The Cape Mendocino, California, Earthquakes of April 1992: Subduction at the Triple Junction

ARTICLE The Cape Mendocino, California, Earthquakes of April 1992: Subduction at the Triple Junction

D. Oppenheimer, G. Beroza, G. Carver, L. Dengler, J. Eaton, L. Gee, F. Gonzalez, A. Jayko, W. H. Li, M. Lisowski, M. Magee, G. Marshall, M. Murray, R. McPherson, B. Romanowicz, K. Satake, R. Simpson, P. Somerville, R. Stein, D. Valentine

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that caused by this sequence is primarily the the North America-Gorda plate boundary is seismogenic and illustrated hazards that could result of low population density and the result from much larger earthquakes forecast for the Cascadia region. The shock occurred predominance of small, wood frame struc- just north ofthe Mendocino Triple Junction and caused strong ground motion and moderate tures in the epicentral area. The sequence damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and caused 356 reported injuries, destroyed 202 propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two buildings, and caused damage to an addi- magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture tional 906 structures primarily in the towns on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and of Petrolia, Ferndale, Rio Dell, Scotia, and faulting at the triple junction suggests that the region is likely to continue experiencing Fortuna (Fig. 1) (3). It also triggered nu- significant seismicity. merous landslides and rock falls and caused widespread liquifaction in local river val- leys. Analysis of 1296 surveys in the north coast area indicate that the Modified Mer- On 25 April 1992 at 18:06 (UTC), a lion to $66 million and President Bush calli intensity peaked at IX in the Petrolia surface wave magnitude (Ms) 7.1 earth- declared the region a major disaster area. region and decreased in approximately a quake occurred near the town of Petrolia, Much of the damage resulted from the main radial pattern around the epicenter (Fig. 1). California (Fig. 1). The main shock was shock; however, fires triggered by the first Both of the two large aftershocks produced on September 7, 2011 followed the next day by two MS 6.6 after- large aftershock destroyed most of the peak intensities of VIII, although the pat- shocks at 07:41 and 11:41, located offshore Scotia shopping district, and both large, tern was somewhat different from the main about 25 km west-northwest of Petrolia. off-shore aftershocks caused additional shock. These three earthquakes and more than structural damage. The relatively low inci- 2000 recorded aftershocks illuminated the dence of injuries and structural damage Tectonic Setting configuration of the Mendocino Triple Junction, where the Pacific, North Ameri- The Cape Mendocino earthquakes are a ca, and southernmost Juan de Fuca (Gorda) response to ongoing plate motions between plates meet. The occurrence of a M 7 the Gorda, North America, and Pacific www.sciencemag.org earthquake is not unusual at the triple plates at the Mendocino Triple Junction. junction; over 60 earthquakes of Modified The Gorda plate is converging on the Mercalli intensity > VI (1) or M > 5.5 North America plate at about 2.5 to 3 have occurred there since 1853 (2). How- cm/year in the direction N50WE to N550E ever, this earthquake sequence may have (4). The seaward edge of Gorda plate sub- provided the first direct evidence of inter- duction is marked by an abrupt change in plate seismicity and thus impacts regional sea-floor topography and by the western Downloaded from hazard assessment. In this article, we de- limit of the accretionary prism imaged in scribe geophysical and seismological obser- seismic reflection profiles (5). Active folds vations and discuss implications for seismic and thrust faults in Franciscan Complex hazards in the Pacific Northwest. and Cenozoic rocks and sediments of the Damage estimates ranged from $48 mil- overriding North America plate are parallel to the seaward edge of the Cascadia subduc- D. Oppenheimer, J. Eaton, A. Jayko, M. Lisowski, G. Marshall, M. Murray, R. Simpson, and R. Stein are with tion zone (6). the U.S. Geological Survey, Menlo Park, CA 94025. G. Rigid plate theory predicts oblique con- Beroza and M. Magee are in the Geophysics Depart- vergence of the Gorda plate with the Pacific ment, Stanford University, Stanford, CA 94305. G. Carver, L. Dengler, and R. McPherson are in the plate at 5 cm/year in the direction N1 15VE Department of Geology, Humboldt State University, Fig. 1. Simplified tectonic map in the vicinity of (4). Translational motion occurs along the Arcata, CA 95521. L. Gee and B. Romanowicz are at the Cape Mendocino earthquake sequence. east-west-trending, vertical, right-lateral the University of California, Seismographic Station, Stars, epicenters of three largest earthquakes; ESB 475, Berkeley, CA 94720. F. Gonzalez is with the Mendocino transform fault, whereas the National Oceanic and Atmospheric Administration's contours, Modified Mercalli intensities (values, convergence results in internal deformation Pacific Marine Environmental Laboratory, Seattle, WA Roman numerals) of main shock; open circles, of the Gorda plate. The attendant Gorda 98115. W. H. Li is with Geological Sciences, AJ-20, strong motion instrument sites (22) (adjacent recorded in the University of Washington, Seattle, WA 98195. K. numbers give peak horizontal accelerations in plate seismicity 17 years Satake is in the Department of Geological Sciences, g). Abbreviations: FT, Fortuna; F, Ferndale; RD, before the Cape Mendocino sequence (7, University of Michigan, Ann Arbor, Ml 48109. P. Som- Rio Dell; S, Scotia; P, Petrolia; H, Honeydew; 8) (Fig. 2) has been concentrated in two erville is with Woodward-Clyde Consultants, Pasade- zones with a combined thickness of na, CA 91101. D. Valentine is in the Department of MF, Mendocino fault; CSZ, seaward edge of parallel Geological Sciences, University of California, Santa Cascadia subduction zone; and SAF, San An- approximately 15 km. In the region of the Barbara, CA 93106. dreas fault. Cape Mendocino earthquake, most seismic-

