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An integrated study of crustal structure and regional wave propagation for southeastern Missouri

Department of Geosciences, 440 Deike BuildingThe Pennsylvania State University, University Park, Pennsylvania 16802

Abstract

Teleseismic receiver function analysis and published refraction studies are used to infer the existence of first-order discontinuities within the crust and mantle of the Ozark uplift at the Incorporated Research Institutions in Seismology (IRIS) station Cathedral Cave, Missouri (CCM). This information is then used to study the nature of regional wave propagation from the New Madrid seismic zone to CCM across the Ozark Dome to interpret broadband seismograms written by small events. Receiver function inversion indicates a crust 40 km thick characterized by smooth-velocity gradients down to the Moho. Secondary Ps conversions late in the receiver functions show the existence of a high-velocity mantle layer 10 to 15 km thick at approximately 60 km depth with P-wave velocity of about 8.5 km/sec. The data resolve both the top and bottom of the layer. Such high velocities suggest rocks with eclogite or dunite compositions. Regional data from the 26 September 1990 Cape Girardeau earthquake (M₁ = 4.7) are used to model average crustal velocities and the nature of the velocity gradient near the Moho through inversion of relative arrival times of observed phases. Regional P and S waveforms are exceptionally simple at ranges of 180 km showing the mantle head waves P_n and S_n with a triplicated P_g and S_g arriving within a few seconds, respectively. sP is quite prominent and is an accurate indicator of source depth for the Cape Girardeau event, which occurred between 14 and 16 km depth. There is no evidence for other major phases, except R_g, after the S_g arrival confirming the observation that the crust is dominated by smooth-velocity gradients. A grid search technique is used to model relative phase amplitudes to obtain the focal mechanism for the Cape Girardeau event. Allowable focal mechanisms are dominated by thrust faults striking NS or NE-SW. The inferred earth structure is used to model the 4 May 1991 Risco event (M₁ = 4.6), but observed differences in relative arrival times between P, S, and Rayleigh phases suggest that crustal velocities are slightly lower within the Mississippi Embayment and that the uppermost mantle has a higher positive gradient than further northwest. Nevertheless, sP constrains the source depth between 7 and 9 km, and P, SV, and SH wave amplitudes yield mechanisms consistent with that obtained by the Portable Array for Numerical Data Acquisition (PANDA) deployment. Waveforms from two small events with poor signal-to-noise ratios are compared with waveforms of the larger events to infer source depths. Modeling broadband waveforms from small local and regional events shows great promise in determining source parameters and inferring details of crustal and upper mantle structure. However, lateral heterogeneity in velocity structure will remain one of the principal obstacles to overcome.

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