Abstract
We studied the complex multiple-faulting pattern of the 40-km-long
rupture zone of the 1999 M7.1 Hector Mine, California, earthquake
with fault zone trapped waves generated by near-surface explosions
and aftershocks, and recorded by linear seismic arrays deployed across
the surface rupture. The explosion excited trapped waves, with relatively
large amplitudes at 3-5 Hz and a long duration of S coda waves, are
similar to those observed for aftershocks but have lower frequencies
and travel more slowly. Three-dimensional finite difference simulations
of fault zone trapped waves indicate a 75- to 100-m-wide low-velocity
and low-Q zone (waveguide) along the rupture surface on the Lavic
Lake fault (LLF) in the Bullion Mountains. The S velocity within
the waveguide varies from 1.0 to 2.5 km/s at depths of 0-8 km, reduced
by \~35-45\% from the wall rock velocity, and Q is \~10-60. The
pattern of aftershocks for which we observed trapped waves shows
that this low-velocity waveguide has two branches in the northern
and southern portions of the rupture zone, indicating a multiple-fault
rupture at seismogenic depth. North of the Bullion Mountains, although
only the rupture segment on the northwest LLF broke to the surface,
a rupture segment on a buried fault also extended \~15 km in the
more northerly direction from the main shock epicenter. To the south,
the rupture on the LLF intersected the Bullion fault (BF) and bifurcated.
The rupture on the south LLF extended \~10 km from the intersection
and diminished while there was minor rupture on the southeast BF,
which dips to the northeast and disconnects from the LLF at depth.
Thus the analysis of fault zone trapped waves helps delineate a more
complex set of rupture planes than the surface breakage, in accord
with the complex pattern of aftershock distribution and geodetic
evidence that the Hector Mine event involved several faults which
may also rupture individually. Our simulations of dynamic rupture
using a finite element code show that generic models are able to
produce the general features of the northern part of the rupture,
including slips on subparallel fault segments. The models indicate
that such a faulting pattern is physically plausible and consistent
with observations.
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