In the Woods Listening for the Langseth
By Geoff Abers
While the R/V Langseth plies the waters offshore the Pacific Northwest, we have been recording its source with seismic equipment on land. Lamont ran seismometers in Washington, deployed by two Columbia graduate students, Helen Janiszewski and Zach Eilon, and myself, and received some “logistical support” (shovels, batteries) from colleagues at the University of Washington. Anne Trehu of Oregon State led a parallel Oregon deployment. Like the Langseth, we are making use of national shared instruments; our gear comes from the PASSCAL Instrument Center in Socorro, NM, a facility of the Incorporated Research Institutions for Seismology and supported by the National Science Foundation. Writing this reminds me that modern science tends toward major collaborations; most field seismologists nowadays have to be masters of logistics. Much of my job was negotiating with myriad landowners to get permission to place our (small) equipment on their land, including timber companies, state agencies, civil safety organizations and even people with big backyards.
Our sensors record the same seismic signals as the ocean bottom seismometers the R/V Oceanus deployed, and we will combine the data later. They can detect Langseth signals up to 100 miles inland! This is something extraordinary, and difficult to believe until seen. The on-land data allows the project to extend over and past the fault zone that underlies the coast off Washington and Oregon, the Cascadia Megathrust. While the existence of the fault has been long recognized, growing evidence suggests that this fault is building up strain, and is capable of generating great (magnitude 9) earthquakes. Still, unlike most other subduction zone faults there are almost no small earthquakes on it, and so we know relatively little about it. The signals from the Langseth will reflect off the fault, at 15 – 20 miles depth near the shoreline, and be recorded on the seismometers we deploy farther west. The reflections should tell us a great deal about the thickness and internal structure of the fault zone, and the nature of the rocks on either side. While we cannot predict earthquakes, these data help test physical models of what active faults are like deep in the earth where we cannot otherwise see them.
In mid-June we recorded data in Washington from the Langseth far offshore, and in early July the Oregon group did the same. The Washington work should be completed in a second phase in mid-July. All of this means lots of trips and a good deal of time driving between the Washington beaches and the Mt Rainier foothills. Most of our sites are in recent clearcuts accessed via logging roads, so we can avoid large trees that occupy the rest of the Northwest (they shake the ground too much and block GPS signals). The clearcuts are old and the roads are not used much, so we spend much time clearing branches and cutting small trees that fell across the road to get to our sites. My students did not expect they would be lumberjacks when they came to grad school in New York!
At the end of the first deployment we met the Langseth in Astoria. We had been driving the biggest SUV that I could find – a large Suburban capable of carrying seismic equipment, big batteries, tools, and people, over any road. Nevertheless, next to the Langseth, the Suburban is very small. Stepping across the shoreline to do science clearly requires a whole other scale of operation.
Geoff Abers is associate director of the Seismology, Geology and Tectonophysics division of the Lamont-Doherty Earth Observatory.