Guide Atlantic Coastal Plain Sedimentation and Basement Tectonics Southeast of Washington, D.C.

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Buildings shift from foundations and collapse. Ground cracked. Underground pipes broken. X Some well-built wooden structures destroyed. Most masonry structures destroyed.

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Ground badly cracked. Landslides on steep slopes. XI Few, if any, masonry structures remain standing. Railroad rails bent; bridges destroyed. Broad fissure in ground.

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XII Virtually total destruction. Waves seen on ground;objects thrown into the air. In , a seismograph station was established at Soldiers Delight in Baltimore County. Measurement of the severity of an earthquake can be expressed in several ways, the two most common being intensity and magnitude. Intensities are ranked on a level scale and range from barely perceptible I to total destruction XII. The lower intensities are described in terms of people's reactions and sensations, whereas the higher intensities relate chiefly to observable structural damage.

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Magnitude is an objective measure of earthquake severity and is closely related to the amount of seismic energy released at the focus of an earthquake. It is based on the amplitude of seismic waves as recorded on standardized seismographs. The standard for magnitude measures is the Richter Scale, an open-ended scale expressed in whole numbers and decimal fractions. The Richter Scale is logarithmic, meaning that an earthquake of magnitude 5. As a first approximation, each whole number increment on the Richter Scale corresponds to a release of about 31 times more seismic, or vibrational, energy.

Actually, there are several different methods of determining Richter magnitude.

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One uses surface waves, another body waves, and so on. However, the differences in results are slight. Although the Richter scale has no upper limit, the greatest magnitude on record is 8. By comparison, the famous San Francisco earthquake of had an estimated magnitude of about 8. It is important to realize that these relationships are only generalizations and can vary for any given earthquake depending upon local geologic conditions. As a general rule of thumb, damage is slight at the magnitude 4. This rela tion may not apply to earthquakes in Maryland, if recent events are any indication. A small tremor in January, , west of Baltimore was assigned an Modified Mercalli Intensity V near the epicenter, but registered only a 2.

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To most people in the United States, damaging earthquakes are a California phenomon, but this is misleading. Even though the greatest seismicity in the United States occurs along the Pacific Coast especially Alaska and Southern California , major earthquakes have also occurred in the central and eastern U. The last earthquake to cause appreciable damage in the eastern United States occurred in near Charleston, South Carolina.

It had an estimated magnitude of 6. Even in Maryland, the felt intensity from this earthquake was IV to V. Perhaps the greatest seismic event ever to occur in North America in historic times was a series of earthquakes that shook the mid-continent around New Madrid, Missouri in the winter of Estimates of the magnitude range as high as 8. Other damaging earthquakes in the eastern U. Figure 2 shows earthquake epicenters in the eastern United States and eastern Canada for a year period, Although numerous, these earthquakes were all low-intensity, low-magnitude events.

Most had a magnitude less than 2. It is definitely worth noting that Maryland seems to be part of a seismically quiet zone. Several earthquakes in adjacent states have been felt in Maryland. Marylanders are more likely to feel one of these out-of-state earthquakes than one within Maryland.

As shown by Figure 2, Southwestern Virginia, central Virginia, and the Atlantic seaboard northward from Wilmington, Delaware have significantly more seismic activity than does Maryland. One out-of-state earthquake that was felt in much of Maryland occurred Easter Sunday, April 22, In fact, it was reported felt in eight states and the District of Columbia, over an area of approximately 19, square miles. Centered about 12 miles south of Lancaster, Pennsylvania, this earthquake registered 4.

Most notable effects in Maryland were in the northeastern part of the state, which generally experienced Modified Mercalli Intensity V effects for example, hanging pictures fell in Conowingo; windows cracked in Elkton and Joppa; and standing vehicles rocked slightly in Union Bridge Stover, Ten aftershocks registering 2 to 2.

The Lancaster earthquake is likely related to Triassic-age structures in the area.


Over the next ten years, that total reached For example, not included in the list was a moderate shock that occurred on January 2, in an area near the Frederick County, Maryland-Loudon County, Virginia border. The maximum intensity was V, with the total felt area covering more than 3, square miles. The first reported earthquake to have actually had its epicenter in Maryland occurred south of Annapolis on April 25, , but no record of its strength is known to exist.

The shock lasted 30 seconds and was preceded by subterranean noises. Additional felt reports were received from a few points in Pennsylvania U. Geological Survey, Maryland's strongest confirmed tremor was a 3. That perhaps was rivaled by an intensity V event unknown magnitude near Phoenix, Baltimore County, in Earthquakes of such magnitudes or intensities are still considered to be minor, and very seldom result in significant damage or injury.

Magnitude estimated from other events in the series. Recent confirmed earthquakes in Maryland were both felt in roughly the same location and, therefore, may possibly be related. The first of these occurred on January 13, at about p. According to reports from nine seismograph stations, the shock's magnitude registered 2. Depth to focus was approximately 2 miles, which indicates a very shallow earthquake. Several first-hand accounts of the event from the Granite-Hernwood area reported that houses shook or windows rattled, both indicative of an intensity IV.

No damage was reported.

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On April 4, , reports of another small earthquake came from the Randallstown-Granite-Hernwood area. However, seismic stations in Delaware and Virginia place the epicenter in western Carroll County Fig. By all accounts, this event was smaller than the January tremor. Preliminary analysis of seismic records indicated a magnitude of about 1. One eyewitness described the event as starting with the sound of distant thunder, getting louder for about 25 seconds, then followed by 5 to 7 seconds of minor rumbling or shaking.

Another resident of this area has reported nearly two dozen similar events, although not confirmed as earthquakes, between October, and May, The earthquake hazard in the United States has been estimated in a variety of ways. Chief among them is the production of "risk maps. Seismic risk maps are based either on relative risk or on the probability of a certain seismic event at a particular time and place.

Two examples of risk maps are shown in Figure 4.

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Figure 4a shows four zones that are assigned risk on a relative scale.