SCIENCE * VOL. 261 * 23 JULY 1993 433 ity locates at depths greater than 17 km; the phy, and seismicity of the Juan de Fuca- focal mechanism determined by the inver- hypocenter zone dips about 60 eastward Gorda plate system with other subducting sion of teleseismic mantle Rayleigh waves between 124.75° and 123.250W, at which plates suggest that it does not subduct aseis- and aftershock locations indicate nearly point the dip increases to about 250 (9). mically but instead is locked and capable of pure thrust motion on a N10TW-striking Most M > 5 earthquakes within the Gorda generating major earthquakes (14). Paleo- fault plane that dips 130 to the east-north- plate exhibit left-lateral motion on steep seismic evidence of large, late Holocene east (18) (Table 1). The location of the northeast-oriented faults (7, 10) that re- subduction earthquakes is present along the hypocenter at the southeast end of the lieve convergence between the Gorda and subduction zone in submerged and buried aftershock zone suggests that the fault rup- Pacific plates through slip on preexisting wetlands (15), raised marine terraces (16), tured unilaterally to the west (19). Most planes of weakness inherited at the Gorda and surface displacement on thrust faults aftershocks <12 km deep (Fig. 3, circles) Ridge (11). that may be genetically related to large occurred within 10 km of the coast in a The San Andreas fault marks the prin- subduction events (17). Radiocarbon dat- region bounded on the east by the main- cipal Pacific-North America plate bound- ing indicates that at least three episodes of shock epicenter, on the south by the Men- ary south of the Mendocino Triple Junc- seismicity of similar age are represented in docino fault, and on the north by a west- tion. Triangulation data and observations the stratigraphy from central Washington northwest trend of earthquakes. The loca- of ground cracks indicate the fault ruptured to northern California in the last 2000 tion, depth, and orientation of the rupture as far north as Point Delgada in 1906 (12), years; the last episode occurred at about plane are consistent with the absence of but its location farther north is uncertain. 1700 A.D. (17). surface faulting onshore. Some studies place it immediately offshore The two Ms 6.6 aftershocks were located (13), but others suggest that it merges with Observations 30 km west of the main shock at depths onshore faults at the triple junction (5). near 20 km, and their mechanisms indicate Geometry requires that the Pacific plate is Seismicity. The hypocenter of the 25 April right-lateral, strike-slip motion on planes also in contact with the North America 1992 main shock was located 4 km east of striking to the southeast (20) (Fig. 3 and plate along the Mendocino fault above the Petrolia at a depth of 10.6 km (Fig. 3). A Table 1). The slip plane of the first after- subducting Gorda plate. Until the Cape Mendocino earthquake, few earthquakes were recorded with focal B Cra a mechanisms that indicated slip on the Cas- cadia subduction zone. However, compari- on September 7, 2011 sons of the age, spreading rates, physiogra- 10 201 30[ 0 2 20 40

0 b b& . 4, - .,. www.sciencemag.org 10 a TL OD 20

30 - .; 'I 1 .. *-: o 10 20 -30 Downloaded from a _ _ _ C Cascadia W E .6-subduction zone c c' U101 p 0HI l& 501 .1 0 100 200 E10 10 Distance (km) " 1 . *.. T . ;- Fig. 2. Seismicity between August 1974 and i20 20 time of main shock (8). (A) Locations of oper- ating and discontinued seismic stations (upward and downward pointing triangles, respec- 30 30 1 A' time of main shock. of tively) at (B) Depths ta 0 10 20 30 earthquakes along cross section aa'. Depths Distance (kmn) Distance (km) west of longitude 1 24040'W are unreliable. Note the gap in seismicity between main shock rup- Fig. 3. (A) Focal mechanisms (lower hemisphere projections) of the main shock and two large ture plane at 10 km depth (Fig. 3B, aa') and aftershocks at their epicentral locations (compressional quadrants in black) and location of other pre-main shock seismicity at depths greater aftershocks for 25 April 1992 to 30 September 1992 (circles for foci < 12 km deep and plus symbols than 17 km. The earthquakes at cross section for deeper foci). (B) Depth of earthquakes on cross sections aa' (perpendicular to main shock distance 200 km occur at depths greater than strike, width + 20 km), bb' (perpendicular to Mendocino fault, width + 20 km), and cc' 35 km and begin to image the region of the (perpendicular to strike of M. 6.6 aftershocks, width + 9 km). Compressional quadrant marked by Gorda plate where the dip increases. Earth- 'T." (C) East-west cross section depicts location of main shock rupture plane (solid line), quakes as deep as 90 km occur near longitude hypocenters (stars), and pre-main shock seismicity with respect to plausible interpretation of 122010'W (9). Gorda-North America plate geometry. 