Earthquakes
Earthquakes johncallahanOverview of Earthquakes in Delaware
Overview of Earthquakes in Delaware johncallahanEvery 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 greater (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.
USGS Earthquake Lists, Maps, and Statistics: https://earthquake.usgs.gov/earthquakes/browse/.
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. 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 currently operates seismic stations in Delaware. Fifty-eight earthquakes have been documented in Delaware since 1871. Refer to Baxter (2000) and http://www.dgs.udel.edu/delaware-geology/catalog-delaware-earthquakes-s… 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
- Catalog of past earthquakes in Delaware
- Special Publication 23: Earthquake Basics
- Map of past earthquakes in Delaware
- USGS Real-time map of M1+ earthquakes recorded
- 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 Spreadsheet
Catalog of Delaware Earthquakes Spreadsheet rockmanThe 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.
The Delaware earthquake data is listed below in table format, can be downloaded in Excel and ESRI shapefile format, and is also displayed as a Google Map at http://www.dgs.udel.edu/delaware-geology/delaware-earthquake-map.
Earthquake Felt Report
Earthquake Felt Report johncallahanDid 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.
Delaware Earthquake Map
Delaware Earthquake Map johncallahanEarthquake data mapped below was taken from the Catalog of Delaware Earthquakes.
More details and information on earthquakes in Delaware can be found in the following:
The Earthquake of August 23, 2011
The Earthquake of August 23, 2011 johncallahanDelaware 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.
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.
The Earthquake of November 30, 2017
The Earthquake of November 30, 2017 johncallahan
The largest measured earthquake to occur within Delaware was recorded on November 30, 2017. The magnitude 4.1 temblor occurred at 4:47 p.m. with an epicenter located 6 miles northeast of Dover in Bombay Hook National Wildlife Refuge, according to data reported by the U.S. Geological Survey. Analysis of the shaking associated with the Dover earthquake indicates that the source was approximately 3 km (10,000 ft) beneath the land surface in deep crystalline basement rocks and had a predominantly strike-slip direction of motion (side-ways movement along a fault zone) with a significant thrust component (some upward movement along the fault), probably along a deep pre-existing fault related to the past tectonic episodes.
How did Delaware Geological Survey scientists confirm that an earthquake had occurred?
DGS operates a network of seismic stations in the state of Delaware to monitor earthquakes and feeds data into the Lamont Doherty Seismographic Network as well as to the U.S. Geological Survey. These stations provide publically available, real-time data on seismic signals (or wave energy generated by earthquakes) that occur. The DGS seismic station located in Greenville in New Castle County was the first station to receive the seismic wave from this earthquake.
How was this earthquake felt in Delaware and surrounding areas?
The Delaware earthquake of 2017 was felt in locations throughout the state and along the eastern seaboard from central Virginia to Massachusetts. Reports compiled on the internet by the USGS and DGS indicate a Modified Mercalli Intensity of IV felt closest to the epicenter and III around most of the region. An intensity of IV is generally associated with light shaking that is felt by many indoors but not as commonly felt outdoors. Dishes, windows, and doors may be disturbed; walls make cracking sound; and the earthquake may have a sensation like heavy truck striking a building. An intensity of III is commonly quite noticeable to persons indoors, especially on upper floors of buildings, but in many people may not recognize it as an earthquake. It may feel similar to vibrations from the passing of a truck.
As of Dec 15, 2017, the Delaware Geological Survey website had received approximately 260 "felt reports" from individuals in and around Delaware, with an average intensity reply between Mercalli III and IV. Higher intensities, commonly VI, were reported closer to the epicenter, mostly in Kent County, many of the reports associated with shaking of dishes, teapots, and lamps. The USGS also has received nearly 17,000 reports through the internet from throughout the northeastern United States.
How is the strength of an earthquake measured?
The strength of an earthquake can be estimated by how it feels to people in the area or measured using seismic equipment. The Mercalli intensity scale is characterized by how a quake feels to the observer. A Mercalli intensity of III is considered “Weak” or "Slight." It is felt by people indoors, especially on the upper floors of buildings. A Mercalli intensity of IV is termed “Light” to "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.
The Richter scale measures the energy released by an earthquake and is determined using seismic equipment. 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 Dover M4.1 temblor fell in the lower end of the “Light” strength range, the class that includes Richter magnitudes 4.0 and 4.9. An interesting fact to remember about the Richter scale is that an increase of one number, such as from 4 to 5, means a 10-time increase in the amplitude of shaking and 33-time increase in energy released. This exponential increase in energy is why the 5.8 event in August 2011 was felt more strongly by most Delawareans than this magnitude 4.1 quake near Dover; a magnitude 5.8 earthquake represents more than 350 times the energy of a magnitude 4.1 quake.
Why did this earthquake occur?
