Reflection and Transmission Imaging of the Upper Crust Using Local Earthquake Seismograms

Abstract

We utilize the characteristic features of primary P - and SH -wave coda and Sp waveforms from local microearthquake data and perform prestacking 1D migration in an attempt to image prominent reflectors in the upper crust of the New Madrid seismic zone (NMSZ). This methodology applies to data from broadband stations, PARM and PENM, of the Cooperative New Madrid Seismic Network (CNMSN). Nearby exploration well log acoustic data of the Wilson 2-14 and Dow Chemical/Wilson number 1 wells are used to constrain the upper 4 km of P -wave velocity model. Despite polarity differences among P , SH , and Sp waveforms, this technique demonstrates that consistent reflectors in the deep sedimentary section can be imaged commonly among the three wave types. There are excellent correlations associated with the base of the upper Cretaceous/Holocene Mississippi Embayment Supergroup and the base of Knox group, which were also reported in a study of nearby seismic-reflection profiles by Hamilton and Zoback (1982). The Bonne Terre formation seems to be a prominent seismic stratigraphic marker associated with an interface displayed in the profiling. We find that earthquake event S - P times must be ∼3 sec or more to resolve reflectors at about 4 km depth. Possible basement at about 4.0 to 4.5 km appears on the reflected P -wave image for PARM and SH -wave image for PENM. We conclude that the velocity structure of the upper 4 km crust beneath PARM can be represented by a constant velocity of ![Graphic][1] and ![Graphic][2] for the unconsolidated sediments and ![Graphic][3] and ![Graphic][4] for the Paleozoic sedimentary rocks. A constant velocity of ![Graphic][5] and ![Graphic][6] for the unconsolidated sediments and ![Graphic][7] and ![Graphic][8] for the Paleozoic sedimentary rocks can represent structure beneath PENM. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif [8]: /embed/inline-graphic-8.gif