Field Investigation of the Aftershocks From the October 1, 1987 Whittier Narrows Earthquake

Dar Way Yi, December 1988
Committee Members: Roswitha B. Grannell (Chair), CSULB Geology; Gerry Simila (Director), Geological Sciences, CSU Northridge; Jack Green, CSULB Geology

Abstract

On October 1, 1987 a damaging earthquake (M=5.9) occurred in the Whittier Narrows region, Los Angeles. A field study was conducted to interpret aftershock activity in terms of a tectonic model of the associated faulting. Sixteen recorders were deployed in the Whittier Narrows area between October 1, 1987, and November 13, 1988. The strong aftershock (M=5.3) at 10:39:38.00 G.M.T. on October 4, 1987, and 62 aftershocks were recorded. Peak acceleration values were 0.07-0.28 g with epicentral distances of 3.2 to 10.9 km. Inversion of 73 P-wave and 35 S-wave readings from 13 events resulted in the crustal velocity model: V(P) = 4.41 + 0.045Z, V(S) = 2.26 + 0.02Z. Station travel time adjustments were also determined and the aftershocks located. The aftershock distribution was aligned along a northwest direction with a hypocentral depth distribution of 11-19 km. The focal mechanism of the (M=5.3) aftershock event indicated right-oblique slip along a northwest trending plane.