Wednesday, October 15, 2008

Rapid-Response Seismic Array Drill

I had my first experience in setting up an array of seismic stations this past Thursday! In conjunction with the Great Southern California ShakeOut - though not directly part of the drill - one of the professors in our department wanted to do a practice rapid response deployment on the San Andreas. She asked for volunteers among students and faculty alike to help with the deployment; I jumped at the chance, initially thinking it would just plain be a good experience. I realized a little later that, if there were to be a significant earthquake in the area, and if part of this professor's job is to go out and deploy stations, those of us who helped with the practice run might actually be enlisted to help out when the real deal happens. Exciting!

Our practice run took us to Whitewater, California, between Palm Springs and Cabazon. This little bitty town sits on top of the active Banning strand of the San Andreas, and part of the fault's trace is indicated by a distinct band of green trees set against the brown of Whitewater Canyon's plant life and rocks. The green part, however, was not the part where we put the stations. We had permission from the owner of a large chunk of the land in the canyon to pepper his property with equipment. (For a House-On-Fault guy, he seemed pretty with it. Not that choosing to live on the fault is the best move, but he knew its general direction relative to the house, had the house retrofitted, and was willing to let scientists do their thing!) We actually had a harder time finding the fault on this guy's land than we would have expected. All of the fault features were subtle, and within our group of about ten people, we had three different guesses, though all were within about 100m of each other. In the end, I think we got pretty darn close to sticking our "midpoint" station on the actual fault, since it was easiest to dig holes there, and the soil got chunkier and with larger rocks the further out from the midpoint we went.

We set up eleven stations: one supposedly on the fault itself, five to the north, and five to the south. The first four of each of those were spaced 50 meters apart, with the final station 100 meters away from the previous one. We did the installation in stages, visiting each station a few times throughout the day, rather than doing each step in one go and leaving the station be. The first step was digging holes - two per station, one for the strong motion sensor and one for the weak motion sensor. I personally dug five holes, and definitely felt it the next in ur fault diggin holez

After the holes were finished, we set out the equipment. Each station included a strong motion sensor (with the brand name EpiSensor - gotta love how punnable seismology really is!), a weak motion sensor, a solar panel, a GPS transmitter, a control box for the solar panel, a control box for the entire setup, and a truck battery. The third pass over all of the stations was to actually hook all of the things up, turn them on, bury the sensors, and try and keep the boxes of equipment and solar panels out of view from the nearby road. This proved to be no small feat for the station furthest to the north, since the site was maybe 50m from said road. But we were resourceful and tricsky, and using various tumbleweed and dead creosote bush parts and various other partially burned dead plants, we constructed a row of bushes where there had only been tufts of grass before. It looked natural from the road, and we made sure to actually secure the wood in the dirt, and to weigh it down with rocks. Here's hoping it doesn't blow away!
Marvel at our artificial bushes! There is a solar panel in this photo, really!

The last step was to make sure all the sensors were recording as they were supposed to. The weak motion ones could be set off with a good hard stomp, but we were having a hard time testing the strong motion ones. Stomping, even in conjunction, barely got a flicker. It required dropping of the largest liftable rocks on site to get a testable reading. "Can You Trip The Strong Motion Detector" definitely sounds like some sort of strange game show, does it not?

The deployment ended up taking most of the day - not exactly the most rapid response ever. Naturally, people who aren't doing this for the first time can get stations up much faster! I'd like to think our little UCR crew would be faster the next time, too. The stations will stay in place for about two months. Since that particular section of the fault has a lot of microseismicity, these stations ought to pick up some events, making them definitely useful for more than just an installation drill. And if the Big One does hit within those two months, well, we'll be a step ahead in the deployment, right? Riiiight?


Silver Fox said...

Sounds pretty involved and heavy duty! Thanks for the photos, too. I can't see the solar panel!

octopod said...

Putting down seismic arrays is fun! Remind me to tell you a story about doing seismic profiles in Western China a couple of years ago...