Аннотация
Regional surface wave observations offer a powerful tool for determining
source properties of large earthquakes, especially rupture velocity.
Supershear ruptures, being faster than surface wave phase velocities,
create far-field surface wave Mach cones along which waves from all
sections of the fault arrive simultaneously and, over a sufficiently
narrow frequency band, in phase. We present the first observation
of far-field Mach waves from the major Kokoxili earthquake (Tibet,
2001/11/14, Mw 7.9) and confirm that ground motion amplitudes are
indeed enhanced on the Mach cone. Theory predicts that on the Mach
cone, bandpassed surface wave seismograms from a large supershear
rupture will be identical to those from much smaller events with
similar focal mechanisms, with an amplitude ratio equal to the ratio
of the seismic moments of the two events. Cross-correlation of 15-25
s Love waves from the Kokoxili event with those from a much smaller
(Mw 5) foreshock indicates a high degree of similarity (correlation
coefficients ranging from 0.8 to 0.95) in waveforms recorded at stations
near the far-field Mach cone. This similarity vanishes away from
the Mach cone. These observations provide further evidence for supershear
propagation of the Kokoxili rupture, and demonstrate how this simple
waveform correlation procedure can be used to identify supershear
ruptures.
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