A High-Frequency Secondary Event During the 2004 Parkfield Earthquake
Bettina P. Allmann* and
Peter M. Shearer
By using seismic records of the 2004 magnitude 6.0 Parkfield
earthquake, we identified a burst of high-frequency seismic
radiation that occurred about 13 kilometers northwest of the
hypocenter and 5 seconds after rupture initiation. We imaged
this event in three dimensions by using a waveform back-projection
method, as well as by timing distinct arrivals visible on many
of the seismograms. The high-frequency event is located near
the south edge of a large slip patch seen in most seismic and
geodetic inversions, indicating that slip may have grown abruptly
at this point. The time history obtained from full-waveform
back projection suggests a rupture velocity of 2.5 kilometers
per second. Energy estimates for the subevent, together with
long-period slip inversions, indicate a lower average stress
drop for the northern part of the Parkfield earthquake compared
with that for the region near its hypocenter, which is in agreement
with stress-drop estimates obtained from small-magnitude aftershocks.
* To whom correspondence should be addressed. E-mail: ballmann{at}ucsd.edu
