What is the Driving Mechanism of Earthquakes in the New Madrid Seismic Zone? – A Study of Shear Wave Splitting and Tomography.
Previously Published Material: Preliminary results of part 1 (tomography) of this work was presented at the annual AGU 2014 meeting in San Francisco California. The results are still being improved with addition of more data from the Flex Array Deployment and in preparation to be submitted to a journal in the near future. The second part (Shear wave splitting study) has not been presented anywhere else.
Abstract ID#: 34259
We use data from January 2011 to date recorded by the broadband USArray Transportable Array (TA), Northern Embayment Lithospheric Experiment (NELE) project stations and Cooperative New Madrid Seismic Network broadband stations. NELE is a lithospheric-scale passive array experiment. Fifty stations of the TA form the basic grid. 51 Flex Array (FA) broadband stations are used to form three profiles over a two-year deployment (July 2013 to June 2015) and an additional 6 FA broadband stations are moved to fill in the TA grid in six month intervals in order to increase the background spatial resolution in velocity structure from 70km to about 35km.
For the tomographic study, we use arrival times from local earthquakes and travel time residuals from teleseismic earthquakes recorded by the three networks. We perform a joint local and teleseismic inversion using the TOMOG3D inversion code of Zhao et al., (1994) to determine the velocity structure. Our results indicate a consistent low velocity anomaly in both the Vp and Vs solutions at about 200-300 km depth. Checkerboard tests show that the spatial resolution is high in the upper mantle especially for the Vp model and fairly high in the crust for most of the study area.
For the SKS splitting study we select events (Mw ≥ 6.5) from the global centroid moment tensor catalog recorded by the three networks. We compare three inversion techniques simultaneously: the rotation-correlation method, minimum energy on the transverse component and the eigenvalue criteria. Preliminary results indicate a complex pattern of anisotropy beneath the study region. We combine the splitting results with the new, detailed P-wave and S-wave velocity models for the upper mantle to further our understanding of the driving mechanism of the NMSZ intraplate earthquakes and allow us to better assess the associated seismic hazard.