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1 Applied Geophysical Science
Laboratories
North Carolina A & T State University
1020 E. Wendover
Ave.
Greensboro, North Carolina
27411
gwang{at}ncat.edu
gtang{at}ncat.edu
(G.-Q.W.,
G.-Q.T., G.B., C.J., Q.L.)
2 College of Architectural and Civil
Engineering
Beijing University of Technology
Beijing, 100022
China
zhouxy{at}bjut.edu.cn
(X.-Y.Z.)
The U.S. Geological Survey Parkfield Dense Seismograph Array
(UPSAR) recorded successfully strong ground motions during the 2004
Parkfield earthquake (M 6.0) and its aftershock series after waiting for
15 years for an anticipated event like this. The array also recorded the 2003
San Simeon earthquake (M 6.5). Because the array covers a very small area
(0.45 km2), these data offer some interesting fresh insights into
intrasite variations of seismic ground motions. In this article, we study
strong-motion data recorded at the UPSAR from the San Simeon event,
the Parkfield event, and its seven aftershocks. We find that the variations of
high- frequency ground motions (e.g., >3 Hz) are very considerable. The
largest horizontal peak ground acceleration (PGA) (P11, 408
cm/sec2) from the Parkfield event is close to three times of the
smallest one (P01, 157 cm/sec2); the largest peak response spectrum
is even over three times of the smallest one. The shortest station-to-station
distance (between P06 and P07) in the array is only 25 m, but three-component
PGAs of the two stations differ from a factor of 1.5 for the
Parkfield event. The coefficient of variation (Cv =
/mean) of Fourier acceleration is about 50% at frequencies
higher than about 3 Hz. We find that Cv depends strongly on
frequency, while it is nearly stable for different earthquakes. The significant
variation of the high-frequency ground motions seems to be brought about mostly
by the local and neighboring topographic effects, which have a larger effect in
horizontal than vertical directions. We also calculate the ratio of vertical to
horizontal response spectrum. Our plotting shows that the ratio is not sensitive
to earthquake magnitude. We compare the observed motions (PGA,
5%-damped pseudoacceleration response spectrum [PSA] from
0.02 to 5 sec) with estimations from four commonly used prediction equations
(Abrahamson and Silva, 1997;
Boore et al., 1997;
Campbell and Bozorgnia, 2003;
and
Sadigh et al., 1997).
The comparisons indicate that the significant station-to-station variation
reduces largely the accuracy with which a site-specific estimation can be
predicted. However, the mean of the observations at the UPSAR
compares reasonably well with these estimations.
This article has been cited by other articles:
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  G.-Q. Wang, D. M. Boore, G. Tang, and X. Zhou Comparisons of Ground Motions from Colocated and Closely Spaced One-Sample-per-Second Global Positioning System and Accelerograph Recordings of the 2003 M 6.5 San Simeon, California, Earthquake in the Parkfield Region Bulletin of the Seismological Society of America, February 1, 2007; 97(1B): 76 - 90. [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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J. B. Fletcher, P. Spudich, and L. M. Baker Rupture Propagation of the 2004 Parkfield, California, Earthquake from Observations at the UPSAR Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S129 - S142. [Abstract] [Full Text] [PDF]
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