Changes in shear wave splitting at Anza near the time of the North Palm Springs earthquake

Conference · Tue Jul 10 00:00:00 EDT 1990 · Journal of Geophysical Research; (United States)

OSTI ID:5446695

Crampin, S; Booth, D C; Evans, R ^[1]; Peacock, S ^[2]; Fletcher, J B ^[3]

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1. British Geological Survey, Edinburgh (Scotland)
2. Univ. of Reading (England)
3. Geological Survey, Menlo Park, CA (USA)

Changes in shear wave splitting are observed at KNW before and after the M = 6 North Palm Springs earthquake of July 8, 1986. KNW is a station of the Anza seismic network monitoring the Anza seismic gap on the San Jacinto fault, southern California. The gradual increase in the delays between the split shear waves over 3 years at KNW, reported by Peacock et al. (1988), ended in June 1986. The further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the North Palm Springs earthquake, 33 km north of KNW. Peacock et al. demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress-aligned fluid-filled inclusions, and speculated that this increase might be the result of a buildup of stress before an impending earthquake. The new data appear to confirm this speculation, but the temporal variations require a more complex interpretation, although they still suggest that the changes in shear wave splitting are caused by earthquake-induced stress changes to the fluid-filled inclusions throughout the rockmass. Central to interpretation of temporal changes in shear wave splitting is the well established existence, throughout at least the uppermost 10 to 20 km of the crust, of small fluid-filled cracks, microcracks, and pores. The existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock. The authors term these distributions of inclusions extensive dilatancy anisotropy or EDA, and the individual inclusions EDA cracks because, although they may include a wide range of shapes, many of the seismic properties can be simulated by distributions of thin parallel cracks.

• OSTI ID: 5446695
• Report Number(s): CONF-880531-; CODEN: JGREA
• Journal Information: Journal of Geophysical Research; (United States), Vol. 95:B7; Conference: 3. international workshop on seismic anisotropy, Berkeley, CA (United States), 31 May - 4 Jun 1988; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CALIFORNIA
GEOLOGIC FAULTS
EARTHQUAKES
SEISMIC S WAVES
BIREFRINGENCE
AFTERSHOCKS
ANISOTROPY
CONTINENTAL CRUST
DATA ANALYSIS
FORESHOCKS
GEOLOGIC FISSURES
GEOLOGIC FRACTURES
INCLUSIONS
POROSITY
RESERVOIR FLUIDS
STRESS ANALYSIS
TIME DELAY
VARIATIONS
WAVE PROPAGATION
DEVELOPED COUNTRIES
EARTH CRUST
FEDERAL REGION IX
FLUIDS
GEOLOGIC STRUCTURES
NORTH AMERICA
REFRACTION
SEISMIC EVENTS
SEISMIC WAVES
USA
580000* - Geosciences