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The effects of spall on teleseismic P-waves: An investigation with theoretical seismograms

1991; American Geophysical Union; Linguagem: Inglês

10.1029/gm065p0141

2328-8779

Jörg Schlittenhardt,

Earthquake Detection and Analysis

The effects of spall on teleseismic P-wave seismograms are investigated using theoretical seismograms calculated with an extended reflectivity method. The equivalent point-source used to model the spall process has been derived following the work of Day et al. [1983] with the modification of a spall rise-time which is introduced to account for the time necessary to bring the total spalled mass into ballistic flight. The effect of spall is studied by the superposition of synthetic seismograms for a pure explosion source with synthetic seismograms for a pure spall source. The source signal for the pure explosion is the von Seggern-Blandford reduced displacement potential. The model parameters for explosion and spall sources used are representative for the Pahute Mesa (NTS) nuclear explosion HARZER and have been determined from close-in (distance 2–7 km) and regional seismograms [Johnson, 1988; Patton, 1988]. Additionally, spall model parameters calculated from scaling relations which have been derived independently of the HARZER event are used. The result of this part of the study is that spall can contribute significantly to the waveforms of teleseismic P-waves. This conclusion still holds if certain ranges for the parameters of both the explosion and spall models are introduced. The effect of spall is to increase the peak-to-peak amplitudes and to enhance the higher frequencies compared to the predictions for the explosion without spall. However, the interference pattern in the composite explosion/spall seismograms is generally complicated and depends critically on the kinematic spall characteristics like the spall dwell- and rise-time. For the spall scaling relations considered here Sobel'ls [1978] and Patton's [1989, 1990] relations predict essential contributions of spall to teleseismic P-waveforms; only that of Viecelli [1973] predicts a minor effect on teleseismic P-waveforms. Finally, a comparison of the theoretical seismograms with observations of HARZER at teleseismic distances (SRO stations MAJO and GRFO) is made. From this comparison it is found that an explosion source without spall explains reasonably well both the maximum peak-to-peak amplitudes and the general frequency content of the data if an earth model with a dissipation time t* of 0.75 s is assumed. Therefore it is argued that the spall-model parameters momentum and spall mass used for the HARZER calculations might be up to an order of magnitude too large.