Earthquakes: Vertical views of faults, horizontal views of strata

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Fig. 1. Study region and fault geometry. ( A) Southern California study region, with shaded relief in background. Thin lines: regional faults; dashed box: region in (B). LA: Los Angeles; SAF: San Andreas fault; SJF: San Jacinto fault; SMF: Sierra Madre fault; CF: Cucamonga fault. ( B) Fault geometry, regional stress orientation, and fault rakes due to regional stress, with shaded relief in background. Heavy solid lines show surface fault traces, dotted lines show surface projections of down-dip geometry. Arrows show fault rakes, with values in degrees. CP: Cajon Pass. ( C) Fault surfaces as viewed from the northeast looking over the SAF toward the Los Angeles metropolitan area.

The northern edge of the densely populated Los Angeles metropolitan region is bounded by the Sierra Madre-Cucamonga (SMF-CF) thrust fault system, which produced the Mw= 6.7 1971 San Fernando earthquake and may generate events up to Mw= 7.5 ( 7). To the east and north lie the San Andreas (SAF) and San Jacinto (SJF) right-lateral strike-slip fault systems; each may slip in events exceeding Mw= 7.0, and events of Mw= 7.8 occurred on the SAF in 1685 and 1857 ( 8). Here, we examine past and potential ruptures on the SAF, SJF, CF, and SMF ( Fig. 1 and Table 1 ) ( 9). We model both static stress transfer between separate events and dynamic rupture propagation during a single event, so that we can understand a wider range of possible fault interactions. In doing so, we find faults whose ruptures encourage ruptures of other faults at later times, as well as sets of faults that may fail in a single, large, complex event.

Anderson, G., Aagaard, B., & Hudnut, K. (2003). Fault Interactions and Large Complex Earthquakes in the Los Angeles Area. Science, 302(5652), 1946-1949.

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