Physics
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pepi...83..225b&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 83, Issue 3-4, p. 225-247.
Physics
4
Scientific paper
Short-period teleseismic P waves may suffer considerable distortion owing to the effects of upper-mantle/crustal heterogeneity. This sensitivity suggests the use of P waveforms as an aid to constraining upper-mantle structure; however, this requires a means of separating the relative contributions from these two regimes. We investigate the nature of short-period waveform distortion as recorded on the Washington Regional Seismograph Network (WRSN). A technique for analyzing short-period telescismic P-waveform distortion is presented to extract the impulse response function of the underlying crust and upper-mantle for a given event-station pair. The technique is shown to yield highly reproducible results and permits direct comparison of response functions for different source locations. Short-period waveform distortion can be strong and is essentially incoherent between stations (minimum separation 20 km). We subsequently confine our attention to a single station where waveform distortion is evidenced by strong defocusing and examine the behavior of this distortion as a function of source location. In addition to crustal defocusing, which persists over a broad range of azimuths and epicentral distances, we also find evidence for more subtle variations in the initial waveform which may be attributable to upper-mantle structure. These observations are modeled in two ways. We employ a parabolic approximation method to simulate distortion by deep-seated, large-scale structure as exemplified by a best estimate model of the upper mantle below WRSN derived from travel-time inversion. The resulting synthetics fail to exhibit strong waveform distortion; the principal effects are focusing/defocusing which accompany geometrical spreading. Crustal heterogeneity is probably stronger and exists at smaller scales comparable to the wavelengths of the short-period waves, hence contributions to waveform distortion through scattering processes become important. These contributions are investigated qualitatively using an acoustic finite difference code and models of crustal heterogeneity comprising volume heterogeneity and interface topography. Waveforms vary markedly over station separations as small as 5-10 km but at a given station are relatively insensitive to changes in source location. Crustal contamination will seriously complicate the retrieval of information on upper-mantle structure from short-period waveform distortion; however, these results suggest that the identification of an upper-mantle signature can be achieved by comprehensive analysis of the impulse response at a given station as function of epicentral distance and back-azimuth.
Present address: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC, WA 20015-1305, USA.
Bostock Michael G.
Vandecar John C.
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