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Hough, S.E., S. Martin, R.
Bilham, and G.M. Atkinson, "The 26 January, 2001 Bhuj, India,
earthquake: observed and predicted ground motions", EOS, Transactions of the American Geophysical Union 82,
Vol. 47, Fall Meeting, Abstract S52G-02, 2001. |
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Abstract: It is
unclear whether or not the 26 January, 2001, Bhuj earthquake
occurred in an intraplate or interplate setting. However, to
understand the damage caused by this earthquake, and the hazard
posed by future similar earthquakes, one must consider not only the
source setting but propagation issues as well. TITLEhough local and
regional instrumental recordings of the devastating January 26,
2001, Bhuj earthquake are sparse, the distribution of macroseismic
effects can provide constraints on the ground motions. We compiled
news accounts describing damage and other effects and interpreted
them to obtain modified Mercalli intensities at over 300 locations
throughout the Indian subcontinent. These values are used to map the
intensity distribution using a simple mathematical interpolation
method. These maps reveal several interesting features. Significant
sediment-induced amplification is suggested at a number of locations
around the Gulf of Kachchh and in other areas along rivers, within
deltas, or on coastal alluvium. The overall distribution of
intensities also reveals extremely efficient wave propagation
throughout the subcontinent: the earthquake was felt at distances as
large as 2400 km and caused light damage at distances upwards of 700
km. This is consistent with earlier
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theoretical
and observational
results suggesting that higher mode surface waves (Lg waves) will
propagate efficiently in intraplate crust, which forms a relatively
uniform, high-Q waveguide. We use fault rupture parameters inferred
from teleseismic data to predict ground motions at distances of
0-1000 km. We convert the predicted peak ground acceleration (PGA)
values to MMI using a relationship between MMI and PGA that assigns
MMI based on the average effects in a region. The predicted MMI's
are typically lower by 1-2 units than the estimated values. We
discuss two factors that probably account for this discrepancy:
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1) a tendency for media
accounts to focus on the most dramatic damage, rather than the
average effects, and 2) a level of high frequency shaking that was
high compared to that of comparable sized California earthquakes.
When we use a relationship between response spectra and MMI to
estimate predicted shaking, we obtain MMI values a full unit higher
than those estimated using the relationship between MMI and PGA. In
any case, our results suggest that, regardless of the tectonic
setting of the source, the ground motions (and therefore hazard)
were consistent with expectations for intraplate earthquakes.
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