Imaging Shallow Crustal Structure in the Upper Mississippi Embayment Using Local Earthquake Waveform Data

Abstract

A 1D normal moveout (NMO)‐corrected and stacked pseudoprofiling method was applied to analyze the characteristic features shown on primary P ‐ and S ‐wave coda and on Sp waveforms from local microearthquakes in an attempt to image prominent reflectors and to resolve shallow crustal velocity structure (∼5 km) in the upper Mississippi embayment. Acoustic well log data were used to constrain the P ‐wave velocity in the upper 5 km. Events at close distances and with clear P and S arrivals were selected to ensure reliable NMO correction for reflections and transmissions. The observed reflections and transmissions are important controlling factors on modeling waveforms. We analyzed local earthquake data recorded at all broadband and one short‐period station of the Cooperative New Madrid Seismic Network. Despite polarity differences among P , S , and Sp waveforms, consistent reflectors in the sedimentary section can be imaged across the three wave types. Correlation with a basement‐penetrating well indicates that reflectors at the base of the Upper Cretaceous–Holocene Mississippi Embayment Supergroup, the base of the Cambrian–Ordovician Knox Group, and the high‐velocity lower Upper Cambrian Bonneterre Formation are shown in pseudoprofiles among stations in the upper Mississippi embayment. Our study finds that a one‐layer homogeneous velocity model of sediments in the ranges of 1.95–2.42 km/s for V P and 0.60–0.73 km/s for V S overlying a half‐space of Paleozoic rocks with velocities in the ranges of 6.0–6.2 km/s for V P and 3.26–3.6 km/s for V S can represent shallow crustal structure in the upper Mississippi embayment. Differential times of P – PpPhp and S – SsShs appear linearly proportional to sediment thicknesses, which best fits a one‐layer sediment structure with average V P 2.042±0.041 km/s and V S 0.709±0.051 km/s, in the least‐squares sense predicted by the wave propagation effects. Online Material: Figures illustrating seismograms, process procedure, imaged reflectors, and resolved velocity structure for six broadband stations and one short‐period station.