Using Earthquakes as Sources for Seismic Investigations of the Crustal Structure using Seismic Interferometry and Tomography Techniques: A Case Study from Oklahoma
Christopher Toth, Austin Holland, Randy Keller, and Stephen Holloway
Over the last few years the frequency of earthquakes in central Oklahoma has increased dramatically. Two regions of seismicity contribute to the majority of this increase. An earthquake swarm in eastern Oklahoma County has been ongoing since October 2009, averaging more than 600 earthquakes per year for the last few years. The second region is the aftershock decay sequence associated with the November 5th, 2011 M5.7 Prague earthquake in Lincoln County, Oklahoma. These two seismic source zones are separated by approximately 50 km. In late January 2012, a seismic receiver array consisting of 156 single channel vertical geophones was deployed spanning these two seismic source regions. More than eight earthquakes were recorded: two were part of the Oklahoma County swarm, and six were part of the aftershock decay. Over 2,500 travel times were picked for direct and refracted P and S phases and for P wave multiples. Conventional seismic tomography allows us to determine a smooth velocity structure within the region, but does not image sharp velocity contrasts adequately. Results of the tomography so far show a low velocity anomaly in the basement that coincides with a comparatively lower seismicity region, as well as negative anomalies in both the gravity and aeromagnetic data. The results of the tomography also show a more complex, laterally varying velocity structure around the Wilzetta fault. The magnitude of the velocity variations suggests that using 1D velocity model is inappropriate for locating earthquake hypocenters within the Prague M5.7 earthquake sequence. Seismic interferometry provides greater resolution of velocity interfaces and much greater resolution of the shallow crustal structure within the study area. The well-resolved interface imaging from seismic interferometry can be used to further constrain our tomographic velocity inversion, yielding a subsurface model of higher resolution than tomography alone can generate.
AAPG Search and Discovery Article #90176©AAPG Mid-Continent Meeting, Wichita, Kansas, October 12-15, 2013