Estimation of Strong Response Spectra from Weak Motion Data for Earthquakes
in Taiwan
Yi-Ben Tsai and Hsiao-Ling Chao
Institute of Geophysics, National Central University
Chung-Li, Taiwan
ABSTRACT
We have developed and tested an empirical approach for rapid estimation of
strong ground motion response spectra that are useful for making timely damage
assessment following a strong earthquake. This approach takes advantage of the
unique features of two operational strong ground motion networks in Taiwan: the
Taiwan Rapid Earthquake Information Release System (TREIRS), currently
consisting of about 80 real-time digital accelerographic stations and the dense Taiwan
Strong Motion Instrumentation Program (TSMIP) network, currently consisting of
about 650 free-field digital accelerographic stations. The TREIRS can quickly provide
complete ground acceleration time histories at its stations after a strong earthquake,
whereas the TSMIP network can provide dense spatial coverage of populated areas at
about 5-km grid spacings. By this approach we can make rapid estimation of dense
spatial patterns of ground motion response spectra within minutes after a strong
earthquake. In this approach, we first calculate the response spectra from recorded
acceleration time histories at the TREIRS stations immediately after a strong
earthquake. We then use pre-established inter-station spectral ratio functions between
a TSMIP station and a nearby TREIRS station to obtain the corresponding response
spectra for individual TSMIP stations. This empirical approach is based on the fact
that for a given station the response spectral shape, after removal of source and
propagation path effects, is highly repeatable from earthquake to earthquake. We use
an average response spectral shape of regional stations to represent the source and
propagation path effects. This regionally averaged spectral shape is next used to
normalize the response spectral shape at individual stations. The resultant spectral
shape represents primarily the local site response effects. We can obtain a smooth
local site response spectral shape for each station by averaging over many earthquakes.
We have obtained an average local site response spectral shape for each of the 650
free-field stations by using recordings of harmless ground motion from more frequent,
moderate earthquakes. Once a strong earthquake takes place, we can use the actual
recording at a TREIRS station to calculate the response spectrum. We then use this
spectrum as a regional reference to estimate the response spectra at nearby free-field
stations. This empirical approach was tested satisfactorily on the recordings of the
1999 Chi-Chi, Taiwan earthquake sequence.
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