Seismicity, crustal structure, and tectonics near the northern termination of the San Andreas fault

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1982

OSTI ID:6240843

Knapp, J S

Further evidence supporting the supposition that a triple junction has existed near Cape Mendocino in Late Cenezoic time was provided by the discovery of a layer of seismic activity dipping 12/sup 0/ beneath the continent north of the Mendocino fracture. A seismic refraction profile shot along the continental slope and onshore velocity modeling confirms that this layer of earthquakes is confined within subducted oceanic layers 2 and 3. In this region, layer 2 appears to be slightly thickened and layer 3 anomalously thin. Travel time delays from offshore explosions and from teleseismically-recorded earthquakes indicate the presence of a large velocity discontinuity as great as 10% across the Mendocino escarpment represents the juxtaposition of two distinct lithospheric plates. The tectonic development of the region seems, however, to be shifting away from a stable triple junction configuration. Because the spreading direction at the Gorda ridge is no longer parallel to the Mendocino fracture, the Gorda plate is breaking up along a complex series of left-lateral, northeast-trending faults, as suggested by the high level of intraplate seismicity and by some aftershock distributions. By asymmetric spreading, the Gorda and Juan de Fuca ridges are undergoing clockwise rotation to bring them into alignment with the San Andreas-East Pacific rise system, eventually leading to the cessation of subduction beneath the northern California, Oregon, and Washington coasts.

• OSTI ID: 6240843
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201* -- Geophysics-- Seismology & Tectonics-- (1980-1989)
CALIFORNIA
CENOZOIC ERA
EARTHQUAKES
FEDERAL REGION IX
GEOLOGIC AGES
GEOLOGIC FAULTS
GEOLOGIC STRUCTURES
NORTH AMERICA
SEISMIC EVENTS
SEISMICITY
TECTONICS
USA