Quaternary stratigraphy, paleoseismicity, and crustal thickening of the southern end of the Cascadia subduction zone, northwestern California, and coastal uplift studies of the 1992 Cape Mendocino earthquake.

摘要

The Cascadia subduction zone (CSZ) in Northwestern California is a complex tectonic environment with a high seismic hazard that experienced a magnitude 7.1 earthquake in 1992. The region is important to seismic hazard investigations of the CSZ. This dissertation examines the recent and paleoseismic earthquake history and Quaternary tectonics southern end of the CSZ, in three sections.; Quaternary stratigraphy of the bays and estuaries were studied for evidence of coseismic subsidence due to strong, to great, earthquakes. A paleoseismic record containing four to six events occurred in the past 2000 contains both smaller, strong to major events on local structures (Mw 6.5--7.2), and possible great events encompassing one or more segments of the subduction zone (Mw > 8.2). Because of differing magnitudes of the events, it is not appropriate to estimate a meaningful recurrence interval.; A relationship between surface uplift and subsidence is examined using data from the CSZ. Crustal thickening along the thrust faults causes surface uplift on folds, and tectonic load subsidence. The tectonic load subsidence is an isostatic response to increased crustal thickness. The rate of surface uplift is equal to the rate of crustal thickening minus the rate of tectonic load subsidence. The anticlines would be significantly higher topographically, if tectonic load subsidence did not reduce the surface uplift.; The April 25, 1992 Cape Mendocino (CM) earthquake (Ms 7.1) caused coseismic uplift that killed intertidal organisms on rocky sections of shore between CM and Punta Gorda. The maximum uplift of 1.4 +/- 0.2 meters occurred near the center of the area that was displaced, and uplift for the central 11 km of the affected coast was about 1 to 1.4 meters.; In the onshore CSZ, deformation is the summation of the active tectonic processes of the region. The area has a high seismic hazard that experiences major to great earthquake events, with the most recent occurring in 1992. Thrust faulting thickens the crust and drives both surface uplift and tectonic load subsidence. Using only surface uplift, such as marine terraces, in seismic hazard estimates may underestimate the seismic hazard.