518 Faults and Earthquakes in the Monterey Bay R

DEPARTME T OF THE I TERIOR TO ACCOMPANY MAP MF- 518 U ITED STATES GEOLOGICAL SURVEY

FAULTS AND EARTHQUAKES IN THE MONTEREY BAY REGIO , CALIFORNIA*

By H. G. Greene , W. H. K. Lee , D. s . McCUlloch, and E . E . Brabb

INTRODUCTIO seismically active San Andreas and the Sur- acimiento fault zones (fig . 1) , which mark the northeast and As urbanization of the Monterey Bay coastal region southwest boundaries , respectively, of the Salinian progresses, city and county planners find a growing need block (Reed, 1933) . The Salinian block consists of for up-to- date information on where geologic hazards are continental crust dominated by granitic rocks and is located and how serious a threat they present . At the f l anked on either side by oce~ t ic crust of the same time , the public , aware of the dangers of active Franciscan assemblage . The Sur- Nacimiento fault zone faulting along the California coast , is bringing pres comprises faults of various kinds and ages in a belt sure for development of earthquake-resistant communities extending southeastward from Point Sur through the and structures. central and southern Coast Ranges of Califor nia; it Faulting and seismic activity are being investigated includes the Sur fault zone and the Nacimiento fault by ·the U. S . Geological Survey along the continental (Page, 1970a). shelf between Point Sur and Point Reyes , California . The Cretaceous granitic basement rocks of the Salinian The Survey's work in Monterey Bay (fig . 1) , the area block, and the overlying Tertiary strata , have been both dealt with in this report, has rev ealed the presence of horizontally and vertically offset by many faults that many previously unrecognized faults . Some of the faults trend southeast from Monterey Bay through the Santa are active and , according to e xisting maps, e x tend Lucia Range. Some of these faults can be traced for onshore. more than 10 krn and appear to have controlled the devel Offshore faults were mapped by geophysical methods. opment of major geomorphic features , such as the Salinas, In 1969 , detailed seismic reflection ("sparker") surveys Carmel , and Palo Colorado Valleys , which formed along were made in Monterey Bay and the shelf areas of Point the King City , Tularcitos, and Palo Colorado faults Sur to Cypress Point and Santa Cruz to the Gulf of (pl. 1) . Farallones. In 1970 , additional surveys were made North of Monterey Bay, faults in the Salinian block across the shelves from Point Sur to Cypress Point , and trend generally northwest and offset the granitic base northern Monterey Bay to Ano Nuevo Point. ment rocks and overlying Tertiary strata (Jennings and Burnet t , 1961) . Several of these faults, the Butano , Procedures and Methods Ben Lomond , and zayante faults , bend around to trend Continuous seismic reflection profiles were obtained east- west as they approach the San Gregorio fault zone . by a high-resolution, . 6 kj sparker system , and an ear ru1o uevo Point , the San Gregorio fault , oriented intermediate- penetration, 23 kj sparker system . The N. 25°W ., e xtends for nearly 30 km onland and cuts geophysical data were collected along approximately across the regional structural grain . 2 , 600 km of track line . See the appendix for detailed Offshore the granitic basement of the Salinian block descriptions of equipment , procedures , and methods " imparts a rigid block-faulting structural style to the used in this investigation. overlying sediments" (Hoskins and Griffiths , 1971 , p. Numerous faults mapped on the basis of interpretations 212) . The seaward e x tension of onland faults has not of the seismic reflection records are shown on plate l . yet been well established . Howe ver , offshore faults All major faults are well defined by several seismic have been discussed in several reports . Curr ay (1966 , criteria giv en in the appendix. p . 342) noted that sediments on the Monterey Bay shelf Acknowledgements have been displaced by many faults , and Martin and The authors thank B. M. Page , Stanford University, Emery (1967) described a northwest- trending fault across and C . M. Wentworth , U. S . Geological Survey , for heir the upper axis of the submarine Monterey Canyon , and two advice and critical comments , and J . R. CUrray , Scripps northwest-trending faults in Carmel Canyon that they Institution of Oceanography , and E . A. Silver, U. S. connected with the San Gregorio fault to the north and Geological Survey , for permitting the use of their with the Palo Colorado and Sur faults to the south . seismic reflection profiles of the Monterey Bay region . Hoskins and Griffiths (1971) interpreted onshore offshore faults southeastward from the San Gregorio Previous work fault through Carmel Canyon in a similar manner but did Unlike the present study , previous geological and not map the southeast end of the fault onshore . geophysical investigations of Monterey Bay have been primarily reconnaissance studies (e . g. , Shepard and OFFSHORE FAULTS Emery, 1941; Shepard , 1948 ; Martin , 1964 ; Curray, 1965 , 1966 ; Rusnak , 1966; Martin and Emery, 1967; Faults in the Monterey Bay region lie prim.ri ly with Hoskins and Griffiths , 1971; and Silver and others , in two major intersecting northwest- trending zones 1971) . The geophysical studies discuss the general (plate 1) , the Palo Colorado-San Gregorio and Monterey regional structure and major faulting as they relate Bay zones . The Palo Colorado- San Gregorio fault zone, to more e x tensive investigations of the central a narrow (approximately 3 krn wide) zone represented in California continental shelf , but do not treat the most places by one or two faults , appears to connect at structure of the bay in detail, as this report does . the south end with the onland Serra Hill and Palo Martin (1964) used detailed bathymetry and geologic Colorado faults (Trask , 1926 ; Jennings and Strand , samples as a basis for a geologic map of Monterey Bay . 1958; Gilbert , 1971) near Kaslar and Hurricane Points , Dohrenwend and Ellsworth used detailed bathymetry to and at the north end with the San Gregorio fault and a describe the geology of the continental shelf between thrust fault on Ano Nuevo Point described by Clark Point Sur and Cypress Point (J . C . Dohrenwend, 1971 , (1970a) and J . G. Evans and K. Lajoie (written c ommun. , and W. L. Ellsworth , 1971) . 1971) . The length of this zone , including onland seg The onland geology of the region has been studied ments , is at least 125 krn; the zone may be much longe r , extensively (for e xample , Johnson , 1855 ; Clark , 1930 , for it may join the faults at Half Moon Bay, which may , 1970a, 1970b; Woodring , 1938; Taliaferro , 1943; in turn, join the 6an Andreas fault northwest of the Allen, 1946; Jennings and Strand , 1958 ; Jennings and Go l den Gate (Cooper , 1970). To f ac ilitate discussion , Burnett, 1961 ; Baldwin, 1963; Page , 1966, 1970a, the zone is d i v i d ed i nto thre e s e gme nts : (1) Cyp r ess 1970b; Compton, 1966 ; Christensen , 1966; Bowen, 1969 ; Po i nt-Point Sur shel f, (2 ) Mon terey Bay , and (3 ) Ano Durham, 1970; Brabb, 1970), and onland faults have bee n u e vo Point-Santa Cruz she lf. the subject of studies by Fairborn (1963) , Sieck (1964), The Monte rey Ba y fault zone (p r e viou s ly c a l led Durham (1965) , Burford (1971) and Gilbert (1971) . Tularcitos fault zone by Gr eene , 1970) in the inner bay Geologic setting between Monte r e y and Santa Cruz , i s a diffuse zone 10 Principal onland faults in this region are the to 15 krn wide o f short en echelo n northwes t-tre nding

*Work done in cooperation with the California S tate De partme nt of \-la t e r Resources, and t he Divi sion o f Re ac tor De ve lo - ment and Technology, U.S . Atomic Energy Commission. 0 I 3715

0 100

3700'

0 5 10 Miles

0 5 10 Km

Figure 1.--Index map of central California coast showing location of study area .

