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Knowing the Depth of a Shallow Seismic Event, How Often Can We Find Depth Phases at Regional Distances?

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Digital data from 14 events magnitude 2.4 - 4.5 occurring during the 1981 Elk Lake, Washington earthquake sequence were examined for evidence of phases that could be used to establish focal depths from single recording stations at regional distances. Hypocentral parameters for the Elk Lake sequence are well known as a result of its occurrence within a dense monitoring network. Paths to stations at distances of 130 to 300 km are usually structurally complex. The P sub n, P sub m P, and P sub g phases were found to propagate poorly, and consequently these phases are not promising candidates for use in depth determination. Phases that exhibit depth dependence were found at some stations, but at no station could two P phases be found whose time difference could be used to estimate focal depth. A pair of stations in the same azimuth showed observable time differences between phases that are a function of event focal depth. The time differences are small, 0.1 to 0.3 s for events having a 4 km range of focal depths. Our results suggest that good determination of focal depths from pairs of stations along the same azimuth is possible for events located in small source regions.

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  • Government and Political Science
  • Seismology
  • Seismic Detection and Detectors
  • Nuclear Weapons

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