Coseismic folding, earthquake recurrence, and the 1987 source mechanism at Whittier Narrows, Los Angeles basin, California

Abstract

Static deformation associated with the October 1, 1987, Whittier Narrows, California, M = 6.0 earthquake was detected by geodetic elevation changes. The earthquake uplifted a 1.5‐km‐high Quaternary fold (the Santa Monica Mountains anticlinorium) by 50 mm but caused no fault rupture at the ground surface. This suggests that folding and faulting of the Los Angeles basin sediments are coincident and continuing. After correction for surveying errors and nontectonic subsidence, we model the 214 geodetic observations with a simple dislocation in an elastic half‐space. A thrust fault with reverse slip of 1.1±0.3 m and dipping 30°±4°N, with an upper edge at a depth of 12±1 km and a lower edge at 17±1 km, fits the geodetic data best and is consistent with the main shock hypocenter, fault plane solution, and initial aftershock distribution. The upper limit on the static stress drop, Δσ, is 17.5±5.0 MPa (175±50 bar). The lower limit on the geodetic moment is 1.0±0.2 × 1025 dyne cm, in accord with seismic estimates, indicating that most of the slip took place during seismic rupture. If earthquakes of M ≤ 6 characterize the blind thrust fault on which the earthquake occurred, we estimate a 200 year repeat time at Whittier Narrows, and a 5–18 year rate of earthquake occurrence within a 150‐km‐long band in the northern Los Angeles basin and Santa Monica Bay to the west. The 25‐year rate of historical occurrence since 1860 is less than this prediction. The deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here.