2 faults . It may be the offshore extension of northwest swings eastward and m y conn ct w~th t he Palo Color o trending f aults i n the Salinas Valley and the Sier ra de fault o n l and (pl . l) . Salinas Moun t ains to the southeast . To the north , the Trask (1926 , p . 164-165) has describe Lhc P lo zone appears to t e rmina t e against the Pal o Colorado- San Colorado fault onshore as a southeast- trending thr s t Gregorio fault zone . that a r allels t he coast or ap roxima ely 2 km , and A thi rd ma j or f aul t , the probable of f s hor e c ontinua then bends to the east . On the northeast s~d e o t he tion of the Sur- acimie nto fault zone , . lies a rallel t o , fault, quartz diorite is hrust over sandstones of but offshore of , the Palo Colorado- San Gr egorio f a ult Cretaceous age . The fault plane robably d~ps 70° E., zone at the southwestern edge of the Mo nter ey Bay and separ a t ion may be as much as a thousand meters regio n . (Trask , 1926 , . 165) . Sur- acimiento faul t zone The probable seaward e xte s ·on of the lo Colora o Several faults occur on the shelf between Poi nt Sur fault a l so a pears o be u thrown on the northe t and Cypress Point . They are a l ined with onl and f a ul t s side , and Doh r e nwend (1971) calculated th t in the Sur- ac imiento fault zone , a major struc ural Pliocene and leistoce e sedimentary rocks at le st 200 feature in the southern Monterey Bay region. This zone, meter s thick have been bro ght into fault contact with described as a belt of faults of various kinds and ages the q uar t z diorite . Howe ver , sandy s~ltstone , rath r extending southeast f rom the Sur fault zone (Page , than quartz dior ite , was dredg on th surf c n near- sur face ar t s of the throw side of the scar 1970a, p . 670) , i s approximate ly 300 km l ong and includes the Sur thrust zone, the acimiento fault , and (Dohrenwend , 1971 ; Ellsworth , 1971) . other faults . side , seismic record s show a refl ctin un1t Along the shelf between Poi nt Sur and Hurricane Poi nt , c r ystalline rocks lying north of the ault . where the offshore e xtension o f the thrust zone shoul d sed imen t overlies the qu rtz diorite east of be found , seismic reflection profiles f ail t o reveal i t mus t lie w i th ~n the bubble ulse (i . e ., i must b the subsurface bedrock structure, perhaps because the l ess than 3 meters t hick) . Ellsworth and Dohrenw n rocks are highly folded and faulted . This complex zone (1971 ) have s uggested that ecause the sc r cons~ s ts of poor reflectivity may be bounded on the south by of easily eroded siltstone , it w s probably produc by recent f a ult offset, rath r than d1ffe renti l faults that are assumed to parallel the northwest tre nd Offshore of Point Lobes , th P lo is of the onshor e faults (pl . 1) . Th e two longest faults may join a fault to the northwest, and both may be part well defined in seismic reflection deeply incised eastern tributary of ( 1 . of a longer fault zone that e xtends southeastward , 3 , sect ion M- M' ) . Thee st w 11 of th parallel to the coast. composed of gra nitic rock . On th we s w 11 , Tightly folded and sheared rocks onshore south of sediment s 120 meters thick overlie a prob ly Point Sur s uggest the existenc e of suc h an o ffshore basement, su ggest~ng a t le st 120 meters o f fault (W. G. Gilbert, oral commun. , 1970) . Another with relative upward movem nt of the A fairly continuous inferred fault strikes . 40° W. from d r edg haul at the base of the west wall of this the zone of poor seismic reflectivity and has been t a ry coll ect ed a quartz dior ~te with ca aclas ic ur projected along the continental slope to connect with a ( tect o nically deformed quart z diorite with broken fault observed on another profile severa l kilometers t o bent phenocrys s) , which also sugg s s h t he f the north (pl . l) . The fault , buried beneath about 200 l ies i n t he t r ibutary valley . meters of late Tertiary(?) sediment , may be the offshor e 2 . Mo nter ey Bay trace of the Sur- acimiento fault , or it may be a f aul t The Palo Color ado- San Gregorio fault zone , whic h that Page (1970a) suggested should pass seaward of the includes the offshore e xtension of t he Serra Hill f ul , Farallon Islands , west of the Sur fault trace , for Page tre nd s appr oximat ely . 25 ° \" . down Carm 1 Canyon in believes that the islands lie within the Salinian bloc k . outer Monte r ey Ba y . Seismic r flection is of li tl Palo Colorado- San Gregorio Fault Zone ass istance in followi ng the faul s in Carmel a nd 1 . Cypress Point-Point Sur shelf Monter ey Ca nyons because s t eep bottom opogra hy re Two faults trend northwest from the coast, north of duces complex seismic r efl ec t ions (s~de echos n the zone of poor reflectivity; the southernmost leaves hyperbolic refl ect ions) that obscure subbottom reflec the coast near Hurricane Point , the northernmost near t ions . However , Ma rti n a nd Eme r y ( 1967 , . 2291 , fig . Kaslar Point (pl . l). The former comprises three s eg 5) map ed a fault i n Carml e Canyon on the basis of ments . The central and northern segments are delimited dredge hauls and unpubl ished g ophysical data ; y where well-defined reflectors southwest of the fault end recovered well-indurated middle Miocene limeston along abruptly against a zone of seismic incoherenc y (pl . 3 , the oute r west wa l l i n contras t t o granodior ite long section -' ) ; the southernmost segment may bend east the ea s t wa l l . On t his ba sis , and t he linear lcm nts ward and join the Ser ra Hill fault . The Serra Hill i n the s ea floor t opography , as det ermined by high fault (Sie rra Hill thrust of Trask , 1926) , as described pr ecision dep t h r ecorde r, the Palo Colorado- S n by Gilbert (1971 , p . 46) , is a major thr ust fault that Gregorio faul t i s project ed along the a xis o Carmel emerges from the ocean just north of Hurricane Point and Canyon . trends southeast . ear Hurricane Point , the fault dips ear the junc t ion o f carmel a nd ~1 onterey Canyons , 50°-60 ° NE . Granodiorit e northeast of the fault is Marti n and Eme ry (1967, p . 2289) m d a narrow band thrust over upper Miocene sandstone . Gilbert (1971) of pre- Cre taceous (?) me t amo r phic r ocks lying betw en estimated that the vertical separation on the fault was two northwest-tre nding par all el faul t s that s par a it probably at least 300 meters . from middle Mi oc ene sili ceous sil s t ones on t h west and Seismic profiles across the probable offshore e xten Cretaceous i ntrusi ve r ocks o n t he east. On pla e l sion also show what appears to be a fault contact these faults are gene r a l i zed as a single f ault , which ~s between well- bedded sedimentary rocks to, the south , and p rojected to the nor thwes , follow i ng linear topogra hie rocks with s eismic r eflect ion t yp i cal of crystalline and elements to the southe rn edge of Ano uevo Point - Santa well- indurated r ocks to the north . The fault trends Cruz shelf , where seismic records clearly indicate northwest down the a xis of the western tribut ary of faulting . Carmel Canyon and may have controlled the location of 3 . Ano uevo Point-Santa Cr uz Shelf this submarine canyon (pl. 1 and 3 , section M- M' ) . Two northwest-tre nding par allel aults cut cross he Northeast of the offshore extension of the Serr a Hill Ano uevo Point-Sant a Cr uz shelf . As far as can b fault , a well- defined , fairly continuous offshore faul t determined by the spacing of the s ismic rofile lines , has been i dentified . The southern part of the fault has t he faults appear to be conti nuous for mo r e t han 26 km been located by s eismic r eflec tion and bathymetri c pro and bound a de formed zone i n which t he r ocks dip more f iles. Li ke t he offshor e e xtension of the Serra Hill steepl y than i n adj acent a r ea s . The charact e r of the f ault, thi s fault lies between strati fied rocks , some s eismic ref lec tion in thi s zone ind icates that i n s ome locally disturbed , and s e ismi c r eflec t ors that indicate areas dips may exceed 35° o r t hat t he rocks may have c rystalline or w e ~l - indurated roc ks (pl . 3 , section been sheared by additi onal faul t i ng (pl . 4 , p ro f iles M-M ' ). The sea floor i s about 2 mete rs higher east of A-A '-D-D ' ) . Hoski ns and Gr i ffiths (1971) indicat ed the fault, form "ng a west facing s carp that may be f aults at about the same locati ons . They suggested r e lated to recent faulti ng. Dohrenwend (1971 ) and that these faults did not affect rocks above a buried Ellsworth (1971) obtained high-resolution (7 . 5 KHz) erosional unconformity of late Miocene age . How ever , ba thymetric profiles which indicate that this scarp both the intermediate- pe netration and high- resolution

