Sense of security on shaky ground; Bay State not immune to earthquake damage.

Byline: Bill Fortier

Earthquakes have been big news recently.

A late-September temblor caused extensive damage to American Samoa and created a tsunami that caused water to rise about a foot as far away as the California coast, and two more earthquakes in the South Pacific were recorded earlier this week.

Most people know about earthquakes in California and Alaska where people use the 1964 Good Friday earthquake that measured a magnitude 9.2 as a major marker in their lives.

So what about Central Massachusetts and New England - how vulnerable is this area to an earthquake?

Massachusetts may never experience an earthquake like the big one scientists have been predicting in California, but we could experience one big enough to cause billions of dollars of damage, according to experts.

Massachusetts Emergency Management Agency spokesman Peter Judge said New England averages about six minor tremors a year; for the most part, they cause only shelf rattling and dish shaking. People usually feel an earthquake when it exceeds a magnitude of 3.2. Damage ordinarily does not occur until a reading of 5.0 is reached.

Studies have shown that New England can expect an earthquake severe enough to cause major damage about once every 300 years, although 122 years between 1633 and 1755 seemed especially active.

A 1633 earthquake with an epicenter in Quebec City was strong enough to knock over chimneys in Boston. Another strong earthquake five years later caused major damage in Boston and inland.

A 1755 earthquake near Cape Ann, believed to have been between magnitude 6.5 and 7.0, caused much damage in Boston, according to John E. Ebel, director of the Weston Observatory, who has written extensively about this area's earthquake history.

The geology of the East Coast means the shock wave from earthquakes can travel over a much wider distance than in the West, which, according to Mr. Ebel, is why an 1886 earthquake in Charleston, S.C., estimated at 7.3, shook areas as far north as Boston.

Mr. Ebel and George C. Klimkiewicz, president of Weston Geophysical Engineers Inc. of Acton, said there is little doubt that more similarly strong earthquakes are in our future.

"It's happened in the past," said Mr. Klimkiewicz, whose company evaluates seismic hazards for large construction projects. "The processes that were ongoing in the past will continue to be ongoing in the future."

"In any given year, there is a low probability we'll be hit with a big earthquake," Mr. Ebel said. "But in my opinion, we could have an even larger one than the 1638 or 1755 earthquakes."

State officials were concerned enough about earthquakes that building codes that went into effect in 1975 called for provisions that would at least lower the amount of damage from a quake, said Edward S. Fratto, executive director of the Northeast States Emergency Consortium.

Among the chief authors of the building code in 1975, and revisions since then, is Norton S. Remmer, a Worcester resident and former Worcester building commissioner.

Mr. Remmer, who was commissioner from 1976 to 1986, is a code consultant. He said major earthquakes in Peru and China in the 1960s which killed thousands of people were the impetus for upgrading safety measures for large buildings erected in the state. For example, he said new construction since 1975 calls for steel reinforcements in walls of new, large buildings.

Earthquakes in California and New England are caused by two different geological processes, experts say.

California is along the edge of two major tectonic plates, Mr. Ebel explained. While Los Angeles is on the eastern edge of the Pacific plate, San Francisco, which is 400 miles to the north, is on the western edge of the North Atlantic plate. People flying into San Francisco or Los Angeles can look out a window and see evidence of previous earthquakes. In some places it is like looking at a shaken-up puzzle, Mr. Ebel said. Earthquakes there happen when the two plates collide or rub together under the earth's surface, he said.

New England is on the same North American plate as San Francisco, Mr. Ebel said, adding that another large tectonic plate is under the middle of the Atlantic Ocean. Our quakes are caused by underground movement within the North American plate. Mr. Ebel used the analogy of squeezing a brick with a vice to the point at which it cracks in a certain area as the main reason quakes occur in this part of the country.

Despite changes in building codes, experts are worried about the damage that could occur if an earthquake, similar in severity to the one in 1755, struck.

Mr. Ebel said damage can be especially severe if an earthquake hits an area built on fill.

"The damage to Boston could be several billion dollars because about two-thirds of the downtown is built on fill," he said.

Mr. Remmer added, however, that while there could be major damage, the city would recover.

"There wouldn't be many total collapses, if any," he said.

Mr. Ebel said the 1755 earthquake caused some damage as far inland as Springfield. When told that one of this area's shopping centers - the Auburn Mall - was built on fill, he said it wouldn't collapse, "but there certainly would be some shaking."

ART: PHOTOS; GRAPH; MAP

CUTLINE: (1) Norton S. Remmer (2) John E. Ebel, director of the Weston Observatory, who has written extensively about New England's earthquake history. (GRAPH) Richter scale (MAP) Earthquakes in Massachusetts

PHOTOG: (2) THE BOSTON GLOBE (GRAPH), MAP) T&G Staff/DON LANDGREN JR.