Accurate locations of nuclear explosions at the Lop Nor test site using alignment of seismograms and IKONOS satellite imagery

Locations are estimated of 11 underground nuclear explosions conducted between 26 May 1990 and 29 July 1996 at the Lop Nor test site in western China. To perform the analysis, seismic recordings were obtained from the Center for Monitoring Research (CMR), U.S. National Data Center, and Incorporated Research Institutions for Seismology (IRIS) Data Management Center. Using recordings with adequate signal-to-noise ratios, P-wave arrivals are picked manually by aligning waveforms of the events at common stations. This provides very precise relative phase timings (i.e., with relative errors typically much less than 0.1 sec). Based on analysis of commercial IKONOS satellite imagery, the locations of three explosions are fixed and used as master events. The other events are located relative to the nearest master event after correcting their travel times by the travel-time residuals (relative to IASP91) of the master events. Locations estimates of the 25 September 1992 and 29 July 1996 nuclear tests are within about 100 m of each other. The 90% confidence error ellipse for the 29 July 1996 event has a semimajor axis of less than 0.3 km. The results indicate that these two tests were likely conducted in the same tunnel. Location estimates of nine explosions conducted at the eastern zone of the Lop Nor test site are within 140 to 490 meters of vertical boreholes and infrastructure to support nuclear testing observed in IKONOS imagery. The location estimates of the eleven Chinese explosions are all considered to be well within one km of the true detonation points and, hence, are categorized as GT1 events (as defined by Yang et al., 2000a). These solutions allow seismic travel times to be calibrated accurately for the Lop Nor region at seismographic stations of the International Monitoring System (IMS).