12/23/2011

Bar Harbor Earthquake!

From http://www.nps.gov/acad/photosmultimedia/Earthquake-Effects.htm

There’s a joke that goes: things are so bad in California, even the earthquakes are moving east. On October 2, 2006 at 8:07 PM a 4.2 magnitude earthquake rocked Mount Desert Island. The epicenter was in the ocean just off Schooner Head, north of Sand Beach. We felt it here at your Acadia vacation apartment, SeaCat’s Rest. It was kind of like a big invisible gravel truck driving through our house. Nothing fell over and I’m sure our west coast friends would laugh at our reaction. One response from San Francisco to the Virginia quake of August 2011 was, “We eat 5.9 for breakfast.”

Reported effects, 10/03/2006, from http://earthquake.usgs.gov

What a shame they can’t let us have a little excitement! But really, the effects were quite mild. The worst damage was done to trails in Acadia; primarily precipice trail, where true to its name there are sections where the steep ascent got littered with rocks, some quite big. Fortunately there were no hikers on this trail at 8:07 PM on October 2.

From http://www.nps.gov/acad/photosmultimedia/Earthquake-Effects.htm

The rocks that fell were part of unstable formations anyway, subject to frequent similar events usually due to freeze/thaw cycles. Besides the precipice trail the only other affected spot was on the Park Loop Road, which was closed for a while until heavy equipment could clear the rubble.

So how common are earthquakes in Maine and what does it say about our plate tectonics? A report in the Seismological Society of America says,

No seismically active geologic structures have yet been confirmed in the northeastern United States (Ebel and Kafka 1991).

According to the report, small tremors in the area can be attributed to remnants of ice age glaciation; layers of rock still rebounding from the weight, which I interpret from their “minor offsets of glacial striations”. But the report goes on to caution,

Nevertheless, the persistence of small-earthquake activity over time and the historic occurrences of past damaging earthquakes (e.g., Ebel 1996; Ebel 2000; Ebel et al. 2000; Ebel 2006) indicate that there must be some seismically active structures in the region that are capable of hosting earthquakes above magnitude 6.0. Because such earthquakes are capable of causing significant damage, there is great incentive to learn which structures are seismically active in this heavily populated region.

Our earthquakes are “intraplate” earthquakes, we have no clashing tectonic plates below us. A map of historical New England quakes looks like a shotgun pattern. The only area which can be considered an exception is Quebec, where the top of Lake Ontario and the Gaspé peninsula show clusters of quakes. We had two other notable quakes, one in 1940 and another in 1755. These were dish-breakers and chimney-topplers, but not killers. Besides these, there were many rumblers, and we can expect many more. If the past is any indication, Mainers and visitors should not be too worried.