Earthquakes

What is an Earthquake?

An earthquake is a sudden rapid shaking of the earth caused by the shifting of rock beneath the earth's surface. Earthquakes can cause buildings and bridges to collapse, disrupt gas, electric and phone lines, and often cause landslides, flash floods, fires, avalanches, and tsunamis. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and are followed by vibrations of gradually diminishing force called aftershocks. The underground point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is measured by the Richter scale and the Mercalli scale.

When are Earthquakes Most Likely?

There is no season for earthquakes. They can occur at any time without warning.

Who Is Most at Risk?

People who live or work in unreinforced masonry buildings built on filled land or unstable soil.

History of Earthquakes in New England

The average person may not associate earthquakes with the eastern portion of the United States, yet some of the largest seismic events have occurred east of the Rocky Mountains. In fact, the recorded history of earthquakes in the New World began shortly after the Pilgrims landed, when in 1638, a damaging earthquake rocked Plymouth Colony.

A recently released report by one of New England's foremost seismologists has shed some light on the possible epicenter and magnitude of the 1638 earthquake. He found that the French colonists on the St. Lawrence River and the English along the Massachusetts coast felt this quake with equal intensity. In order for this to happen he believes the most likely epicenter would be central New Hampshire with a magnitude of 6.5 to 7.

The first well documented, damaging earthquake to affect New Hampshire occurred the night of October 29, 1727. This earthquake, with its epicenter located off the New Hampshire and Massachusetts coast, caused damage from Boston, Massachusetts to Portland, Maine. Brick buildings "shattered" in Haverhill, Massachusetts, chimneys toppled and large quantities of sand were ejected from the ground in Newbury, Massachusetts and in Hampton, New Hampshire.

Twenty-eight years later there occurred the Cape Ann Earthquake. This earthquake, with an estimated magnitude of 6.0, caused widespread damage along coastal New England on the night of November 18, 1755. The crew of a vessel sailing 200 kilometers offshore thought they had hit a rock. They "hove-to and cast the lead," only to find plenty of water under the keel. In Boston, twelve to fifteen hundred chimneys toppled, gable ends of brick buildings broke off and fallen bricks blocked the streets. According to John Hyde, a Boston writer, damage was particularly heavy "on the low, loose ground made by encroachments on the harbor . . ."

Did You Know?

• New England experiences an average of 30-40 earthquakes per year although most are not felt.
• Due to the solid bedrock geology of New England, an earthquake will affect a much larger area than an earthquake of similar magnitude in California.
  Courtesy of NESEC (Northeast States Emergency Consortium)

Web Resources

• Earthquake History of RI
• Seismic Hazard Map of RI
• NESEC - Earthquakes
• Earthquake Activity Channel -USGS
• Earthquakes in the North East
• Earthquake Preparedness