A balanced cross-section of the 1994 Northridge earthquake, southern California

Abstract

THE Northridge earthquake of 17 January 1994¹ was the latest in a series of very damaging, thrust-fault-generated earthquakes to strike California, following the San Fernando² 1971, Coalinga³ 1983, and Whittier Narrows^4,5 1987 events. Like the last two of these, the Northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic-hazard methods^6,7. Balanced cross-sections^8,9, which flatten and remove the crustal deformation, can be used to identify and quantify the seismic hazard posed by thrust faults. Here we present a balanced cross-section through the Northridge portion of the Transverse Ranges fold-and-thrust belt¹⁰, which shows that the earthquake occurred on what we call the Pico thrust. A cross-section of this sort constructed before the earthquake would have revealed the fault, although it would not have predicted the earthquake. Cross-sectional modelling of the Pico thrust yields an average slip rate of 1.4–1.7 mm yr⁻¹ and a recurrence interval of Northridge-sized (M_w 6.7) earthquakes every 1,500–1,800 years. We show that the Pico thrust is the back thrust to the 170-km Elysian Park thrust^3,4 which underlies some of the most densely urbanized portions of the Los Angeles basin.