“What strainmeters tell us about volcanoes and eruptions; the view from below.”

At Wednesday’s DTM seminar, Dr. I. Selwyn Sacks presented data based on the instrument he and his team use to measure changes in the strain of rocks. Sacks is a co-developer of this device, the borehole strainmeter.

The relative sensitivity of the sites was determined quite accurately from the recorded amplitudes of long period surface waves generated by great earthquakes. The absolute sensitivities are constrained by comparing recorded Earth tides with model calculations.

In winter 2002-2003, Sacks and his team implanted a network of borehole strainmeters in Montserrat, a volcanic island in the Caribbean. Using a small network of such sensitive instruments, they were able to evaluate the critical pressure changes in a complicated magma plumbing system that comprises two reservoirs (one at about 11 km, the other at about 5 km depth), a shallower dike, and a small conduit connecting the dike to the surface.

One year ago, with support from the Brinson Foundation, Selwyn Sacks and fellow DTM staff Michael Acierno, Michael Crawford, Alan Linde and Brian Schleigh traveled to Iceland to install a new borehole strainmeter close (~5 km) to the volcano Hekla. This year they returned to upgrade older electronics at existing sites. The new site is much closer to Hekla than the earlier strainmeter sites, so the expectation is that data for the next eruption will allow a better understanding of the eruption and also provide earlier warning