I. Seismotectonics of the San Jacinto fault zone and the Anza seismic gap. II. Imaging the shallow crust in volcanic areas with earthquake shear waves

Thesis/Dissertation · Wed Dec 31 23:00:00 EST 1986

OSTI ID:5953808

Sanders, C O

Details are presented of the small earthquake stress release near Anza. Earthquakes define an 18-km quiescent segment of fault which is bounded by areas of high seismicity. Moderate earthquakes with few aftershocks indicate that the fault is seismogenic and highly stressed but locked, possibly due to high normal stresses. An earthquake swarm occurred recently near the gap in an area which was previously active before two nearby M greater than or equal to 6 earthquakes. This suggests that the swarm area may signal the presence of high stresses. Variations are investigated in the depths of earthquakes in the San Jacinto-southern San Andreas fault region. The maximum depth of earthquakes progressively shallows from 20 to 10 km towards the Imperial Valley region of high heat flow. Earthquakes occur predominantly in a band along the bottom of the seismogenic zone; few earthquakes occur in the shallower fault zone. This implies that shear stresses are greater deep in the brittle fault zone, and that loading of the brittle crust occurs along aseismic extensions of the fault zones. Deeper earthquakes in the region occur principally in the major fault zones, and shallow earthquakes occur principally in the adjacent crust. In Part II-S-wave seismograms are used to image attenuation structure in the crust of the Coso geothermal region of California. Forward modeling and inversion illuminate regions in the shallow crust which severely attenuate S waves. Surprisingly, these lie south of Coso beneath Indian Wells Valley. They are coincident with the epicentral locations of migrating earthquake swarms. No attenuating effects were seen beneath Coso above 5 km depth.

• Research Organization: California Inst. of Tech., Pasadena (USA)
• OSTI ID: 5953808
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
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58 GEOSCIENCES
580201* -- Geophysics-- Seismology & Tectonics-- (1980-1989)
CALIFORNIA
COSO HOT SPRINGS
DEPTH
DIMENSIONS
EARTHQUAKES
FEDERAL REGION IX
GEOLOGIC DEPOSITS
GEOLOGIC FAULTS
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
GEOPHYSICAL SURVEYS
GEOTHERMAL RESOURCES
IMAGES
MEASURING INSTRUMENTS
NORTH AMERICA
RELAXATION
RESOURCES
SEISMIC ARRAYS
SEISMIC DETECTORS
SEISMIC EVENTS
SEISMIC S WAVES
SEISMIC SURVEYS
SEISMIC WAVES
SEISMICITY
SEISMOGRAPHS
STRESS ANALYSIS
STRESS RELAXATION
STRESSES
SURVEYS
TECTONICS
USA
VARIATIONS