Earthquakes, faulting, and stress in the Los Angeles Basin

Since 1920 fourteen moderate‐sized (ML = 4.9–6.4) earthquakes have been reported in the Los Angeles basin. These events are associated with both mappable surficial faults and concealed faults beneath the basin sediments. To determine the style of faulting and state of stress in the basin, single‐event focal mechanisms for 244 earthquakes of M ≥ 2.5 that have occurred during 1977–1989 have been calculated. Fifty‐nine percent of the events are strike‐slip and are mostly located near two of the major, northwest striking right‐lateral strike‐slip faults in the basin, the Newport‐Inglewood fault and the Palos Verdes fault. The 1988 Pasadena and the 1988 Upland earthquakes showed left‐lateral strike‐slip on northeast striking faults. Numerous small earthquakes in the eastern part of the basin show left‐lateral strike‐slip faulting and form a northeast trend near Yorba Linda. Thirty‐two percent of the events have reverse mechanisms and are distributed along two broad zones. The first, the Elysian Park fold and thrust belt, coincides with anticlines along the eastern and northern flank of the Los Angeles basin extending into Santa Monica Bay. The second, the Torrance‐Wilmington fold and thrust belt, coincides with anticlines mapped on the southwest flank of the basin and extends from offshore Newport Beach to the northwest into Santa Monica Bay. Oblique faulting that could be inferred by the merging of strike‐slip and compressional tectonics does not occur in the basin. Instead, the coexistence of zones of thrusting and large strike‐slip faults in the basin suggests that the thrust and strike‐slip movements are mostly decoupled. A few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines. The trend of the maximum horizontal stress varies from N1°W to N31°E across the basin and consistently forms high angles with the fold axes. The stress field that exists along the flanks of the basin has a vertical minimum stress axis. This stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin. Today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north‐south compression across the basin.