Article,
Application of 3-D crustal and upper mantle velocity model of North America for location of regional seismic events
Pure and Applied Geophysics, 158 (1-2): 79--103 (Feb 1, 2001)
DOI: 10.1007/PL00001169
Abstract
Seismic event locations based on regional 1-D velocity-depth sections
can have bias errors caused by travel-time variations within different
tectonic provinces and due to ray-paths crossing boundaries between
tectonic provinces with different crustal and upper mantle velocity
structures. Seismic event locations based on 3-D velocity models
have the potential to overcome these limitations. This paper summarizes
preliminary results for calibration of IMS for North America using
3-D velocity model. A 3-D modeling software was used to compute Source-Station
Specific Corrections (SSSCs(3-D)) for Pn travel times utilizing 3-D
crustal and upper mantle velocity model for the region. This research
was performed within the framework of the United States/Russian Federation
Joint Program of Seismic Calibration of the International Monitoring
System (IMS) in Northern Eurasia and North America. An initial 3-D
velocity model for North America was derived by combining and interpolating
1-D velocity-depth sections for different tectonic units. In areas
where no information on 1-D velocity-depth sections was available,
tectonic regionalization was used to extrapolate or interpolate.
A Moho depth map was integrated. This approach combines the information
obtained from refraction profiles with information derived from local
and regional network data. The initial 3-D velocity model was tested
against maps of Pn travel-time residuals for eight calibration explosions;
corrections to the 3-D model were made to fit the observed residuals.
Our goal was to find a 3-D crustal and upper mantle velocity model
capable predicting Pn travel times with an accuracy of 1.0-1.5 seconds
(r.m.s.). The 3-D velocity model for North America that gave the
best fit to the observed travel times, was used to produce maps of
SSSCs(3-D) for seismic stations. The computed SSSCs(3-D) vary approximately
from +5 seconds to m5 seconds for the western USA and the Pre-Cambrian
platform, respectively. These SSSCs(3-D) along with estimated modeling
and measurement errors were used to relocate, using regional data,
an independent set of large chemical explosions (with known locations
and origin times) detonated within various tectonic provinces of
North America. Utilization of the 3-D velocity model through application
of the computed SSSCs(3-D) resulted in a substantial improvement
in seismic event location accuracy and in a significant decrease
of error ellipse area for all events analyzed in comparison both
with locations based on the IASPEI91 travel times and locations based
on 1-D regional velocity models.
