In general, there is no clear association among seismicity, geologic structure, and surface displacement, in contrast to a common association in the western U. The mid-Atlantic and central Appalachian region, including Maryland, is characterized by a moderate amount of low-level earthquake activity, but their cause or causes are largely a matter of speculation. In Maryland, for example, there are numerous faults, but none is known or suspected to be active.
Because of the relatively low seismic energy release, this region has received relatively little attention from earthquake seismologists Bollinger, In the Atlantic Coastal Plain, it is now thought that earthquakes may be associated with nearly vertical faults that formed during the opening of the present Atlantic Ocean during the Triassic period about million years ago Hanks, Such faults would occur in the "basement" bedrock, and not in the overlying, younger Coastal Plain sediments themselves.
Recent evidence suggests that earthquakes in the Valley and Ridge Province and in the Piedmont Province occur at shallow depths usually less than 15 miles in the Precambrian crystalline basement rocks Wheeler and Bollinger, The geologic structure that may be responsible for earthquake activity in these areas is a nearly horizontal fault that formed during continental collision and closing of a proto-Atlantic Ocean during late Paleozoic time approximately million years ago.
It is also possible that some earthquakes in the Piedmont are in some way related to igneous dikes that were intruded into surrounding bedrock during the Triassic and Jurassic periods roughly million years ago.
TABLE 1. The Modified Mercalli Intensity Scale of abridged. I Not felt except by very few people under especially favorable conditions. II Felt by a few people, especially those on upper floors of buildings. Suspended objects may swing. III Felt quite noticeably indoors. Many do not recognize it as an earthquake. Standing motorcars may rock slightly.
IV Felt by many who are indoors; felt by a few outdoors. At night, some awakened. Dishes, windows and doors rattle. V Felt by nearly everyone; many awakened. Some dishes and windows broken; some cracked plaster; unstable objects overturned. VI Felt by everyone; many frightened and run outdoors. Some heavy furniture moved; some fallen plaster or damaged chimneys. VII Most people alarmed and run outside. Damage negligible in well constructed buildings; considerable damage in poorly constructed buildings.
VIII Damage slight in speciall designed structures; considerable in ordinary buildings; great in poorly built structures. Heavy furniture overturned. Chimneys, monuments, etc. IX Damage considerable in specially designed structures.
Buildings shift from foundations and collapse. Ground cracked. Underground pipes broken. X Some well-built wooden structures destroyed. Most masonry structures destroyed. Ground badly cracked. Landslides on steep slopes. XI Few, if any, masonry structures remain standing. Railroad rails bent; bridges destroyed.
Broad fissure in ground. 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.
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. 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. 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 To get breaking news alerts, download our app now.
Baltimore News. Maryland School Closings. That Fall football for sure. It was at a depth of 2. Was there some type of earthquake just now in the city? And watch WJZ news for the latest on the earthquake. Stay with WJZ on this developing story. To get breaking news alerts, download our app now. Baltimore News. Maryland School Closings. That Fall football for sure.
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