The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects

Abstract

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 control of earthquake sequences on hillslope stability

“…While it is effectively impossible to know the stress history of any slope, our results suggest that local fault characteristics will ultimately generate characteristic slope responses at a landscape scale evident over decades to centuries. Empirical studies documenting transient changes in landslide rates for alternative fault mechanisms are limited, particularly those that generate different earthquake sequences characterized by foreshock sequences, for example [Hauksson et al, 1995;Jones and Molnar, 1979;Scheingross et al, 2013;Vidale et al, 2001]. Fault mechanisms that generate specific and characteristic types of earthquake sequences may affect how a landscape responds to and recovers from a high-magnitude earthquake.…”

Fine structure of the rupture zone of the April 26 and 27, 1997, Northridge aftershocks

“…Moreover, there is no simple explanation for the lower geodetic strain rates as compared to the higher geologically measured strain rates in this region [Jackson and Molnar, 1990;Donnellan et al, 1993;Yeats, 1993]. In their study of the Northridge aftershock sequence, Hauksson et al [1995] see a diffuse distribution of aftershocks in this western part of the aftershock zone. They suggest that these aftershocks occurred on faults that did not rupture in the mainshock.…”

The Seismogenic Thickness of the Southern California Crust

“…The bottom of the slip model is shallower even than the hypocenter of the earthquake at 18.7 km (Hauksson et al, 1995). We therefore exclude the N-HU model from our calculation of the appropriate value of percent for the prediction of seismogenic thickness in southern California.…”