
January 22

What's The Risk Of More Connecticut Quakes?

Plainfield earthquakes just Earth's way of saying hello

Our living planet is not dead yet. I just love it when the solid Earth communicates with us directly, rather than indirectly through its more volatile derivatives of air, water and life. Like last week, when a tectonic jolt below Plainfield sped to the surface with a velocity of several miles per second to tell us that something really big was breaking down below.

That surface news then moved with the speed of light through excited cellphone conversations, local and state media stories, and finally national television. The "Land of Steady Habits" had acted out of character. Anxious and curious residents filled an auditorium to find out why.

I also love it when we can experience seismicity without anyone getting hurt, and without much damage. Having a few things tipped over and a few foundations cracked is a small price to pay for the little bit of fear we need to keep alert.

That bit of fear peaked at 6:37 p.m. local time on Monday of last week. The largest earthquake within a recent — and perhaps ongoing — swarm struck with a magnitude somewhere between 3.1 to 3.3. Though its depth is not yet known, a good estimate is somewhere between one and four miles.

Within a day, my seismology colleague at UConn's Center for Integrated Geosciences, Vernon Cormier, sent me the data from the nearest permanent seismic station, located just south of campus on Spring Hill. Mounted on a stable concrete pad below the ground is station UCCT, a compact seismograph that continuously records ground movements in three directions — east-west, north-south and up-down — at the micrometer scale. It's part of a network used to monitor the seismicity of the Northeastern United States. Think of it as an underground weather station.

So what's our seismic climate? We live on ancient, thick, cold and strong continental crust in the middle of the North American tectonic plate. It's moving westerly at less than an inch per year. It's being very slowly tilted to the south, a legacy of ice sheet loss about 20,000 years ago. It's being squeezed by a nearly uniform compression oriented east-northeast. That colossal stress is being distributed through a hodgepodge of rock formations, many of which were emplaced along old thrust faults.

The most significant rock boundary in the vicinity of Plainfield is the Lake Char Fault, separating older rocks to the east from younger rocks to the west. The Quinebaug Valley manifests this fault line, as does I-395, which follows the valley. And now there's an earthquake swarm in the vicinity, the densest concentration in the state since reliable monitoring began in 1990.

Clearly visible on the UConn seismograph record are a pair of foreshocks preceding the main event by four and two minutes, and a single aftershock 13 minutes later. In context, this sequence tells the story of a highly localized patch of rock strained to the point of rupture either on or near this old fault zone. From that rupture, news of the main shock traveled to Storrs in about five seconds and to Hartford in about 10. Then, everything went quiet again.

Quiet is the right word. Indeed, the most compelling thing about Plainfield's seismic record is not the recent swarm but its persistent tranquility. There's no tearing of the crust down below, no new rift being ripped open. There's nothing but the occasional snap, crackle, pop of a slow crunch, the background noise of a rocky crust being squeezed by a vice of continental proportions.

Bottom line, there's no cause for alarm. Mega-thrusts like those that force one part of the Pacific Rim beneath another cannot occur. Slicing lurches like those that slide one part of southern California or Haiti against another cannot occur.

Nevertheless, it's prudent for us to keep alert for locally damaging earthquakes, and construct buildings accordingly. Connecticut's strongest recorded earthquake threw down stone walls, knocked chimneys over and ruptured the ground in East Haddam in 1791. Something like that will happen again.

We just don't know when or where.


Copyright © 2017, Hartford Courant