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1 U.S. Geological Survey
345
Middlefield Road, MS 977
Menlo Park, California 94025
(R.W.S., J.L.,
J.R.M., M.J.R.)
2 Invisible Software
San Jose,
California
(M.B)
In map view, aftershocks of the 2004 Parkfield earthquake lie along a line
that forms a straighter connection between San Andreas fault segments north and
south of the Parkfield reach than does the mapped trace of the fault itself. A
straightedge laid on a geologic map of Central California reveals a
50-km-long asymmetric northeastward warp in the Parkfield reach of the
fault. The warp tapers gradually as it joins the straight, creeping segment of
the San Andreas to the northwest, but bends abruptly across Cholame Valley at
its southeast end to join the straight, locked segment that last ruptured in
1857. We speculate that the San Andreas fault surface near Parkfield has been
deflected in its upper 6 km by nonelastic behavior of upper crustal rock
units. These units and the fault surface itself are warped during periods
between large 1857-type earthquakes by the presence of the 1857-locked segment
to the south, which buttresses intermittent coseismic and continuous aseismic
slip on the Parkfield reach. Because of nonelastic behavior, the warping is not
completely undone when an 1857-type event occurs, and the upper portion of the
three-dimensional fault surface is slowly ratcheted into an increasingly
prominent bulge. Ultimately, the fault surface probably becomes too deformed for
strike-slip motion, and a new, more vertical connection to the Earth’s
surface takes over, perhaps along the Southwest Fracture Zone. When this happens
a wedge of material currently west of the main trace will be stranded on the
east side of the new main trace.
This article has been cited by other articles:
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  B. P. Allmann and P. M. Shearer A High-Frequency Secondary Event During the 2004 Parkfield Earthquake Science, November 23, 2007; 318(5854): 1279 - 1283. [Abstract] [Full Text] [PDF]
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 R. A. Harris and J R. Arrowsmith Introduction to the Special Issue on the 2004 Parkfield Earthquake and the Parkfield Earthquake Prediction Experiment Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S1 - S10. [Abstract] [Full Text] [PDF]
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M. J. Rymer, J. C. Tinsley III, J. A. Treiman, J R. Arrowsmith, K. B. Clahan, A. M. Rosinski, W. A. Bryant, H. A. Snyder, G. S. Fuis, N. A. Toke, et al. Surface Fault Slip Associated with the 2004 Parkfield, California, Earthquake Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S11 - S27. [Abstract] [Full Text] [PDF]
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A. Shakal, H. Haddadi, V. Graizer, K. Lin, and M. Huang Some Key Features of the Strong-Motion Data from the M 6.0 Parkfield, California, Earthquake of 28 September 2004 Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S90 - S118. [Abstract] [Full Text] [PDF]
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J. J. Lienkaemper, B. Baker, and F. S. McFarland Surface Slip Associated with the 2004 Parkfield, California, Earthquake Measured on Alinement Arrays Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S239 - S249. [Abstract] [Full Text] [PDF]
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