:500,000. Machette, M. N., 1978, Geologic map of the Socorro 1° by 2° 1977c, Generalized map of recorded joint systems in quadrangle, New Mexico: U.S. Geological Survey Open- New York: New York State Museum Map and Chart File Report 78-607, 1 plate, scale 1:250,000. Series 31F, 1 sheet, scale 1:1,000 000. 1980, Seismotectonic analysis, Rio Grande Rift, New -1977d, Sources of information for preliminary brittle Mexico: Semiannual progress report, in Summaries of structures map of New York and 'generalized map of re technical reports, v. 9: U.S. Geological Survey Open-File corded joint systems in New York: New York State Report 80-6, p. 56-57. Museum Map and Chart Series 31G, 1 sheet, scale Malde, H. E., 1971, Geologic investigation of faulting near the 1:1,000,000. National Reactor Testing Station, Idaho, with a section Isachsen, Y. W., and Geraghty, E. P., 1979, Ground investiga on Microearthquake studies, by A. M. Pitt and J. V. tions of projected traces of focal mechanisms for earth Eaton: U.S. Geological Survey Open-File Report 71-338, quakes at Blue Mountain Lake, Raquette Lake, and Chazy 167 p. Lake, Adirondack uplift, New York: Available from Na Mauk, F. J., Coupknd, D., Kimball, J., and Ford, P., 1979, tional Technical Information Service, Springfield, VA Geophysical investigations of the Anna, Ohio earthquake 22161 as report NUREG/CR-0888, 33 p. zone: Available from National Technical Information Ser Kafka, A. L., Schlesinger-Miller, E. A., and Sykes, L. R., vice, Springfield, VA 22161 as report NUREG/CR-1065, 1980, Recent seismic activity in the New York region: 182 p. Seismological Society of America, Eastern Section, Pro McHone, J. G., 1977, Dike types and distribution in central gram and Abstracts, p. 20. New England [abs.]: Geological Society of America Kane, M. F., 1977, Correlation of major eastern earthquake Abstracts with Programs, v. 9, p. 300. centers with mafic/ultramafic basement masses, in Rankin, McHone, J. G., Black, W. W., and Pergram, W. J., 1976, Ori D. W., ed., Studies related to the Charleston, South gin of northwestern New England lamprophyres Sr Carolina earthquake of 1886 A preliminary report: U.S. isotopes and geochemical evidence [abs.]: Geological Soci Geological Survey Professional Paper 1028, p. 199-204. ety of America Abstracts with Programs, v. 8, p. 227. Kirkham, R. M., 1977, Quarternary movements on the Golden McKeown, F. A., 1978, Hypothesis; Many earthquakes in cen fault, Colorado: Geology, v. 5, p. 689-692. tral and southeastern United States are causally related Kirkham, R. M., and Rogers, W. P., 1978, Earthquake poten to mafic intrusive bodies: U.S. Geological Survey Journal tial in Colorado; a preliminary evaluation: Colorado of Research, v. 6, no. 1, p. 41-50. Geological Survey Open-File Report 78-3, 135 p., 3 plates, 1982, Investigations of the New Madrid, Missouri earth scales 1:100,000,1:62,500. quake region Overview and discussion, in McKeown, F. 1981, Earthquake potential in Colorado; A preliminary A., and Pakiser, L. C., eds., Investigations of the New evaluation: Colorado Geological Survey Bulletin 43, 171 p., Madrid, Missouri earthquake region: U.S. Geological Sur 3 plates, scales 1:100,000, 1:62,500. vey Professional Paper 1236, 201 p. Klitgord, K. D., and Behrendt, J. C., 1979, Basin structure Mudge, M. R., 1970, Origin of the disturbed belt, Montana: of the U.S. Atlantic continental margin, in Watkins, J. Geological Society of America Bulletin, v. 81, no. 2, p. S., Montadert, L., and Dickerson, P. W., eds., Geological 377-392. and geophysical investigations of continental margins: Muehlberger, W. R., Belcher, R. C., and Goety, L. K., 1978, American Association of Petroleum Geologists Memoir 29, Quaternary faulting in Trans-Pecos Texas: Geology, v. 6, p. 85-112. p. 337-340. Koons, E. D., 1945, Geology of the Uinkaret Plateau, northern