EarthquakesEarthquakes johncallahan Fri, 06/25/2010 - 09:45
Overview of Earthquakes in DelawareOverview of Earthquakes in Delaware johncallahan Sat, 06/27/2009 - 00:10
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 â 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#tabl…).
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-spreadshe… 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.
- List and interactive map of earthquakes in Delaware
- Real-time map of M1+ earthquakes recorded by USGS
- USGS Earthquake Hazards Program
- Seismic Hazard Map of Delaware (USGS)
- RI39 Earthquakes in Delaware and Nearby Areas, June 1973 - June 1984
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.
Catalog of Delaware Earthquakes SpreadsheetCatalog of Delaware Earthquakes Spreadsheet rockman Fri, 07/31/2009 - 15:11
The occurrences of earthquakes in northern Delaware and adjacent areas of Pennsylvania, Maryland, and New Jersey are well documented by both historical and instrumental records. Over 550 earthquakes have been documented within 150 miles of Delaware since 1677. One of the earliest known events occurred in 1737 and was felt in Philadelphia and surrounding areas. The largest known event in Delaware occurred in the Wilmington area in 1871 with an intensity of VII (Modified Mercalli Scale). The second largest event occurred in the Delaware area in 1973 (magnitude 3.8 and maximum Modified Mercalli Intensity of V-VI). The epicenter for this event was placed in or near the Delaware River. Sixty-nine earthquakes have been documented or suspected in Delaware since 1871.
Interactive Map of Delaware EarthquakesInteractive Map of Delaware Earthquakes johncallahan Thu, 07/30/2009 - 13:34
Earthquake Felt ReportEarthquake Felt Report johncallahan Thu, 07/30/2009 - 10:39
Did you feel an earthquake? If so, please complete our Earthquake Felt Report below.
Please answer every question to the best of your ability. Either fill in the blanks where called for or check the response that best describes the event. If a particular question does not apply or if you don't know how to respond, simply skip it and go on to the next. You can review and modify answers to all the questions at any time. Feel free to include any additional information in the Additional Comments box at the end of this form.
The Earthquake of August 23, 2011The Earthquake of August 23, 2011 johncallahan Tue, 08/23/2011 - 22:58
Delaware and surrounding areas experienced an earthquake event on the afternoon of Tuesday, August 23, 2011. According to the US Geological Survey, a magnitude 5.8 earthquake struck at 1:51 p.m. in central Virginia, in an area referred to as "the Central Virginia Seismic Zone" because of its relatively active earthquake activity for the region. The epicenter was located five miles south-southwest of Mineral, Virginia, with the quake was focused at a depth of 6 km (3.7 miles) below the surface. The Virginia Geological Survey reports that this is the largest Virginia earthquake known in historic times. A few small aftershocks have occurred in the hours afterward. (http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/se082311a.html)
How was this earthquake felt in Delaware?
In Delaware, the earthquake was felt in many locations from northernmost New Castle County to coastal and inland Sussex County. The Delaware Geological Survey website had "felt reports" completed by more than 300 respondents as of 5:00 pm on Aug 23 and over 500 respondents by the evening of Aug 24 (thank you to all of you who replied!). Preliminarily, these responses indicate a Mercalli Intensity of III to IV. The average intensity of building shaking reported by respondents was "moderate." The majority of respondents noted movement or shaking of furniture such as, bookcases, chairs, and computer equipment, some window shaking, and a few overturned items such as picture frames, bottles, and sculptures.
As of 4:00 pm, the US Geological Survey website had received reports from 108 individuals in Delaware with an average intensity reply of 3.3, so between Mercalli III and IV. In the area of central Virginia near the earthquake epicenter, intensities of VII or "Very Strong" were reported, a level strong enough to make standing difficult and to damage poorly built structures. The USGS also received numerous responses from throughout the eastern United States.
How is the strength of an earthquake measured?
The intensity can be gauged using the Mercalli intensity scale, which is characterized by how a quake feels to the observer. A Mercalli intensity of III is considered "Slight." It is felt by people indoors, especially on the upper floors of buildings. A Mercalli intensity of IV is termed "Moderate". It is felt indoors and, by some people, outdoors. Dishes and windows are commonly disturbed and rattle; it may give the sensation of a heavy truck striking a building. These two categories fit most of our reports from Delaware.
The Richter scale measures the energy released by an earthquake. Geophysical sensors called seismometers measure the amount of shaking of the ground at numerous locations; in Delaware, we have five seismic stations that cover all three counties. The August 23 temblor fell in the upper end of the Moderate strength range, between Richter magnitudes 5.0 and 5.9. An interesting fact to remember about the Richter scale â an increase of one number, such as from 5 to 6, means a 10-time increase in the amplitude of shaking and 33-time increase in energy released. This 5.9 event had considerably more energy than the magnitude 3.8 quake that occurred in Delaware in 1973.
