Path-Dependent Regional Phase Travel-Time Corrections for the International Monitoring System in North America

The software developed for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Data Centre (IDC) in Vienna, Austria, uses the IASPEI91 seismic travel-time tables as default. However, in reality, travel times as a function of source location are different for each station, especially at regional distances. This is not modeled by a global one-dimensional model such as IASPEI91. The software therefore allows the application of a suite of travel-time corrections for improved travel-time prediction and consequently, more accurate hypocentral locations. To account for path dependency, we have developed gridded source-specific station corrections (SSSCs) for Pn, Sn, Pg , and Lg phases for 22 International Monitoring System (IMS) stations in North America. In deriving the SSSCs and associated modeling errors, we used a methodology that had been successfully applied previously for Fennoscandia. We first compiled regionalized one-dimensional travel times for North America. Then we assigned travel times as weighted sums of the travel times in each region to calculate SSSCs. Validation testing of the SSSCs was conducted using two separate sets of North American ground truth (GT) events with location accuracies within 10 km (GT0–GT10). These events were recorded at IMS stations between 1995 and 1999 and distributed throughout North America. Test event location results with SSSCs applied show reductions in both mislocations and error ellipses. For GT events with location accuracies better than 2 km, the median change in location was a 5-km improvement, and the median area of the 90th percentile error ellipses was reduced from 2550 km 2 to 910 km 2 . Using SSSCs for both data sets, the median improvement is less than 5 km. The testing emphasized that GT events with accuracies 5 km or better are crucial for validating SSSCs. Using SSSCs for all 51 GT0–GT10 events used in the validation testing, the median area of the error ellipses was reduced to less than 1000 km 2 , and the error ellipse reductions were achieved without significant degradation of coverage. The modeling errors used in this study may be somewhat underestimated, however, leaving the target goal of error ellipses less than 1000 km 2 still somewhat in question. We expect the SSSCs, event locations, and uncertainties can be improved by future refinement of travel times and modeling errors. These corrections were implemented in the operational Prototype IDC system located in Arlington, Virginia, in March 2000 and have been delivered to the Vienna IDC in January 2001.

[1]  István Bondár,et al.  Development of calibration techniques for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) international monitoring system , 1999 .

[2]  T. C. Bache,et al.  Locating events with a sparse network of regional arrays , 1988 .

[3]  B. G. Brooks,et al.  Travel times and amplitudes of principal body phases recorded from gnome , 1962 .

[4]  R. Kovach,et al.  Upper mantle structure in the Basin and Range province, western North America, from the apparent velocities of S waves , 1969 .

[5]  Freeman Gilbert,et al.  The Effect of Small, Aspherical Perturbations on Travel Times and a Re-examination of the Corrections for Ellipticity , 1976 .

[6]  B. Kennet.  Iaspei 1991 Seismological Tables , 1991 .

[7]  R. Massé.  Compressional Velocity Distribution Beneath Central and Eastern North America in the Depth Range 450–800 km , 1973 .

[8]  Jack F. Evernden,et al.  Precision of epicenters obtained by small numbers of world-wide stations , 1969 .

[9]  G. Keller,et al.  Chapter 28: Seismic properties of the crust and uppermost mantle of the conterminous United States and adjacent Canada , 1989 .

[10]  I. Bondár,et al.  Teleseismic slowness-azimuth station corrections for the International Monitoring System Seismic Network , 1999 .

[11]  N. Vasiliev.  Methods for the IMS Regional Kinematic Calibration , 2000 .

[12]  A. L. Hales,et al.  The travel times of P waves to 30° in the central United States and upper mantle structure , 1968 .

[13]  H. M. Iyer,et al.  Chapter 29: Upper-mantle velocity structure in the continental US. and Canada , 1989 .

[14]  T. Cook,et al.  Stratigraphic atlas of North and Central America , 1978 .

[15]  K. Priestley,et al.  Shear wave structure of the Southern Volcanic Plateau of Oregon and Idaho and the Northern Great Basin of Nevada from surface wave dispersion , 1982 .

[16]  Thomas H. Jordan,et al.  Teleseismic location techniques and their application to earthquake clusters in the South-Central Pacific , 1981 .

[17]  Gabi Laske,et al.  CRUST 5.1: A global crustal model at 5° × 5° , 1998 .

[18]  J. Hunter,et al.  Crustal and upper mantle structure under the Canadian Shield from Project Early Rise data , 1969 .

[19]  Y. Xiaoping,et al.  Source specific station corrections for regional phases at IMS stations in North America and Fennoscandia , 1998 .

[20]  K. Priestley,et al.  Higher-Mode Surface Waves and Structure of' the Great Basin , 1980 .

[21]  Xiaoping Yang,et al.  Location Calibration Data for CTBT Monitoring at the Prototype International Data Center , 2001 .

[22]  E. A. Flinn,et al.  Fine structure of the upper mantle , 1969 .

[23]  E. A. Flinn,et al.  Confidence regions and error determinations for seismic event location , 1965 .