My research interests center on understanding why earthquakes occur in continental intraplate seismic zones. I approach this interesting problem using local travel time tomography to investigate the seismic velocity structure of the crust hosting the seismic activity. To date, P wave and S wave velocity models and Vp/Vs ratios have been determined for four intraplate zones: the Charlevoix seismic zone located near Quebec, Canada, the 2001Bhuj India aftershock zone, the eastern Tennessee seismic zone (ETSZ) and the New Madrid seismic zone (NMSZ). A common observation in all four zones is that strong velocity contrasts are present and that these contrasts appear to control the distribution of seismicity. The results suggest that earthquakes in intraplate seismic zones tend to occur in rocks where strain energy is concentrating. The Vp/Vs results for the NMSZ are particularly interesting. Low Vp/Vs ratios are associated with the main branches of seismicity to depths of at least 9 km. The cause for the low ratio values remains an open question. I am also using gravity and magnetic data to investigate crustal structure associated with the ETSZ. Potential field data along several cross sections were inverted to determine possible fault parameters, rock densities and susceptibilities, and plausible source rocks. The models suggest that the seismicity is associated with a Grenville-age suture zone that juxtaposed crustal masses with different lithological properties. Ongoing research projects involve application of double-difference tomography to determine source locations and velocity structure in the ETSZ and the NMSZ.
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