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Seismic Velocity Structure and Event Relocation in Kazakhstan from Observation and Modeling of Secondary P Phases

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Scientific rept. no. 1

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Data from presumed explosions in Kazakhstan recorded by NRDCSAS stations were analyzed to model the crustal structure of the region and to examine the use of arrival times of secondary P phases in event location. Data from 20 events were modeled using a reflectivity algorithm. A good fit to the first five seconds of the P wave was obtained in the epicentral distance range of 100 to 300 km. The best fitting crustal model has a velocity in the upper crust increasing from 4.5 kmsec near the surface to over 6 kmsec at 10 km depth and a mid-crustal velocity of 6.5 kmsec at 15 km depth increasing to 6.95 kmsec at 40 km depth. The model is similar to previous estimates of P velocity structure in the region, though with a gentler gradient in the upper crust and steeper gradient in the lower crust. We used the derived crustal model and the primary and secondary P wave arrival times to relocate events in the region. Utilizing the phase PmP substantially decreases the focal depth uncertainty for many events. Virtually all of the events analyzed are concluded to be surface explosions.

Subject Categories:

  • Seismology
  • Seismic Detection and Detectors

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