Event Characterization Development and Analysis at the Prototype IDC

Abstract : This report describes work implementing seismic event characterization capabilities at the prototype International Data Centre (PIDC) and quantifying the performance of these capabilities with regard to monitoring the Comprehensive Nuclear Test-Ban Treaty (CTBT). Seismic event-screening procedures and criteria are described, based on standard event characterization parameters computed at the PIDC, with the objective of screening out events that are consistent with natural seismic phenomena. The screening capabilities are based on focal depth, Ms:mb, determination of events with offshore locations, and comparisons of events to regional event populations using high-frequency regional Pn/Lg and Pn/Sn. Uncertainties in estimates of the event characterization parameters are included in the analyses so events may be screened out at a specified confidence level. The screening criteria performance is assessed for all events above mb 3.5 in the Reviewed Event Bulletin (REB) for 1996 and l997. During this period there were over 30,000 such events, including two underground nuclear tests at the Lop Nor test site. The screening criteria are also applied to 45 and 48 historical underground nuclear explosions at the Nevada Test Site and Semipalatinsk, respectively. These results are also compared to those obtained using alternative screening criteria. Also described are in-depth analyses of some interesting seismic events, including the two underground nuclear explosions at the Lop Nor test site in China, an event in Southern India, and an event in the Kara Sea on 16 August 1997. Also, preliminary region-specific distance corrections are presented for regional seismic amplitude ratios, Pn/Lg and Pn/Sn in the 2-4, 4-6 and 6-8 Hz bands, for the existing IMS seismic stations. The results utilize regional data from 50 IMS seismic stations, a parameterization of the distance dependence including geometrical spreading and attenuation terms, and investigation of subregional effects.c