3 profiles indicate that these f ults also cut the over near canyon del Rey. The fault also has a vertical lying younger rocks, and in some places closely approach separation of possibly 300 meters , with the upthrown the modern ocean floor . block on the southwest . If it connects with one of the Onshore faults that are the robable continuation of continuous offshore faults, the Chupines fault is over these offshore faults also indicate relatively recent 26 km long . displacement. The easternmost fault l~es directly on Another major onshore fault that may be continuous trend with the San Gregorio fault, which juxtaposes the with offshore faults in southern Monter ey Bay is the Pliocene Purisima Formation and the upper Miocene Santa Tularcitos fault . Plate 1 shows the Tular citos fault Cruz Mudstone (Clark, 1970a). The western fault lies bending northwest , under Carmel Valley (Bowen , 1969) , on trend with a fault on Ano uevo Point along which the but it may inst ead continue in a more nor therly direc Miocene Monterey Shale has been thrust (northeast side tion across the Meadow Tract area of the mountains of up) over Pleistocene marine terrace deposits (Clark , Monterey Peninsula , then pass under the alluvium at the l970a; J . G. Evans and K. R. LaJoie, written commun ., northern base of the mountains near Seaside , and 1971). A shear zone several meters wide on Ano uevo finally trend out to sea near Laguna del Rey (R. R. Po·n , between the thrust fault and the San Gregorio Thorup, consulting geologist , Monterey , oral commun ., fault, suggests a fault trending . 30° W. (K. R. 1972) . If it does continue uninterrupted into Monterey Lajoie , G. E. Weber and J. C. Tinsley , written commun . , Bay and joins the southern continuous offshore fault , 1971) hat probably continues offshore into the deformed the Tularcitos fault is over 42 km long. T. W. Dibblee zone, but is not discernible on the high- resolution (oral commun . , 1973) suggests that rather than continu records. Thus the deformed zone a pears to come ashore ing across the Meadow Tract area , the Tularcitos fault , in what might be called the San Gregorio fault zone . which is a southwest- dipping reverse fault where locally To the south, the easternmost offshore fault bounding exposed , bends slightly to the west , then dies out this narrow fault zone either ends or changes its northwestward in Carmel Valley . Howe ver , he s uggests character so that it is unrecognizable by seismic profil that a branch of the fault may extend as a zone of ing. Hoskins and Griffiths (1971) mapped the end of the discontinuous faults northwest across the Meadow Tract fault in about the same location as on plate 1 . The area toward Seaside . westernmost offshore fault that bounds the western side In southern 1onterey Bay, the northeastern boundary of of the zone (pl. 1) is e x tended across Monterey Canyon the Monterey Bay fault zone is gradational and lies along linear bottom topographic features , and is joined along a continuous fault and several en echelon discon with the carmel Canyon fault . If this interpretation tinuous faults (pls. 1 and 3 , section P- P ' , fix 10 .5). is correct, the San Gregorio fault zone joins the Palo The northeastern boundary may be the offshore e xtension Colorado fault . of a major fault here referred to as the King City fault (B . L. Clark , 1929; R. D. Reed , 1933). Onland projec Monterey Bay Fault Zone 1 . Southern Monterey Bay tion of the more continuous fault is alined with an South of Monterey Canyon , the Monterey Bay fault zone inferred hidden trace of the King City fault (also comprises many en echelon faults identified from both called Gabilan fault) as projected from the base of the high-resolution and intermediate-penetration seismic Sierra de Salinas to the ocean just south of the town of reflection profiles (pls . 1 and 4, i ntermediate-penetra Marina. tion sections J - J ' through L- L' ). A third of the faults On land the King City fault is a major structural are at least 1.6 km long , or longer , and have been feature , a high-angle reverse fault along which granitic correlated between two or more track lines on the basis rocks of the Sierra de Salinas were uplifted to form the of similar structural characteristics . Two-thirds of western border of the Salinas Valley (Reed , 1925 , 1933; the faults , however , were identified on one track line Clark , 1929; and Sieck , 1964 , p. 20). The vertical only and have b en mapped as parallel to the adjacen separation along this fault decreases to the north , more continuous faults . toward Monterey Bay , where it may die out . To the Onshore and offshore faults in southern onterey Bay southeast , the King City fault was presumed to e xtend cannot be correlated owing to insufficient data in the concealed under the southwest margin of the Salinas onshore coastal region. However , among the generally Valley (Clark, 1929 , p . 204 ; Reed, 1933 , p . 43 , 44) . discontinuous faults in southern Monterey Bay , the three Schombel (1943 , p . 467- 470) projects the King City fault most continuous ap ear to e xtend onshore between Sand only a short distance i nto the Salinas Valley west of City and Marina . Two are about 9 km long , and the third Greenfield and Durham (oral commun ., 1972) suggests that is about 3 km . One of the longer faults may be the off the fault e xtends up the Salinas Valley from Monterey shore e xtension of the Chupines fault , which may enter Bay to j ust west of Greenfield where it apparently dies the ocean north of Monterey near Seaside. out . Along the southern margin of the Salinas Valley , Interpretation of offshore seismic reflection profiles beneath the alluvial fans near the mouth of Pine Canyon , and onland geophysical data indicates that the two gravity data (Fairborn , 1963) suggest about 2500 meters longer continuous offshore faults and the onland separation , whereas at the west end of the Sierra de Chupines fault e xhibit the same sense of separation . At Salinas , gravity data (Sieck , 1964) indicate between 900 depth, all three faults have Ter tiary sedimentary rocks and 1200 meters separation . If the offshore correlation downthrown on the northeast against Mesozoic granite on is correct , the vertical separation has decreased to the southwest ( 1 . 3 , section K- K', fix 14 . 5 and sections approximately 240 meters where the fault crosses section K-K' and L-L' , between fixes 11 and 12) . The high P- P ' offshore ( ls . 1 and 3) . resolution profiles across the nearshore part of the The Reliz fault , first mapped by Schombel (1943 , p . southernmost continuous fault , where the fault offsets 467) acr o s s Arr oyo Seco Cr eek , was interpr eted to be a the modern sea floor , show drag folding that indicates branch of the King City fault; however , Durham (1970 , the opposite sense of displacement . The drag folding may pl . 2) terminates the Reliz faul t near Olsen Ranch . be the result of recent motion on the fault , which might Gribi (1967 , p . 91) and Walrond and others (1967) , on differ from the predominant displacement , or may indicate the other hand , e x tend the Reliz fault northwest into that the fault has some strike-slip component . the fault previously designated as the King City fault On land, the Chupines fault trends northwestward from along the base of Sierra d e Salinas and call the entire the Sierra de Salinas Mountains and extends beneath an combination the Reliz fault . Dibblee (1972) concurs area of alluvial deposits several kilometers wide near with this designation , on the basis of the linear posi the coast . Southeast of the alluvial cover , the fault tion of the steep front of the Sierr a de Salinas and its is well defined in the mountains where the Miocene near alinement with the Reliz fault of Schombel (1943) Monterey Formation is faulted against the lower Pleisto south of Olsen Ranch . Dibblee (1972) also alines the cene Aromas Red Sands (Allen , 1946) and younger Reliz fault with the Rinconada fault , which he considers surficial alluvial deposits (Jennings and Strand , 1958; to e x tend for over 110 km to the southeast . Bowen, 1969; Calif . Dept . Water Resources, 1970) . At The projection of the King City fault beneath the depth the Monterey Formation is in fault contact with alluvial cover near the coast along the lower Salinas the granitic basement rocks . H. C. Sieck interpreted Valley is inferred from water wells by the California gravity data (1964) as indicating that the Department of Water Resources (1970) . From well data , Chu ines fault lies beneath the alluvium at the base of within 3 km of the coast near the town of Marina, it the mountains and suggested that strike-slip movement appears that the King City fault has a probable post along the fault may have displaced the Monterey Formation Pleistocene vertical separation of 30 meters , south side

4 up (Cal~f . Dept . Water Resources, 1970, sheet 5, pl . 2) . for the trough in the basement beneath Elkhorn Slough Movement along the fault has brought the liocene that Starke and Howard (1968) showed. Monterey Formation , on the southwest, against the upper The coarse definition of the shallower reflectors in Pliocene and lower Pleistocene Paso Robles Formation . the 160-kj seismic rofiles makes it difficult to At Fort Ord , wells penetrate the Paso Robles Formation determine the stratigraphically highest strata the faul at an altitude of 30 meters above sea level southwest cuts. However , it appears to extend u to the base of of the fault . Across the fault , to the northeast, only the modern canyon fill. Faulting in the granites, and post-Paso Robles alluvial materials are penetrated; the possibly in overlying strata, has probably influenced Paso Robles on this side is downdropped more than 160 the developmen of Monterey Canyon . meters below sea level, below the depth of the deepest (180 meters) wells (R. S. Ford , written commun., 1972). SEA-FLOOR SCARPS AND DISCO T UITIES OT DUE TO FAULT! G The southwestern limit of the Monterey Bay fault zone in southern Monterey Bay is represented by a series of Several sea floor scarps mapped in the Monterey Bay parallel faults trending northwestward from Cypress region resemble faults in some respects but pear to Point (pl . 1) . Three of these faults displace the sea have had other origins. These features are gener lly floor by 1 to 5 meters; two show relative uplift on restricted to central Monterey Bay, in and round the southwest, the other shows relative uplift on the Monterey Canyon near Moss Landing, and along the shelf northeast. The most continuous fault along this bound break at the top of the continental slope. The most ary may connect with the onland Cypress Point fault pronounced and abundan features are slump sc rps that that extends southeastward from Cypress Point to lie along most of the walls in the headward art of Pescadero Point and across Carmel Bay to connect with a Monterey Canyon (dashed single-hachured line on late l; fault on a point, "Abalone Point , " just north of the plate 3 , section 8-8 ' ) . Similar scarps occur at epth mouth of the Carmel River (Bowen, 1969). Bowen (1969) of about 125 meters near the edge of the continen 1 continues this fault from Abalone Point across the mouth shelf . Hummocky topography downslope from some of these of Carmel Valley, beneath terrace and alluvial deposits, scarps indicates that they are related to slum ing of and joins it to the Tularcitos fault . unconsolidated sediment . 2 . orthern Monterey Bay Other seaward-facing scarps at the edg of the conti North of lonterey Canyon , the !onterey Bay fault zone nental shelf appear to be erosional features at th is composed of many faults identified primarily from landward edge of young sediments deposited w~ h forese high-resolution seismic records (pls . 1 and 3 , high bedding, on a prograding continental slope (dash resolution sections A-A' through E-E ' ) . A high-powered double-hachured line, pl . l; pl. 3, sec ion E-E', fix seismic survey made in Monterey Bay late in 1972 appears 14) . Eli Silver (oral commun ., 1971) attribut s simi! r to show deeper faults in this area than can be seen in features along the northern california coast to wav the high-resolution and intermediate- penetration pro erosion and accompanying deposition that occurred whil files discussed here. Most faults are downthrown on the sea level was lower , and Dietz (1952, p. 1809) advanc d landward side . A third are 1.6 km long , or longer , and a similar ex_lanation . However, some of the sc r s my have been correlated between two or more track lines on be associated with downslope cree of contin nt 1 shelf the basis of similar separation of the same seismic sediments. reflectors and from similar associations with other sub An arcuate subsurface break, covered with about 10 surface structural features (i .e ., anticlines , synclines, meters of unbrok n sediment, lies on the south side of drag folds , etc . ) . Two-thirds of the faults , however , the head of Monterey Canyon (do -dash lin , 1. 1) . I cannot be correlatea between two track lines and have roughly parallels the canyon , and in the subsurface th been mapped as oriented parallel with adjacent continu north side has been downdro ped an unknown amount. Th~s ous faults . subsurface break appears to be the head of an incipien The longest fault lies in the center of the zone and slump that has moved toward Monterey Canyon . It may extends at least 9 .6 km (pls . 1 and 3 , sections C-C ' and have become stable and covered with lat Holocene sedi E- E', between fixes 9 and 10) . High-resolution records ment, or the slump may still be active and th br k show that it is accompanied by drag folding and off may be propagating toward the surface . setting of gently dipping reflectors that can be corre lated from line to line . In areas where high- resolution SEISMICITY A D EVIDE CE FOR RECE T FAULT! G profiles do not give good , correlatable seismic charac teristics , the fault has been continued from observations Earthquakes in the onterey Bay region indic te hat of similar seismic characteristics in intermediate the Palo Colorado- San Gregorio and Monterey Bay f ul penetration profiles. zones are seismically active . Historical accounts o The northeastern limit of the Monterey Bay fault zone earthquakes in the region date back o 1836 . Informa in northern Monterey Bay is a gradational boundary tion prior to the installation of seismographs in 1 34 represented by a decreasing number of discontinuous is based on reports of "felt" earthqu kes, locations o faults . The fault zone does not appear to cross the which cannot be accurately determined . (For a review o Palo Colorado- San Gregorio fault zone . the earthquake history of the region .from 1836 to 1968, Faults probably continue beneath the canyons, but they see Griggs, 1973.) could not be identified by the seismic-reflection method Sites of reliably located earthquakes that occurred used in this invest igation , for reasons noted earlier . from 1926 through ovember 1972 in th Mon er y Bay area However , linear trends in the topography of the bay floor are shown on late 2. E icenters of earthquak s e st of within the Monterey Bay fault zone parallel and may be the San Andreas fault are not shown, nor are epicenters controlled by the faulting ; the large northwest-south east of the Salinas Valley in the south and he Zayan e east meander in Monterey Canyon is parallel to the trend fault in the north. Epicenters of all arthquak s a L r of the faulting, as ar the channels of small valleys June 1972 are ap roximately located (R . L. W sson , tributary to the canyons . written commun., 1973). A 1972 geophysical survey along the central California The largest recorded earthquakes in the Mon er y Bay continental shelf produced evidence for the "Monterey region occurred in 1926 . Steinbrugge (1968, p . 77) Canyon fault , " which had been previously inferred to described them as follows: lie beneath , and parallel to , the headward a xis of "1926 October 22 , 4:35a.m . C nter on he contin ntal Monterey Canyon (Greene, 1970) . The sparker used in shelf off Monterey Bay. In ens~ty VIII a Santa Cruz , this survey (160 kj) was more powerful than seismic where many chimneys were thrown down; VII t C i ol , systems discussed in this re or , and thus able to Monterey, Salinas and Soquel . Felt from Healdsburg to penetrate dee er . In the higher powered sparker Lompoc [a distance of 250 miles or 450 kilometers] and rofiles the acoustic basement (Cretaceous granites) east to the Sierra , an area of nearly 100 , 000 square generally lies at a shallower depth on the sou h side of miles [180,000 square kilometers]. Another shock one the canyon than on the north and suggests a vertical hour later was similar to the first in almost every se aration of approximately 60 o 150 meters, south side respect." up (fig. 2) , w~th greater apparent offset offshore . Detection of earthquakes and determination of their The Monterey Canyon fault is about 10 km long. It location and focal depth (hypocenter) in the Monterey follows the axis of the canyon from the meander to the Bay area are difficult because the area lies largely mouth of Elkhorn Slough and may extend onland (pl . 1). outside the network of seismographic stations . Epicenter If the fa lt does e xtend onshore, it may be responsible locations for earthquakes that occurred before 1969 are

