Complexities in high‐frequency seismic waveforms due to three‐dimensional structure in the New Madrid Seismic Zone

Abstract

Effects of three‐dimensional structure on 2 to 10 Hz seismic signals in the New Madrid Seismic Zone (NMSZ) are investigated using waveforms from similar microearthquakes that occurred near Ridgely, Tennessee. Slowness power spectral (SPS) analysis is used to study the plane wave composition of P, S, and coda waves. The observed SPS at four stations suggest that the early S coda is composed mainly of wavelets leaving the source with slownesses of the direct S wave. At greater lapse times, coda is increasingly affected by random scattering. SPS observed on the vertical components and/or near the S wave nodes are more comlplex due to practical limitations of the algorithm. P coda and early S coda at a fifth station, located southwest of the junction, show the most complex SPS. Analysis of that complexity, together with waveform synthetics and travel time analysis, strongly suggests that scattering/multipathing due to source zone velocity heterogeneities occurs near the path, probably due to dilatancy occurring southwest of the Ridgely junction, or due to internal fault zone complexities. Coda Q, when averaged over three components, show no station variations. We infer that (1) coda at 1 Hz is mainly caused by scattering in the sedimentary layer where Q is frequency independent, (2) at higher frequencies the coda contains an increasing amount of scattered body waves, and (3) the source radiation pattern and path variable scattering/multipathing effects in early coda do not affect coda Q.