434 SCIENCE * VOL. 261 * 23 JULY 1993 ------shock is unknown because of the paucity of west trend of seismicity may define the ed with the second aftershock may explain aftershocks. However, the second after- position of the Mendocino fault. some of the differences in the intensity shock was located within a trend of smaller Source properties. The mechanism and patterns for the main shock and two after- aftershocks at depths of 14 to 30 km on a location of the two aftershocks were simi- shocks. Although the second aftershock southeast-striking plane that dips about 800 lar, but the second aftershock exhibited a had 25% of the moment of the main shock, to the southwest (Fig. 3B, cc'); this orien- strong variation of amplitude with azimuth it has larger velocity amplitudes at stations tation is consistent with the focal mecha- (Fig. 4). The seismic moment of the second to the southeast, such as ISA. The differ- nism. The depths and mechanisms of the aftershock was approximately twice that of ence in both Modified Mercalli intensity two large aftershocks provide evidence that the first, but amplitudes of the P wave for and broad-band velocity records between rupture took place on faults in the Gorda this event were as much as 10 times as large the main shock and the second aftershock plate, distinct from the main shock fault. near an azimuth of 1300. This variation is was probably enhanced by rupture propaga- Although no large shocks ruptured the most easily attributable to enhancement of tion to the west during the main shock as Mendocino fault during this sequence, the amplitude in the direction of rupture, inferred from the location of the hypo- many aftershocks occurred on the eastward known as directivity (21). Directivity in P center at the down-dip end of the rupture projection of the fault (Fig. 3). The after- waves is surprising because it requires rup- plane. shock activity was bounded on the south ture velocities that are a large fraction of The strong ground motions of the main where the distribution of hypocenters is the P wave velocity. The high amplitudes shock and two aftershocks were recorded on near vertical and extends to a depth of 25 and strong high-frequency content associat- 14 instruments at epicentral distances of 5 km (Fig. 3B, bb'). If this marks the bound- to 130 km (Fig. 1), and the peak accelera- ary between the Gorda and Pacific plates, tions were some of the highest ever record- then the lack of any aftershocks in the ed (22). Recordings of the main shock at Pacific plate suggests that the main shock Petrolia and Cape Mendocino (Fig. 5) at represented strain release between the epicentral distances of 5 and 10 km, respec- Gorda and North America plates. The tively, have absolute time, facilitating the mapped location of the Mendocino fault in analysis of rupture evolution on the fault. this region is uncertain (5), and this east- Modeling of the large, long-period pulse that occurred 1 s after the main shock began (Fig. 5, Ep1 and Es1) with generalized ray theory indicates that this pulse originat- on September 7, 2011 ALE 0(X ) HRV (x4) ed from slip that occurred about 5 km ifV~~lv'n up-dip from the hypocenter, beneath Petro- 1 lia. This result is consistent with the arrival I times and polarities of the vertical P waves and horizontal S waves at both stations. In addition, the P wave first motions were ( 0) MAJO ISA upward and northwestward at Cape Men-

t ] 16.49 102x docino. These motions indicate that the 090/ source was southeast of the station. This www.sciencemag.org

01+0 1.59 x 102 C1/s2 source location is consistent with the west- U Time (s) - southwest direction of rupture determined waves al- 2s0 from teleseismic surface (19), Fig. 4. Broad-band velocity records from four though that study inferred that the rupture stations (ALE: epicentral distance = 5185 initiated offshore. A large, high-frequency km, azimuth = 90; HRV: A = 4366 km, azimuth pulse (Fig. 5, E2) followed the long-period = 690; MAJO: A = 8029 km, azimuth = 3050; Fig. 5. Strong motion recordings and three ig pulse at Cape Mendocino and exceeded Downloaded from ISA: 1 = 745 km, azimuth = 1320) for the main components of the peak accelerations from on both horizontal components. This pulse shock (top trace) and the first (middle) and stations of the California Strong Motion Instru- was not discernible at the neighboring second (lower) aftershocks. The amplitudes of mentation Program (CSMIP) at Cape Mendo- the seismograms at ALE, HRV, and MAJO are cino and Petrolia (22). Discernible first motion Petrolia station; thus, it cannot be ex- increased relative to ISA for display. The large directions of the P and S waves for long-period plained simply by source effects but may amplitudes of the second aftershock relative to event (Ep1 and ES1) are indicated, as is the represent motions that were generated or the other two events in the along-strike azimuth large, high-frequency pulse at Cape Mendo- amplified locally near the Cape Mendocino (ISA) is attributable to rupture directivity. cino (E2). station.

