Every year approximately 3,000,000 earthquakes occur worldwide. Ninety eight percent of them are less than a magnitude 3. Fewer than 20 earthquakes occur each year, on average, that are considered major (magnitude 7.0 – 7.9) or great (magnitude 8 and greater). Between 2000 and 2009, the United States experienced approximately 32,000 earthquakes; six were considered major and occurred in either Alaska or California (http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php#table_us).
Earthquakes do not occur exclusively in the western United States. Seven events with magnitudes greater than 6.0 have occurred in the central and eastern sections of the United States since 1811 (http://earthquake.usgs.gov/earthquakes/states/historical_mag.php). Of these, four occurred near New Madrid, Missouri between 1811 and 1812, and one occurred in Charleston, South Carolina, in 1886. The largest event in Delaware occurred in 1871 and had an estimated magnitude 4.1. The largest recorded event in Delaware occurred in 1973 and had an estimated magnitude of 3.8.
In 1997, Delaware was reclassified from being a low seismic risk state to being a medium seismic risk state by the U.S. Geological Survey (USGS) and the Federal Emergency Management Agency (FEMA).
The Delaware Geological Survey (DGS) currently operates a network of five seismic stations throughout Delaware. Fifty-eight earthquakes have been documented in Delaware since 1871. Refer to Baxter (2000) and