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Structure and seismotectonics of the Mendocino Triple Junction, California

Record type: Article

We analyze and interpret two marine multichannel seismic reflection/onshore-offshore/ocean bottom seismometer (OBS) seismic profiles that cross in the region of the Mendocino Triple Junction over the fault plane of the 1992 Petrolia, California, M-s=7.1 earthquake. From the offshore reflection data and the wide-angle constraints provided by the onshore- offshore and OBS data we have determined a rough three- dimensional view of crustal and uppermost mantle structure beneath the triple junction region. This structure is also consistent with gravity data. The upper and mid crust has seismic velocities and densities consistent with accretionary complex rocks of the Franciscan terrane; a ubiquitous lower crustal layer with velocities of 6.5-7.1 km s(-1) is 5-7 km thick throughout most of the region. Over an area extending at least 25 km to the south and 60 km to the west of the triple junction the lower crustal layer is up to 12 km thick; this area is overlain by undisturbed pre-11 Ma sediments, indicating little deformation since that date. The thick lower crust extends beneath the coast, suggesting that the deep boundary between Pacific and North American terranes lies onshore. The northeastern edge of the thick lower crustal layer is marked by concentrated seismicity, with the hypocenter of the 1992 Petrolia earthquake lying close to the NE corner. We speculate that at depth the Pacific lower crust acts an asperity acting to concentrate stress, from which the Petrolia earthquake ruptured.

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Citation

Henstock, Timothy J. and Levander, Alan (2003) Structure and seismotectonics of the Mendocino Triple Junction, California Journal of Geophysical Research, 108, (B5), 2260-[20pp]. (doi:10.1029/2001JB000902).

More information

Published date: 20 May 2003
Keywords: mendocino triple junction, gorda plate, vizcaino block, coast ranges, seismicity, seismic velocity structure

Identifiers

Local EPrints ID: 1397
URI: http://eprints.soton.ac.uk/id/eprint/1397
ISSN: 0148-0227
PURE UUID: 38f79741-df97-4f8a-8f01-c963be5dcc43
ORCID for Timothy J. Henstock: ORCID iD orcid.org/0000-0002-2132-2514

Catalogue record

Date deposited: 06 May 2004
Last modified: 17 Jul 2017 17:16

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Contributors

Author: Alan Levander

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