By KIM SMITH DEDAM
---- — AUSABLE FORKS — The chair starts to roll. The windows rattle.
Water in a glass ripples, and the table might jolt. A giant noise sometimes sounds like a train coming down the street.
But nothing visible is out there.
People in this region have come to recognize the hidden, unexpected shakes and groans of a moving Earth.
Five midsize tremblors rumbled the region in the past month. They actually are the largest and most widely felt among 22 recorded in the region by Lamont-Doherty Cooperative Seismographic Network at Columbia University since mid-July.
On Sept. 22, a 3.1-magnitude earthquake, measured on the Richter scale, sent waves rolling from an epicenter near Clarence, Quebec.
On Oct. 4, a 2.5-magnitude jolt struck north of Huntingdon, Quebec, followed nearly an hour later by a 3.9 centered in Beloeil, some 65 miles to the northeast.
At 12:19 a.m. Oct. 10, a 3.9-magnitude quake centered about 21 miles north of Montreal could be felt across the area.
And at 7:12 p.m. Oct. 16, a 4.0-magnitude quake centered at Waterboro, Maine, reverberated across much of New England and Eastern New York. The shaking was clocked as a “V” out of “X” in what scientists measure using Roman numerals on the Modified Mercalli Intensity scale. Intensity is a measurement other than magnitude on the Richter scale to indicate damage. The evening’s rattling was felt as far south as Long Island.
The seemingly tight sequence of events has left many in the area wondering what startled the usually quiet Earth underfoot: The Northeast section of North America doesn’t sit on the edge of a fault zone.
But there are ancient pieces of old faults.
Many of the ruptures stretch north to south, and other fractures — not faults — align at roughly 45-degree angles like great striations across the entire mass of Adirondack bedrock.
The Adirondack rock structures share a narrow bridge under the St. Lawrence River near Niagara Falls and continue into the Laurentians and Ontario and Quebec.
The faults are lined around it, tell-tale tracks of plate motion left in geologic layers eons ago as the Earth’s crust cooled and heaved.
Dr. Won-Young Kim is the senior research scientist at the Lamont-Doherty Cooperative Seismographic Network. He gathers and correlates data from an array of seismic stations and calculates earthquake waveforms.
In the past 20 years, his research has given seismic science, geologists and other scientists a far more detailed look at how earthquakes affect a relatively stable, but heavily populated, Northeastern America.
Seismic-monitoring equipment is strategically placed at about 40 sites, including SUNY Plattsburgh, Middlebury College, the University of Vermont and SUNY Potsdam.
Tremblors here are called “intraplate” events, he explained.
“Here, it is still an active area in the sense that every piece of the ground is slowly moving or adjusting to movement in other parts of the Earth,” Kim said.
“It is a whole, lively system. In one part, the (Earth’s crust) crumbles into the sea, as in Japan. In California, it is sliding. We are not quiet here in the Northeast; the Atlantic ocean is trying to push us to the West.”
Geologists have tracked great shifts in the Earth’s crust dating back 250 million years when all the continents were pressed together in one land mass.
“It is continuously moving,” Kim said of the planet’s surface.
“What we understand nowadays is that the crust, or earth’s surface, (is) plates sort of critically balanced with the stress between them. So if it becomes unbalanced, it will just crack.
“Here, in the Northeast, we find the east-to-west (tectonic-plate) compression is squeezing, so the ground is shortening as faults going north to south are breaking.”
To visualize east-west compression, think of the land mass as two arrows crossed in a single plane.
The north-south arrow would rise as it is squeezed in east-west compression, opposite to the gravity-induced slide.
“And the ground has to adjust for that,” Kim said.
“It adjusts at old, broken fault lines. If the fault is going northeast on a 45-degree angle, each side is squeezed (uniformly), and the motion is very small.”
The three largest earth-shaking events in New York date to 1944, starting with a magnitude-5.8 earthquake centered between Massena and Cornwall.
On Oct. 7, 1983, a 5.3-magnitude quake struck Blue Mountain Lake.
On April 20, 2002, a 5.1 quake shook AuSable Forks. That quake proved a scientific windfall.
Kim helped lead the team of scientists who monitored motion after the 2002 AuSable Forks event. They gathered data there intensively for months.
According to the Lamont scientists’ research papers, the first portable seismograph was in place near AuSable Forks within about 12 hours of the first big tremor. Four more were deployed the next day, and five international and regional research centers assisted with personnel, equipping some 15 stations surrounding AuSable Forks by April 27.
By November 2002, the group had detected 69 small aftershocks.
“Some aftershocks,” Kim reported, “are located a significant distance from the rupture, suggesting … the activation of secondary faults.”
Research measurements detected a rupture area 10-kilometers to- 13-kilometers deep in AuSable Forks, “dipping westward with reverse slip,” Kim said.
It is one of the ancient faults awakened by crust compression as the Earth turns.
Geologic maps show the Adirondack bedrock is cordoned off to the east by a large, old fault, running the length of Lake Champlain and into the St. Lawrence seaway.
The ancient seam — along the eastern edge of Lake Champlain — is called the Champlain Thrust.
Doug Wolfe, consulting manager of the Atmospheric Sciences Research Center operated by SUNY at Whiteface Mountain, said the first seismology equipment was installed there around 1971.
“We had operated a seismic station for Lamont observatory,” Wolfe said.
“It was part of the original study in the 1970s funded by the Department of Defense. They were monitoring underground nuclear tests. In addition, they were getting quite a bit of data from small earthquakes in the area. An observatory at Blue Mountain Lake ... was also generating a lot of data. They also had a sensor set up in Mineville, monitoring the moans and groans of the mine shafts.”
Earthquake monitoring at Whiteface continued for six or seven years, before ending in the mid-1970s, Wolfe said.
“I specifically remember an event we were monitoring, the Amchitka (Island), Alaska, nuclear test blast, in 1971. Waves would go straight out from the initial impact,” Wolfe said.
“The secondary waves were snakelike, winding back and forth. They are the ones that bring everybody’s house down.”
MILLIONS EACH YEAR
The Northeast State Emergency Consortium reports 40 to 50 earthquakes are recorded in New York and New England per year, or about four per month.
Though there is really no way to predict one, any time the earth rolls, real-time earthquake information is posted online within minutes by the U.S. Geological Survey.
“We have a series of networks all over the world supported by USGS and other organizations, including universities,” said U.S. Geological Survey spokeswoman Diane Noserale, who is based in Reston, Va.
“All of the information is fed into headquarters at Golden, Colo. USGS has a geophysicist on duty 24 hours a day, seven days a week. They make a determination; the information is prepared and posted online.”
Earthquakes do happen more than people realize, Noserale said.
”Earthquakes can occur any place on Earth. There are millions each year.”
Email Kim Smith Dedam:
email@example.comTO LEARN MORE ▶ Real-time information about earthquakes worldwide and in this area is available at earthquake.usgs.gov/earthquakes/map ▶ For the Lamont-Doherty Earth Observatory, visit ldeo.columbia.edu/LCSN/recenteqs