Abstract
It is well documented that lateral heterogeneities in the earth can
introduce serious errors in seismic event locations derived from
radially symmetric earth models, in particular, for small events
recorded only by sparse networks of regional stations. It should
be possible to significantly improve the seismic location accuracy
for such events by applying travel-time calibration information specific
to the regions where the sources and stations are located. This article
describes a research investigation directed toward regional travel-time
calibration of 30 International Monitoring System (IMS) stations
in eastern Asia. For this calibration an initial 3D velocity model
covering the entire region was constructed from several submodels.
Seismic travel times in this complex 3D velocity model have been
computed by using a raytracing algorithm based on a finite-difference
approximation to the eikonal equation, which is believed to be accurate
to within 0.3-0.5 sec over the regional distance range of interest.
A new regional tomography algorithm has also been implemented which
solves the fully nonlinear problem by iterating over linear inversion
steps in which event hypocenters, model velocities, and ray paths
can all be updated. Extensive testing and validation of the final
tomographically refined velocity model have been performed with data
from numerous ground truth (GT) explosions and earthquakes throughout
the region, and it has been demonstrated that this model predicts
P-wave travel times with associated root-mean-square (rms) errors
on the order of 1 sec across the study region. Finally, a completely
new multistation kriging method, which satisfies seismic reciprocity
constraints, has been formulated and applied to derive empirical
corrections to account for remaining unmodeled error, further reducing
the rms error to approximately 0.7 sec. These results support the
conclusion that our new 3D velocity model for this region represents
a significant improvement over the default IASP91 model and will
provide improved seismic location capability, in particular, for
events recorded by sparse networks of regional stations. 10.1785/0120040087
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