Muller, 0. H., Diment, W. H., and Lavin, P. M., 1980, Trans Arizona: Geologicial Society of America Bulletin, v. 56, verse gravity features and seismicity in New York and p. 151-180. Pennsylvania [abs.]: Geological Society of America Krausse, H. F., and Treworgy, C. G., 1979, Major structures Abstracts with Programs, v. 12, p. 74. of the southern part of the Illinois Basin, in Palmer, J. Nishenko, S. P., and Kafka, A. L., 1980, Focal mechanisms E., and Dutcher, R. R., eds., Depositional and structural of two Bermuda Rise earthquakes [abs.]: Seismological So history of the Pennsylvanian System of the Illinois Basin; ciety of America, Eastern Section, Program and Part 2, Invited papers: Illinois State Geological Survey Abstracts, p. 24. Guidebook Series, No. 15, pt. 2, p. 115-120. Nuttli, 0. W., 1973, The Mississippi Valley earthquake of 1811 34 and 1812; intensities, ground motion and magnitudes: Seis- see: Geological Society of America Bulletin, pt. 1, v. 90, mological Society of America Bulletin, v. 63, no. 1, p. 227- p. 1013-1018. 248. Ryall, A. S., 1977, Earthquake hazard in the Nevada region: 1974, Magnitude-recurrence relation for central Missis Seismological Society of America Bulletin, v. 67, no. 2, sippi Valley earthquakes: Seismological Society of America p. 517-532. Bulletin, v. 64, no. 4, p. 1189-1207. Ryall, A. S., and Van Wormer, J. D., 1980, Estimation of -1979, Seismicity of the central United States, in Hathe- maximum magnitude and recommended seismic zone way, A. W., and McClure, C. R., Jr., eds., Geology in changes in the western Great Basin: Seismological Society the siting of nuclear power plants: Geological Society of of America Bulletin, v. 70, no. 5, p. 1573-1581. America Reviews in Engineering Geology, v. 4, p. 67-93. Sanders, C. 0., and Slemmons, D. B., 1979, Recent crustal Nuttli, 0. W., and Herrmann, R. B., 1978, State-of-the-art movements in the central Sierra Nevada-Walker Nevada; for assessing earthquake hazards in the United States; Part III, the Olinghouse fault zone: Tectonophysics, v. 52, Credible earthquakes for the central United States: U.S. p. 585^597. Army Corps of Engineers, Waterways Experimental Sta tion, Miscellaneous Paper S-73-1, no. 12, Vicksburg, Mis Sanford, A. R., Olsen, K. H., and Jaksha, L. H., 1979, Seis sissippi, 99 p. micity of the Rio Grande Rift, in Riecker, R. E., ed., Pardee, J. T., 1926, The Montana earthquake of June 27, 1925: Rio Grande Rift, tectonics and magmatism: American U.S. Geological Survey Professional Paper 147-B, p. 7-23. Geophysical Union, p. 145-168. 1950, Late Cenozoic block faulting in western Montana: 1981, Seismicity of New Mexico, in Hays, W. W., ed., Geological Society of America Bulletin, v. 61, p. 359-406. proceedings of the conference on evaluation of regional Pavlis, T. P., and Smith, R. B., 1980, Slip vectors on faults seismic hazards and risk, August 25-27, 1980, Santa Fe, near Salt Lake City, Utah, from Quaternary displacements New Mexico: U.S. Geological Survey Open-File Report and seismicity: Seismological Society of America Bulletin, 81-437, p. 74-89. v. 70, no. 5, p. 1521-1526. Sbar, M. L., Armbruster, J., and Aggarwal, Y. P., 1972, The Perkins, D. M., Thenhaus, P. C., Hanson, S. L., Ziony, J. Adirondack, New York, earthquake swarm of 1971 and I., and Algermissen, S. T., 1980, Probabilistic estimates tectonic implications: Seismological Society of America of maximum horizontal ground motion on rock in the Bulletin, v. 62, p. 1303-1317. Pacific Northwest and the adjacent Continental Shelf: U.S. Sbar, M. L., and Sykes, L. R., 1973, Contemporary stress Geological Survey Open-File Report 80-471, 39 p., 7 maps, and seismicity in eastern North America; An example of scale 1:5,000,000. intra-plate tectonics: Geological Society of America Bulle Perkins, D. M., Thenhaus, P. C., Wharton, M. K., Diment, tin, v. 