The Landers earthquake, which had a moment magnitude (M(w)) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its M(w) 6.1 preshock and M(w) 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

The Southern California Seismic Network (SCSN) has produced the SCSN earthquake catalog from 1932 to the present, a period of more than 77 yrs. This catalog consists of phase picks, hypocenters, and magnitudes. We present the history of the SCSN and the evolution of the catalog, to facilitate user understanding of its limitations and strengths. Hypocenters and magnitudes have improved in quality with time, as the number of stations has increased gradually from 7 to ∼400 and the data acquisition and measuring procedures have become more sophisticated. The magnitude of completeness (M c ) of the network has improved from M c ∼3:25 in the early years to M c ∼1:8 at present, or better in the most densely instrumented areas. Mainshock-aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470,000 events. The earthquake frequency-size distribution has an average b-value of ∼1:0, with M ≥ 6:0 events occurring approximately every 3 yrs. The three largest earthquakes recorded were 1952 M w 7.5 Kern County, 1992 M w 7.3 Landers, and 1999 M w 7

The (MW6.1, 7.3, 6.2) 1992 Landers earthquakes began on April 23 with the MW6.1 1992 Joshua Tree preshock and form the most substantial earthquake sequence to occur in California in the last 40 years. This sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long. The faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the San Andreas fault. The MW6.1 Joshua Tree preshock at 33°N58′ and 116°W19′ on 0451 UT April 23 was preceded by a tightly clustered foreshock sequence (M≤4.6) beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks. The aftershocks extended along a northerly trend from about 10 km north of the San Andreas fault, northwest of Indio, to the east‐striking Pinto Mountain fault. The Mw7.3 Landers mainshock occurred at 34°N13′ and 116°W26′ at 1158 UT, June 28, 1992, and was preceded for 12 hours by 25 small M≤3 earthquakes at the mainshock epicenter. The distribution of more than 20,000 aftershocks, analyzed in this study, and short‐period focal mechanisms illuminate a complex sequence of faulting. The aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults, and 40 km to the south, crossing the Pinto Mountain fault through the Joshua Tree aftershock zone towards the San Andreas fault near Indio. The rupture initiated in the depth range of 3–6 km, similar to previous M∼5 earthquakes in the region, although the maximum depth of aftershocks is about 15 km. The mainshock focal mechanism showed right‐lateral strike‐slip faulting with a strike of N10°W on an almost vertical fault. The rupture formed an arclike zone well defined by both surficial faulting and aftershocks, with more westerly faulting to the north. This change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west. A 20‐km‐long linear cluster of aftershocks occurred 10–20 km north of Barstow, or 30–40 km north of the end of the mainshock rupture. The most prominent off‐fault aftershock cluster occurred 30 km to the west of the Landers mainshock. The largest aftershock was within this cluster, the Mw6.2 Big Bear aftershock occurring at 34°N10′ and 116°W49′ at 1505 UT June 28. It exhibited left‐lateral strike‐slip faulting on a northeast striking and steeply dipping plane. The Big Bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north. The Landers mainshock occurred near the southernmost extent of the Eastern California Shear Zone, an 80‐km‐wide, more than 400‐km‐long zone of deformation. This zone extends into the Death Valley region and accommodates about 10 to 20% of the plate motion between the Pacific and North American plates. The Joshua Tree preshock, its aftershocks, and Landers aftershocks form a previously missing link that connects...

Abstract. The Mw 6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north-northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km. depth in the lower crust. It had a thrust faulting focal mechanism with a rake of 100° on a fa.ult plane dipping 35° to the south-southwest and striking N75°W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges. The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970. The mainshock was followed by an energetic aftersh<>ek sequence. Eight aftershocks of M 2: 5.0 and 48 aftershocks of 4 ~ M < 5 occurred between January 17 and September 30, 1994. The aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illunrinating a previously unmapped thrust ramp, extending from 7-10 km. depth into the lower crust to a depth of 23 k:m. No flattening of the aftershock distribution is observed near its bottom. At shallow depths, above 7-10 km, the thrust ramp is topped by a dense distnbution of aftershock hypocenters bounded by some of the surlicial faults. The dip of the ramp increases from east to west. The west side of the aftershock zon.e is characterized by a dense, steeply dipping, and north-northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Ca.nyon lateral ramp. Along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks, with some strike-slip and normal faulting in the smaller aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north-south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of the faults and the depth of faulting. The 1971 northnortheast trend of left-lateral faulting (Chatsworth trend) was not activated in 1994.Introduction .

The 1999 M w 7.1 Hector Mine mainshock showed right-lateral strikeslip faulting, with an initial strike of N6ЊW and vertical dip. The mainshock was preceded within 20 hours by 18 recorded foreshocks of 1.5 Յ M Յ 3.8 within a few kilometers distance of the mainshock hypocenter. The aftershocks delineate how the Hector Mine earthquake ruptured with strike N6ЊW to the south for a distance of 15 km, and possibly to the north for a distance of several kilometers. The two largest aftershocks of M 5.9 and M 5.7 occurred near the north and south ends of the first mainshock rupture segment. The second segment of rupture, starting 15 km to the south away from the mainshock hypocenter, delineated by strike-slip and thrustfaulting aftershocks, extends 10 km farther away with a strike of S140ЊE along the Bullion fault. The aftershocks also outline an unusual third rupture segment, extending from about 5 km south of the hypocenter with a strike of N30ЊW to N35ЊW for a distance of 20 km. Approximately 10 to 25 km farther to the north and west of the mainshock epicenter, several clusters form a complex aftershock distribution. Threedimensional Vp and Vp/Vs models of the region exhibit only small regional changes, as is typical for the Mojave region. Nonetheless, the mainshock rupture started within a region of rapidly varying Vp, and at least three regions of low Vp/Vs are imaged within the aftershock zone. The rate of decay for the Hector Mine earthquake sequence has been slightly above the mean for both p-values and b-values in southern California. The focal mechanisms of the aftershocks and the state of stress are consistent with strike-slip faulting, including a component of normal faulting most prominent to the north. The orientation of the regional maximum horizontal stress, the variation in orientation of the mainshock fault segments by 30Њ, and scattered distribution of aftershocks suggest that the mainshock and aftershock deformation field exhibit volumetric shear deformation accommodated by complex conjugate sets of strike-slip faults.