For nearly 40 years, the former Soviet Union has carried out an extensive program of seismic studies of the Earth's crust and upper mantle, known as “Deep Seismic Sounding” or DSS [Piwinskii, 1979; Zverev and Kosminskaya, 1980; Egorkin and Pavlenkova, 1981; Egorkin and Chernyshov, 1983; Scheimer and Borg, 1985]. Beginning in 1939–1940 with a series of small‐scale seismic experiments near Moscow, DSS profiling has broadened into a national multiinstitutional exploration effort that has completed almost 150,000 km of profiles covering all major geological provinces of northern Eurasia [Ryaboy, 1989].
Ð Seismic event locations based on regional 1-D velocity-depth sections can have bias errors caused by travel-time variations within dierent tectonic provinces and due to ray-paths crossing boundaries between tectonic provinces with dierent 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 Speci®c 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 dierent 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 pro®les 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 ®t the observed residuals. Our goal was to ®nd 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 ®t 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 )5 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 signi®cant 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.
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