Multitaper Cross-Spectral Analysis of Closely Located NTS Explosions

摘要

Location relative to a reference event is often more useful and precise thanabsolute event location. Such location requires relative times for pairs of events observed at a common station, which can be obtained with high precision for similar events by waveform cross-correlation. The precision can be further boosted by applying the cross-spectral analysis method, which can provide relative delay times with resolution up to an order of magnitude better than the seismogram sampling interval. Software for cross-spectral analysis based on use of both single cosine taper and the significantly more sophisticated multitaper was developed and tested on real data. Analysis of data from six closely-located Yucca Flat explosions provided relative locations with mean location error of only about 1 km; an impressive result if one considers the large epicentral distances and the complex geology of the Nevada Test Site. Mean velocities along the four source-receiver paths were found to be stable, and significantly different, suggesting considerable anisotropy. The cross-spectrum method was also applied to regional data from two pairs of closely located NTS explosions recorded at common stations. Our results indicate that the analysis can detect small differences (of the o 0.1%) in the propagation velocities of regional phases generated by the two shots.... Multitaper, Relative location, Cross-spectral analysis, Inter-source coherence, Regional phases, Colocated explosions.