The Geology of Delaware

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Overview of Earthquakes in Delaware

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DGS Seismic Recorder

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 http://www.dgs.udel.edu/datasets/catalog-delaware-earthquakes-spreadsheet for more details about the DGS Seismic Network, Delaware Earthquake Catalog, and for documentation of earthquakes.

An earthquake occurred in Delaware on October 9, 1871, and caused severe property damage. In Wilmington, Delaware's largest city, chimneys toppled, windows broke, and residents were quite bewildered by the unusual event. Lighter damage was sustained in northern Delaware at Newport, New Castle, and in Oxford, Pennsylvania. Earth noises, variously described as "rumbling" and "explosive," accompanied the shock in several areas.

A tremor in March 1879 beneath the Delaware River, not far from Dover, was felt "strongly" in that area according to old seismic records. The records, however, do not describe the "strong" effects.

On May 8, 1906, a shock occurred in Delaware just three weeks after the noted San Francisco earthquake in California. Records state this shock was strong at Seaford, in southwest Delaware, but list no details concerning the event.

Two tremors, both below intensity V, occurred in Delaware, one on the Lower Delaware in December 1937, and one near Wilmington in January 1944.

Parts modified from Earthquake Information Bulletin, May - June 1971, Volume 3, Number 3.

More Information

More information about the frequency, locations, and science of earthquakes can be found in the adjacent pages on this site and in the DGS report: Special Publication 23: Earthquake Basics.