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Poster Abstracts | Group 2 – Seismology
Next we discuss on the moment functions in a log-log graph. Seismic moment of each event
increases along with the common growth curve, Mo(t) = 2 × 10^17 t^3, decelerates, and stops
growing. The moment function of Mw 6.0 event also grows along with the same growth curve until
1 s after the onset, but gentler after 1 s . That is because the thickness of seismogenic layer limits the
width of earthquakes. According to the relocated hypocenter distribution [Thurber et al., 2006], the
thickness of the seismogenic layer around the hypocenter of Mw 6.0 event is ~ 5 km. In our slip
model the rupture front reach at the edge of the seismogenic layer in 1 s.
The fundamental image of earthquake growth processes suggested by this study is a limited selfsimilarity
as followings. Each earthquake grows self-similarly, along the common growth curve,
Mo(t) = 2 × 10^17 t^3, independent on its eventual magnitude. The growth curve can be
suppressed by the thickness of a seismogenic layer. The earthquake rupture growth decelerates by
some chance, its moment function runs off the common growth curve, and the rupture terminates.
2-117
VARIATIONS OF VELOCITY CONTRASTS AND FAULT ZONE DAMAGE ALONG
THE PARKFIELD SECTION OF THE SAN ANDREAS FAULT USING FAULT ZONE
TRAPPED WAVES Lewis MA, Ben-Zion Y, Peng Z, Shi Z, and Zhao P
We investigate variations of velocity contrasts and damage zones in the Parkfield region of the San
Andreas fault (SAF), using fault zone trapped waves generated by the total internal reflection of
waves within a low velocity fault zone layer. The trapped waves follow the S or P wave arrival, are
dispersive, characterized by high amplitudes and lower frequencies, and are particularly apparent
in the vertical or fault parallel components of motion. The specific features of these waves are
highly dependent on the fault zone structure within which they are generated and hence they can
be used to obtain high resolution fault zone images. This study is part of a larger project on
imaging material interfaces and damage zones in the Parkfield region using data from multiple
seismic networks. Much of the previous work has concentrated on using fault zone head waves.
These observations have established the strength and variations of the contrast in velocity across
the SAF. Direct wave travel time inversion and head wave moveout studies indicate that the
northeast side of the fault is generally slow and the southwest side is generally fast with a ~5-10%
difference in velocity. However, towards the southeast, near both the seismic station at Gold Hill
and the hypocenter of the 2004 Parkfield earthquake, the velocity contrast is reduced to ~0-2%.
Here we expand and develop upon the results from the head waves studies with observations of
fault zone trapped waves. We make observations of trapped waves in near fault stations of the
high resolution seismic network and other networks in the Parkfield region. Synthetic waveforms
generated using a model of two quarter spaces with differing velocities separated by a low velocity
layer, show a separation between the direct body (S or P) wave and trapped wave onset in a fault
zone station. In contrast, when the velocities in the quarter spaces are the same, the trapped and
body waves are continuous. We find strong variation in the strength and character of trapped
waves at the different stations and events along the fault and will attempt to correlate the
variations with changes of the velocity contrast indicated by the head wave analysis.
2-118
HIGH-FREQUENCY BURSTS DURING STRONG MOTION – TRIGGERED OR
“DRIVEN”? Fischer AD, and Sammis CG
High-pass filtering (>20Hz) of acceleration records from the 1999 Chi-Chi Taiwan and 2004
Parkfield, California earthquakes reveal a series of bursts that occur only during strong shaking.
Initially interpreted as originating from asperity failure on the Chelungpu fault [Chen et. al,
204 | Southern California Earthquake Center