84, p. 1861-1882. W. K., Hanson, S. L., and Algermissen, S. T., 1979, Prob 1977, Seismicity and lithospheric stress in New York abilistic estimates of maximum seismic horizontal ground and adjacent areas: Journal of Geophysical Research, v. motion in rock on the East Coast and the adjacent outer 82, p. 5771-5786. continental shelf: U.S. Geological Survey Interagency Re Shakal, A. F., and Toksoz, M. N., 1977, Earthquake hazard port to the Bureau of Land Management, 18 p., 7 plates, in New England: Science, v. 195, p. 171-173. scale 1:2,500,000. Slemmons, D. B., Van Wormer, D., Bell, E. J., Silberman, Pomeroy, P. W., and Fakundiny, R. H., 1976, Seismic activity M. L., 1979, Recent crustal movements in the Sierra and geologic structure in New York and adjacent areas: Nevada-Walker Lane region of California-Nevada; Part I, New York State Museum and Science Service Map and rate and style of deformation: Tectonophysics, v. 52, p. Chart Series 27, 2 sheets, scale 1:500,000. 561-570. Prinz, M., 1970, Idaho rift system, Snake River Plain, Idaho: Smith, R. B., 1972, Contemporary seismicity, seismic gaps, Geological Society of America Bulletin, v. 81, p. 941-948. and earthquake recurrences of the Wasatch Front, in Hil- Pulli, J. J., and Toksoz, M. N., 1980, Focal mechanisms of pert, L. S., ed., Environmental geology of the Wasatch some recent northeastern United States earthquakes Im Front, 1971: Utah Geological Association Publication 1, p. plications for the state of stress in the crust [abs.]: Seis 11-19. mological Society of America, Eastern Section, Programs 1978, Seismicity, crustal structure, and intraplate tec and Abstracts, p. 29. tonics of the interior of the western Cordillera, in Smith, Qamar, A., and Hawley, B., 1979, Seismic activity near the R. B., and Eaton, G. P., eds., Cenozoic tectonics and re Three Forks Basin, Montana: Seismological Society of gional geophysics of the Western Cordillera: Geological So America Bulletin, v. 69, no. 6, p. 1917-1929. ciety of America Memoir 152, p. 111-144. Ratcliffe, N. M., 1971, The Ramapo fault system in New York Smith, R. B., and Sbar, M. L., 1974, Contemporary tectonics and adjacent northern New Jersey; A case history of tec and seismicity of the western United States with emphasis tonic heredity: Geological Society of America Bulletin, v. on the intermountain seismic belt: Geological Society of 82, p. 125-142. America Bulletin, v. 85, p. 1205-1218. Revetta, F. A., and Diment, W. H., 1971, Simple Bouger grav Smith, W.E.T., 1962, Earthquakes of eastern Canada and adja ity anomaly map of western New York: New York State cent areas 1534-1927: Ottawa, Canada, Publication of the Museum and Science Service, Geological Survey, Map and Dominion Observatory, v. 26, p. 271-301. Chart Series 17, scale 1:250,000. 1966, Earthquakes of eastern Canada and adjacent areas Russ, D. P., 1979, Late Holocene faulting and earthquake re 1928-1959: Ottawa, Canada, Publications of the Dominion currence in the Reelfoot Lake area, northwestern Tennes Observatory, v. 32, p. 87-121. 35 Stauder, W., 1982, Present day seismicity and identification U.S. Geological Survey, 1970, National Atlas of the United of active faults of the New Madrid seismic zone, in States of America: Washington, D.C., 418 p. McKeown, F. A., and Pakiser, L. C., eds., Investigations Van Wormer, J. D., and Ryall, A. S., 1980, Sierra Nevada- of the New Madrid, Missouri, earthquake region: U.S. Great Basin boundary zone; Earthquake hazard related to Geological Survey Professional Paper 1236, 201 p. structure, active tectonic processes, and anomalous pat Stevens, A. E., 1980, Reexamination of some larger La Mal- terns of earthquake occurrence: Seismological Society of baie, Quebec, earthquakes (1924-1978): Seismological Soci America Bulletin, v. 70, no. 5, p. 1557-1572. ety of America Bulletin, v. 