Magnitude 3.4 - POTOMAC-SHENANDOAH REGION
2010 July 16 09:04:47 UTC
- This event has been reviewed by a seismologist.
|Depth||5 km (3.1 miles) set by location program|
|Distances||15 km (10 miles) NW of Rockville, Maryland|
25 km (15 miles) ENE of Leesburg, Virginia
40 km (25 miles) NW of WASHINGTON, D.C.
75 km (45 miles) WNW of ANNAPOLIS, Maryland
|Location Uncertainty||horizontal +/- 13.3 km (8.3 miles); depth fixed by location program|
|Parameters||NST= 15, Nph= 15, Dmin=44.7 km, Rmss=1.49 sec, Gp=133°,|
M-type="Nuttli" surface wave magnitude (mbLg), Version=6
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Felt (IV) at Cooksville, Maryland; (III) throughout Montgomery County and in much of central Maryland, in northern District of Columbia and in much of Loudoun County, Virginia. Felt (II) from Charlottesville, Virginia to Lancaster, Pennsylvania and Newark, Delaware and from Martinsburg, West Virginia into the Eastern Shore of Maryland. Also felt in parts of Connecticut, New Jersey, New York and Ohio with isolated felt reports as far away as Indiana, Massachusetts and South Carolina.
Earthquakes in Maryland and Northern Virginia are uncommon but not unprecedented. The earthquake on July 16th, 2010 occurred in a part of the Eastern Seaboard that is less seismically active than central Virginia, New England, and the area surrounding New York City. Since 1980, 14 earthquakes have been felt within 80 km (about 50 miles) of the July 16th earthquake. All were smaller than this event. Other earthquakes have been reported in that area as far back as at least 1758.
The most recent earthquake to have been widely felt in the Washington area occurred west of Richmond, Virginia on December 9, 2003, in the Central Virginia Seismic Zone. It had a magnitude of 4.3 and was felt throughout the Washington-Baltimore area.
The largest earthquake on record in Virginia is a magnitude 5.9 in Giles County on May 31, 1897. More recently in the broader area, a magnitude 4.1 earthquake struck Lancaster County, Pennsylvania on April 22, 1984. This Pennsylvania earthquake caused slight damage at Conestoga, Lampeter, Mt. Nebo and New Providence and was felt from West Virginia to Connecticut. A series of small, felt earthquakes spanning March to July 1993 occurred near Columbia, Maryland, within 33 km (20 miles) of the July 16, 2010 earthquake. The largest in this series was a magnitude 2.7 in March, 1993. On May 5, 2003 a magnitude 3.9 event struck near Cartersville, Virginia about midway between Charlottesville and Richmond. That earthquake was felt widely in central Virginia and in parts of Maryland. The most recent earthquake in this region was a magnitude 2.0 event on May 6, 2008 near Annandale, Virginia.
Earthquakes occur on faults. However, east of the Rocky Mountains it is very difficult to determine which fault generated a particular earthquake with much confidence. As a result, it is unlikely that the fault responsible for the July 16th, 2010 earthquake will be identified.
EARTHQUAKES IN THE STABLE CONTINENTAL REGION
Most of North America east of the Rocky Mountains has infrequent earthquakes. Here and there earthquakes are more numerous, for example in the New Madrid seismic zone centered on southeastern Missouri, in the Charlevoix-Kamouraska seismic zone of eastern Quebec, in New England, in the New York - Philadelphia - Wilmington urban corridor, and elsewhere. However, most of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S. states have never reported a damaging earthquake. The earthquakes that do occur strike anywhere at irregular intervals.
Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).
Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most of the region's bedrock was formed as several generations of mountains rose and were eroded down again over the last billion or so years.
At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. All parts of this vast region are far from the nearest plate boundaries, which, for the U.S., are to the east in the center of the Atlantic Ocean, to the south in the Caribbean Sea, and to the west in California and offshore from Washington and Oregon. The region is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even most of the known faults are poorly located at earthquake depths. Accordingly, few earthquakes east of the Rockies can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake. In most areas east of the Rockies, the best guide to earthquake hazards is the earthquakes themselves.
- Preliminary Earthquake Report
- U.S. Geological Survey, National Earthquake Information Center:
World Data Center for Seismology, Denver