AN INTEGRATED STUDY OF CRUSTAL STRUCTURE AND REGIONAL WAVE-PROPAGATION FOR SOUTHEASTERN MISSOURI

Teleseismic receiver function analysis and published refraction studie s are used to infer the existence of first-order discontinuities withi n the crust and mantle of the Ozark uplift at the Incorporated Researc h Institutions in Seismology (IRIS) station Cathedral Cave, Missouri ( CCM). This information is then used to study the nature of regional wa ve propagation from the New Madrid seismic zone to CCM across the Ozar k Dome to interpret broadband seismograms written by small events. Rec eiver function inversion indicates a crust 40 km thick characterized b y 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-wav e velocity of about 8.5 km/sec. The data resolve both the top and bott om of the layer. Such high velocities suggest rocks with eclogite or d unite compositions. Regional data from the 26 September 1990 Cape Gira rdeau earthquake (M1 = 4.7) are used to model average crustal velociti es and the nature of the velocity gradient near the Moho through inver sion of relative arrival times of observed phases. Regional P and S wa veforms are exceptionally simple at ranges of 180 km showing the mantl e head waves P(n) and S(n) with a triplicated P(g) and S(g) arriving w ithin a few seconds, respectively. sP is quite prominent and is an acc urate indicator of source depth for the Cape Girardeau event, which oc curred between 14 and 16 km depth. There is no evidence for other majo r phases, except R(g), after the S(g) arrival confirming the observati on that the crust is dominated by smooth-velocity gradients. A grid se arch technique is used to model relative phase amplitudes to obtain th e focal mechanism for the Cape Girardeau event. Allowable focal mechan isms 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, an d Rayleigh phases suggest that crustal velocities are slightly lower w ithin the Mississippi Embayment and that the uppermost mantle has a hi gher positive gradient than further northwest. Nevertheless, sP constr ains the source depth between 7 and 9 km, and P, SV, and SH wave ampli tudes yield mechanisms consistent with that obtained by the Portable A rray for Numerical Data Acquisition (PANDA) deployment. Waveforms from two small events with poor signal-to-noise ratios are compared with w aveforms of the larger events to infer source depths. Modeling broadba nd 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 str ucture will remain one of the principal obstacles to overcome.