Table 1. Earthquake parameters.

Origin tie*time* (UTC) Latitude* Longitude* Depth* Centroid Monmentt SrktStriket DpDipt Raketae Orign (TC) (North) (West) (kin) deptht mbt Mst (dyn~cm) (kin) Main shock 25 April 18:06:05.16 40019.94' 124013.69' 10.6 20 to 25 6.3 7.1 4.45 x 1026 349.70 13.00 105.60 First after- 26 April 07:41:39.98 40026.13' 124034.43' 19.3 20 to 25 5.9 6.6 6.35 x 1025 122.30 75.90 175.20 shock Second after- 26 April 1 1:18:25.82 40023.38' 124034.30' 21.7 30 to 35 6.5 6.6 1.20 x 1Qo26 311.50 89.60 181.80 shock *Hypocentral location determined from the local seismic network of the U.S. Geological Survey (8). tFrom surface-wave inversion (18, 20). *National Earthquake Information Center, Preliminary Determination of Epicenters.

SCIENCE * VOL. 261 * 23 JULY 1993 435 Coseismic displacement. The elastic strain ceptable models is 2.5 x 1026 to 3.5 x 1026 3.5 hours), and Point Reyes (65 min and 3 released by the main shock caused signifi- dyn-cm, about 60% of the main shock hours). cant horizontal and vertical deformation in seismic moment (Table 1). The model pre- the epicentral region. The main shock ele- dicts a maximum uplift along the coast that Motion on the Plate Boundary vated about 25 km of the coast from 3 km is consistent with but somewhat less than south of Punta Gorda to Cape Mendocino the observed uplift. More complex models Interplate main shock. The main shock fault (Fig. 6). Many intertidal organisms inhab- that use nonuniform slip to describe the projects to the sea floor within 5 km of the iting rocky reefs perished in the 3 weeks rupture may improve these estimates of seaward edge of the Cascadia subduction after the main shock. Maximum uplift was uplift and geodetic moment. zone (25) (Fig. 3), suggesting that the main 140 + 20 cm at Mussel Rock and 40 to 50 Tsunani. The main shock generated a shock ruptured the Gorda-North America cm at the northernmost reef at Cape Men- small tsunami recorded by tide gauges along plate boundary. In contrast, the upper docino (23). The lack of rocky intertidal the California, southern Oregon, and Ha- boundary of the pre-main shock seismicity, environments farther north precluded the waii coastlines (Fig. 7). The largest tsunami which is 7 km deeper than the main shock precise location of the northern limit of the amplitudes were recorded at Crescent City, rupture plane (Figs. 2 and 3), projects to uplift, but several near-shore rocks located California, where two well-defined packets the surface about 85 km west of the Casca- about 7 km north of Cape Mendocino of wave energy were recorded within the dia subduction zone and thus does not showed no evidence of uplift. first 5 hours with maximum positive heights appear to define the plate boundary. The Coseismic horizontal and vertical site of 35 and 53 cm. Neither the precise arrival seismicity gap between the slip plane of the displacements in a regional geodetic net- time nor the polarity of the first wave are main shock and the pre-main shock seis- work (Fig. 6) were determined from Global clear because of the presence of background micity is about the same thickness as the Positioning System (GPS) surveys in 1989, noise. However, the first packet of wave Gorda crust and overlying accretionary sed- 1991, and 1 month after the main shock. energy is consistent with the predicted trav- iments, as determined from refraction ex- The relative positions of most sites near the el time of 47 min for a wave ray path that periments 10 km east of the seaward edge of epicenter were measured shortly after the traversed