The geology of the Dover area is characterized by soft sediments and sedimentary rocks near the earth’s surface and hard basement rocks below approximately 4000 ft depth. The hard basement rocks are thought to be similar to the rocks exposed at the land surface in the hilly Appalachian Piedmont region of northern Delaware. Those basement rocks are thought to contain old faults formed by tectonic movements of the earth’s crustal plates. The tectonic episodes that formed eastern North America include a series of continental collisions that built the Appalachian Mountains in several phases between 440 and 280 millions of years ago. This was followed, between around 230 and 190 million years ago, by the separation of North American tectonic plate from an adjacent plate, creating rift faults along the edges and beginning the opening of the Atlantic Ocean. Since this rifting ended around 190 million years ago, the east coast has been fairly quiet tectonically. Ancient faults related to these tectonic episodes exist beneath us and are mostly quiet but in some areas can experience a gradual accumulation of tectonic stress that is occasionally released in the form of an earthquake. However, in contrast with the tectonically active western coast of the United States, Delaware does not have a major fault line, like the well-known San Andreas fault system in California. Instead, Delaware is situated away from the edge of the North American Continental Plate, which is located a few thousand miles from Delaware in the Atlantic Ocean, placing the region at less risk for earthquake. Thus, tremblors that occur in Delaware are termed intraplate earthquakes as they occur within a tectonic plate.
Why was this earthquake felt beyond Delaware?
The geology of the Middle Atlantic region of the east coast favors the travel of earthquake energy for great distances. The earthquake source was centered near Dover, approximately 10,000 feet beneath the Delaware Coastal Plain, deep in hard, ancient basement rocks of the North American continental crust. The basement rocks are overlain by a blanket of softer sediments that progressively thins toward the Piedmont region in northern Delaware, to where the basement rocks are exposed at the surface as the hard rocks we see in the Piedmont Province, which extends along the east side of the Appalachian Mountains, including the Baltimore area, northernmost Delaware, and the Philadelphia area. Earthquake energy can travel well through the hard, cold ancient rocks that underlie our region, in contrast with areas such as California where there is abundant faulting and warmer, softer rocks that absorb the energy more quickly.
The ability of the Piedmont rocks to efficiently transfer earthquake energy is likely responsible for the northeast-trending line of “felt reports” that extend from northern Virginia to New England, and roughly follows the trend of the Piedmont. The hard basement rocks of our region have been in generally the same configuration, with gradual sinking, for the last 180 million years.
How often do earthquakes occur in Delaware?
The earthquake on Nov 30, 2017 was the fifty-eighth documented event in Delaware since 1871, but it is only about every decade or so that there is an earthquake that people would feel. To put that into perspective, approximately 3 million earthquakes occur worldwide each year, but ninety eight percent of them are less than a magnitude 3.
The 2017 Dover earthquake matched the previous largest event in Delaware, which occurred in 1871 and was estimated to have had a magnitude 4.1 based on the historical accounts of shaking. The largest previously recorded (by instrumentation) event in Delaware occurred in 1973 and had an estimated magnitude of 3.8.
Real-time Earthquake Maps
Real-time Earthquake Maps johncallahan- https://earthquake.usgs.gov/earthquakes/map/ (USGS, by default, the above map will show M2.5 earthquakes that occurred in the past 1 day.)
- https://earthquaketrack.com/p/united-states/delaware/recent (Earthquake Track, this map shows recent earthquakes M1.5 or greater.)
- http://ds.iris.edu/seismon/? (IRIS Seismic Monitor)
- http://seismo.berkeley.edu/seismo.real.time.map.html (Berkeley Seismology Lab)
More resources on current earthquake monitoring
Investigating the Causes of Earthquakes in Delaware
Investigating the Causes of Earthquakes in Delaware johncallahanDGS Seismic Network
DGS Seismic Network johncallahanSeismic Network Map
Seismic Network Map johncallahanThe 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.
Greenville (GEDE) Seismic Station
Greenville (GEDE) Seismic Station johncallahanThe 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.
Instruments:
- Quanterra Q330 Datalogger
- Streckeisen STS-2 Broadband Seismometer
- Setra 278 Microbarometer
- NCPA Infrasound Microphone
- MEMS Barometric Pressure Gauge
Latitude: 39.8113
Longitude: -75.6358
Elevation: 110 meters (NAVD88)
39.8113, -75.6358
Milford (MIDE) Seismic Station
Milford (MIDE) Seismic Station johncallahanThe 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 one of the following sites:
Instruments:
- Quanterra Q330 Datalogger
- Nanometrics Trillium 240 v2 Broadband Seismometer
- Setra 278 Microbarometer
- NCPA Infrasound Microphone
- MEMS Barometric Pressure Gauge
Latitude: 38.8799
Longitude: -75.3256
Elevation: 10 meters (NAVD88)
