CRUSTAL SHEAR WAVE ANISOTROPY STUDY
ALONG THE CHELUNGPU FAULT BY USING THE
DATA OF 1999 CHI-CHI,TAIWAN,EARTHQUAKE
Xiu-Fen Zheng1)、Chau-Huei Chen2) and Chia-June Lu2)
1)
2)
2)
(鄭秀芬 、陳朝輝 、盧佳君 )
1) Institute of Geophysics, China Earthquake Administration, Beijing, P.R.C.
2) Institute of Seismology, National Chung Cheng University, Chiayi, Taiwan, R.O.C.
ABSTRACT
The 1999 Chi-Chi(Taiwan) earthquake brought about 100-km ruptures along the
Chelungpu fault. On the two sides of the fault, the seismograms recorded by 16
free-field strong-motion stations during the period of 1998-2000 will be used to
analyze the crustal shear wave anisotropy. The purpose of this study is to investigate
the preseismic、coseismic and postseismic variations of fast polarization directions
and time delays between fast and slow shear waves. We also want to know if the time
delays can monitor the stress build up before large earthquake and the stress release
as earthquake occur. If the variations in shear-wave splitting can be used for short
term stress-forecasting, it can provide meaningful information of the source
processes in earthquake preparation zones.
Fig. 1 Blue solid line indicates the Chelungpu fault, solid triangles are denoted
as the locations of the strong-motion station and the epicenter of mainshock is
denoted by red solid star
Figure 1 shows the locations of station used in this study. These stations are
operated by CWB(Central Weather Bureau,Taiwan). There are 9 stations on the
hangingwall of Chelungpu fault, and other 7 stations on the footwall.
INTRODUCTION
Seismic shear wave splits into two approximately orthogonal polarizations when
travelling through anisotropic media. Stress-aligned shear wave splitting is widely
observed in the Earth’s crust. It appears to be caused by propagation through the
fluid-saturated stress-aligned vertically oriented parallel grain-boundary cracks and
intergranular pores present in almost all rocks (Crampin,1994,1999). For propagation
within about 45° of the vertical, the polarization direction of the faster split shear
wave is approximately parallel to the direction of maximum horizontal stress
indicating fluid-saturated microcracks oriented(Gao et al.,2003). The source region
of the Chi-Chi earthquake is in the upper crust that is mainly composed of Tertiary
marine sediments, which should have incorporated a large amount of fluid during
their formation. It’s assumed that the Chi-Chi source region is practically saturated
with fluid. The high Vp/Vs values(Chen et al.,2001) indicate that the upper crust of
the source region is fluid saturated and highly fractured.
Temporal changes in time-delays before earthquakes have been observed from
several places in America and Europe (Peacock et al., 1988, Crampin et al., 1990,
Crampin et al., 1991, Crampin et al., 1999, Booth et al., 1990, Liu et al., 1997, Volti
and Crampin, 2003a and Volti and Crampin, 2003b). On one occasion, the time and
magnitude of a (M=5) earthquake in SW Iceland was successfully stress-forecast in
real time during an extensive study of shear-wave splitting in Iceland (Crampin et al.,
1999, Volti and Crampin, 2003a and Volti and Crampin, 2003b). Gao(Gao et
al.,2003) have demonstrated similarities between laboratory and field observations of
changes in shear-wave splitting before earthquakes. This tends to confirm that
shear-wave splitting in the crust is caused mainly by stress-aligned fluid-saturated
grain-boundary cracks and low aspect-ratio pores.
The key for stress-forecasting is the need for persistent swarms of small
earthquakes. It can provide shear-wave source signals for monitoring shear-wave
splitting by local seismic networks which have stations within the shear-wave
window of the seismic activity. The problem is that such persistent swarms are
uncommon and sporadic(Gao et al.,2003). Fortunately, CWB operates more than 650
free-field strong-motion stations in Taiwan, and with such dense distribution of
stations, the 1999 Chi-Chi earthquake(M=7.6) may provide great opportunity for us
to practice the topics of shear wave splitting and stress-forecasting.
REFERENCES
Booth, D. C., Crampin, S., Lovell, J. H., and Chiu, J.-M., 1990. Temporal changes in
shear wave splitting during an earthquake swarm in Arkansas. J. Geophys. Res.,
95, 11151–11164.
Chen, C. -H., and Teng, T. -L., 2001. 3D velocity structure around the Source Area
of the 1999 the Chi-Chi, Taiwan, Earthquake: Before and After the Mainshock.
Bull. Seism. Soc. Am., 91, 1013-1027.
Crampin, S., Booth, D. C., Evans, R., Peacock, S., and Fletcher, J. B., 1990. Changes
in shear wave splitting at Anza near the time of the North Palm springs
earthquake. J. Geophys. Res., 95, 11197–11212.
Crampin, S., Booth, D. C., Evans, R., Peacock, S., and Fletcher, J. B., 1991.
Comment on "Quantitative measurements of shear wave polarizations at the
Anza seismic network, southern California: implications for shear wave
splitting and earthquake prediction" by R.C. Aster, P.M. Shearer and J. Berger.
J. Geophys. Res., 96, 6403–6414.
Crampin, S., 1994, The fracture criticality of crustal rocks. Geophys. J. Int., 118
428–438.
Crampin, S., 1999. Calculable fluid–rock interactions. J. Geol. Soc., 156, 501–514.
Crampin, S., Volti, T., and Stefánsson, R., 1999. A successfully stress-forecast
earthquake. Geophys. J. Int., 138, F1–F5.
Gao, Y., and Crampin, S., 2003. Temporal variations of shear-wave splitting in field
and laboratory studies in China. J. App. Geophysics., 54, 279–287.
Liu,Y., Crampin, S., and Main, I., 1997. Shear-wave anisotropy: spatial and temporal
variations in time delays at Parkfield, Central California. Geophys. J. Int., 130,
771–785.
Peacock, S., Crampin, S., Booth, D. C., and Fletcher, J. B., 1988. Shear-wave
splitting in the Anza seismic gap, southern California: temporal variations as
possible precursors. J. Geophys. Res., 93, 3339–3356.
Volti, T., and Crampin, S., 2003a. A four-year study of shear-wave splitting in
Iceland: 1. Background and preliminary analysis. In: D.A. Nieuwland, Editor,
New insights into structural interpretation and modellingGeol. Soc. Lond., Spec.
Publ., 212, 117–133.
Volti, T., and Crampin, S., 2003b. A four-year study of shear-wave splitting in
Iceland: 2. Temporal changes before earthquakes and volcanic eruptions. In:
D.A. Nieuwland, Editor, New insights into structural interpretation and
modellingGeol. Soc. Lond., Spec. Publ., 212, 135–149.