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High Resolution Topography-Derived Offset Along the 1857 Fort Tejon Earthquake Rupture Trace, San Andreas Fault

Olaf Zielke, J Ramon R. Arrowsmith, Lisa B. Grant Ludwig, & Sinan O. Akciz

Published June 2012, SCEC Contribution #1505

The great Fort Tejon earthquake of 1857, with a ~350km long surface rupture, was the most recent major earthquake along the south-central San Andreas Fault (SAF). Prior reconstructions of its surface slip distribution and reconstruction of preceding earthquakes along the 1857 rupture trace have contributed to formulation of the characteristic earthquake and uniform slip model 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 re-evaluate 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 two. 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 4.0x1020 Nm (~28% less than prior estimates) assuming 10 km rupture depth and 30GPa shear modulus. Reconstruction of pre-1857 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 non-unique reconstructions of pre-1857 earthquake slip 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.

Zielke, O., Arrowsmith, J. R., Grant Ludwig, L. B., & Akciz, S. O. (2012). High Resolution Topography-Derived Offset Along the 1857 Fort Tejon Earthquake Rupture Trace, San Andreas Fault. Bulletin of the Seismological Society of America, 102(3), 1135-1154. doi: 10.1785/0120110230.