Publication Date:
2017-04-04
Description:
What can be learned about absolute site effects on ground motions, with
no geotechnical information available, in a very poorly instrumented region? In addition,
can reliable source spectra be computed at a temporary deployment? These
challenges motivated our current study of aftershocks of the 2001 Mw 7.6 Bhuj
earthquake, in western India, where we decouple the ambiguity between absolute
source radiation and site effects by first computing robust estimates of coda-derived
moment-rate spectra of about 200 aftershocks in each of two depth ranges. Crustal
attenuation and spreading relationships, based on the same data used here, were
determined in an an earlier study.
Using our new estimates of source spectra, and our understanding of regional wave
propagation, for direct S waves we isolate the absolute site terms for the stations of
the temporary deployment. Absolute site terms for each station were determined in
an average sense for the three components of the ground motion via an L1-norm
minimization. Results for each site were averaged over wide ranges of azimuths and
incidence angles.
The Bhuj deployment is characterized by a variable shallow geology, mostly of
soft sedimentary units. Vertical site terms in the region were observed to be almost
featureless (i.e., flat), with amplifications slightly 1.0 within wide frequency ranges.
As a result, the horizontal-to-vertical (H/V) spectral ratios observed at the deployment
mimic the behavior of the corresponding absolute horizontal site terms, and
they generally overpredict them. This differs significantly from results for sedimentary
rock sites (limestone, dolomite) obtained by Malagnini et al. (2004) in northeastern
Italy, where the H/V spectral ratios had little in common with the absolute
horizontal site terms.
Spectral ratios between the vector sum of the computed horizontal site terms for
the temporary deployment with respect to the same quantity computed at the hardest
rock station available, BAC1, are seriously biased by its nonflat, nonunitary site
response. This indicates that, occasionally, the actual behavior of a rock outcrop may
be far from that of an ideal, reference site (Steidl et al., 1996).
Description:
Published
Description:
456-466
Description:
JCR Journal
Description:
reserved
Keywords:
Aftershocks
;
Bhuj Earthquake
;
04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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