What’s all this talk about (interplate) Earthquakes?

The Virginia Earthquake – An Intraplate Event

The Virginia Earthquake (M5.8) occurred at roughly 2 pm on August 23, 2011 in the Virginia Seismic Zone (central Virginia). In Mineralogy lab on the fifth floor at Virginia Tech, my microscope nearly toppled over. After the shaking ceased, students rushed down the stairs to check out the live stream from nearby seismographs. Needless to say, this earthquake struck close to home.Screen Shot 2015-05-04 at 8.46.36 PM

(USGS)

A surprise to many, the Virginia Earthquake was an unexpected, relatively large East Coast intraplate event. Unlike most of the events that occur in the North America (Californa, Alaska, etc.) this event happened far from any active tectonic boundary. Events like this remind us that the stresses and strains we use to describe such boundaries, are actually present all over.

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Figure: Seismic events (Mw > 4.0) since 1980 for eastern North America. Location of the Virginia Earthquake is shown in blue. (USGS)

Intraplate earthquakes are technically unrelated to plate boundaries and have an interval on the order of a thousand years or much, much more. Intraplate earthquakes have different source properties than interplate earthquakes. For example, while both types of events have a moment relationship to length of

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the equivalent relationship between slip and length is

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and includes different values for alpha. For interplate earthquakes, values of alpha are usually around 1×10-5 whereas for intraplate events values for alpha fall closer to 6×10-5. So while both sets of earthquakes “obey the same scaling laws, intraplate earthquakes have, on average, 6 times more slip on the fault.” This further indicates that stress drop is much larger for intraplate earthquakes.

These differences are likely a direct result of differences in frictional properties between the two types of earthquakes. Those occurring away from plate boundaries have more slip on shorter lengths with slower slip velocities.

Another interesting aspect of East Coast intraplate earthquakes is the distance from which they are able to be felt. For similar magnitude events in California, shaking is usually felt within the region, like most middle magnitude earthquakes in the East Coast, the Virginia Earthquakes was felt at much larger distances. This is a result of crustal difference between the eastern United States and the West Coast. The older, denser crust in the East allows seismic energy to propagate much further.felt-comparisons

For more information, see Scholz et al. JGR 1986

 

3 thoughts on “What’s all this talk about (interplate) Earthquakes?

  1. Ashley Nichole Grijalva

    Another great example of an intraplate earthquake that could be felt for hundreds of miles away is the New Madrid Seismic Zone earthquake, which ruptured in 1811. The event, believed to have a magnitude somewhere between 7.5 and 8, rung church bells in Boston, Massachusetts, ~1,000 miles away. There was another event in 1895, magnitude 6.8, where the shaking was felt in over 15 states. The NMSZ appears to be about 50 years overdue for a magnitude 6.3 quake because the last quake of this size occurred ~120 hundred years ago at Charleston, Missouri, on Oct. 31, 1895.

    http://dnr.mo.gov/geology/geosrv/geores/techbulletin1.htm

  2. Kerry Lynn Ryan

    I was in Bayonne, NJ (close to New York City) the day of the 2011 Virginia earthquake and was able to feel it. I also didn’t realize it was an earthquake at the time. There was construction going on outside my house so I assumed that was the cause of the shaking.

    Reading this prompted me to look up historical earthquakes in NJ. The largest event on record for the USGS was a magnitude 5.3 in 1783. In the past year there have been 9 recorded events, the largest with a magnitude of 2.7. At earthquaketrack.com you can look at recent earthquakes in your (or any) area.

    http://earthquaketrack.com/p/united-states/new-jersey/recent

  3. Kyle A Homman

    Very interesting stuff. I’m glad to say that I was able to ‘feel’ the VA earthquake (in Dubois PA)! I did not know it was an earthquake until about 20 minutes after the fact though. It just felt like somebody was kicking my chair.

    There was an earthquake in Southwest Michigan on 5/2/2015. Near Kalamazoo. A magnitude 4.2 at a depth of 8km. This is the 2nd largest earthquake in Michigan’s history. The beach ball and ‘did you feel it’ map are <a href = https://docs.google.com/document/d/1VrwmnoxnHVR7_AsnKtRylJy9eeZY94xlaBGhVIxjiOw/edit?usp=sharing&quot; here! I’ll be interested to see if the shake map extends any farther in the coming days. I’m not sure of the geology around Michigan. I’d hazard a guess and say that it is probably similar to the east coast.

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