The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis

Abstract

The October 1, 1987, Whittier Narrows earthquake (ML=5.9) was located at 34°2.96'N, 118°4.8_6'W, at a depth of 14.6 ±0.5 km in the northeastern Los Angeles basin. The focal mecharusm of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks defme an approximately circular surface that dips gently to the north, centered at the h~center of the mainshock with a diameter of 4-6 km. Hence the spatial distribution of the mamshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confmed to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by.~e thrust faulting obs~~ in the mainshock. A cluster of reverse faulting events on north strikmg planes occurred wtthin hours after the mainshock, 2 km to the west of the mainshock. The _largest aftershock (ML=5.3) occurred on October 4 and showed mostly right-lateral faulting on the same north-northwest striking plane within the hanging wall. Similarly, several left-lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the ne~ models and delays, the absolute hypocentral location bias is less than 0.5 km. The mamshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence. INrR.onucnoNThe moderate-sized (ML=5.9) Whittier Narrows earthquake occurred in the east Los Angeles metropolitan area in ~outhern California at 1442 (UT) on October 1, 1987 (Ftgure 1), and caused three direct fatalities and substantial damage to structures in many communities in Los Angeles and Orange counties .The data analyses carried out in this paper are more detailed than those from Hauksson et al. [1988]. The blast arrival time data [Perkins, 1988] that were not available in the previous study are used here to constrain new velocity models. The new velocity models and station delays provide 0.5-1.0 km shallower focal depths for most of the aftershocks, while the epicenters are almost identical within 0.5 km. Detailed aftershock statistics are also derived to facilitate comparison with other sequences, although such a comparison is not the subject of this paper. To analyze the deformation caused by this sequence, 57 new single-event focal mechanisms are determined. The results of this more detailed study mostly confirm the conclusions reached by Hauksson et al. [1988] with three exceptions. First, Hauksson et al.

Active thrusting and folding along the northern Tien Shan and Late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan

“…This, as well as the temporal clustering of those earthquakes (i.e., the last five of 60 years of instrumental recording [Hauksson and Jones, 1991] suggests to us that they might be precursors of an impending, larger and deeper event originating on the crustal ramp beneath the mountains. If so, the greatest seismic hazard for the Los Angeles region would not lie on the shallow blind thrusts south of the San Gabriel Mountains but on that deep ramp, which probably merges with the San Andreas fault at depth and may be kinematically coupled with it.…”

“…Southwest of Upland, the Elysian Park fold zone may merge with the subsurface San Jose fault [Hauksson and Jones, 1991]. Together, the June 1988, ML=4.6 and February 1990, ML=5.2 Upland earthquakes ruptured 14 km of that 18-km-long fault, to a depth of 13 km, but relieved only a fraction of the stress stored on the fault [Hauksson and Jones, 1991]. The focal mechanisms and aftershock locations of both shocks indicate left-lateral motion on the NE striking, 70°NW dipping fault, which may thus be interpreted to represent a leftlateral ramp linking the Cucamonga and Elysian Park thrusts [e.g., Hauksson and Jones, 1991].…”

“…Where the Los Angeles basin is over 11 km deep, this active crustal thrust appears to have flattened under the foreland sediments and propagated to ramps underlying the Elysian Park-santa Monica Mountains anticlinal zone [e.g., Davis et al, 1989;Hauksson and Jones, 1991]. In three dimensions, the transfer of shortening from the deep ramp under the San Gabriel Mountains to that zone is complex and may take place at different crustal levels but occurs mostly in two discrete steps, along the San Jose lateral ramp and the deeper Raymond lateral ramp, that ruptured during the December 1988, ML=4.9 Pasadena earthquake [e.g., Hauksson and Jones, 1991]. We view these two lateral ramps as analogous to those between A3 and A6, and across f6, WSW of Shawan (Plate 2 and Figure 22).…”

“…Blind thrusting on a 10-15 km deep, 25°N dipping ramp and incremental folding of the surface occurred along a 6 km long segment of that zone during the October 1, 1987, ML=5.9 Whittier Narrows earthquake [Hauksson and Jones, 1989;Lin and Stein, 1989]. Southwest of Upland, the Elysian Park fold zone may merge with the subsurface San Jose fault [Hauksson and Jones, 1991]. Together, the June 1988, ML=4.6 and February 1990, ML=5.2 Upland earthquakes ruptured 14 km of that 18-km-long fault, to a depth of 13 km, but relieved only a fraction of the stress stored on the fault [Hauksson and Jones, 1991].…”

Dislocation modeling of blind thrusts in the eastern Los Angeles basin, California

“…However, a set of anticlines within the northeast part of the basin trend east-west, parallel to the Transverse Ranges. The focal mechanism of the Whittier Narrows earthquake also revealed an east-west striking fault plane [Bent and Helmberger, 1989;Hauksson and Jones, 1989], an echo of the Transverse Ranges to the north. The folds, all sites of oil fields [Wright, 1991], are from west to east, the Santa Fe Springs, West Coyote, East CoyoteHualde, East Coyote-Anaheim, Richfield, and Kraemer anticlines.…”