5 SEISMIC PROF ES s A A 0

<• DIP CORREC 10 er oco exoggero •on opproxomotel 5 :

HORIZO TAL SCALE ___ _j- 700 5600 A -- Second!. Meers Fee

:,;~8~ to-1 I 1A-l _____:_: ____ ~.....J Seconds

______;-D'o

Con tour s '" rothom' (100 fm t,. • l8Jm)

0 \.8 3.6 Km I 0 m~l~':' ~ 10-]1

1. Seconds

Figure 2.--Deep penetration (160 kj) seismic profiles and geologic interpretations across Monterey Canyon showing location of Monterey Canyon fault. 6 probably accurate to ±10 km . Epicenters of earthquakes possible planes along which the fault displacement might since 1969, the year that additional seismographs were have occurred, and these planes lie at right angles to inst.alled, are probably accurate to ±5 km. Focal depths each other . are probably accurate to ±10 km, and most appear to be Analysis of the epicenter cluster in the Monterey Bay shallow--within 15 km of the surface. fault zone (epicenters 6-8, pl. 2) indicates that the In the offshore area and the narrow onland fringe of fault planes are nearly vertical and that strike-slip the bay (as defined by lat 36°30' to 37°00' and long (horizontal) movement has occurred along a fault that 121°45'W to 122°30'W) there were 82 earthquakes of trends northeast or northwest (as indicated by the magnitude 0.9 to 6.1 from 1926 through November 1972. lines that divide each corresponding first-motion The number of small earthquakes reported has greatly diagram into the four quadrants on the insert on plat increased owing to improved methods of detection, as 2) . The reflection profiles show evidence of faults the list of earthquakes in table 1 shows: 2 earthquakes parallel to the northwest trend, but none parallel to of magnitude 6 . 1 in 1926 , 15 earthquakes of magnitude 4 the northeast trend, so we interpret the first-motion or greater from 1934 through 1961, 14 earthquakes of studies as indicating that right-1 ter 1 strike-slip magnitude 2 . 5 or greater from 1962 through 1968, 45 displacement is occurring on northwest-trending f ults . earthquakes of magnitude 0 .9 or greater from 1969 The locations of two large earthquakes of magnitud through 1971, and 6 earthquakes of magnitude 1 . 0 or 6.1 that occurred in 1926, noted earlier, are placed greater from 1971 through November 1972. within the Monterey Bay fault zone by Richter (1958). Distribution of Epicenters However, these epicenters were probably not located The distribution of epicenters in space and time accurately enough for them to be assigned with certainty indicates where and how often faulting occurs . Using to a particular fault zone, whether the Monter y Bay seismic records (fault- plane solutions), the direction zone, the Palo Colorado-S n Gregorio zone, or lsewhere. of movement can be determined on some faults. Most faults in the Monterey Bay faul zone displac e In the Monterey Bay area, many epicenters lie within late Tertiary and Pleistocene sediments and ar thus discrete zones associated with faults, whereas others geologically young . North of Monterey Canyon, f ults are more widely dispersed. The greatest concentration e xtend to within 6 meters of the ocean floor; most lies along the San Andreas fault; the more recent epi displace late Pliocene strata , som displace Pleistocene centers (solid circles on plate 2) are closer to the deposits, and a few may displace Holocene deposits . fault than those previously located. This apparent South of t-1onterey Canyon, most faults close to shore near difference in distribution may reflect the recent Monterey also extend to within 6 meters of the ocean refinement in the determinations of epicenter locations. floor and cut Pleistocene deposits. Some cut Holocene Epicenters. are also concentrated in two groups in deposits as well. Farther offshore, in southern Mont rey central Monterey Bay, where the southwestern edge of Bay, faults appear to be older because they cut only the Monterey Bay fault zone abuts the Palo Colorado- San upper Tertiary strata and are covered with about 100 Gregorio fault zone, and in a linear zone that trends meters of unfaulted sediment . A few faults cut only to the northwest along the Palo Colorado-San Gregorio Cretaceous granites and lower and middle T rtia y str ta. fault zone. There is also a small group of epicenters Scarps on the modern sea floor occur on seven faul s to the south where the Palo Colorado-San Gregorio fault in the l-1onterey Bay fault zone (indicated by a bar and zone comes ashore. box symbol on pl . 2) : three along the southwestern In the southeastern onland area there are three limit of the zone , two on the shel~ around the h ad of discrete groups of epicenters . One group of 13 (magni Soquel Canyon , one just offshore of the town o S aside, tude 1 . 0-4 . 5) , possibly related to the King City fault , and one on the southern shelf near the mean er in lies at the junction of the Salinas Valley and the north Monterey Canyon . All but two of the scarps face land eastern edge of the Santa Lucia Range , a few kilometers ward , and the rocks appear to be similar on both sides southwest of the town of Gonzales . orth of the first of the scarps. Thus , five are not wave-cut scarps group is a smaller group of epicenters (magnitude 1 . 2- associated with a lower stand of se level, nor ar they 2 . 4) of six earthquakes that occurred in 1972 and may be likely due to differential erosion . associated with movement along the King City fault. 2. Palo Colorado-San Gregorio fault zone Seven earthquake epicenters (magnitude 1 . 0-3.5) are A cluster of 14 epicenters in the Palo Colorado-S n located in the vicinity of Basin Creek and may be related Gregorio fault zone (pl. 2) represents earthquakes th t to movement along faults mapped by Durham (1970) between ranged in magnitude from 1 . 0 to 4.4. Three, in the Bruce Ranch and Reliz fault (pl . 2) . magnitude 4.1 to 4 . 4 range, occurred in the spring of In the northwestern onland area the earthquake epi 1971. Fault-plane solutions for five earthquakes s~nce centers are generally scattered, and most appear to be 1969 (inset on pl. 2) are interpreted as showing n rly associated with the San A.ndreas fault . However , some vertical fault planes and right-lateral strike-slip of the earthquakes that have occurred along a narrow motion . Solutions for four indicate a trend of approx (2 km wide) northwest-trending zone west of the san imately N. 20° W. parallel to the Palo Colorado-San Andreas fault , between San Juan Bautista in the south Gregorio fault zone. The solution for the fifth indi and Redwood Estates in the north , may be related to cates a trend parallel to the strike of the Monterey Bay movement along the Vergeles, Zayante , and Butane faults . fault zone (N. 60° W., ±10°) , suggesting a relatively Two earthquakes (magnitude 2 . 4-2 .5) occurred near the close spatial tie between the two fault zones . base of Sugarloaf Mountain, on the southwestern side of North of this cluster, 14 more epicenters lie in , or the Gabilan Range, in October and November , 1972 . close to the fault zone offshore and along its onland Eleven earthquakes have occurred in Happy Valley , just equivalent , the San Gregorio fault. The earthquakes north of Capitola , 10 (magnitude 4 . 0- 4 . 5) between 1934 ranged from magnitude 1.0 to 2 .3, and two occurred in and 1956 and one (magnitude 1.6) in 1969 . The 10 events July and September, 1972. are located to the nearest 1/4 degree of latitude and The two faults mapped between AOo uevo Poin nd the longitude, thus their e xact location is unknown. ep_icenter cluster appear in the seismic reflection pro 1 . Monterey Bay fault zone files to extend up to within 6 meters or less of the ocean floor (pls . 1 and 4, sections A-A', B-B ', nd Eight epicenters are clustered in the Monterey B~y fault zone just north of Monterey Canyon. The earth C-C') . These faults cut latest Tertiary strata and quakes ranged from 0 . 9 to 4.7 in magnitude . Three probably also Holocene deposits . Much of the eas ern (magnitude 3.0 , 3 . 7, and 4 . 7) occurred in August 1970 . most of these two faults coincides with a topogr phic Seismic records have been analyzed to determine the break in the ocean floor separating flat-lyin9 young direction of movement of the rocks on either side of the sediment on the west from higher standing bedrock on the fault during these earthquakes. If the movement is east . This topographic break might result in part from toward a seismograph , the first motion of seismic waves recent fault displacement .. Southeast of the epicenter cluster, the Palo Colorado is directed upward (indicating the ground is undergoing San Gregorio fault zone has been relatively quiet compression); if away, it is directed downward (indica ting the ground is undersoing dilatation). Knowing the seismically throughout the period of record , but there location of the hypocenter and the direction of first is no guarantee that it will remain so. Indeed, the segment of the San Andreas fault that generated the motion that reaches several seismographs, it is possible 1906 San Francisco earthquake is also quiet, although to define two quadrants of compression, and two of dila most agree that it will probably generate another large tation. However , this kind of analysis produces two earthquake .