Why did this earthquake occur?
The Virginia Geological Survey notes that the earthquake activity in the area is associated with old faults related to movements of the earthâs tectonic plates related to ancient mountain building episodes that were followed by the opening of the Atlantic Ocean about 150 million years ago. The opening of the Atlantic Ocean was accompanied by cracking of the earth's crust, or faulting, along eastern North America. Since the period of this rifting ended, the east coast has been fairly quiet tectonically, in contrast with the tectonically active western coast of the United States. Nevertheless, there are areas that still experience a gradual accumulation of tectonic stress that is occasionally released in the form of an earthquake.
Why did we feel this earthquake in Delaware so far from its central Virginia source?
The geology of the Middle Atlantic region of the east coast favors the travel of earthquake energy for great distances. The earthquake epicenter was located in the Virginia Piedmont, an area underlain by hard basement rocks that predate the opening of the present-day Atlantic Ocean. To the west lie ancient hard rocks of the Appalachian Mountains. To the east, the hard basement rocks are overlain by a blanket of softer sediments that thickens toward the Atlantic Ocean. The hard basement rocks of our region have been in generally the same configuration, with gradual sinking, for the last 150 million years. Earthquake energy can travel well through these hard, cold ancient rocks, in contrast with areas such as California where there is abundant faulting and softer rocks, which absorb the energy more quickly.
The DGS will update this page with additional information, as it becomes available, on the earthquake of August 23, 2011.
Earthquake intensity map from the US Geological Survey at http://earthquake.usgs.gov/earthquakes/dyfi/events/se/082311a/us/index…
USGS Real-time Earthquakes MapUSGS Real-time Earthquakes Map johncallahan Mon, 02/15/2010 - 11:02
The above map shows M1+ earthquakes that occurred in the past 7 days. This data stream is served by the United States Geological Survey. However, the seismic observations are collected from numerous organizations withing the US and around the world. The Delaware Geological Survey is a contributing agency and maintains its own seismic network.
More resources on current earthquake monitoring
Investigating the Causes of Earthquakes in DelawareInvestigating the Causes of Earthquakes in Delaware johncallahan Fri, 07/31/2009 - 00:09
DGS Seismic NetworkDGS Seismic Network johncallahan Fri, 06/25/2010 - 09:52
Seismic Network MapSeismic Network Map johncallahan Thu, 07/30/2009 - 11:10
The DGS maintains its own network of seismometers to detect local earthquake activity. Following an earthquake swarm in 1972, the DGS established its first seismometer station in Newark. The network now consists of five seismic stations spread across the state: three stations in the Newark-Wilmington area, one at the DEMA office in southern New Castle County, and one at the Sussex County Emergency Operations Center. Signals are recorded on paper and captured digitally using Earthworm, a seismic processing system developed by the USGS. These records are shared nationally through participation in the Advanced National Seismic System Network, the Lamont-Doherty Cooperative Seismographic Network, the Northeast U.S. Seismic Network, and the Southeast U.S. Seismic Network. Small local earthquakes and larger earthquakes from other parts of the country and the world are clearly resolved by the network.
Bellevue State Park (BVD) Seismic StationBellevue State Park (BVD) Seismic Station johncallahan Tue, 07/28/2009 - 00:49
The seismometer located at Bellevue State Park is placed on the Brandywine Blue Gneiss. The Brandywine Blue Gneiss is the new name given to the granulite-facies felsic gneisses in the Wilmington Complex. Informally called the âblue rocks,â this unit is a medium- to coarse-grained, lineated (pinstriped), two-pyroxene gneiss with variable quartz content and thin, discontinuous mafic layers, pods, and schlieren. The pinstriping and the massive nature of the rock suggest the rock is intrusive; however, deformation and recrystallization have obscured original igneous features.
The unit appears on the Wilmington North, Wilmington South, and Marcus Hook quadrangles where it underlies the city of Wilmington and its northeastern suburbs.
Seismometer:Mark Products L-4C Short period Vertical
Amplifier: Sprengnether AS-110 Gain 68dB
VCO: TC-10 1700 Hz
Elevation: 58 meters above sea level
Brandywine Creek State Park (BWD) Seismic StationBrandywine Creek State Park (BWD) Seismic Station johncallahan Tue, 07/28/2009 - 00:59
The seismometer located at Brandywine Creek State Park is positioned on the Wissahickon Formation. The Wissahickon Formation is an extensive sequence of pelitic and psammitic gneisses interlayered with amphibolites. With few exceptions, most of the amphibolite layers are less than 30 feet thick. The rocks have been metamorphosed to upper amphibolite facies and isoclinally folded.
