2D Modeling of Lower Mantle Structure with WKM Synthetics

[摘要] The Earth is very heterogeneous, especially in the region close to the surface of theEarth, and in regions close to the core-mantle boundary (CMB). The lowermostmantle (bottom 300km of the mantle) is the place for fast anomaly (3% faster Svelocity than PREM, modeled from Scd), for slow anomaly (-3% slower S velocity thanPREM, modeled from S,ScS), for extreme anomalous structure (ultra-low velocityzone, 30% lower inS velocity, 10% lower in P velocity). Strong anomaly with largerdimension is also observed beneath Africa and Pacific, originally modeled from traveltime of S, SKS and ScS. Given the heterogeneous nature of the earth, more accurateapproach (than travel time) has to be applied to study the details of various anomalousstructures, and matching waveform with synthetic seismograms has proven effectivein constraining the velocity structures. However, it is difficult to make syntheticseismograms in more than 1D cases where no exact analytical solution is possible.Numerical methods like finite difference or finite elements are too time consumingin modeling body waveforms. We developed a 2D synthetic algorithm, which isextended from 1D generalized ray theory (GRT), to make synthetic seismogramsefficiently (each seismogram per minutes). This 2D algorithm is related to WKBapproximation, but is based on different principles, it is thus named to be WKM, i.e.,WKB modified. WKM has been applied to study the variation of fast D" structurebeneath the Caribbean sea, to study the plume beneath Africa. WKM is also appliedto study PKP precursors which is a very important seismic phase in modeling lowermantle heterogeneity. By matching WKM synthetic seismograms with various data,we discovered and confirmed that (a) The D" beneath Caribbean varies laterally, andthe variation is best revealed with Scd+Sab beyond 88 degree where Sed overrunsSab. (b) The low velocity structure beneath Africa is about 1500 km in height, atleast 1000km in width, and features 3% reduced S velocity. The low velocity structureis a combination of a relatively thin, low velocity layer (200 km thick or less) beneaththe Atlantic, then rising very sharply into mid mantle towards Africa. (c) At theedges of this huge Africa low velocity structures, ULVZs are found by modeling thelarge separation between S and ScS beyond 100 degree. The ULVZ to the easternboundary was discovered with SKPdS data, and later is confirmed by PKP precursordata. This is the first time that ULVZ is verified with distinct seismic phase.

[发布机构] University:California Institute of Technology;Department:Geological and Planetary Sciences

[关键词] Geophysics and Computer Science