7 Table 1.--Earthq akes in the !".onterey Bay area (withln lat 36"30' to 37"00 ' and long l21"45'W to 122"30 ' W), October 22 , 1926 , through November 30 , 1972

Ongin T1me (Greenwlch Mean T1me) Month ~ Hour Minu e Lati ude ( Longl tude (W) Focal Depth (Km) Magnitude Refer ence

1926 OCT 22 12 35 11.0 36°45 . 0 ' 122° 0.0 ' 6.1 Richter, 1958 1926 OCT 22 13 35 27 .o 36" 4 5 . 0 ' 122° 0.0 ' 6.1

1934 APR 23 16 8 0.0 37" 0 . 0 ' 122° 0.0 ' 4 .0 Ca liform.a Depar tment of 1935 JU 18 15 0.0 37° 0 . 0 ' 122° 0 . 0 ' 4 . 0 Wa t er Resour ces (1964 ) 1936 SEP 24 14 12 0.0 37° 0 . 0 ' 122° 0.0 ' 4 .0 937 OCT 27 15 53 0.0 37° 0 . 0 ' 122" 0 . 0 ' 4. 5 1937 OV 12 50 0.0 37° 0.0' 122° 0 . 0 ' 4 . 0 1938 FEB 12 20 0 0 . 0 37° 0 . 0 ' 122" 0 . 0 ' 4. 5 1939 JUL 17 9 24 0 . 0 37" 0 . 0 ' 122 ° 0. 0 ' 4 . 5 1940 MAR 13 27 0.0 37° 0 . 0 ' 122° 0 . 0 ' 4. 0 1941 APR 14 16 16 54. 0 36" 48 .0 ' 121 " 48 . 0' 4. 0 194 MAY 28 6 23 18 . 0 37 ° 0 .o ' 122" 0.0 ' 4 .5 1946 AUG 5 8 44 .o 36"51.0' 121"47 . 0 ' 4. 1 947 JUN 22 23 30 0.0 37° 0.0' 121"46 . 0 ' 4. 7 1947 NOV 15 22 30 0 . 0 36°4 7 .o' 122° 7 .0 ' 4.1 1956 OV 22 16 43 50 . 0 37° 0 . 0 ' 122° 0 . 0 ' 4. 2 1958 ov 7 21 33 24 . 0 36"52 . 0 ' 121"53 . 0 ' 4 . 3

1962 AUG 10 58 26.3 36"52 . 7 ' 122"17 . 5' 2. 7 Bulletin of Seismographic 1962 SEP 14 0 49 49 . 9 36° 38.4 ' 121"46 . 7 ' 3 .o Stat ion s, University of 1962 SEP 17 7 16 34. 3 36° 35.2 ' 121"52.6 ' 2 . 6 California , Be r keley (1962- 1962 DEC 24. 0 16 23.4 36"50 . 9 ' 121"47 . 4 ' 3. 7 1968) 1963 MAR 19 18 30 5 . 0 36"56 .6 ' 121°45 . 5 ' 3 . 0 1964 AUG 31 17 10 19 . l 36° 59 . l ' 121°47 .0 ' 2.5 1965 APR 6 10 31 59 . l 36°48 . 0 ' 121" 58 .o ' 3. 3 965 APR 6 11 23 10 . 4 36°49.6 ' 122° 6.7 ' 2. 5 1966 APR 21 17 58 . 5 37° 0 . 0 ' 121"46 . 0 ' 2.6 1966 MAY 15 7 10 . 5 36°56 . 0 ' 122"13 . 0 ' 2.5 1966 JUN 13 12 20 . 4 36°58 . 0 ' 122"12 . 0 ' 2.9 1966 OCT 25 20 54 42 . 5 36" 55 . 8 ' 121°48 . 2 ' 2 . 9 1967 FEB 5 7 36 23.3 36°47.5 ' 122" 7 .4' 2 . 6 1968 MAR 6 14 6 . 0 36"58 . 0 ' 122"10 . 0 ' 2.5

1969 JA 13 12 32.9 36"4 5 .9 ' 122° 5 . 6 ' 5.0 2.0 Lee a nd other s , 1972a 1969 MAY 6 7 8 43 .4 36°46 . 0 ' 122° 1. 4' 12 . 6 0 . 9 1969 JUN 19 6 46 4.3 36" 36 . 2 ' 121°4 7 . 6 ' 5 . 0 1. 1 1969 JUL 29 21 48 17.2 36"53.7 ' 1 22" 8 . 9 ' 11 . 6 1.9 1969 OCT 14 15 58.6 36°45 . 2 ' 122° 2 . 6 ' 7.2 1.7 1969 NOV 6 19 40 26 . 6 36°51.8 ' 122° 8 . 5 ' 9. 7 2 . 6

1970 FEB 19 23 17.5 36"59.0 ' 122"14. 1 ' 8. 7 2. 2 Lee a nd other s , 1972b 1970 FEB 27 23 21 4 5 . 6 36"46 . 0 ' 121"58 . 9 ' 5 . 0 2. 2 1970 APR 15 21 0 14 . l 36°44. 9 ' 121°51. 5 ' 5 . 0 1.2 1970 MAY 14 8 26 6 . 6 36°44. 5 ' 122° 3 . 7 ' 10 . 0 1. 0 1970 AUG 14 23.4 36° 4 5 . 0 ' 122° 3 . 5 ' 9 . 0 4 . 7 1970 AUG 44 7 . 4 36° 4 5 . 0 ' 122° 2 . 6 ' 9 . 2 3 .o 1970 AUG 48 53 .a 36° 4 5.8 ' 122° 1.8 ' 13 .3 1. 6 1970 AUG 4 4 59.6 36"58. 4' 122° 6 .4' 5 . 0 1.0 1970 AUG 7 8 15 17.1 36° 59 .4' 122° 7 . 1 ' 5.0 1.1 1970 AUG 23 17 53 48 . 6 36" 4 5 .4' 122° 2. 8 ' 9 . 6 3 . 7 1970 SEP 29 10 58 32 .4 36° 59.3 ' 121 " 47 .4' 12.6 2 . 0 1970 SEP 29 11 l 28.7 36"59. 4' 121"47 . 8 ' 13 . 2 1.3 1970 SEP 29 11 58 33 . l 36°59 .4' 12 1 " 47 . 5 ' 9.8 1.3 1970 SEP 29 23 1 7 2 . 7 36"59 . 2 ' 121 "48 . 7 ' 13 . 7 1. 0 1970 SEP 30 13 4 1 56 .a 36° 59.3 ' 121°4 7 . 6 ' 11 . 9 1.3 1970 SEP 30 18 27 3 . 6 36°59 . 6 ' 121"4 7 .4' 11 . 7 1. 3 1970 OCT 0 3 51.7 36"59 . 3 ' 121 " 48 . 7 ' 13.9 1. 2 1970 OCT 17 8 28 4 1.1 36° 59 . 1 ' 121°48 . 0' 12 .a 1.6 1970 DEC 0 14 46 . l 36°49.6 ' 122° 7 . 0 ' 10 . 0 1. 1

1971 J AN 15 55 . 6 36°4 6.5 ' 122° 1.8 ' 10 . 0 1.3 Lee a nd oth e r s , l972c 1971 MAR 3 41 38 .a 36"51.9 ' 121°56 . 3 ' 11.1 1.1 1971 MAR 8 9 10 18 . 9 36°48 .5 ' 1 22" 6 .4' 8 . 5 2 . 3 1971 MA R 8 18 3 1 46 . 5 36"48 . 1 ' 122°7 . 2 ' 8 . 6 4. 1 1971 MAR 9 15 35 16 . 3 36"48.4' 122°7 . 5 ' 9 . 0 4. 4 1971 MA R 23 7 0 . 6 36° 48 . 1 ' 122° 5 .8' 10 . 0 1. 4 1971 MA R 10 4 31 22 . 0 36" 48 . 8 ' 122° 7 . 2 ' 9 . 2 1.9 1971 MA R 10 9 15 42. 8 36°4 7.9 ' 122° 7 .1' 8 .4 2.6 1971 MAR 11 12 1 2 17.8 36°4 8 . 3 ' 122° 7 . 0 ' 9. 3 1.8 1971 MAR 11 16 33 37.8 36° 48.2 ' 122° 7 . 2 ' 9 . 8 1. 6 1971 MAR 26 1 40 . l 36 "48. 4' 122° 6 .4 ' 8. 2 2. 3 1971 MAR 26 4 1 18.8 36°48.5 ' 122° 6 . 4' 9.3 2 .o 1971 AP R 16 12 58 32.2 36°49 . 2 ' 122° 6 . 6 ' 8 . 2 4. 4 1971 APR 16 14 11 18 . 7 36°49.4' 122°6 . 7 ' 10.0 1.8 1971 APR 16 16 15 50 .s 36°49.4 ' 122° 6. 4 ' 7 . 2 1.2 1971 APR 16 16 19 33 . 3 36°49 . 1' 122° 6.6 ' 9 . 3 1. 8 1971 MA Y 21 20 10 30 . 0 36"59 . 4' 1 21 "47.4' 13 .4 1.7 1971 MAY 28 8 57 33 . 2 36~ 4 7.8 ' 122° 6 .1' 8. 2 1. 8 1971 JUL 13 1 4 17 31.9 36" 41.9 ' 121 " 47 . 8 ' 12 . 9 1. 2 1971 SEP 18 28 52 . 9 36°36.1 ' 1 21"56 . 9 ' 3 . 0 1. 0