deep water. The second wave the subduction zone (26). The gap may 17 August 1991 Honeydew earthquake packet probably represents coastal trapped reflect a ductile subducted Gorda crust, and [body wave magnitude (mb) 6.0], which waves, or edge waves, having much slower the inception of seismicity at a depth of 17 occurred 6 km south of the Cape Mendo- velocities and amplitudes that rapidly de- km may reflect brittle behavior of the cino epicenter. The coseismic displace- crease with distance offshore. Because the Gorda upper mantle (27). Tabor and Smith ments were determined by comparison ofthe tsunami arrival nearly coincided with low (28) reached a similar conclusion from their on September 7, 2011 1989 to 1992 observations, except in the tide at Crescent City, the wave did not observations of seismicity and velocity vicinity of the Honeydew event where the cause any damage. The tsunami at Crescent structure of the Juan de Fuca plate beneath 1991 survey was referenced. All displace- City had an 8-hour duration; wave heights the Olympic peninsula of Washington. ments were corrected for secular strain accu- reached a maximum 3 to 4 hours after the However, an inversion for the three- mulation estimated from Geodolite trilater- first arrival. Tide gauges also recorded the dimensional velocity structure of the region ation measurements made between 1981 and initial arrival and subsequent edge waves at indicates that velocities typical of Gorda 1989. A site 13 km northeast of the epicen- North Spit (Eureka, California) (20 min crust are evident at depths greater than 15 ter had the largest measured coseismic dis- and 2.5 hours), Arena Cove (35 min and to 20 km (29). Moreover, modeling of placement, moving 40 ± 2 cm to the west- www.sciencemag.org southwest and subsiding 16 ± 8 cm. Our preferred uniform-slip fault model Fig. 6. Observed and predicted (24), estimated from the coseismic site coseismic displacements for the observa- Cape Mendocino main shock (epi- displacements and coastal uplift center located at star). The vec- tions, indicates 2.7 m of nearly pure thrust tors are horizontal displacements motion occurred on a gently dipping fault relative to a site located at plane. This model, chosen from a suite of 4109.20'N, 123052.92'W. Ob- acceptable models (24), is consistent with served displacements derived Downloaded from the main shock focal mechanism, the hy- from GPS and Geodolite mea- pocenter location, and the distribution of surements; ellipses enclose re- aftershocks (Fig. 6 and Table 2). The range 40'40' - gions of 95% confidence. Predict- of geodetic moment inferred from the ac- ed displacements are from a model of uniform slip on the north- (~" east-dipping rectangular fault Table 2. Displacement model. plane, indicated by its surface projection. Rounded rectangles Parameter Value show vertical displacements measured by GPS that are great- Width 16 km er than their standard deviations. Length 21.5 km Contours are elevation changes Depth to top edge 6.3 km 40 2O' - in millimeters predicted by the Latitude origin* 40018.08'N model. Abbreviations: CM, Cape Longitude origin* 124°11 .80'W Mendocino; MR, Mussel Rock; Depth at epicenter 9.2 km and PG, Punta Gorda. (Inset) Up- location lift measurements and their stan- Momentt 2.79 x 1026 dyn'cm dard deviations from the die-off of Strike/dip/rake 3500/12.00/940 51) marine organisms at coastal sites Slip 2.7 m (open circles on map) and pre- *Southeast corner of fault plane. tAssumes uni- dicted uplift projected along form rigidity of 3 x 10l dyn/cm2. N1 0°W.

