Huntsville Earthquake Report
From Steve Jones at Alabamaquake.com:
Earthquake, M2.3, Northern Alabama, 29 April 2013
A small magnitude 2.3 earthquake was registered today by the USGS at 13:26:57 UTC (8:26:57 am CDT) in northern Alabama at a location approximately 18km (11 mi) from the AlabamaQuake seismic station in Huntsville, Alabama. The focal depth of the quake was approximately 13.5 km (8.4 mi) underground. There have been no reports of damages in the area due to the small tremor.
A large part of northern Alabama lies in the Southern Appalachian Seismic Zone (SASZ), which roughly follows the Appalachian Mountains from southwestern Virginia into the northeastern corner and central part of Alabama. The USGS says that, “the Southern Appalachian Seismic Zone is one of the most active earthquake zones in the eastern United States.” The SASZ has released moderate levels of seismic energy for many hundreds of years. The tectonic setting for SASZ is crustal uplift/compression. No active seismic faults are known to reach the surface in the SASZ, although the area is laced with many ancient faults that developed as the Appalachian Mountains formed several hundred million years ago. While many surface and buried faults have been located in the region, numerous unknown smaller and more deeply buried faults remain undetected, and these all have an ongoing potential for randomly located small-to-moderate earthquakes.
At the seismic station in Huntsville, the initial P (pressure; primary) seismic body wave arrival from the small M2.3 quake was seen approximately 3 seconds after the quake occurred, and the initial S (shear; secondary) body waves arrived at +6 seconds, producing a maximum vertical ground movement of approximately 452 nanometers measured at the station.
The local seismogram and earthquake epicenter location map have been posted at the AlabamaQuake website (http://alabamaquake.com) on the “Huntsville Seismograms” and “Recent Earthquake Maps” pages.