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Science 12 November 1976:
Vol. 194. no. 4266, pp. 691 - 695
DOI: 10.1126/science.194.4266.691
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Articles

Episodic Strain Accumulation in Southern California

Wayne Thatcher 1

1 Geophysicist at the National Centre for Earthquake Research, U.S. Geological Survey, Menlo Park, California 94025

Reexamination of horizontal geodetic data in the region of recently discovered aseismic uplift has demonstrated that equally unusual horizontal crustal deformation accompanied the development of the uplift. During this time interval compressive strains were oriented roughly normal to the San Andreas fault, suggesting that the uplift produced little shear strain accumulation across this fault. On the other hand, the orientation of the anomalous shear straining is consistent with strain accumulation across northdipping range-front thrusts like the San Fernando fault. Accordingly, the horizontal and vertical crustal deformation disclosed by geodetic observation is interpreted as a short epoch of rapid strain accumulation on these frontal faults. If this interpretation is correct, thrust-type earthquakes will eventually release the accumulated strains, but the geodetic data examined here cannot be used to estimate when these events might occur. However, observation of an unusual sequence of tilts prior to 1971 on a level line lying to the north of the magnitude 6.4 San Fernando earthquake offers some promise for precursor monitoring. The data are adequately explained by a simple model of up-dip aseismic slip propagation toward the 1971 epicentral region. These observations and the simple model that accounts for them suggest a conceptually straightforward monitoring scheme to search for similar uplift and tilt precursors within the uplifted region. Such premonitory effects could be detected by a combination of frequenlty repeated short (30 to 70 km in length) level line measurements, precise gravity traverses, and continuously recording gravimeters sited to the north of the active frontal thrust faults. Once identified, such precursors could be closely followed in space and time, and might then provide effective warnings of impending potentially destructive earth-quakes.


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
The Santa Barbara, California, earthquake of 13 August 1978.
E. J. CORBETT and C. E. JOHNSON (1982)
Bulletin of the Seismological Society of America 72, 2201-2226
   Abstract »    PDF »
Strain on the San Andreas Fault Near Palmdale, California: Rapid, Aseismic Change.
J. C. Savage, J. C. SAVAGE, W. H. PRESCOTT, M. LISOWSKI, and N. E. KING (1981)
Science 211, 56-58
   Abstract »    PDF »
Speculations on the nature of the southern California uplift.
J. B. RUNDLE and W. THATCHER (1980)
Bulletin of the Seismological Society of America 70, 1869-1886
   Abstract »    PDF »
Tectonic Tilt Rates Derived from Lake-Level Measurements, Salton Sea, California.
M. E. Wilson, M. E. WILSON, and S. H. WOOD (1980)
Science 207, 183-186
   Abstract »    PDF »
Strain-Softening Instability Model for the San Fernando Earthquake.
W. D. Stuart and W. D. STUART (1979)
Science 203, 907-910
   Abstract »    PDF »
Strain in Southern California: Measured Uniaxial North-South Regional Contraction.
J. C. Savage, J. C. SAVAGE, W. H. PRESCOTT, M. LISOWSKI, and N. KING (1978)
Science 202, 883-885
   Abstract »    PDF »
Recent seismicity in the San Fernando region and tectonics in the west-central transverse ranges, California.
D. HADLEY and H. KANAMORI (1978)
Bulletin of the Seismological Society of America 68, 1449-1457
   Abstract »    PDF »
The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves.
C. A. LANGSTON (1978)
Bulletin of the Seismological Society of America 68, 1-29
   Abstract »    PDF »


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