By Katie Curley

The Daily News - Newburyport, MA

AMESBURY — The quake that rumbled through the region Sunday night has been officially categorized as 1.8 on the Richter Magnitude Scale, with measured waves recorded at 2.3.

That's a small quake, but seismologists say the active fault line that runs through this region has the potential to someday produce a big earthquake, like the one that shook the area in 1727, toppling chimneys, rattling houses, and creating "liquifaction" plumes that sent geysers of sand shooting into the air. In some areas the land shifted, changing dry land into soggy quagmires.

"Most earthquakes are very small and happening all the time," Dr. John Ebel, Director of Weston Observatory and Professor of Geophysics said. "Most of the ones felt in the area are 1.7 to 2.0. Occasionally we get a 3.0, very occasionally a 4.0, and very, very occasionally a 5 as in 1727, which was the last major earthquake."

Ebel said there is no special significance to the earthquake that rattled homes on Sunday and that the Clinton-Newbury fault line has been averaging two to three minor earthquakes a year.

"The Newbury, Amesbury, Haverhill areas going south have been steadily active for 30 years," Ebel said. "Littleton, Mass., seems to have roughly the same rates. Central N.H. in Concord and Central Maine are also comparable."

Ebel noted we are living in the middle of a tectonic plate. The eastern boundary of the North American Plate is in the middle of the Atlantic Ocean, where there is a separation between Europe and Africa.

"We know from geological history that the eastern edge of the North American Plate boundary has mountains, faults and remnants of cool volcanic rocks where there has been past activity," Ebel said. "This is probably why we have earthquakes today. As plates are moving over the surface of the earth, they get squeezed a bit and there is cracking in places and tectonic activity. Every 20 to 30 years there are a couple of earthquakes felt. There is potential in the area for a larger and even possibly damaging one."

Seismologists from Weston Observatory researched the data and fielded calls from curious locals yesterday who described the quake as a large explosion that lasted two to three seconds.

The quake, which had an epicenter in Haverhill, was one of many which usually go undetected in the region each year.

Sunday at 4:35 p.m., Amesbury Middle School's seismograph detected a spike. In the moment following the rumble, calls poured into local police dispatch center alerting police of a large "explosion," which one person suggested could have been a structure collapse in Rowley.

Local cities and towns in the area sit on the Clinton-Newbury fault line, which trends east to west to northeasterly at its northeast end near Newbury.

Gale Regis and her sixth-grade science class at Amesbury Middle School have the ability to pick up the waves stemming from earthquakes as far as Japan and New Zealand. But the one that rumbled through the region Sunday night was one of the most memorable.

"It's pretty exciting; there are quite a few around here actually," Regis said. "That's why the seismograph is here. Weston Observatory recruited us and wanted to have one here because there is activity."

Regis says her students have been fascinated with the tiny instrument since it arrived in her classroom in 2006, paid for as part of a grant from the Amesbury Educational Foundation's Innovative Grant. The $10,000 grant was put toward starting The Boston College Educational Seismology Project.

As part of the project, seismologists from Weston make visits to the region to check on activity. Another seismograph is located at Amesbury Town Hall.

The seismograph is located on a classroom counter and is specially designed to pick up just tremors that shake the building. When a quake occurs in Japan, Regis said she and her class watch the seismograph hours later for the waves to reach Amesbury and cause a blip in the reading.

"The kids love it," Regis said. "I check on where there have been earthquakes each morning and then we watch to see what the seismograph looks like. When there is one in Japan, it creates a beautiful picture (on the seismograph); when they are closer, it's just a couple of spikes."

Ebel said the difference in the readings depends on how far away the earthquake is.

"An earthquake is an earthquake," Ebel said. "Vibration waves look and feel different depending on how close you are to the epicenter."

Ebel said those closest to the epicenter often hear a "boom" sound. Those farther away will feel rolling waves.

"If you are far away, you feel longer, rolling waves," Ebel said. "When you are near the epicenter it is a sharp shake; it all depends on where you are."

http://www.newburyportnews.com/punews/local_story_357232542.html

http://www.standeyo.com/NEWS/08_Earth_Changes/081223.MA.EQ.html