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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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