A Plan for Location Calibration of Ims Stations in and near Kazakhstan

For purposes of monitoring compliance with the Comprehensive Nuclear Test-Ban Treaty, it is desirable to be able to locate seismic events routinely to within an uncertainty not greater than 1000 square km. From more than five years of experience with publication of the Reviewed Event Bulletin (REB) by the Prototype International Data Centre (PIDC), resulting in estimated locations for more than 100,000 seismic events, it is apparent that improved location accuracy is needed in order to reduce uncertainties below 1000 square km. In this paper, we outline a threeyear program of applied research which commenced in March 2000 and which has the goal of achieving improved REB locations based upon data to be contributed to the International Data Centre from 30 IMS stations in Eastern Asia. Our first efforts will focus on the four IMS seismographic stations in Kazakhstan (AKT, BRV, KUR, MAK), together with IMS stations ZAL in Russia and AAK in Kyrgyzstan. Following the recommendations of two “IMS Location Calibration Workshops” held in Oslo, Norway, in 1999 and 2000, our approach is to generate station-specific travel times for each observable seismic phase, as a function of distance and azimuth (and depth, where possible). Such travel times are obtained on the basis of (i) early studies based mainly on earthquake data (e.g. Nersesov and Rautian, 1964), (ii) Deep Seismic Sounding, and (iii) recent studies of nuclear and chemical explosions. We are also using (iv) an empirical approach in which phases are picked at IMS stations, for so-called Ground Truth events whose location is known quite accurately on the basis of additional data, obtained for example from local and regional networks. INTRODUCTION Major users of seismic data include: (1) those engaged in earthquake engineering and earthquake hazard mitigation; (2) researchers who improve our scientific knowledge of Earth's internal structure and the physics of earthquake processes; and (3) the national and international groups now being organized to monitor compliance with the Comprehensive Nuclear Test-Ban Treaty (CTBT). Although the most basic data in seismology for all these users are seismograms, in practice the great majority of those in (1) -(3) who work with seismic data do not use seismograms directly. Instead they mostly use data products derived from seismograms. The most important of these products, are bulletins of seismicity. In the last 20 years there have been enormous improvements in the quality and quantity of seismograms, associated with the development of feedback sensors and techniques of digital recording to permit high dynamic range across wide bands of frequency. And there is ongoing revolutionary improvement in access to seismogram data, as satellite communications and the Internet spread even to remote locations. It has therefore been frustrating to find that the quality of the principal data product derived from seismograms, the global bulletin of seismicity, has not yet seen the types of radical improvement needed by any of the user communities (1) -(3). The US Geological Survey (USGS) and the International Seismological Centre (ISC) publish their bulletins months to years in arrears, using volunteered data of variable quality and methods of analysis that essentially have not changed for sixty years.