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You call that an earthquake? Maine has 4.0 shaker, is fine

October 17, 2012|By Amy Hubbard

Those of us who live in California can get a little snarky about earthquakes. Maine had a 4.0 earthquake? Oh, that's tough, quake-nonchalant Californians might be tempted to say. 

Heard of Loma Prieta, 1989? San Fernando, '71? San Francisco, 1906?

But, Californians, here's something you should know: Earthquakes are  different on the East Coast.  A 4.0 back East might jolt more people over a much wider area and, by some measures, could be a much more intense experience.

"On the East Coast and in the Midwest, where they have an earthquake that they feel once every seven or eight years, it 's a big deal to them," California state geologist John Parrish told the Los Angeles Times in an interview Wednesday.

A quake that might not bestir Angelenos, for instance, could "break water mains, break glass, put cracks in houses," he said.  It could do a lot more visible damage in places where "earthquake-safe" isn't such a priority.

Plus, it's just more shocking.

"Their attitude," Parrish said, "is the same as if we had a tornado in Los Angeles: 'This is something special.' "

The earthquake that hit southern Maine at 7:12 p.m. Tuesday caused no major damage, according to the Associated Press, but it did send people running into the streets, including patrons at a Portland, Maine, pizza place. 

"It was loudest bang you ever heard in your life," Jessica Hill, owner of Waterboro House of Pizza, told the AP. "We actually thought it was an explosion of some type."

That's pretty intense.  But measuring a quake can be subjective. One type of measurement scale, called the Modified Mercalli Scale, provides a measure of quake intensity. As Parrish explained, the Mercalli deals in observations of structural damage, which tends to be worse in places with more lax building codes, plus how people describe that a quake felt. 

This Maine quake, the AP notes, was the most powerful tremor in New England since Oct. 2, 2006, when a magnitude-4.2 quake was centered on Maine's Mount Desert Island. That earthquake sent boulders tumbling around in Acadia National Park.

It was widely felt in New England, which is typical of an East Coast earthquake. Why? It's the old rocks.

"In the New England area, those rocks are ... 300 million to 500 million years old," Parrish said. They're extremely hard, tend to be contiguous and also tend to fracture near the surface.

So quakes are usually shallow, and seismic waves travel long distances, he said:  "A Maine quake can be felt in New York and Boston, hundreds of miles away."

In California, the rocks are younger. 

California is at the "suture line" of the North American and Pacific tectonic plates, which rub constantly, building up and relieving stress.  The rock is young and soft, Parrish said, with lots of fractures everywhere. When trembling happens, it just doesn't go as far.

But what we may lack in observed intensity or widespread shaking, we make up for in frequency. The West Coast has about 500 to 700 earthquakes a week, Parrish said, whereas a once-a-month temblor "would be pretty standard" for spots on the East Coast.

"We have a magnitude 6 earthquake about every three years in California," the geologist said.

"In California," Parrish said, "most people who have lived here for long or were born here have felt some kind of an earthquake. ... We don't worry too much; we don't see too much damage from a magnitude 4 to 5 earthquake."

That doesn't mean we're not realistic. As Parrish said: "We're very preparedness-oriented. We are very aware we live in so-called earthquake country."

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