COMPLEXITIES IN HIGH-FREQUENCY SEISMIC WAVE-FORMS DUE TO 3-DIMENSIONAL STRUCTURE IN THE NEW-MADRID SEISMIC ZONE

Effects of three-dimensional structure on 2 to 10 Hz seismic signals i n the New Madrid Seismic Zone (NMSZ) are investigated using waveforms from similar microearthquakes that occurred near Ridgely, Tennessee. S lowness 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 time s, coda is increasingly affected by random scattering. SPS observed on the vertical components and/or near the S wave nodes are more comlple x due to practical limitations of the algorithm. P coda and early S co da at a fifth station, located southwest of the junction, show the mos t complex SPS. Analysis of that complexity, together with waveform syn thetics and travel time analysis, strongly suggests that scattering/mu ltipathing due to source zone velocity heterogeneities occurs near the path, probably due to dilatancy occurring southwest of the Ridgely ju nction, or due to internal fault zone complexities. Coda Q, when avera ged over three components, show no station variations. We infer that ( 1) coda at 1 Hz is mainly caused by scattering in the sedimentary laye r where Q is frequency independent, (2) at higher frequencies the coda contains an increasing amount of scattered body waves, and (3) the so urce radiation pattern and path variable scattering/multipathing effec ts in early coda do not affect coda Q.