An earthquake is defined as being a sudden motion of the ground which may result in surface faulting (ground rupture), ground shaking and ground failure. This complex motion is caused by a sudden shifting or breaking of subsurface rock to relieve built up stress. The energy released at the center produces a variety of seismic waves that travel out in all directions through the surrounding rock. Some of these waves make their way to the surface to the surface and travel out.

The effects of an earthquake depend on its strength measured by the Richter scale and the concentration of population and extent of development in the impacted area. If an earthquake is strong enough, utility services could be interrupted, transportation routes cut off in places, the chance of fire increase if gas lines rupture live power lines could break and emergency responders could be unable to reach scene of incidents. Personal injury can also be caused by falling debris or infrastructures.

Maine has not experienced any substantial structural damage from earthquakes, but there have been numerous measurable earthquakes recorded around the state. All of the earthquakes in Maine and New England are intra plate earthquakes. Between 1747 and 1992 there were 507 earthquakes, the largest being a 5.9 magnitude near Eastport in 1904 (PDF).

The most seismically active regions of Maine are the areas along the eastern side of Passamaquoddy Bay, the Dover-Foxcroft/Milo area, and southwestern Maine, specifically the Portland/Lewiston region. However, the Maine Geological Survey says all of Maine has a moderate risk for earthquake.

Mitigation: Buildings in high risk areas should adhere to standards of safety for earthquakes. The Maine Geological Survey maps, interprets, and publishes geologic information, and provides advisory and interpretive information for planning and regulatory agencies.

Emergency planning for earthquakes should include search and rescue, medical assistance, crisis counseling, evacuation, sheltering and mass care, and reduction of the potential problems caused by falling debris, fire, utilities, and hazardous materials.