This you already know about earthquakes: They are among the most devastating and terrifying of natural hazards. While floods, tornadoes and hurricanes account for much greater annual loss in the United States, severe earthquakes pose the largest risk in terms of sudden loss of life and property.

Under Pressure
Earthquakes occur when great stresses building up within the earth are suddenly released. This sudden release of stored energy causes movement of the earth’s crust along fractures separating blocks of rock, called faults. Faults can be just a few inches in length or stretch for thousands of miles; larger faults like the San Andreas Fault in California are known the world over.

During an earthquake, the rock on one side of the fault suddenly slips, generating seismic waves that radiate in all directions from the focus – the point of an earthquake’s origin within the earth – much as ripples radiate outward when a pebble is dropped into a pool of water. The two basic types of seismic waves are body or primary waves, which travel through the interior of the earth, and surface waves, which travel along the earth’s surface and are believed to be responsible for most damage.

How destructive an earthquake will be depends on many factors, chief among them being the amount of seismic energy released, duration of shaking, depth of focus (the shallower the focus usually the greater the potential for destructive shock waves reaching the earth’s surface), distance from epicenter (the point of ground directly above the focus), population and building densities, and time of day that the earthquake takes place.

Ninety percent or more of all earthquakes are shallow (0-40 miles to the focus) and occur along boundaries between large, slowly moving slabs, or plates, of the earth’s crust and upper mantle. Most very shallow earthquakes can be attributed to the fracturing of brittle rock in the crust or relief of internal stresses due to frictional resistance locking opposite sides of a fault. But they can also be triggered by volcanic activity, large landslides, and even some types of human activity. However, in areas not known for frequent earthquakes, pinpointing the cause of the rare tremor can be very difficult.

Measuring A Quake’s Strength
Since its invention in 1880, the modern seismograph has been able to detect and record the vibrations produced by earthquakes. With data received from seismograph stations positioned around the world, seismologists and geologists are able to determine the time of occurrence, the duration of shaking, the locations of the epicenter and focus, and estimates of the energy released from earthquakes both large and small.

Information taken directly from the USGS Earthquake Hazards Program website earthquake.usgs.gov
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Magnitude is related to the amount of seismic energy released at the focus of an earthquake. It is based on the amplitude, or size, of seismic waves as recorded on standardized seismographs. The standard for magnitude measures is the Richter scale expressed in whole numbers and decimal fractions. As a general rule of thumb, damage is slight at the magnitude 4.5 level, becomes moderate at about 5.5, and above 6.5 or so can range from considerable to nearly total.

Since 1900, the strongest earthquake on record was a monster 9.5 magnitude quake that ravished the South American country of Chile on May 22, 1960. A 9.2 magnitude earthquake that rocked Alaska on March 28, 1964 holds the distinction of being the largest to ever hit the United States. By comparison, the famous San Francisco earthquake of 1906 had an estimated magnitude of a mere 7.7.

Maryland - Shaken and Stirred
In the United States, the greatest seismicity occurs along the Pacific Coast, particularly Alaska and Southern California. In contrast, the Mid-Atlantic region is characterized by a moderate amount of low-level earthquake activity. Because of the relatively low seismic energy releases, this region has historically received relatively little attention from earthquake seismologists. Although there are numerous known faults in Maryland, none is suspected to be active.

The last earthquake to cause appreciable damage in the eastern United States occurred well over 100 years ago, in 1886 near Charleston, South Carolina. It had an estimated magnitude of 6.5-7, an intensity of X, and was felt over an area of 2million square miles.

Marylanders are more likely to feel an out-of-state quake than one centered within the state’s borders, with some originating as far away as the St. Lawrence Valley and Timiskaming, Canada. As evidenced by the origin of the quake felt this past December, southwestern and central Virginia and the Atlantic seaboard northward from Wilmington, Delaware have significantly more seismic activity than Maryland.

One out-of-state earthquake that was felt in much of Maryland occurred Easter Sunday, April 22, 1984. In fact, it was felt in eight states and Washington, D.C., an area of approximately 19,000 square miles. Registering 4.1 on the Richter scale, the quake was centered about 12 miles south of Lancaster, Pennsylvania. Its most notable effects here were recorded in the northeastern part of the state where hanging pictures fell in Conowingo, windows cracked in Elkton and Joppa, and standing vehicles rocked slightly in Union Bridge. The quake was preceded by a 3.0-magnitude tremor four days earlier, and 10 aftershocks were reported over the following four days, registering 2 to 2.5 magnitudes.

As of late 1993, 47 earthquakes were known to have been centered within the state’s borders. Over the next 10 years that total reached 61. Of these quakes, two occurred in the Valley and Ridge region of Allegany and western Washington Counties, 36 were in the Piedmont region of northern and central Maryland, and 10 were in the tidewater or Coastal Plain Province.
The first reported earthquake to have its epicenter in Maryland occurred south of Annapolis on April 25, 1758. No record of its strength is known to exist; however, the shock reportedly lasted 30 seconds and was preceded by subterranean noises. The state’s strongest confirmed tremor was a 3.1-magnitude event near Hancock, Washington County, in 1978. This perhaps was rivaled by an intensity V event (unknown magnitude) near Phoenix, Baltimore County, in 1939. Earthquakes of such magnitudes or intensities are still considered to be minor, and very seldom result in significant damage or injury.

Assessing the Threat Within
Although earthquakes have been the subject of study and superstition for centuries, the modern science of seismology really gained momentum after the San Francisco earthquake of 1906. Since then geologists have learned much more about the structure and composition of the earth’s interior and, more recently, have made progress in earthquake forecasting and in hazard and risk mitigation. In order to evaluate and monitor this risk locally, the Maryland Geological Survey (MGS) has developed the Maryland Seismic Network, providing high quality, real-time data on local earth movements and earthquakes in Maryland and other more distant earthquakes around the globe.

In 2002, MGS established a seismograph station at the Department of Natural Resources (DNR) Soldiers Delight Natural Environmental Area in western Baltimore County. The site offers a number of critical advantages for the placement of the sensitive equipment. First off, it is located in an area relatively isolated from heavy traffic, construction and other vibration-producing activities that interfere with seismic measurements. Due to the unique geology of the Soldiers Delight area, soil cover is thin and poor in quality so tree cover is relatively sparse. (Trees transmit wind energy to the ground, creating seismic noise that can degrade the quality of the seismic signal.) In addition, the site is close enough to the MGS facilities in Baltimore and has sufficient elevation to broadcast data back to the Survey. And finally, because DNR’s State Forest and Parks Service manages the land, the site offers long-term security and infrastructure stability. Such an association also offers excellent opportunities for educational outreach and public involvement in the seismic project.

So what was it like, the tremor from that 4.5 quake that rippled through the Annapolis region back in December? I compare it to the sensation I’ve had when stopped in traffic high atop the Bay Bridge, when a big gust of wind seems to get up under your car. It makes you catch your breath, wondering whether what you feel is real or just a product of an overactive imagination. It also excites an instinctive desire to move – quickly! - as if one could easily flee what is going on below. But most of all, it awakened me to the vulnerabilities of the earth beneath my feet, making me ever more grateful to be on Maryland’s relatively steady shores.