70, p. 529-558. Wallace, R. E., 1975, Fault scarp geomorphology and seismic Stickney, M. C., 1978, Seismicity and faulting in western Mon history, north-central Nevada [abs.]: Geological Society of tana: Northwest Geology, v. 7, p. 1-9. America, Abstracts with Programs, v. 7, p. 385. Stover, C. W., Reager, B. G., and Algermissen, S. T., 1977, 1977, Profiles and ages of young fault scarps, north-cen Seismicity map of the state of Maine: U.S. Geological Sur tral Nevada: Geological Society of America Bulletin, v. 88, vey Miscellaneous Field Studies Map MF-845, scale p. 1267-1281. 1:1,000,000. 1978, Geometry and rates of change of fault-generated Street, R. L., and Turcotte, F. T., 1977, A study of northeast range fronts, north-central Nevada: U.S. Geological Sur ern North America spectral moments, magnitudes and in vey Journal of Research, v. 6, no. 5, p. 637-650. tensities: Seismological Society of America Bulletin, v. 67, -1979, Map of young fault scarps related to earthquakes p. 599-614. in north central Nevada: U.S. Geological Survey Open-File Sumner, J. S., 1976, Earthquakes in Arizona: Field notes, v. Report 79-1554, 2 sheets, scale 1:125,000. 6, no. 1, p. 1-5. Weichert, P. H., and Milne, W. G., 1979, On Canadian Suppe, J., Powell, C., Berry, R., 1975, Regional topography, methodologies of probabilistic seismic risk estimation: Seis seismicity and Quaternary volcanism, and the present-day mological Society of America Bulletin, v. 69, p. 1549-1566. tectonics of the western United States: American Journal Wetmiller, R. J., 1975, The Quebec-Maine border earthquake, of Science, v. 275-A, p. 397-436. 15 June 1973: Canadian Journal of Earth Science, v. 12, Swan, F. H., Ill, Schwartz, D. P., Cluff, C. S., 1980, Recur p. 1917-1928. rence of moderate to large magnitude earthquakes produc Wheeler, R. L., and Bollinger, G. A., 1980, Types of basement ed by surface faulting on the Wasatch fault zone, Utah: faults probably responsible for seismicity in and near Giles Seismological Society of America Bulletin, v. 70, no. 5, County, Virginia [abs.]: Seismological Society of America, p. 1431-1462. Eastern Section, Program and Abstracts, p. 38. Sykes, L. R., 1978, Intraplate seismicity, reactivation of Witkind, I. J., 1975a, Preliminary map showing known and preexisting zones of weakness, alkaline magmatism and suspected active faults in Wyoming: U.S. Geological Sur other tectonism postdating continental fragmentation: Re vey Open-File Report 75-279, 1 plate, scale 1:500,000. views of Geophysics and Space Physics, v. 16, p. 621-688. 1975b, Preliminary map showing known and suspected Thenhaus, P. C., Perkins, D. M., Ziony, J. I. and Algermissen, active faults in Idaho: U.S. Geological Survey Open-File S. T., 1980, Probabilistic estimates of maximum seismic Report 75-278, 1 plate, scale 1:500,000. horizontal ground motion on rock in coastal California and 1976, Preliminary map showing known and suspected the adjacent continental shelf: U.S. Geological Survey active faults in Colorado: U.S. Geological Survey Open- Open-File Report 80-924, 70 p., 7 maps, scale 1:5,000,000. File Report 76-154, 1 plate, scale 1:500,000. Thompson, G. A., and Burke, D. B., 1973, Rate and direction -1977, Major active faults and seismicity, northwestern of spreading in Dixie Valley, Basin and Range Province, Montana: U.S. Geological Survey Miscellaneous Field Nevada: Geological Society of America Bulletin, v. 84, p. Studies Map MF-923,1 plate, scale 1:500,000. 627-632. Zoback, M. D., Hamilton, R. M., Crone, A. J., Russ, D. P., Tweto, 0., 1979, The Rio Grande Rift System in Colorado, McKeown, F. A., and Brockman, S. R., 1980, Recurrent in Riecker, R. E., ed., Rio Grande Rift: Tectonics and intraplate tectonism in the New Madrid seismic zone: Sci Magmatism, American Geophysical Union, p. 33-56. ence, v. 109, p. 971-976. 36