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The Effects of Mississippi Embayment Sediments on Local Earthquake Tomography

The effects of velocity and thickness variations in thick, unconsolidated Mississippi Embayment sediments on local earthquake tomography for the New Madrid seismic zone must be understood before the tomography solutions can be interpreted in terms of deeper structure. Sediment effects are investigated using normal station corrections, synthetic modeling, and application of a sediment correction based upon independent knowledge of sediment thickness and velocity structure. Synthetic modeling indicates that smearing from strong velocity perturbations in the unconsolidated sediments can influence the uppermost, Paleozoic portion of the P-wave velocity solution and can extend into deeper portions of the S-wave velocity solution. However, there is little evidence for smearing due to velocity perturbations in the unconsolidated sediments in inversion results obtained using real arrival time data. The clustered distribution of hypocenters makes station corrections sensitive to velocity variations located deeper than the unconsolidated sediments; application of station corrections removes useful information from the portion of the inversion volume located below the unconsolidated sediments. The sediment correction also appears to remove information from the inversion solution but is an improvement for the P-wave solution over removal of station corrections. There is no evidence that the sediment correction is removing smearing effects produced by velocity perturbations in the unconsolidated sediments. The unconsolidated sediments do not appear to be exerting a first-order effect on local earthquake tomography solutions.