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' U110 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 (up- ward and downward pointing triangles, respec- 30 30 1 A' time of main shock.