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The vertical fingerprint of earthquake-cycle loading in Southern California

Samuel M. Howell, Bridget R. Smith-Konter, Neil Frazer, Xiaopeng Tong, & David T. Sandwell

Published 2016, SCEC Contribution #6117

The San Andreas Fault System (SAFS), one of the best-studied transform plate boundaries on Earth, is well known for its complex network of locked faults that slowly deform the regional crust in response to large-scale plate motion1-8. Horizontal interseismic motions of the SAFS are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Dense Global Positioning System (GPS) vertical velocity data in southern California should theoretically observe vertical velocity fluctuations from earthquake cycle loading. However, this signal is often masked by velocity variations across small distances (<10s km) in both magnitude and direction induced by non-tectonic signals9. We show that when carefully treated for spatial consistency, GPS-derived vertical velocities expose a small amplitude (±2.5 mm/yr), but spatially considerable (200 km), coherent pattern of uplift and subsidence straddling the SAFS in southern California. This vertical velocity field shows remarkable agreement with the sense of vertical motions predicted by physical earthquake cycle loading models of the SAFS spanning the last few centuries6,10. For the first time, this study reveals the subtle but identifiable tectonic fingerprint of far-field flexure due to 300+ years of fault-locking and creeping depth variability along the SAFS. Understanding this critical component of interseismic tectonic deformation at a complex strike-slip plate boundary has important implications for quantifying seismic hazards in Southern California and beyond.

Howell, S. M., Smith-Konter, B. R., Frazer, N., Tong, X., & Sandwell, D. T. (2016). The vertical fingerprint of earthquake-cycle loading in Southern California. Nature Geosciences, 9, 611-614.