High resolution earthquake locations and velocity models in the New Madrid Seismic Zone

High resolution earthquake relocations and velocity models have been calculated for the New Madrid Seismic Zone (NMSZ) using the double-difference algorithm (?). The NMSZ is the most active intraplate seismic zone in the United States, famous for three large earthquakes that occurred in 1811-1812. On average approximately 200 earthquakes occur along the four major arms of seismicity each year. The four major arms of seismicity are examined through earthquake relocations using the double-difference program hypoDD (?). HypoDD uses differential catalog and waveform cross correlation (WCC) times to calculate relative earthquake locations. The data set consisted of earthquakes recorded between 2000-2006 by the New Madrid Cooperative Seismic Network on three component stations. The relocations provide a sharper image of the individual faults at depth.Geologic features and rock properties that could affect the generation of present day seismicity in the NMSZ are investigated through travel time tomography. The double-difference algorithm was expanded to include not only differential times but also absolute P and S wave travel times to infer velocity structure, implemented in the program tomoDD (?). The hypoDD data set was expanded to include network earthquakes recorded in 2007, as well as the addition of earthquakes recorded between 1989-1992 using the temporary PANDA deployment. Results of the velocity inversion focused on the Reelfoot Fault (RF), a reverse fault which produces a majority of earthquakes in the NMSZ. Results indicate normal P and S wave velocities along the northern portion of the RF fault relative to the 1-D starting model. P and S wave velocity models along the southern portion of the RF indicate a decrease in velocities, suggesting an increase in pore fluid pressure and fracturing. A mafic intrusion that has been imaged in previous tomography studies, as well as gravity and magnetics, is also imaged.