1972 APR 30 12 4 3 26 .o 36°4 0 . 3 ' 122 ° 0 . 2 ' 5 . 2 2 . l We sson a nd others , 1973 1972 JUN 29 15 9 4 .a 36°33 .4' 1 21 "50.6 ' 7 . 5 1. 6

1972 JUL 18 54 56.2 36°32 . 2 ' 121"56 . 7 ' 2 . l 1.0 Wesson, wr i tten c orranun . , 1972 JUL 22 16 47.5 36° 59 . 2' 122"13.2 ' 10.0 1.5 1973 (preliminary 1972 AUG 26 2 53 27.1 36°46 . 0 ' 121"48 . 7 ' 8.9 1.5 loca tions) 1972 SEP 19 20 19 31.3 36° 59 . 9 ' 122" 13 .a ' 8.3 1.8 Two epicenters lie offshore of Kasler Point , and one estimates do not preclude the possibility that the Palo is on the Palo Colorado fault onshore near Rocky Point , Colorado-San Gregorio fault can produce an earthquake of but the probable inaccuracy in epicenter location does even greater magnitude than indicated. First, the not allow a definite tie to mo vement on a specific magnitudes listed in table 2 are derived from least fault . There is, however, geologic evidence for rela squares approximations (that is , some earthquakes have tively recent fault displacement in this area. The larger and smaller magnitudes for a given rupture previously discussed sea- floor scarp on the Palo length); second, t e rupture length might exceed half Colorado-San Gregorio fault zone that lies just north the mapped length; and, finally, the Palo Colorado of Kasler Point can be followed for 5 km and cuts fault might extend farther south than shown on the map. Holocene deposits . Furthermore , recent onshore An estimate of how large an earthquake can occur on reconnaissance mapping by Greene along the Point Sur the Palo Colorado-San Gregorio fault zone can also be Cypress Point coast has revealed a highly faulted and ~ta by a comparison with. the eometry and seismic sheared zone in the coastal terrace between Rocky Point h~story of the nearby Hayward fault. Without a consid and Kasler Point, in which many of the faults may erably better understanding than is presently avai.lable extend into deposits of probable Pleistocene age. Some of the structural relations of these two faults to each faults appea.r to have offset an elevated Pleistocene other and to the San Andreas fault, and the driving wave- cut platform and may have displaced overlying forces that produce earthquakes on each fault , there are Holocene alluvial deposits . valid objections to concluding that because an earth Onland north of the Palo Colorado fault three earth quake of a certain magnitude occurred on the Hayward quakes (magnitude 1.0-2 . 3) occurred in 1972; two are fault it should be expected to occur on the other fault . alined with a possible extension of the Church Creek However, the comparison may be valid for establishing fault . One (magnitude 1 . 4) occurred in August 1972 the possibility that an earthquake of similar magnitud near the Big Sur fault in the Sur thrust zone. can occur on the Palo Colorado-San Gregorio fault . Faults on the Cypress Point- Point Sur shelf vary in These faults are similar in gross aspect . Both age . Those west and southwest of Hurricane Point , approach the San Andreas at one end (Hayward t the including the offshore projection of the Sur fault, south, Palo Colorado-San Gregorio at the north). Both extend to between 50 and 200 meters of the ocean floor are long: the Hayward is 160 km long, including th on and cut late Tertiary strata; they appear to be covered trend strands of the active Rodgers Creek and Healdsburg with unfaulted Quaternary sediments. Northwest of faults (Brown, 1970) and the Palo Colorado-San Gregorio Hurricane Point the two faults mapped on the shelf is 205 km long. Both faults exhibit right-lateral appear from seismic-reflection profiles to generally strike-slip displacement . Finally, earthquake hypo extend to within 6 meters of the ocean floor , and centers are at approximately the same depth long both locally cut Holocene deposits . The probable offshore faults. continuation of the Serra Hill fault seems to become The Hayward and the Palo Colorado-San Gregorio f ults younger toward the north; it extends up to within 10 are also both part of the San Andreas fault system, an meters of the ocean floor in the south , cutting only thus respond to stress-generating forces that produce late Tertiary strata , and up to 6 meters in the north, earthquakes on the San Andreas fault . The Hayward cutting late Pleistocene and possibly Holocene strata. fault, which has been called an "active branch" of h San Andreas fault (Richter, 1958 , p . 476) , undergoes ESTIMATE OF HOW LARGE AN EARTHQUAKE COULD OCCUR tectonic creep (Radbruch and others , 1966) and has ON THE PALO COLORADO - SAN GREGORIO FAULT long history of seismic activity . An indication that the Palo Colorado-San Gregorio fault also acts in In order to anticipate the seismic forces that the response to stress along the San Andreas fault is man-made structures in an area might be subject to, it suggested by the following. Burford (1971) noted that is necessary to estimate how large an earthquake can a sequence of fault creep and small earthquakes on the occur on nearby faults . At present there is no absolute San Andreas fault near Pinnacles ational Monument way of determining how large an earthquake may be started at about the time of a magnitude 2 . 6 earthquake expected in a given area. One technique that has been (July 22, 1970) near San Juan Bautista , and ended with used is an analysis of the historic record in which the a magnitude 4.3 earthquake (August 3, 1970) in Monterey length of surface breakage on faults is compared with Bay. the magnitudes of the associated earthquakes (Tocher, Two major earthquakes have occurred on the Hayw rd 1958; Iida , 1965; Albee and Smith, 1967; Bonilla, fault during historic time, in 1836 and 1868. There are 1967; Bonilla and Buchanan, 1970) . no direct measurements of the magnitudes of these earth Estimating the possible magnitude of a large earth quakes , but the damage they caused suggests that they quake by this empirical relation necessitates making an were of large magnitude. Lawson (1908, p. 434) noted assumption as to what part of the fault might rupture that observers regarded the 1868 earthquake as severe as in a single event . In a study done for the Atomic the 1906 earthquake . Steinbrugge (1968, p. 73-74) Energy Commi ssion , Wentworth and others (1972) used a summarizes these earthquakes as follows: rupture length equal to half the fault length in esti "1836 June 10, 7:30a.m. One of the five largest mating the magnitude of earthquakes that might occur earthquakes centered in the San Francisco Bay region on several faults in California. They argued that in historic times . Ground breakage along the line of rupture is not likely to occur along the entire length the Hayward fault at the base of the hills e st of the of a fault in a single event. After comparing fault bay, e xte nding from Mission San Jose to San Pablo . As length and fault rupture length for data from southern strong or stronger than the shock of Oc tober 21, 1868, California (Allen and others , 1965) and also comparing which had its center along the same fault . At 1 ast data from Bonilla (1967) and original literature that one foreshock; numberous aftershocks for at 1 ast a indicates surface rupture length for 10 historic orth month. American events in which 2 to 75 percent of the fault 1868 Oc tober 21, 7:53a.m. One of California ' s grea length ruptured , they suggested that the half length shocks , and second of the two large Bay Area shocks of be used but cautioned that it must be considered only the 1860 ' s. Surface breakage was observed on the approximate at best. Haywa rd fault from Warm Springs to San Leandro , a Application of this empirical relation to the Palo distance of about 20 miles. The maximum horizontal Colorado-San Gregorio fault zone also necessitates offset was about 3 feet . Intensity X at Hayward , defining the fault length . The continuously mapped where every building was damaged, and many demolished . fault zone is about 135 km long, from the south end of Intensity IX at San Francisco , where, as in earlier the Palo Colorado fault to the northern end of the San large shocks, damage was chiefly confined to buildings Gregorio fault. However , the fault may be as much as on filled ground along the bayshore. About 30 persons 205 km long , for as noted earlier , it may continue to lost their lives in this shock. This earthquake was the north and include the Seal Cove fault at Half Moon felt at places 175 miles from the source . " Bay and its offshore extension , which Cooper (1971) Slemmons (1967) has assigned magnitudes of 7 ± .5 to mapped as joining the San Andreas fault zone at Bolinas. these two earthquakes. This is in agreement with For this reason, two different half lengths, 65 and 100 magnitudes estiamted in table 2 . Insofar as the analogy km, were used in calculating the estimates of magnitude between the Hayward and the Palo Colorado- San Gregorio listed in table 2. There are several reasons why these fault is valid, it appears that the latter is capable of

9 Table 2 . --Est.imates of eartilqual<.e magnitude for the Palo Colorado-San Gregorio fault zone Estimated magnitude

l. 2. 3 . 4. 5. Tocher Iida Albee and Smith Bonilla Bonilla and Buchanan Fault half lensth (k:m) (1958) (1965) (1967) (1967) (1970) Ave rase

65 7. 2 7. 4 7.6 7 . 6 7.4 7. 4

100 7 .4 7 . 6 7 . 8 7 . 8 7.9 7. 7

Magnitudes were es tahlished as follows'

l. M • 0.9 X log surface rupture length (L) in k:m + 5 .6

2. M • 0 . 76 (log L k:m) + 6 . 07

3. Least-squares fit of M vs L for California , Nevada and Baja California earthquakes on Albee and Smi til's figu.re 4.

4. M • 1.51 (log L miles) + 5.14

5. Read from graph of M vs L for worldwide strike-slip earthquakes.

Table 3. --General specifications of seismic systems used i n the 1969 and 1970 surveys

Fundamental Sound source H;,::dro12hones source Depth Distance towed Depth Separation High-resolution Energy Filters Sweep rate Fire rate frequency towed behind vessel towed fro.m source s12arker s;,::s tem ~ Hi cut~Lo cut (sec) (sec) (Hz) _!.&_ (m) _!.&_ (m) ~Re=m=ar~k~s~------

1969 survey 0 .6 645/250 0 . 25 0. 75 1000 0.6 1.5 30 . 5 in line A 6-metu , non- preamplified hydro phone cable with 11 crystal elements spaced 0. 3 m apart was used. A single multi point electrode sound source was used (see fig. 3).

