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The vertical fingerprint of earthquake cycle loading in southern California


The San Andreas Fault System, one of the best-studied transform plate boundaries on Earth, is well known for its complex network of locked faults that slowly deform the crust in response to large-scale plate motions1,2,3,4,5,6,7,8. Horizontal interseismic motions of the fault system are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Here we show that when carefully treated for spatial consistency, global positioning system-derived vertical velocities expose a small-amplitude (±2 mm yr−1), but spatially considerable (200 km), coherent pattern of uplift and subsidence straddling the fault system in southern California. We employ the statistical method of model selection to isolate this vertical velocity field from non-tectonic signals that induce velocity variations in both magnitude and direction across small distances (less than tens of kilometres; ref. 9), and find remarkable agreement with the sense of vertical motions predicted by physical earthquake cycle models spanning the past few centuries6,10. We suggest that these motions reveal the subtle, but identifiable, tectonic fingerprint of far-field flexure due to more than 300 years of fault locking and creeping depth variability. Understanding this critical component of interseismic deformation at a complex strike–slip plate boundary will better constrain regional mechanics and crustal rheology, improving the quantification of seismic hazards in southern California and beyond.

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Figure 1: Southern California vertical GPS velocity data.
Figure 2: GPS and physical model comparison.
Figure 3: Simplified physical model vertical velocity pattern.
Figure 4: Simplified schematic of SAFS locking depth variations and historical earthquake sequence.


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We thank P. Wessel and J. Foster for their preliminary review of this manuscript, J. Foster for clarifying sources of data noise, and G. Thornton for his initial modelling efforts that contributed to this work. This study was supported by National Science Foundation grants EAR-0838252 and EAR-1424374. This material is based on data provided by the Plate Boundary Observatory operated by UNAVCO for EarthScope ( and supported by the National Science Foundation No. EAR-0350028 and EAR-0732947.

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S.H. performed the statistical GPS data analysis and comparisons between statistical and physical models, prepared figures, and wrote the paper. B.S.-K. produced vertical velocity field for physical numerical models. N.F. offered guidance on the statistical methods. B.S.-K. and X.T. identified spurious GPS data using InSAR. D.S. offered guidance on regional geology and deformation, as well as the scope and presentation of the study. All authors discussed the study results and improved the manuscript.

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Correspondence to Samuel Howell.

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The authors declare no competing financial interests.

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Howell, S., Smith-Konter, B., Frazer, N. et al. The vertical fingerprint of earthquake cycle loading in southern California. Nature Geosci 9, 611–614 (2016).

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