436 SCIENCE * VOL. 261 * 23 JULY 1993 thermal effects on the strength of the sub- reduces north-south compressional stress tive hypocentral locations of the two large ducting oceanic lithosphere (30) suggests and down-dip tension in the Gorda plate, aftershocks, slip on the northeast-oriented that the double seismic layers observed at but rupture of the northwest-oriented plane plane of the first aftershock would have depths of 20 and 30 km (Fig. 2) reflect, may have been favored because of the static added minor left-lateral shear to the north- respectively, the brittle upper crust and stress changes imposed by the main shock west-oriented plane of the second after- upper mantle of the Gorda plate; the inter- and, perhaps, the first aftershock. shock but would have greatly decreased the vening, relatively aseismic region would To test this hypothesis, we modeled the normal stress on this plane. This scenario correspond to the ductile lower crust. Con- changes in static stress (31) imposed by the provides a simple mechanism in which the sequently, these studies suggest that the main shock (Table 2) on three vertical fault first aftershock helped trigger the second, Cape Mendocino main shock was an intra- planes: the east-west-oriented Mendocino similar to the scenario proposed for the plate event in the North America plate. fault, the possible N40E-oriented slip Elmore Ranch-Superstition Hills pair of Whether the main shock was an inter- or plane of the first large aftershock, and the earthquakes (32). intra-plate event, the Cape Mendocino main N500W-oriented fault of the second large Hazard implications. The Cape Mendo- shock clearly relieved strain resulting from the Gorda aftershock. Large regions of the cino earthquake sequence provided seismo- relative Gorda-North America plate motion. northwest-oriented fault and the Mendo- logical evidence that the relative motion We note, however, that the main shock cino fault received an increase in right- between the North America and Gorda ruptured a region of the plate boundary that lateral shear stress greater than 3 bars and plates results in significant thrust earth- differs considerably from the boundary farther equally large increases in normal extension quakes. In addition to the large ground north, as indicated by the change in its resulting from the main shock. Both the motions generated by such shocks, they can orientation from north-northwest to north- increase in right-lateral shear and the in- trigger equally hazardous aftershock se- west (5), the relatively narrow width of the creased extension would bring both of quences offshore in the Gorda plate and on plate, the likely presence of subducted sedi- these right-lateral faults closer to failure the Gorda-Pacific plate boundary. This se- ments in the region of main shock rupture, under a Coulomb failure criterion for non- quence illustrates how a shallow thrust and its younger age (4). Thus, this earth- zero coefficients of friction. About 90% of event, such as the one of moment magni- quake may not be typical of other Cascadia the aftershocks within 4 km of the Men- tude (Mv) 8.5 that is forecast for the entire subduction zone earthquakes. docino fault and the northwest Gorda Cascadia subduction zone (14), could gen- Intraplate aftershocks. The location, fault occur where the modeling predicts erate a tsunami of greater amplitude than depth, and focal mechanisms of the two the stress changes should load these faults the Cape Mendocino main shock. Not only large aftershocks indicate that they rup- toward failure for coefficients of friction would this tsunami inundate communities on September 7, 2011 tured the Gorda plate. The seismic data ranging from 0.0 to 0.75. Thus, static along much of the Pacific Northwest coast indicate that right-lateral slip occurred on a stress changes may have helped trigger within minutes of the main shock, but it vertical, northwest-oriented fault plane for aftershocks along these two faults. could persist for 8 hours at some locales. at least the second event. For most earlier The model also indicates that static The 25 April 1992 main shock ruptured Gorda shocks, rupture occurred as left- stress changes induced by the main shock only a small part of the plate boundary and lateral slip on a northeast-oriented plane, are slightly more favorable for failure on a apparently did not trigger slip on any of the perhaps because this orientation may allow northeast-striking, left-lateral fault plane in Holocene shallow thrust faults observed reactivation of normal faults formed at the the location of the first large aftershock. onshore in the triple junction region (17). Gorda spreading ridge (11). From a consid- primarily because of a decrease in normal Thus, given the high level of historical www.sciencemag.org eration of stress release, either orientation stress. In consideration of the present rela- seismicity and the emerging picture ofmany active faults, the region is likely to continue experiencing significant seismicity. Fig. 7. Tsunami measurements at 3 tide gauge stations along the E - REFERENCES AND NOTES coasts of California, Oregon, Ha- _' H waii, and Johnston Island. Tidal 3 2 I 1. H. 0. Wood and F. Neumann, Bull. Seismol. Soc.

signal has been removed from all * F J1 Am. 21, 277 (1931). Downloaded from data except for Crescent City, f 2. L. Dengler, G. Carver, R. McPherson, Calif. Geol. California (inset) (note change of IL. 45, 40 (1992). 0 3. California Office of Emergency Services Situation scale). Time of main shock is 0 Report, 4 May 1992; American Red Cross Disas- hours. The vertical line marks the ter Update, 8 May 1992. expected tsunami arrival time. 4. R. Riddihough, J. Geophys. Res. 89,6980 (1984); Contour lines (dotted) represent C. Nishimura, D. S. Wilson, R. N. Hey, ibid., p. ocean depth in kilometers, except 10283; D. S. Wilson, ibid., in press. for the single, unlabeled 500-m 5. S. H. Clarke, Jr., Am. Assoc. Pet. Geol. Bull. 76, 199 (1992). bathymetric contour line nearest 6. H. M. Kelsey and G. A. Carver, J. Geophys. Res. the coast. 93, 4797 (1988). 7. R. C. McPherson, thesis, Humboldt State Univer- sity (1989). 8. Earthquakes for the period August 1974 to De- cember 1984 were recorded by the Tera Corpo- ration. Since 1983, the seismicity has been recorded by local networks operated by the U.S. Geological Survey and the University of California at Berkeley. All earthquake locations are based on a one-dimensional velocity model with station corrections developed from a joint hypocenter- velocity inversion of travel-time data and located with Hypoinverse [F. W. Klein, U.S. Geol. Surv. Open File Rep. 89-314 (1989)]. Calibration explo- sions at Punta Gorda and Cape Mendocino were located to the northwest, 1.5 and 2.7 km, respec- SCIENCE * VOL. 261 * 23 JULY 1993 437 ......

