By Dorcas Coleman
For the typical East Coast resident, the word “earthquake” doesn’t present itself in conversation very often, except for those occasions when lamenting the misfortunes befallen citizens of the so-called Left Coast. I certainly don’t lose any sleep fretting over whether my house and belongings could withstand a good jolt.

That being said, I’m sitting at my desk mid-afternoon on December 9 of last year when I feel the building that I’m in (DNR headquarters in Annapolis), slightly sway. The sensation I experienced was the result of an earthquake that originated about 175 miles away in central Virginia. The relatively minor tremor registering 4.5 on the Richter scale was centered about halfway between Charlottesville and Richmond, southeast of the small town of Columbia.

An earthquake? In Maryland? Can’t be.

Oh yes it can. Although Maryland – or the East Coast in general – is not an area known for its seismic activity, mild tremors, most barely perceptible, are more commonplace than one might guess.

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.

Q: Can animals predict earthquakes?

Changes in animal behavior cannot be used to predict earthquakes. Even though there have been documented cases of unusual animal behavior prior to earthquakes, a reproducible connection between a specific behavior and the occurrence of an earthquake has not been made. Animals change their behavior for many reasons and given that an earthquake can shake millions of people, it is likely that a few of their pets will, by chance, be acting strangely before an earthquake.

Q: Will California eventually fall off into the ocean?

No. The San Andreas Fault System, which crosses California from the Salton Sea in the south to Cape Mendocino in the north, is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving in northwest with respect to the North American Plate at approximately 46 millimeters per year (the rate your fingernails grow). The strike-slip earthquakes on the San Andreas Fault are a result of this plate motion. The plates are moving horizontally past one another, so California is not going to fall into the ocean. However, Los Angeles and San Francisco will one day be adjacent to one another!

Q: What is the biggest earthquake ever?

Since 1900, the earthquake in Chile on May 22, 1960, is the biggest in the World with magnitude 9.5 Mw.

Q: What is the biggest earthquake in the United States?

Since 1900, the earthquake in Alaska on March 28, 1964, is the biggest earthquake in the United States, with magnitude 9.2 Mw. This earthquake is also the second biggest earthquake in the World.

Q: Which States have the most earthquakes?

Alaska and California.

Q: I want to move to a place that doesn’t have earthquakes. Where can I go?

Antarctica has the least earthquakes of any continent, but small earthquakes can occur anywhere in the world.

Information taken directly from the USGS Earthquake Hazards Program website

Measurement of the severity of an earthquake is expressed in terms of intensity and magnitude. Intensities, reported on the Modified Mercalli Intensity (MMI) Scale, are based on eyewitness accounts and ranked on a 12-level scale ranging from barely perceptible (I) to total destruction (XII). Lower intensities are described in terms of people’s reactions and sensations, whereas higher intensities relate mainly to observable structural damage as well as such extreme phenomena as visible surface waves and objects being tossed into the air.

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.

Mercalli Earthquake Intensity Scale of 1931 (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 over- turned. Chimneys, monuments, etc. may topple.

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 ruined. 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.

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