eat the crust

Every two years, UNAVCO hosts a workshop for the greater geodetic community.  This consists of a three-day meeting filled with plenary sessions and breakout sessions, discussion over meals and drinks, meetings between international collaborators, and the next big thing.  The UNAVCO workshop is where I first learned about slow earthquakes, more formally known as “episodic tremor and slip” (ETS), events originally thought to be electrical glitches in instruments but now recognized as sliding along subduction zone interfaces without an actual rupture.  (For more, check out the Pacific Northwest Geodetic Array site at Central Washington University.)  These ETS events may influence when large earthquakes are likely to occur at subduction zones like the one off the coast of Canada and the U.S.’s Pacific Northwest, and certainly tell us something about how stress is accommodated along these important plate boundaries.  So, the UNAVCO workshop is a small meeting (there were 196 registrants last week), especially compared to behemouths like AGU, but it packs a punch.

This year, I was only able to attend scattered sessions, as I had ten hours of lab time scheduled on the three days of the meeting.  Still, I made it to some interesting talks and discussions.  The biggest thing I was exposed to this year: underwater geodesy.  I had spoken several months ago with a friend of mine about using pressure sensors to measure vertical movement at an underwater volcano, but didn’t know the extent of underwater crustal motion measurements.  The pressure sensors on the volcano work by measuring, indirectly, the change in the water column above the instruments:  When the volcano’s surface moves up, it displaces the overlying water.  Thus, the pressure measured at the sensor decreases.  And vice versa.  But pressure sensors can only measure vertical movements.

For horizontal measurements, underwater arrays with acoustic transmitters are deployed and measured periodically from a ship.  With the right instrumentation and equations, scientists can determine the precise location of their ship, and then the location of the sensors relative to the ship.  Make a round of measurements as a baseline and then remeasure, say, after the Tokohu earthquake, which is what Japan’s coast guard did.  On land, scientists measured up to about 5 meters (16 feet) of displacement, or motion, as a result of the Tokohu event.  Underwater, scientists measured up to about 25 meters (82 feet!) of displacement, more than four times the displacement measured on land.

Subduction zones, by nature, consist of land on one side and sea on the other.  If we only measure on land, we’re only observing one side of the process.  If we can measure on the watery side as well, we can learn much more about how subduction and, perhaps most importantly, subduction earthquakes, work.  In some places in the world, such as Indonesia, islands on the seaward side allow land measurements on both plates.  But in most places, we’re limited to seeing what happens on the overriding plate only.  That is, unless we start making more measurements underwater.

Will seafloor geodesy be the next big thing?  I think it will.  I think the more we see the results of underwater networks, the more we’ll want to see.  There is already a sparse network proposed for Cascadia, off the coast of the Pacific Northwest.  Where’s next?