A Day of Unusual Earthquake Activity

Today two earthquakes of magnitude greater than 8.0 (8.6 and 8.2) took place in the Indian Ocean off the coast of Sumatra in a span of just over two hours.  One might expect only one or two earthquakes of this size worldwide in a year but here were two that were close together in both time and space.   The helicorder display on my seismograph was unreadable.  I normally keep it set to a gain of 4 but dropped it back to 1 this evening to view  a Magnitude 6.5 earthquake in the Michoacan, Mexico that had been originally reported as Magnitude 7.0.  Embedded in the signal in the lower part of the display is a Magnitude 5.9 earthquake off the coast of Oregon.  The image  above shows the helicorder display as recorded in northeastern Ohio.

As I looked at the tectonic setting in Sumatra this morning I noticed that the convergence rate of the subducting plate in the Indian Ocean where today’s large earthquakes occurred was 52 mm/yr.  That compares to 77 mm/yr off the coast of Peru where we spent a week last month and which experienced a magnitude 8.0 earthquake and an associated tsunami in 2007.  The devastation there was still very much in evidence five years later.

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2 thoughts on “A Day of Unusual Earthquake Activity

  1. The question begs to be asked;
    What comes first – the chicken or the egg?
    Is our planet going berserk because we are?
    Or are we going berserk because our planet is?
    Dan Gilfry
    Swedish Liberation Party

  2. In this case the question might be better framed as “Which comes first, the chicken or the watermelon?” The tectonic plates in the region of Sumatra are moving relative to each other at a rate of 52 mm/yr along the subduction zone stretching for a thousand kilometers or more. Earthquakes happen. Human beings have no effect on that strain accumulation and release.

    As far as the other part of the question, why human beings act the way they do is beyond me.

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