Interseismic lithospheric response of the southern end of the Cascadia subduction zone since the 1992 Cape Mendocino m 7.1 earthquake

Graduation Date

2016

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Geology, 2016

Committee Chair Name

Mark Hemphill-Haley

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

GPS, Uplift, Singley Flat, Subduction, Mussel Rock, Earthquakes, Subsidence, Cape Mendocino, Mendocino triple junction, Cascadia subduction zone, Humboldt State University -- Theses -- Geology, Interseismic, Intertidal organism

Abstract

The Cascadia subduction zone (CSZ) in the Pacific Northwest, where the Gorda plate is subducting beneath the North American plate, may be capable of producing M 9 earthquakes. At its southern end, the CSZ terminates at the Mendocino triple junction in northern California, a region of frequent seismic activity. Unique among this seismicity was the 1992 M 7.1 Cape Mendocino earthquake, which caused up to 1.4 m of measured coseismic uplift and may have been a segmented rupture of the southern end of the CSZ. The coseismic deformation was measured using Vertical Extent of Mortality of intertidal organisms, as well as a first order NGS leveling survey. Using static GPS relocation of leveling benchmarks and the position of intertidal organisms I measured vertical crustal deformation over the 23 years since the 1992 event. If this earthquake had occurred on the megathrust interface between the Gorda and North American plates, I expected to see 10 – 20 cm of subsidence near the peak of coseismic uplift. However, this earthquake could have occurred along a subsidiary fault within the accretionary wedge or upper plate, in which case the deformation rate should be much lower. Benchmark relocation and intertidal organism relocation yield maximum vertical deformation of 1 mm/yr. These low interseismic deformation rates measurements indicate that the 1992 M 7.1 Cape Mendocino earthquake was not rupture of the subduction zone interface, but likely occurred within the upper plate accretionary complex on a subsidiary fault.

https://scholarworks.calstate.edu/concern/theses/gh93h171m

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