A cross section of the Los Angeles Area: Seismically active fold and thrust belt, The 1987 Whittier Narrows earthquake, and earthquake hazard

Abstract

Retrodeformable cross sections across the Los Angeles area interpret the Pliocene to Quaternary deformation to be a developing basement‐involved fold and thrust belt. The fold and thrust belt is seismically active as evidenced by the 1987 Whittier Narrows earthquake (ML = 5.9) and the 1971 San Fernando earthquake (MW = 6.6). The structural geology of the Los Angeles area is dominated by three major compressional uplift trends: (1) the Palos Verdes anticlinorium and western shelf, (2) the Santa Monica Mountains anticlinorium, and (3) the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains. These trends result from major thrust ramps off a detachment(s) at 10–15 km depth. Thrusts of the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains reach the surface; the other two uplifts are associated with blind thrusts. Compressional seismicity is concentrated along these thrust ramps. The 1987 Whittier Narrows earthquake probably occurred on the Elysian Park thrust which underlies the Santa Monica Mountains anticlinorium. The thrust interpretation accounts for the geometry of the anticlinorium, the seismological characteristics of the earthquake, and the geometry of coseismic uplift. The earthquake and aftershocks occurred within a structurally complex, narrow zone of Miocene and Pliocene northwest trending faults that cross the anticlinorium at a high angle. These northwest trending faults are interpreted to be reactivated faults now behaving as tears in the Elysian Park thrust and not the result of active right‐lateral deformation extending into the Whittier Narrows area. Our analysis suggests the Whittier Narrows earthquake sequence occurred within a structurally weakened zone along the Elysian Park thrust. We also suggest that the Whittier fault is not an important Quaternary structure and may not be seismogenic. The regional cross section is a nonunique solution, and other possible solutions are considered. Multiple solutions arise from the presence of two intersecting compressional belts in the Los Angeles area: the Transverse Ranges and the northern Peninsular Ranges. The belts may be due to one or more regional detachments and the northern Peninsular Ranges may be northeast or southwest vergent. The deformed top of the crystalline basement along the regional cross section requires a minimum of 15.0 km of north‐south convergence between the Palos Verdes Hills and the San Andreas fault regardless of the structural solution. Restoration of our cross section solution requires 21.4 km of north‐south convergence on top of the crystalline basement (including 6.4 km of slip continuing offshore to make structures of the continental borderland) and 29.7 km of convergence on the basal detachment. Geologic relationships suggest major shortening started between early and late Pliocene time (2.2–4.0 Ma) which yields a minimum convergence rate of 3.8–6.8 mm/yr between the Palos Verdes Hills and the San Andreas fault. Convergence rates for our solution range from 5.4 to 13.5 mm/yr between the edge of the...

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