Researchers: Samuel Howell, Bridget Smith-Konter, and Neil Frazer, University of Hawaii at Manoa; Xiaopeng Tong, University of Washington; and David Sandwell, Scripps Institute of Oceanography, University of California, San Diego.
Written by Linda Rowan
26 October 2016
The San Andreas Fault System (SAFS), a well-studied and major transform plate boundary, has been captured flexing between large earthquakes in southern California with EarthScope’s Plate Boundary Observatory (PBO). The vertical motions of GPS stations, isolated from hydrological and other effects, show northeast and southwest quadrants of uplift opposed by northwest and southeast quadrants of subsidence on opposite sides of the SAFS in southern California. The flexure is related to 300 years of fault locking and creeping at different fault depths. Quantifying the flexure will improve estimates of seismic hazard.
The SAFS is a complex network of faults that mark the plate boundary between the North American plate and the Pacific plate. The fault system runs from south to north through the state of California and is mostly a strike-slip fault. A physical model of the fault system shows creeping sections of the fault bounded by shallow locked sections that creep at depth.
PBO was designed to observe any changes along this major plate boundary with millimeter-level precision. Most of the changes in the horizontal components can be explained by the two plates sliding past each other, however, the vertical changes are more complicated and uncertain. Here the vertical motions associated with tectonics are separated from other causes, such as hydrologic influences. The tectonic sources of small vertical motions are filtered using a statistical model and compared to a physical model of the earthquake cycle in the region over the past 300 years.
The statistical model matches very well with the physical model. The PBO data show a small flexing of 2 millimeters per year, up or down on either side of the SAFS over a 200-kilometer area in southern California. These observations are useful for constraining the structure of the fault system and the dynamics of the earthquake cycle. The results show that north of Parkfield and south of the Salton Trough, the entire fault can be modeled as creeping all the way down. Between these two regions, the fault is locked at a depth of about 10-20 kilometers, below which the fault is creeping. Ultimately, these results can help better determine the seismic risks faced by the large populations in southern California and elsewhere.
The vertical fingerprint of earthquake cycle loading in southern California, Samuel Howell, Bridget Smith-Konter, Neil Frazer, Xiaopeng Tong and David Sandwell, Nature Geoscience 9, 611–614 (2016) doi:10.1038/ngeo2741.
strike-slip fault, fault flexure, creeping
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