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They "Shoot" Earthquakes — Don’t They?
Released: 5/29/1998

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Pat Jorgenson 1-click interview
Phone: 650-329-4011

It will be open season on earthquakes in northern Delaware, as scientists from the U.S. Geological Survey, the Delaware Geological Survey and the University of Delaware conduct a high-resolution seismic imaging investigation in the New Castle-Wilmington area during the first two weeks of June. The project commences in New Castle on June 1.

The primary objective of the investigation is to locate active faults that are responsible for moderate (up to magnitude 3.6) earthquakes in the greater Wilmington area, to determine if they are capable of generating much larger earthquakes, and to determine the lateral extent to faulting. This project is jointly funded by the Delaware Geological Survey, the University of Delaware, the Federal Emergency Management Agency (FEMA), and the USGS Earthquake Hazards Program.

Rufus Catchings, a USGS geophysicist who is coordinating the project said the objectives of the proposed work are to locate possible faulting within the Potomac Formation that may be associated with moderate (up to magnitude 3.6) earthquakes; to locate faults within the crystalline basement rocks that may be capable of generating damaging earthquakes; and to determine lateral variations in the stratigraphy of sand and clay units of the Potomac Formation.

The survey will produce a "cat scan" image the earth’s subsurface in an area from New Castle, Del., northward toward the city of Wilmington for a total of 2.9 kilometers (1.8 miles). The images will be obtained by setting off small explosions and recording the seismic waves from these explosions as the waves travel downward and are reflected back to the surface, where they will be recorded on portable seismographs.

The nature of the recorded waves also contains information about the soils and rocks through which they pass. "We expect to obtain reflection images from the near surface, about three meters down, to several hundred meters depth," Catchings said. "The imaging depths will vary depending with the local geology, but we expect to image to depths in excess of 500 meters. Velocity measurements will also vary with the local geology, local noise conditions, and the length of the seismic recording array, but we expect to obtain velocity information to depths on the order of 300 meters deep."

The scientists will produce the seismic waves by firing eight-gauge blank shotgun shells into the ground at a depth of approximately 12 to 18 inches below the surface, or by detonating one-pound explosive charges at a depth of approximately 10-15 feet below the surface. Both seismic sources will be recorded by an array of approximately six seismographs, systems, with a maximum of 360 active channels.

During the past 35 years the USGS has conducted numerous seismic investigations in California and other states, but this is the first time that the procedure has been conducted in any of the mid-Atlantic states. Because of the small amount of explosives used, it is doubtful that anyone other than the scientists in the immediate area of the shot hole will hear or feel anything.

Editors: The attached backgrounder on Delaware earthquakes is supplied by the Delaware Geological Survey. During the seismic imaging project, Rufus Catchings and other scientists will be available for on- site interviews. These interviews may be arranged by contacting Dr. Catchings at 415-722-7826, or Dr. Thomas McKenna at 302-831-8257.

Backgrounder on Delaware Earthquakes

The occurrence of earthquakes in northern Delaware and adjacent areas of Pennsylvania, Maryland, and New Jersey is well documented by both historical and instrumental records. More than 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 the Philadelphia and surrounding areas. The largest known earthquake in Delaware occurred in the Wilmington area in 1871, and had an intensity of VII on the Modified Mercalli Scale. That scale measures the amount of shaking, on a scale of one to 10, usually based on eye-witness reports, rather than the amount of energy released by the earthquake, as expressed in magnitude.

The second largest seismic event occurred in Delaware in 1973, and had a magnitude 3.8 and a maximum intensity of V-VI. The epicenter for this event was placed in or near the Delaware River in northern Delaware. Eighty-three earthquakes have been documented or suspected in Delaware since 1871. Since the installation of the first seismometer by the Delaware Geological Survey (DGS in 1972, at least 55 earthquakes have been recorded in Delaware.

Based on this past history, Delaware has recently been classified as having "moderate" seismic risk by the U. S. Geological Survey (USGS) and the Federal Emergency Management Agency (FEMA). Over the past several years, there has also been an increase in public awareness of, and sensitivity to, earthquakes and the potential effects that earthquakes can have on public health, safety, and welfare, as well as on infrastructure. The northern portion of New Castle County is a bottleneck through which pass the major railroads, roads, pipelines and lifelines along the East Coast. Two major bridges carrying trucks with hazardous materials and commuters cross the Delaware River in this area which connects Washington, DC to Philadelphia and New York City.

The DGS is a member of the Northeast (NEUSSN) and the Southeast U. S. Seismic Networks (SEUSSN). The seismic station in Greenwood is part of the Council of the National Seismic System (CNSS) which is operated by the USGS. The Delaware seismic network is strategically located between stations in northern New Jersey and New York, and southwestern Virginia thereby providing a vital technological link between stations in those areas. Through the provision and sharing of seismic data, scientists have been able to make some progress in their ur understanding of seismicity in the middle Atlantic area. In addition, when opportunities occur, they are able to take advantage of resources throughout the networks to develop an understanding of their need to update the seismic network through the application of new technology that enables them to record, store, retrieve, analyze, and share seismic information in more detail and more efficiently and effectively.

The DGS is currently developing a joint cooperative agreement with the USGS to further evaluate seismicity in Delaware. As part of this effort, they are conducting a seismic survey along an abandoned railroad grade within and northwest of the City of New Castle in New Castle County. The 2.9-kilometer survey extends from Boulden Road southeastward under Route 273 (Frenchtown Road) to Young Street. in the City of New Castle.

With high-resolution seismic profiling, the scientists expect to obtain reflection images of the subsurface from as shallow as three meters to several hundred meters in depth. The seismic data will be processed and interpreted at the USGS lab in Menlo Park, California. The USGS is the only organization in the world that conducts this type of high-resolution seismic profiling which includes both refraction and reflection techniques, and Delaware will be the first state on the east coast to have this caliber of seismic imaging performed by the USGS. The objectives of the survey are to:

  1. Locate possible faulting within the Potomac Formation that may be associated with moderate (up to magnitude ~3.6) earthquakes;

  2. Locate faults within the crystalline basement rocks beneath the Coastal Plain that may be capable of generating earthquakes;

  3. Determine lateral variations in the sand and clay units of the Potomac Formation;

  4. Determine the structural variation in the crystalline basement from the "fall zone," southward toward the Delaware River (e.g., determine if it is a planar structure or one with multiple down-dropped faults);

  5. Determine seismic velocities of the crystalline basement rocks and sands and clays of the Potomac Formation to high-grade time-depth conversions and interpretation of the seismic image and facilitate future determinations of earthquake epicenters; and

  6. Investigate the thickness of the saprolite (weathered zone) overlying the crystalline basement.

This project is an important investigation for the purposes of understanding seismicity patterns and seismic hazards along the east coast in general and the Delaware area including Pennsylvania, Virginia and New Jersey. This project is a great step in realizing the goal of the seismic program at DGS to better understand seismicity and potential seismic hazards in the area. Since the installation of the first seismometer in 1972 at least 55 earthquakes have been recorded in Delaware. The DGS continues to play a very important role in earthquake detection and analysis by providing both raw and processed data pertaining to earthquakes not only in Delaware, but also in nearby parts of Maryland, Pennsylvania, New Jersey, and Virginia.

The USGS serves the nation by providing reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.

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