Interpretation of P seismograms from shallow undersea earthquakes

摘要

The time between direct P and the surface reflections (pP and/or sP) on teleseismic P seismograms is one of the most reliable methods of estimating the depth of a seismic source. However, correctly interpreting the P seismogram in terms of P, pP, and/or sP is critical to the success of the method. Using observations from the North Pacific intraplate earthquake of 7 March 1988 (NPE) we show that the interpretation of P seismograms from shallow undersea earthquakes can be far from straight forward, even in the absence of strong bathymetry gradients above the source. The effect of an unconsolidated sedimentary layer on the seafloor and interference between the water reverberations pnwP and snwP (where n indicates the nth reverberation) can give apparently anomalous polarities on seismograms recorded by narrow band seismograph systems. For cross-correlation-type analyses, care must be taken when choosing the reference pulse because the pulse shape, due to the water reverberations, depends on the acoustic impedance contrast at the seafloor, as well as on the relative amplitudes of upward-radiated P and S. [References: 12]