tively, at the surface. Locations of the offshore boundary of the background Gorda seismicity the main shock using a rectangular dislocation earthquakes are not as accurate as those onshore dips 60 east at a depth of 17.5 km at the coordi- surface (Table 2) in an elastic half-space [Y. because of the network geometry. nates of the main shock hypocenter. Okada, Bull. Seismol. Soc. Am. 82, 1018 (1992)] 9. R. S. Cockerham, Bull. Seismol. Soc. Am. 74, 569 26. G. G. Shor, P. Dehlinger, H. K. Kirk, W. S. French, to stimulate the earthquake rupture. The results of (1984); S. R. Walter, ibid. 76, 583 (1986). J. Geophys. Res. 73, 2175 (1968); S. W. Smith, J. the model are insensitive to small variations in the 10. T. V. McEvilly, Nature220, 901 (1968); J. P. Eaton, S. Knapp, R. C. McPherson, ibid. 98, 8153 (1993). assumed fault geometries or slip distributions, Eos 62, 959 (1981); T. J. Lay, J. W. Given, H. 27. W. P. Chen and P. Molnar, ibid. 88, 4183 (1983); although the maximum values and spatial details Kanamori, Bull. Seism. Soc. Am. 72, 439 (1982). P. Molnar and P. England, ibid. 95, 4833 (1990). of the stress fields are. 11. D. S. Wilson, J. Geophys. Res. 94, 3065 (1989). 28. J. J. Tabor and S. W. Smith, Bull. Seismol. Soc. 32. K. Hudnut et al., Bull. Seismol. Soc. Am. 79, 282 12. A. C. Lawson, Camegie Inst. Washington Publ. Am. 75, 237 (1985). (1989). 87, 54 (1908); W. Thatcher and M. Lisowski, Eos 29. D. Verdonck and G. Zandt, Lawrence Livermore 33. We thank B. Ellsworth, C. Weaver, D. Wilson, and 68,1507 (1987). Nati. Lab. Rep. UCRL-JC-1 11629(1992). D. Merritts for reviews of the manuscript. The 13. J. R. Curray and R. D. Nason, Geol. Soc. Am. Bull. 30. K. Wang and G. Rogers, Eos 73, 504 (1992). measurement of coastal uplift was supported by a 78, 413 (1967); L. Seeber, M. Barazangi, A. Now- 31. We estimated static stress changes caused by grant from the Pacific Gas and Electric Company. roozi, Bull. Seismol. Soc. Am. 60,1669 (1970). 14. T. H. Heaton and H. Kanamori, Bull. Seismol. Soc. Am. 74, 993 (1984); T. H. Heaton and S. H. Hartzell, ibid. 76, 675 (1986). 15. B. F. Atwater, Science 236, 942 (1987); M. E. * RESEARCH ARTICLE Danenz and C. D. Peterson, Tectonics 9, 1 (1990); A. R. Nelson, Quat. Res. 38, 74 (1992); G. S. Vick, thesis, Humboldt State University (1989); D. W. Valentine, thesis, Humboldt State University (1992). 16. D. Merritts and W. B. Bull, Geology 17, 1020 (1989). NMR Structure of a Specific DNA 17. S. H. Clarke, Jr., and G. A. Carver, Science 255, 188 (1992). 18. We inverted for the main shock source parame- Complex of Zn-Containing DNA ters using the formalism of B. Romanowicz and T. Monfret [Ann. Geophys. 4, 271 (1986)] and the spherical-Earth model PREM [A. M. Dziewonski Binding Domain of GATA-1 and D. L. Anderson, Phys. Earth Planet. Inter. 25, 297 (1981)] to correct for propagation effects. Rayleigh waves with periods between 140 and 320 s from a global distribution of stations were James G. Omichinski, G. Marius Clore,* Olivier Schaad, sampled at 10 frequencies. Gary Felsenfeld, Cecelia Trainor, Ettore Appella, 19. An analysis of broad-band surface waves indicates that the main shock ruptured to the south- Stephen J. Stahl, Angela M. Gronenborn* on September 7, 2011 west (azimuth = 2300) with the largest moment release beginning 5 km from and extending to 20 The three-dimensional solution structure of a complex between the DNA binding domain of km offshore [C. J. Ammon, A. A. Velasco, T. Lay, the chicken erythroid transcription factor GATA-1 and its cognate DNA site has been Geophys. Res. Lett. 20, 97 (1993)]. 20. For the aftershocks, we computed path-depen- determined with multidimensional heteronuclear magnetic resonance spectroscopy. The dent amplitude and phase corrections based on DNA binding domain consists of a core which contains a zinc coordinated by four cysteines analysis of the main shock. The approach of and a carboxyl-terminal tail. The core is composed of two irregular antiparallel (3 sheets and path-dependent corrections is advantageous for use of data at regional distances as well as an a helix, followed by a long loop that leads into the carboxyl-terminal tail. The amino- shorter periods [M. Pasyanos and B. Romano- terminal part of the core, including the