1970 survey .6-.8 590/100 0.25 0.50 800 0.6 1.2-2.4 1.5 1. 5 in line A 6 -meter, non- pre amplified hydro phone st.reamer with 11 crystal elements spaced 0. 3 m apart was used. Two electrode sound sources spaced l. 2 m apart and towed abeam were used. Hydrophone streame r was towed between sparker electrodes (see fig. 4) .

Intermediate-

1969 survey 8-12 125/40 1.0 3.0 85 4 .s Abeam 4 . 5 55 ' 70 A 61-meter active section, preamplified hydrophone cable with ·100 crystal elements spaced "- . 6 m apart was used. Four 3-electrod EG'G sparker cage soun sources towed in a planar a.rray were used (see fig. 3) .

1970 survey 23 125/20 1.5 3 . 0 80 4.5 7.6 4.5 53.4 & 68.4 A 42 . 6 m active sectio preamplified hydrophon cable with 100 crystal elements spaced . 3 to .6 m apart and a so phone, 22.8 m active section preamplified streamer were used. Only 1 streamer was used at a time. A single point sparker electrode called a "ladder ," a name coine because of wooden step like braces that separ ate the positive elec trode from the ground electrode , was used (see fig. 4) . 10 producing major earthquakes . Bowen, 0. E ., 1969 , Geologic map of Monterey quadrangle : Calif. Div. Mines and Geology open-file map, scale SUMMARY AND CONCLUSIO S 1:62,500. Brabb, E. E., compiler, 1970, Preliminary geologic map 1 . There a r e two major fault zones in the offshore of the central Santa cruz Mountains, California: Monterey Bay area . The longest, the Palo Colorado u .s. Geol. Survey open-file report. San Gregorio fault zone, is a narrow, northwest Brown, R. D. , Jr ., 1972, Faults that are historically trending zone that joins onland faults--the Palo active or that show e vidence of geologically young Colorado fault south of Mon~erey , and the San surface displacement, San Francisco Bay Region, a Gregorio fault zone to the north at Ano Nuevo Point. progress report: October 1970: U.S. Geol. Survey The other zone, the Monterey Bay fault zone, lies on 1isc. Field Studies Map MF-331, 2 sheets, scale trend with the Salinas Valley and faults in the 1:250 ,000 . Salinas Valley and the Sierra de Salinas . It is a Burford, R. o ., 1971 , Fault creep and related seismicity wide belt of faults that crosses the bay floor and along the San Andreas fault system between Pinnacles Monterey submarine canyon and closely approaches, ational Monument and San Juan Bautista [abs.]: but does not appear to cross, the Palo Colorado-San Geol . Soc. America Abs. with Program, v. 3, no. 2, Gregorio fault zone. p . 90-91. California Department of Water Resources, 1964, crustal 2 . The offshore part of the Palo Colorado-San Gregorio strain and fault movement investigation: f ults nd fault zone has been mapped by other i nvestigators, earthquake epicenters in California: Calif . Dept. who used marine geophysical surveys and bottom Water Resources Bull. 116-2, 96 p. dredge hauls as their source of data. The southern 1970 , Lower Salinas Valley salt water intrusion part of the Monterey Bay fault zone has also been -----study: Calif . Dept . Water Resources open-fil previously mapped. report . Christensen, M. . , 1966, Quaternary of the California 3 . New detailed geophysical data (continuous subbottom Coast Ranges: in Bailey, E. H. , ed., Geology of acoustic profiles) are interpreted as indicating northern California: Calif . Div . Mines and Geology that these fault zones have had a long history . Bull. 190, p. 305-314. Evidence of fairly recent movement is seen in the Clark, B. L . , 1929, Tectonics of the Valle Grande of displacement of young sediment on the sea floor and California: Am. Assoc. Petroleum Geologists Bull . , the presence of scarps on the modern sea floor on v. 13, no. 3, p. 199-238. some segments of these faults. 1930, Tectonics of the Coast Ranges of middl ----C-alifornia : Geol. Soc. America Bull . , v. 41, p. 4 . Both zones are seismically active, as the earth 747-828. quakes that have occurred on them indicate . Clark , J. C., 1970a, Geologic map of the Davenport area, Santa Cruz County , California: U. S. Geol . Survey 5 . Fault-plane solutions of eight recent earthquakes open-file report , scale 1:24,000. in these fault zones indicate that the accompanying 1970b , Preliminary geologic and gravity maps of fault displacement is similar to that on the San -----the Santa Cruz--san Juan Bautista area, Sant cruz, Andreas fault . Movement on these nearly vertical Santa Clara, Monterey and San Benito COunties, faults has been horizontal (strike-slip), with rocks California: U.S . Geol. Survey open-file report. on the seaward side displaced to the north. Compton, R. R., 1966, Granitic and metamorphic rocks of the Salinian block, California Coast Ranges, in 6 . The proximity of these fault zones to areas of grow Bailey, E. H. , ed . , Geology of northern Calif;;nia: ing population necessitates estimating how large an Calif . Div . Mines and Geology Bull . 190, p . 277-287. earthquake might be produced by the Palo Colorado Cooper , Alan, 1971, Structure of the continental shelf San Gregorio fault zone, the longer of the two. west of San Francisco, California : U.S . Geol. Judging from empirical relations between fault rup Survey open-file rept . , 65 p. ture length and magnitude of associated eacthquakes Curray , J. R. , 1965, Structure of the continental margin on other faults, an earthquake of at least magnitude off central California : New York Acad. Sci . Trans., 7 . 2 to 7 . 9 could occur on the Palo Colorado-San ser. II, v . 27, no . 7, p . 794-801 . Gregorio fault zone . Similarities b~tween this zone 1966, Geologic structure of the continental margin and the Hayward fault support the suggestion that -----from subbottom profiles, northern and central Cali the Palo Colorado- San Gregorio fault zone can pro fornia , in Bailey, E. H. , ed., GeolOgy of northern duce earthquakes of large magnitude (estimated California : Calif . Div. Mines and Geology Bull . magnitude 7 ±0 . 5) . 190 , p. 337-342. Dibblee, T . w., Jr . , 1972, The Rinconada fault in the REFERENCES southern Coast Ranges, California , and its signifi cance, in Morrison , R. R. , ed . , Pacific geology, Albee, A. L., and Smith , J . L. , 1967 , Geologic criteria basis for new exploration: Pacific Sec ., AAPG- SEPM for nuclear power plant location: Soc. Mining Annual Meeting , Bakersfield , Calif. Engineers Trans . , v . 238 , p . 430-434. Dietz , R. S ., 1952, Geomorphic evolution of continental Allen, J. E ., 1946 , Geology of the San Juan Bautista terrace (continental shelf and slope): Am. Assoc. quadrangle , California : Calif . Div . Mines and Petroleum Geologists Bull . , v. 36, no. 9 , p . 1802- Geology Bull. 133 , 112 p. 1819 . Allen , C. R. , St. Amand , Pierre , Richter , C. F ., and Dohrenwend, J . C. , 1971 , Marine geology of the continen Nordquist, J. M. , 1965 , Relationship between tal shelf between Point Lobos and Point Sur , Cali seismicity and geologic structure in the southern fornia: A reconnaissance: M.S . thesis , Stanford California region: Seismol . Soc . America Bull ., Univ. , Stanford , Calif ., 60 p. v. 55 , no. 4 , p . 753- 797 . Durham, D. L., 1965 , Evidence of large strike-slip Baldwin, T. A., 1963 , Land forms of the Salinas Valley , displacement along a fault in the southern Salinas California , in Guidebook to the geology of the Valley, California: U.S. Geol . Survey Prof . Paper Salinas Valley and the San Andreas fault: Pacific 525-D, p . Dl06- Dlll . Sec ., Am. Assoc . Petroleum Geologists - Soc . Econ . 1970, Geology of the Sycamore Flat and Par aiso Paleontologists and Mineralogists Annual Spring -----Springs quadrangles , Monterey County , California: Field Trip, p. 11-13 . U.S . Geol . Survey Bull. 1285 , 34 p . Bonilla, M. G., 1967, Historic surface faulting in Ellsworth, w. L., 1971, Geology of the continental shelf , continental United States and adjacent parts of Point Lobos to Point Sur, Cal ifornia : M.S . thesis , Mexico1 U. S. Geol . Survey open-file report; also Stanford Univ ., Stanford, Calif ., 33 p . u . s . Atomic Energy Comm. Rept . TID-24124, 36 p . Fairborn, J . w., 1963 , Gravity survey and int erpretat ion Bonilla, M.G . , and Buchanan , J . M. , 1970, Interim of the northern por tion of the Salinas Valley: M. S . report on worldwide historic surface faulting: U.S . thesis , Stanfor d Univ . , St anf ord , Calif. 3 pls., Geol . Survey open-file report , 32 p . 33 p .

