SEISMICITY IN THE MID-CONTINENTAL U.S. AS RECORDED BY THE EARTHSCOPE USARRAY TRANSPORTABLE ARRAY

As the EarthScope Transportable Array (TA) stations started rolling eastward in mid 2007, it has been possible to get a more complete view of seismic activity in the USArray deployment region. The seismic sensors of the USArray Transportable Array, with a 70km station spacing and attached infrasound and meteorological package, will be in the Eastern North America region until 2014 and should help reduce the overall magnitude threshold for recorded events across the region and we should identify areas of seismicity that have never been monitored before. The Array Network Facility (ANF) acquires and locates seismic events in real-time followed by later analyst review. All events are associated with corresponding solutions from available regional network and global seismic bulletins.

In regions were the USArray stations have been deployed, the ANF has been able to locate about 50% more events than those reported by any other regional seismic bulletin. However, many of these could be quarry blasts that are excluded from published regional network bulletins. Within the continental US, the ANF seismic bulletin is complete to Ml ~ 2.0 and does not distinguish between earthquakes and blasts. We expect completeness of the catalog to have a higher threshold as the USArray moves eastward due to more cultural noise and deployment in softer ground. Many of the events located by the ANF in the intermontane plateaus and Appalachian regions are located near coal and metal mine operations, and given that these events occur mainly during local day-time hours suggests that they may be blasts. However, numerous events located by the ANF in the mid-continental region are tectonic. The USArray has recorded many felt minor earthquakes in Colorado, Oklahoma, Texas, Arkansas, South Dakota and Ohio as well as the swarms in Yellowstone, Oklahoma and Arkansas. Seismic picks and origins are available from http://anf.ucsd.edu.