Waveform inversion for one-dimensional near-surface structure in the New Madrid Seismic zone


Waveform inversion is applied to P and S waveforms selected from microearthquakes of duration magnitude (Md) 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 PS wave shapes, P-PS travel time, and P=PS 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 VP and VS 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 PS 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.

Publication Title

Bulletin of the Seismological Society of America