helix, is similar in structure, although not in sequence, wicz, Eos 73, 372 (1992)]. to the amino-terminal zinc module ofthe glucocorticoid receptor DNA binding domain. In the www.sciencemag.org 21. H. Benioff, Calif Div. Mines Geol. Bull. 171, 199 (1955); A. Ben Menahem, Bull. Seismol. Soc. Am. other regions, the structures of these two DNA binding domains are entirely different. The 51, 401 (1961). DNA target site in contact with the protein spans eight base pairs. The helix and the loop 22. A. Shakal et al., Calif. Div. Mines Geol. Rep. connecting the two antiparallel P sheets interact with the major groove of the DNA. The OSMS 92-05 (1992). carboxyl-terminal tail, which is an essential determinantofspecific binding, wraps around into 23. Where colonies of organisms were completely killed, the vertical range of the organisms repre- the minor groove. The complex resembles a hand holding a rope with the palm and fingers sents a minimum measure of the uplift. On some representing the protein core and the thumb, the carboxyl-terminal tail. The specific inter- reefs, only the upper parts of some colonies of actions between GATA-1 and DNA in the major groove are mainly hydrophobic in nature, plants and sessile animals, such as mussels, died. The vertical extent of mortality in these which accounts for the preponderance of thymines in the target site. A large number of Downloaded from colonies provided the best basis for estimates of interactions are observed with the phosphate backbone. the amount of uplift. The mean and standard deviation of the vertical extent of mortality for each colony (Fig. 6, inset) is estimated from multiple measurements made by a laser total station with an instrument precision of 1 mm. The 24. We assumed that slip is uniform on a rectangular, erythroid-specific transcription factor and is an essential component required for planar fault embedded in an elastic half space. GATA-1 is responsible for the regulation of the generation of the erythroid lineage (1). We performed a Monte Carlo search of the pa- transcription of erythroid-expressed genes GATA-1 binds specifically as a monomer to rameters describing the geometry of the fault the asymmetric consensus plane and, for each geometry, used the horizontal target sequence and vertical GPS and coastal-uplift data to esti- J. G. Omichinski, G. M. Clore, 0. Schaad, and A. M. (T/A)GATA(AIG) found in the cis-regu- mate the magnitude and rake of the slip vector. Gronenborn are in the Laboratory of Chemical Phys- latory elements of all globin genes and most The scatter of the residuals for the best fitting ics, Building 5, National Institute of Diabetes and other erythroid-specific genes that have models, including our preferred model (Table 2), Digestive and Kidney Diseases, National Institutes of is 2.2 times the a priori observational errors. An Health, Bethesda, MD 20892. G. Felsenfeld and C. been examined (2). GATA-1 was the first F-ratio test indicates other models with residual Trainor are in the Laboratory of Molecular Biology, member of a family of proteins, which now scatter less than 2.6 times the a priori errors do Building 5, National Institute of Diabetes and Digestive includes regulatory proteins expressed in not differ significantly from the best fitting models and Kidney Diseases, National Institutes of Health, at the 95% confidence level. Bethesda, MD 20892. E. Appella is in the Laboratory other cell lineages, characterized by their of Cell Biology, Building 37, National Cancer Institute, recognition of the GATA DNA sequence 25. In the projections, we assumed that faults are National Institutes of Health, Bethesda, MD 20892. S. planar, the ocean depth is 2.3 km at the seaward J. Stahl is in the Protein Build- and by the presence of two metal binding edge of the Cascadia subduction zone at the Expression Laboratory, regions of the form latitude 40'27'N, and the mean elevation of the ing 6B, Office of the Director, National Institutes of Cys-X-X-Cys-(X)17- seismic network is 0.9 km. Main-shock parame- Health, Bethesda, MD 20892. Cys-X-X-Cys separated by 29 residues (2, ters are from Table 1. We assumed the upper *To whom correspondence should be addressed. 3). Mutation and deletion studies on 438 SCIENCE * VOL. 261 * 23 JULY 1993