DISCUSSION
TECTONIC SUMMARY
This earthquake occurred on a relatively straight section of the San
Andreas Fault near Parkfield, California. The well-known San Andreas
fault is a right-lateral strike-slip fault that stretches 1300 km from the
Gulf of California to Cape Mendocino in Northern California. The fault
is part of the principal plate boundary between the North American
plate and the Pacific plate. The Pacific plate moves northwest with
respect to the North American plate at a velocity of 46 mm/year. At
the latitude of Parkfield, approximately seventy percent of the overall
relative plate motion is accommodated by slip on the San Andreas.
In the Parkfield section of the San Andreas (Parkfield to Goldhill),
the fault transitions from a creeping zone in the north to a locked zone
in the south. The creeping section stretches north of the Parkfield
section to San Juan Bautista. Along the creeping section, the motion
between the Pacific and North American plates is accommodated by
continuous non-seismic slip along the fault and numerous small
earthquakes with magnitudes generally less than 5. To the south of
the Parkfield section, the San Andreas has been locked since the
magnitude 7.9 Fort Tejon earthquake of 1857. In this section of the
San Andreas, relative plate motions have caused elastic strain to
accumulate for almost a century and a half. When the strength of
the fault is finally exceeded, the ensuing earthquake will be much
larger than the shock of September 28, 2004.
The Parkfield section has been characterized for a century and a
half by frequent minor and moderate shocks. Earthquakes of
comparable size to this recent quake (magnitude 6) occurred in 1857,
1881, 1901, 1922, 1934, and 1966. There is substantial overlap
between the zone of the aftershocks of this recent earthquake and
the aftershock zone of the 1966 earthquake. The fault segment
that ruptured in the 1966 earthquake had a length of about 25 km
and a width of about 5 km; the average displacement of the fault
rupture was about 50 cm.
