Abstract
The Mw 7.9 Wenchuan earthquake of 12 May 2008 was the most destructive
Chinese earthquake since the 1976 Tangshan event. Tens of thousands
of people were killed, hundreds of thousands were injured, and millions
were left homeless. Here we infer the detailed rupture process of
the Wenchuan earthquake by back-projecting teleseismic P energy from
several arrays of seismometers. This technique has only recently
become feasible and is potentially faster than traditional finite-fault
inversion of teleseismic body waves; therefore, it may reduce the
notification time to emergency response agencies. Using the IRIS
DMC, we collected 255 vertical component broadband P waves at 30-95
deg from the epicenter. We found that at periods of 5 s and greater,
nearly all of these P waves were coherent enough to be used in a
global array. We applied a simple down-sampling heuristic to define
a global subarray of 70 stations that reduced the asymmetry and sidelobes
of the array response function (ARF). We also considered three regional
subarrays of seismometers in Alaska, Australia, and Europe that had
apertures less than 30 deg and P waves that were coherent to periods
as short as 1 s. Individual ARFs for these subarrays were skewed
toward the subarrays; however, the linear sum of the regional subarray
beams at 1 s produced a symmetric ARF, similar to that of the groomed
global subarray at 5 s. For both configurations we obtained the same
rupture direction, rupture length, and rupture time. We found that
the Wenchuan earthquake had three distinct pulses of high beam power
at 0, 23, and 57 s after the origin time, with the pulse at 23 s
being highest, and that it ruptured unilaterally to the northeast
for about 300 km and 110 s, with an average speed of 2.8 km/s. It
is possible that similar results can be determined for future large
dip-slip earthquakes within 20-30 min of the origin time using relatively
sparse global networks of seismometers such as those the USGS uses
to locate earthquakes in near-real time.
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