Soccer Dad

Did you feel it?

Map courtesy of the USGS

The above map shows the areas where Tuesday's earthquake centered near Annandale Virgina was felt. People actually felt it not far from where I work, but as far as I can tell no one at work felt it.

The USGS reports on the history of earthquakes experienced in Virginia. The earliest recorded one was:

On February 21, 1774, a strong earthquake was felt over much of Virginia and southward into North Carolina. Many houses were moved considerably off their foundations at Petersburg and Blandford (intensity MM VII). The shock was described as "severe" at Richmond and "small" at Fredericksburg. However, it "terrified the inhabitants greatly." The total felt area covered about 150,000 square kilometers.

Still quakes in the area are rare:

They occur about once per decade, although some decades have none and the 1990s had three. None are known to have caused damage since the arrival of European colonists. The corridor is between more seismically active regions to the southwest and northeast, and residents of Washington or Baltimore have felt several earthquakes that caused damage in those other, more active regions.

Because quakes around here are rare, it's hard for scientists to pinpoint exactly which fault is at fault.

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. The Washington - Baltimore urban corridor is far from the nearest plate boundaries, which are in the center of the Atlantic Ocean and in the Caribbean Sea. The urban corridor is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths. Accordingly, few, if any, earthquakes in the urban corridor 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. As in most other areas east of the Rockies, the best guide to earthquake hazards in the Washington - Baltimore urban corridor is the earthquakes themselves.

UPDATE: The Learning Channel has an earthquake simulator called Make a Quake that allows you to test different building and ground configurations with different level earthquakes.

earthquakes.