Publication Date:
2011-01-26
Description:
Waveform inversion is applied to P and S waveforms selected from microearthquakes of duration magnitude (M (sub d) ) between 1.8 and 2.4 recorded at station PARM in Stahl Farm, Missouri, operated by the Cooperative New Madrid Seismic Network in the upper Mississippi Embayment to resolve the one-dimensional (1D) velocity structure of uppermost near-surface soils. Constrained by vertical seismic profiling (VSP) and standard cone penetration test (SCPT) data at nearby sites, we begin by defining a gradient velocity structure in the soils. A reference velocity model is constructed by grid search in which the observed P and P (sub S) wave shapes, Formula travel time, and P/P (sub S) amplitude ratio are the controlling factors in the systematic selection from among 136,000 models. A waveform inverse scheme is then implemented to obtain the least spectral misfit and best waveform correlation between synthetic and observed data. Four simultaneous inversions of joint waveforms from P and S wave types are performed. Resolved four 1D final models with associated uncertainties are listed in this article. The uppermost 7 m of near-surface soil exhibit very low velocities, through which V (sub P) and V (sub S) are in the ranges of 140 m/s to 470 m/s and 95 m/s to 215 m/s, respectively. This noninvasive technique demonstrates that the observed high-frequency reverberations (1-17 Hz) of P, S, and P (sub S) waveforms from local microearthquakes can be utilized to provide a view of the near-surface soil structure. Although there are trade-offs between layer thickness and slowness, this method explains much of the high-frequency site response due to microearthquake wave propagation through the very low-velocity, near-surface soils.
Print ISSN:
0037-1106
Electronic ISSN:
1943-3573
Topics:
Geosciences
,
Physics
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