Paleoseismologic studies suggest that it may also have produced Mw ≥ 7.0 Holocene earthquakes on three extensive north dipping thrust faults, the Sierra Madre, Puente Hills, and Compton faults (Leon et al., 2007, 2009 Rubin et al., 1998 Figure 3). This shortening has been linked to the damaging 1971 Mw ~ 6.7 San Fernando, 1987 Mw ~ 5.9 Whittier Narrows, and 1994 Mw = 6.7 Northridge thrust earthquakes (Figure 1a e.g., Argus et al., 1999 Dolan et al., 1995). Geologic, seismologic, and strain data indicate that this shortening is the principal strain in the Los Angeles region (Davis et al., 1989 Hauksson, 1990 Li, 1996 Yang & Hauksson, 2013 Zoback et al., 1987). In Los Angeles, Global Positioning System (GPS) data (Figure 1a) show ~6 mm/year of north-south shortening between the Palos Verdes peninsula and the San Gabriel Mountains or 8–9 mm/year if Santa Catalina, San Clemente, and San Nicolas Islands are included (e.g., Argus et al., 1999, 2005 Davis et al., 1989 Feigl et al., 1993 Shen et al., 1996 Walls et al., 1998 Figures 1a and 2b). North of Los Angeles, however, the San Andreas makes a leftward bend and is misaligned by ~20° with the relative plate motion direction for ~200 km (Figure 1c), resulting in north-south shortening. The San Andreas Fault generally strikes subparallel to the relative plate motion direction and accommodates the majority of the relative motion through right-lateral slip (e.g., Argus & Gordon, 2001 Lisowski et al., 1991). In California, the Pacific plate moves northwest at ~50 mm/year relative to the North American plate (e.g., Kreemer et al., 2014). The depth distribution of moment deficit accumulation rate matches that of seismicity rates in Los Angeles to first order, in part, because the models incorporate the blind nature of the Puente Hills and Compton Faults. This locking implies an annual deficit of seismic moment, 1.6 + 1.3/−0.5 × 10 17 Nm/year in total, which is presumably balanced over the long-term average by the moment released in earthquakes. We infer that the three faults slip at 3–4 mm/year over the long term and are currently partially or fully locked and accruing interseismic strain on their upper sections. We explore the impact of the assumed material model, strain accumulation on faults to the west and east, and other model assumptions. We formally invert the geodetic data for the pattern of interseismic strain accumulation on the north dipping Sierra Madre, Puente Hills, and Compton thrust faults and a master decollement. We find that strain accumulation on local strike-slip faults likely contributes no more than 1–2 mm/year of the shortening. To better characterize this seismic hazard, we assess how this shortening is being accommodated by interseismic strain accumulation on subsurface faults, incorporating detailed seismology- and geology-based models of fault geometry and the low-stiffness Los Angeles sedimentary basin. This shortening has been linked to multiple damaging twentieth century thrust earthquakes as well as possible Mw ≥ 7.0 Holocene thrust events beneath central Los Angeles. Geodetic data show that the Los Angeles metropolitan area is undergoing 8–9 mm/year of north-south tectonic shortening associated with the Big Bend of the San Andreas Fault.
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