Using SP precursor waves to detect upper-mantle discontinuities
In: Geophysical Journal International. Wiley: Oxford. ISSN 0956-540X; e-ISSN 1365-246X, more
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Structure of the Earth; Time-series analysis; Body waves |
| Authors | | Top |
- Reiss, A.-S.
- Thomas, C.
- Lecocq, T., more
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| Abstract |
We investigate the usability of the converted phase SP and its precursors, which reflect off the underside of upper-mantle seismic discontinuities. In contrast to PP and SS waves, the SP phases do not reflect midway between source and receiver but about three quarters on the great circle path. This leads to extended data coverage, especially in oceanic regions, where usually few receivers are deployed. Due to similar traveltimes and incidence angles, SP is difficult to distinguish from the PS phase. One feature that makes it possible to separate these two waves is their polarization. Therefore, we developed a polarization filter, which allows detecting precursor signals of SP in vespagrams. For this feasibility study, we analysed events from all azimuthal directions with Mw ≥ 5.8 and ranging between 80° and 140° epicentral distance recorded at the western part of the Transportable Array in the United States. Even though this method has several restrictions like limited distance and depth ranges for which the precursor signals are clearly identifiable, this study resulted in 52 events showing signals reflected off the underside of the 410 and/or 660 km discontinuity. Our averaged results show a deepened 410 km discontinuity beneath the Gulf of Alaska, central Alaska and western Canada and a shallower 410 km discontinuity beneath the northern east Pacific Ocean and the coast of Mexico. For the 660 km discontinuity we find fewer reflections. This discontinuity seems to be slightly elevated in central Alaska and beneath the Pacific shore of Mexico. The southern coast of Alaska and parts of Canada show a deepened 660 km discontinuity. These observations agree with previous results of PP-precursors, SS-precursors and receiver function studies. We show that SP precursors are a successful new approach to map upper-mantle seismic discontinuities. |
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