Section 20
Chapter 19,861

Recent geomorphic and paleoseismic investigations of thrust faults in Santa Clara Valley, California

Finton Clark, H.; Hitchcock Christopher, S.

Cryofront 12: 239-257


Accession: 019860149

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Santa Clara Valley is an intermontane valley bounded by the San Andreas and Calaveras/Hayward right-lateral strike-slip fault systems. Restraining bends in the strike-slip faults and partitioning of the oblique convergence between the Sierran and Pacific plates result in net shortening across this region. This crustal shortening is accommodated by belts of northeast- and southwest-vergent thrust faults on the southwest and northeast valley margins, respectively, resulting in uplift of the flanking ranges of the Santa Cruz Mountains and East Bay Hills. Although the marginal thrust faults exhibit much lower slip-rates than adjacent strike-slip faults, they are located within a major urban center and, thus, pose a potentially significant seismic hazard. Recent studies of these structures, most notably the Monte Vista-Shannon fault on the western side of the valley, and the Evergreen fault on the eastern side, indicate that they experienced movement during late Pleistocene and Holocene time. Detailed geomorphic and paleoseismic investigations indicate that not all faults pose a surface-faulting hazard; however, deformation of fluvial terraces and alluvial deposits indicate that they accommodate a significant portion of the contractional strain across the region. The proximal association with the faults of the San Andreas system indicates that the thrust faults may not be independent seismic sources, but they may accommodate triggered slip during large earthquakes on the nearby strike-slip faults. Without further detailed study of the thrust faults bordering Santa Clara Valley, we may be underestimating the seismic hazard in this region.