Regional Seismic Event Characterization Using a Bayesian Kriging Approach

This effort focuses on collecting and analyzing regional seismic data for earthquakes and underground explosions to improve seismic event characterization capabilities with regard to monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Digital data have now been acquired and processed for 267 regional recordings of 160 underground nuclear (UNE) and chemical (UCE) explosions. The explosion data set includes 99 Nevada Test Site (NTS) UNE’s recorded by MNV and KNB; 6 Lop Nor UNE’s recorded by NIL, ZAL, ULN, WMQ, BRVK and MAKZ; 22 Semipalatinsk (STS) UNE’s recorded by WMQ and BRVK; 6 Novaya Zemlya (NZ) UNE’s recorded by ARCES and KEV; and one UNE at the Indian Test Site recorded by NIL. In addition, the set includes 20 Former Soviet Union (FSU) peaceful nuclear explosions (PNE) recorded by BRVK and KEV; and 6 STS UCE’s recorded by ZAL and BRVK. (Note: not all of these explosions have recordings by all stations listed.) Over 5300 regional recordings, between 3 and 17 degrees, by existing seismic stations of the International Monitoring System (IMS) are also used in this study. The regional recordings correspond to 4173 presumed earthquakes above mb 3.5 in the Reviewed Event Bulletin (REB) that were processed at the prototype International Data Center (pIDC). Maximum amplitude measurements of Pn, Pg, Sn and Lg have been computed in several frequency bands ranging from 2 Hz up to 14 Hz (depending on the Nyquist frequency of a given station). The data for the presumed earthquakes have been used to estimate attenuation corrections for Pn/Sn and Pn/Lg amplitude ratios in the 2–4, 4–6, 6–8, and 8–10 Hz passbands for tectonic and stable region types. The regional phase amplitude ratios are further corrected for path variations using a Bayesian Kriging algorithm. A correction surface is determined for each type of amplitude ratio at each station as an optimal linear combination of existing amplitude-ratio data at the station, giving greater weight to data nearer to the location of the event to be corrected. A corresponding uncertainty surface is also estimated in terms of the residual variance of the data and a calibration variance. For well-calibrated locations, the correction converges to the mean of nearby data and the uncertainty converges to the residual variance. For locations far from calibration data, the correction surface converges to the worldwide average, with larger uncertainty due to both residual and calibration variances. Using these correction and uncertainty surfaces, corrected values of Pn/Smax(6–8 Hz) are then used to define a hypothesis test that fixes the significance level with respect to misclassifying explosions. A score is computed for each event such that events with negative scores are not screened out at the specified significance level. The significance level used here has been set such that none of the available underground explosions are screened out. Events with scores greater than zero are screened out (i.e., are considered to be inconsistent with the hypothesis that the event belongs to the explosion population). Screening rates for earthquakes above mb 3.5 are estimated and improvements in event characterization performance due to Kriging are demonstrated. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE SEP 2000 2. REPORT TYPE 3. DATES COVERED 00-00-2000 to 00-00-2000 4. TITLE AND SUBTITLE Regional Seismic Event Characterization Using A Bayesian Kriging Approach 5a. CONTRACT NUMBER