ll Gilbert, W. G., 1971, Sur fault zone, Monterey County, Reed, R. D., 1925, The post-liocene disturbance in the California: Ph . D. thesis, Stanford Univ ., Stanford , Salinas Valley, California : Jour. Geol., v . 33 , no. Calif . , 80 p. 6, p. 588-607. Greene, H. G., 1970 , Geology of southern Monterey Bay 1933, Geology of California : Tulsa, Okla ., Am . and its relationship to the ground water basin and ----A-ssoc . Petroleum Geologists , 355 p. salt water intrusion; u.s. Geol. Survey open- file Richter, C. F., 1958, Elementary seismology: San rept., 50 p . Francisco, w. H. Freeman & Co ., 768 p . Gribi , E . A. , 1967, Ancient shorelines of the Gabilan Rusnak , G. A., 1966, The continental margin of northern uplift, in Guidebook , Gabilan Range and adjacent and central California , in Bailey, E. H., ed., San Andreas fault : Pacific Sec ., AAPG-SEPM Annual Geology of northern California : Calif. Div . Mines Field Trip, p . 87- 91. and Geology Bull . 190, p. 325-335 . Griggs, G. B., 1973 , Recent earthquake activity between Schombel , L. F . , 1943 , Soledad quadrangle [Calif.) : Monterey and Half Moon Bay , California : Calif. Calif . Div. Mines and Geology Bull. 118 , p . 467-470 . Geology , Calif . Div . Mines and Geology [in press). Shepard , F. P., 1948 , Investigation of the head of Hoskins, E . G., and Griffiths , J . R., 1971 , Hydrocarbon Monterey Submarine Canyon: Scripps Inst . Oceanogra potential of northern and central California off phy, Submarine Geology Rept. 1 , 15 p. shore, in Cram, I. H., ed ., Future petroleum Shepard , F. P . , and Emery , K. 0 . , 1941 , Submarine topo provinces of the U. S . - Their geology and potential : graphy off the California coast: Geol . Soc . America Am . Assoc . Petroleum Geologists Mem. 15 , v . 1, Spec. Paper No . 31, p . 103-112. p . 212-228. Sieck, H. C., 1964 , A gravity investigation of the Iida, Kurnizi , 1965 , Earthquake magnitude , earthquake Monterey-Salinas area , California: M. S. thesis , fault, and source dimensions : agoya Univ ., Jour . Stanford Univ ., Stanford , Calif., 40 p. Earth Sci ., v . 13, no. 2, p . 115-132. Silver , E . A. , Curray , J. R., and Cooper, A. K., 1971, Jennings , C. W., and Burnett, J . L., 1961 , San Francisco Tectonic development of the continental margin off sheet, Geologic map of California , Olaf P . Jenkins central California : Geol . Soc. Sacramento , Ann . edition: Calif. Div. Mines and Geology , scale Field Trip Guidebook , p . 1-10 . 1:250 , 000 . Slemmons , D. B., 1967 , Pliocene and Quaternary crustal Jennings , C. W., and Strand , R. G., 1958 , Santa Cruz movements of the Basin and Range Province , USA : sheet, Geologic map of California , Olaf P . Jenkins Osaka City Univ ., Jour. Geo-Science , v. 10 , Art . 1- edition : Calif . Div. Mines and Geology , scale 11 , p . 91-103 . 1:250, 000. Starke , G. w., and Howard , A. D., 1968 , Polygenetic ori Johnson , W. M., 1855 , Extracts from a report on the gin of Monterey Submarine Canyon: Geol. Soc . features of the country between Pajaro and Salinas America Bull. v . 79 , p . 813-826 . River , California : Ann. Rept . U.S. Coast Survey Steinbrugge , K. V. , 1968 , Earthquake hazard in the San 1854 , app . 22 , p. 31-32 . Francisco Bay area: a continuing problem in public Lawson, A. C. , 1908 , The California earthquake of policy: Inst . Governmental Studies , Univ . Califor April 18 , 1906, in Report of the State Earthquake nia , Berkeley , 80 p . Investigation Commission , A. J. Lawson , chm.: Suppe , J . E., 1970 , Offset of Late Mesozoic basement Carnegie Inst . Washington, v. 1 , pt . 1. terrains by the San Andreas fault system: Geol . Soc. Lee , W. H. K. , Roller, J . C., Bauer , P . G. , and Johnson , America Bull. , v. 81 , p . 3253-3258. J . D. , 1972a, Catalog of earthquakes along the San Taliaferro , N. L., 1943 , Geologic history and structure Andreas fault system in central California for the of the central Coast Ranges of California: Calif . year 1969: U. S. Geol . Survey open-file rept., 48 p . Div . Mines and Geology Bull . 118 , p . 119- 163 . Lee, w. H. K. , Roller , J . C., Meagher, K. L., and Tocher , Donald , 1958 , Earthquake eoergy and ground Bennett , R. E. , 1972b, Catalog of earthquakes along breakage: Seismol . Soc . America Bull. , v. 48 , no. 2 , the San Andreas fault system in central California p. 147-153 . for the year 1970 : U.S . Geol . Survey open- file Trask , P . D. , 1926, Geology of Point Sur quadrangle , rept ., 73 p . California : Univ. Calif. Publ. Bull . Dept. Geol . Lee , w. H. K., Meagher , K. L., Bennett, R. E., and Sci ., v . 16 , no . 6 , p . 119-186 . Matamoros , E . E ., 1972c , Catalog of earthquakes Walrond , Henry , Thorup , R. R., Gribi , E . A., Jr ., and along the San Andreas fault system in central Cali Rogers , T. H. , 1967 , Geologic map of the Gabilan fornia for the year 1971: U.S. Geol . Survey open uplift and adjacent areas , Monterey , San Benito , file rept ., 67 p . Fresno and Santa Cruz Counties , in Guidebook, Louderback, G. D., 1942 , History of the Uhiversity of Gabilan Range and adjacent San Andreas fault : California seismographic stations and related acti Pacific Sec ., AAPG-SEPM Annual Field Trip. vities : Seismol . Soc . America Bull ., v. 32 , p. Wentworth , C. M., Bonilla, M. G. , and Buchanan , J . M., 205-230. 1972 , Seismic environment of the sodium pump test 1947 , California earthquakes of the 1830 's: facility at Burro Flats , Ventura County , California: ----S-eismol . Soc. America Bull., v. 37, p. 33-74 . U. S. Geol . Survey open-file report, 42 p . Martin, B. D., 1964 , Monterey submarine canyon , Cali Wesson R. L., Bennett , R. E. , and Meagher, K. L., 1972 , fornia : Genesis and relationship to continental Catalog of earthquakes along the San Andreas fault geology : Ph . D. thesis , Univ. Southern California , system in central California, January-March 1972: 249 p . U.S . Geol . Survey open- file report, 60 p . Martin, B. D., and Emery , K. 0 ., 1967 , Geology of Wesson , R. L., Bennett , R. E., and Lester, F. W., 1973 , Monterey Canyon , California : Am . Assoc. Petroleum Catalog of earthquakes along the San Andreas fault Geologists Bull. , v. 51 , p. 2281-2304 . system in central California , April-June 1972: U.S . Oakeshott, G. B., 1951 , Guide to the geology of Pfeiffer Geol . Survey open- file report . Big Sur State Park , Monterey County , Ca l ifornia: Wood , H. 0 ., and Heck , N. H., 1966 , Earthquake history Calif. Div . Mines Spec. Rept. 11 , 16 p . of the United States , Part II = stronger earthquakes Page , B. M. , 1966, Geology of the Coast Ranges of Cali of California and western Nevada : U.S . Govt . Print fornia , in Bailey, E. H., ed ., Geology of northern int Office , Washington, D.C., 48 p . California : Calif. Div. Mines and Geology Bull . Woodring, W. P ., 1938 , Pliocene and Pleistocene deforma 190 , p . 255- 276 . tion in the California Coast Ranges [abs . ] : Geol. 1970a, Sur- Nacimiento fault zone of Ca lifornia: Soc . Ame.rica Bull . , Proc . for 1937, v . 49, p . 212 . ----c-ontinental margin tectonics : Geol. Soc. America Bull. , v. 81 , p. 667-690 . 1970b , Time of completion of underthrust ing of ----F-ranciscan beneath Great Valley rocks west of Salinian block , California: Geol . Soc. America Bull. , v. 81 , p. 2825-2933 . Radbruch , D. H., Bonilla , M.G., Lennert , B. J., Blanchard , F. B., Laverty , C. L., Cluff , L . S ., and Steinbrugge , K. V., 1966 , Tectonic creep in the Hayward fault zone, California : U.S. Geol. Survey Circular 525 , 13 p .

12 Orientation of faults i s princi pally determined from APPE DIX the correlation of a fault from one line to another . Faults are correlated from one trac k to the next ma1nly Equipment, Procedures, and 1ethods by the i dentificati on of similar structural and seismic features in adj acent profiles . These features may con Both high- resolution and intermediate- penetration s i st of similar reflectors displaced in the same direc seismic profiles were· gathered. The high-resolution tion, with drag or other folding exhibited in a like system obtained 60 meters of penetration with about manner, or they may consist of contrasting seismic char 1 meter resolution , and the intermediate-penetration acteristic s on either s i de of the fault that is common system obtained about 1,500 meters of penetration with to both features . Those faults that cannot b correlated approximately 5 meters resolution. General specifica from one line to another are oriented parallel to contin tions of seismic systems used in the 1969 and 1970 uous faults that are close ogether (within 2 km) . If surveys are given in table 3. Towing arrangement of the there are no adjacent continuous faults by which strike sound sources and hydrophone streamers are shown in can be inferred, the fault is re resented by an o en figures 3 and 4 . diamon

13 1-----30'---~·1 (9ml

- 200'-250' <61-7Gm)---l 110 (33.5ml ----l LP. Rece~ver H.A. fle

I.P. Source

H . A. · H igh Aeso4utton

Figure 3 . --Towing arrangement of seismic sound sources and hydrophone streamers used in the 1969 survey. Not drawn to scale.

Fi gure 4 . --Towing arrangement of seismic sound sources and hydrophone str eamers used i n the 1970 s urvey . Not drawn t o scale .