Thursday, March 24, 2011

Was the Japanese Earthquake unusual?

Not from a geological perspective.

From headlines in the media over the past few weeks, you'd think the Japanese earthquake was some highly unusual and unexpected event.  While the exact location of large earthquakes is always a surprise (to a certain extent), Japan is well suited for occassional megaquakes like this.

Look at a map of Japanese seismicity since 1990!  The purple line is the subduction zone where the Pacific Plate dives beneath Japan and the different color dots represent earthquakes of different depths (deeper as you move west of the trench).  Earthquakes occur all along that subducting plate and some of them will be very large.

Here's a summary of how many earthquakes occur each year above magnitude 5.0 since 1990.  The data was obtained from the United States Geological Survey (USGS) Earthquake Hazards Program.  Click to enlarge.

On average, you'd expect a magnitude 8+ earthquake every year and a magnitude 9+ earthquake every decade.  It was Japan's turn.  It could have occurred in Chile, Alaska, Indonesia, or off the coast of Washington State!  Wherever there's a subduction zone, you have the potential for a large quake, disruption of the seafloor, and a tsunami.

One will eventually occur on the Cascadia subduction zone and send a tsunami crashing into the Oregon and Washington coast.  Geologically guaranteed.  Communities like Ocean Park, WA (see map below) - built on a spit of land just north of the mouth of the Columbia River - would be toast when that occurs.  May not happen in our lifetimes (but could happen today).

People have short lifespans compared to the recurrence of geologic phenomena like large earthquakes and volcanic eruptions.  We could say the same thing about residents of Tacoma and Mount Rainier - sooner or later their number will come up.

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