High‐Resolution Topography‐Derived Offsets along the 1857 Fort Tejon Earthquake Rupture Trace, San Andreas Fault
byOlaf Zielke, J Ramón Arrowsmith, Lisa Grant Ludwig, Sinan O. Akçiz
Bulletin of the Seismological Society of America. June 2012, Vol. 102, Issue 3, Pages 1135–1154.
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.