High-resolution topography-derived offsets along the 1857 Fort Tejon earthquake rupture trace, San Andreas fault

Abstract

The great Fort Tejon earthquake of 1857, with an ~350-km-long surface rupture, was the most recent major earthquake along the south-central San Andreas fault (SAF). Prior reconstruction of its surface-slip distribution and reconstruction of preceding earthquakes along the 1857 rupture trace have contributed to formulation of the characteristic earthquake (CEM) and uniform-slip models (USM) for earthquake recurrence that find wide application in seismic hazard assessment and earthquake forecasting. We used the high-resolution B4 light detection and ranging (LiDAR) topographic data set-sufficient for depiction of meter-scale tectonic landforms-to reevaluate the distribution of surface displacement along the 1857 rupture trace. We present ~450 offset measurements with displacements below 60 m, increasing observation density relative to previous studies by a factor of 2. Our results show that the 1857 earthquake had overall an average displacement below 3.5 m with 4-6 m released along the northwestern half of the rupture. Its along-fault slip distribution is smooth at > 10-km length scales. At < 1 -km length scales, displacement distribution exhibits high-frequency fluctuation of ± 1 m or more, attributable to rupture variation and measurement uncertainty. In contrast to prior interpretations of the 1857 event, we find no evidence for fault segmentation expressed in the surface-slip distribution. The total moment in the earthquake was 4:0 × 10 20 N·m (~28% less than prior estimates) assuming a 10-km rupture depth and a 30-GPa shear modulus. Reconstruction of pre1857 earthquakes is impeded by an exponential decrease in observation density with increasing displacement amount. Offset observation density soon reaches a level where only tentative and nonunique reconstructions are permitted. The results of this study question prior surface-slip reconstructions of pre-1857 rupture(s) that were based on a distinctly lower number of offset observations, challenging the applicability of the aforementioned earthquake recurrence models to explain SAF rupture behavior.