Romancing the seismometers

Yesterday’s post introduced the whole point of this trip: science is a constructive process, whether you are doing it or teaching it. Today’s post focuses on the practice of science…both the romantic bit about walking 2 miles in to a site with snowshoes only to realize there’s a snow-free road coming in from the other side, and the unromantic bit about how dehydration of amphibole at 100 km depth is no longer the accepted explanation for the volcanic front at subduction zones. The explanation of the first observation is easy (our maps were right but they didn’t have the most recent data); the explanation of the second observation is not easy (although it also has to do with not having the most recent data).

So first the romantic part. Geoff and I have been checking up on some 60 seismic monitoring stations he and his colleagues have deployed in western and eastern Washington. Geoff and his colleagues aren’t interested in the earthquakes themselves-they’re interested in the sound-the seismic waves-these earthquaked produce. As we’ll discover, these waves act much like x-rays in a CAT scan, revealing the detailed structure of the earth. In a CAT scan, the machine provides its own x-rays. In the earth, we need to wait for earthquakes. A lot of earthquakes. A CAT scan takes a few uncomfortable minutes-recording enough earthquakes to map the structure of the earth takes many years and many seismometers.

Our task this week is to go to each seismometer in the network and give it a 100 day check up. The stations are pretty basic: a buried seismometer measures ground motion 50 times per second, then sends this data to a mess of electronics in a glorified Styrofoam cooler. After some fascinating filtering and data compression, the record of ground motion is recorded on 2 Gigabyte flash drives. The whole apparatus is powered by truck batteries and solar panels. The arrangement of parts at a station works great. A few stations have required more maintanenace, mostly because people with rifles somehow mistake the metallic blue solar panels for dull brown deer. For the most part, though, the primary enemies are snow and insects.

The check up itself is generally a brief affair, involving a checklist and the careful swapping of flash drives. The thoroughness with which we do this work is surprising, until you consider that this thoroughness is essential if the data are to be free of systematic error. Good data requires significant energy.

My favorite part of the 15 minute routine is checking to see if the seismometer is working. Geoff hits a button on a PDA, and I start tapping my foot on the ground a few meters from the seismometer. In a few seconds, the PDA displays the seismometer’s output, with a clear record of the shaking I have induced in the earth. The key is to constantly invent a new way of making the ground shake in a pleasing way. Geoff has the best so far: A metal pole near one of the stations produced an exceptional example of a decaying exponential signal with a super-imposed 2 Hz sinusoid I’ve ever seen. Alas we can’t download the data, or I would share it with you!

The array of stations begins nigh on the coast near Ocean Shores, Washington and then fans out over most of the state. At Ocean Shores we could actually hear the surf, and see the surf on the PDA display. A mile from the beach, the seismometers recorded a lovely 3 second period to the noise. As we moved eastward to Copalis and then Humptulips (really), the surf noise lengthened to 6 and then 12 seconds. Even 60 miles inland, with the beach far out of earshot, the seismometers dutifully reported the earth’s propagation of surf noise to the outskirts of the Olympic Mountains.

The seismometers work best in quiet environments far from frequently traveled roads, but they need to be placed in a pattern optimized to give good results, and close enough to a road so the 300 pounds of equipment can be lugged in by foot . (This map shows the layout of the seismometers as black circles.) This combination of requirements means we spend a lot of time in clear cut forests managed by the Department of Natural Resources. (Yes, clear cutting is pretty destructive, and it does mar the landscape and ecosystem. WE’ll return to this topic!) We spend many, many hours every day driving along the best kept logging roads in the world. For the most part, the Department of Natural Resources of Washington does a great job of keeping these roads cleared and graded-even during winter. Yesterday afternoon, after driving for about an hour into the Black Hills at Capitol State Park we ran out of road. well, we actually ran into a few feet of snow, so after digging out the ridiculous SUV we rented, we put on our snowshoes and walked in the last 2 miles to the station. We’d been walking for about 30 minutes or so when the forest opened up to reveal the crater of Mt. St. Helens staring at us. (How could we see into the crater? Because it looks like it is missing. Because it is missing. Where were you in May of 1980?) Sure, it was only 70 miles away, but still,it was so BIG. Oh, and there’s Mt. Adams, oh and over to the left Mt. Rainer. A trifecta on the last station of the day. It made the fact that you could drive into the station from the other side much less painful.

Tomorrow: The Mounds of Mima and why the volcanoes I just mentioned all line up. Mostly.