The formation is located within the Wilmington North, Kennett Square, West Grove, Newark West, and Newark East quadrangles.
Seismometer: HS101-A Short period Vertical
Amplifier: Emtel 6243 Gain 36dB
VCO: 2720 Hz
Elevation: 63 meters above sea level
Delaware Emergency Management Agency (DEMA) Seismic StationDelaware Emergency Management Agency (DEMA) Seismic Station johncallahan Tue, 07/28/2009 - 14:23
The seismometer, located at the Delaware Emergency Management Agency, is located on the Columbia Formation. The Columbia Formation is a fine to coarse, feldspathic quartz sand with varying amounts of gravel. It is primarily a body of glacial outwash sediment deposited in a cold climate during the middle Pleistocene.
Seismometer:Geospace Corp. HS-10 Short period
Amplifier: Emtel 6242 Gain -18dB
VCO: 2380 Hz
Elevation: 14 meters above sea level
Sussex County Emergency Operation Center (SCOM) Seismic StationSussex County Emergency Operation Center (SCOM) Seismic Station johncallahan Tue, 07/28/2009 - 14:30
The seismometer, located on the property of the Sussex County Emergency Operation Center, is positioned on the Beaverdam Formation. The Beaverdam Formation is a very coarse sand with beds of fine to medium sand. It is interpreted to be a Pliocene fluvial to estuarine deposit.
Seismometer: Geospace Corp HS-10-1/B Short period Vertical
Amplifier: Emtel 6242 Gain -18dB
VCO: 1020 Hz
Elevation: 15 meters above sea level
White Clay Creek State Park (NED) Seismic StationWhite Clay Creek State Park (NED) Seismic Station johncallahan Tue, 07/28/2009 - 13:58
The seismometer, located in White Clay Creek State Park, is positioned on the Wissahickon Formation. The Wissahickon Formation is an extensive sequence of pelitic and psammitic gneisses interlayered with amphibolites. With few exceptions, most of the amphibolite layers are less than 30 feet thick. The rocks have been metamorphosed to upper amphibolite facies and isoclinally folded.
The formation is located within the Wilmington North, Kennett Square, West Grove, Newark West, and Newark East U. S. Geological Survey 7.5-minute quadrangles.
Seismometer:Teledyne Geotech S-13 Short period Vertical
Amplifier: Emtel 6243 Gain 24dB
VCO: 2380 Hz
Elevation: 90 meters above sea level
Greenville (GEDE) Seismic StationGreenville (GEDE) Seismic Station johncallahan Wed, 10/01/2014 - 11:46
The seismic instruments located at the Greenville, DE location were adopted by DGS from the Earthscope Transportable Array, which consists of a network of 400 high-quality, portable broadband seismometers that are being placed in temporary sites across the United States. DGS adopted two of these Earthscope stations, P60A in Greenville, DE and Q61A in Milford, DE. This program provided an outstanding opportunity for Delaware to enhance its seismic monitoring capabilities in the future, and upgrade current antiquated equipment.
The equipment at this station was installed on 2013-04-28.
For more info, visit the station page at the Array Network Facility.
- Quanterra Q330 Datalogger
- Streckeisen STS-2 Broadband Seismometer
- Setra 278 Microbarometer
- NCPA Infrasound Microphone
- MEMS Barometric Pressure Gauge
Elevation: 110 meters (NAVD88)
Milford (MIDE) Seismic StationMilford (MIDE) Seismic Station johncallahan Wed, 10/01/2014 - 12:07
The seismic instruments located at the Milford, DE location were adopted by DGS from the Earthscope Transportable Array, which consists of a network of 400 high-quality, portable broadband seismometers that are being placed in temporary sites across the United States. DGS adopted two of these Earthscope stations, P60A in Greenville, DE and Q61A in Milford, DE. This program provided an outstanding opportunity for Delaware to enhance its seismic monitoring capabilities in the future, and upgrade current antiquated equipment.
The equipment at this station was installed on 2013-05-28.
For more info, visit the station page at he Array Network Facility.
- Quanterra Q330 Datalogger
- Nanometrics Trillium 240 v2 Broadband Seismometer
- Setra 278 Microbarometer
- NCPA Infrasound Microphone
- MEMS Barometric Pressure Gauge
Elevation: 10 meters (NAVD88)