Tuesday, May 17, 2011

UNH now has a seismometer!

On Friday, April 29th, Mike Hagerty, from the New England Seismic Network came up to UNH to help us install our very own seismometer. It is installed in the basement of James Hall in a vault that has been dug out beneath the building:

The seismic vault in the basement of James Hall

Our location in New Hampshire means we don't have to dig very deep to hit bedrock, and so the vault is only a couple feet deep. The seismometer sits in the vault, coupled to the bedrock, while the data cables are kept safe and secure in the PVC pipe running from the vault to the cabinet. The seismometer is a Trillium 120 P with a RefTek 130 broadband seismic recorder.

Reftek 130 seismic recorder
Trillium 120P seismometer

We still have to properly align the seismometer so that its north axis is actually pointing north, but in the meantime we can have some fun looking at the data. James Hall is located rather close to the train tracks; fortunate for those wishing to take the train, but unfortunate for seismometers (unless you wish to study seismograms of passing trains). Below is a high-frequency seismogram from our station, the big spike in blue is a passing freight train:

click to see full-size

You can also see when the building's HVAC system kicks in. This means that eventually we'll probably try to find the UNH seismometer a new home, so that we don't have to worry about local small earthquakes getting lost in the noise of trains and building maintenance.

The nice thing is that building noise and passing trains get filtered out when you just look at the long-period signal. This is excellent for looking at teleseisms (a tremor cause by an earthquake more than 1000 km from the station). Below we can see two teleseisms in the seismic recordings from 5/15/2011:

click to see full size

The spike in red is a teleseism from a Mw 6.0 earthquake (event time = 13:08 UTC) located on the St. Paul transform fault system in the mid-Atlantic. The much larger teleseism in green -- if you look you can see that it actually starts in blue around 18:57 and continues on through the black wave until about 20:55 or so -- is from a Mw 6.5 quake off Papua New Guinea. The Papua New Guinea quake occurred at 18:37 UTC, but it took about 20 minutes for the first p-waves to hit our stations. The s-waves appear kick in around 18 minutes after the p-waves, and the surface waves start to hit at 19:37, a full hour after the event time. This may seem like a long time, but remember that seismic waves can take a while to travel through the earth, particularly as they reflect and refract through different layers in the earth's mantle. Surface waves take the longest to arrive, as they alternatively speed up and slow down through different surface materials such as rock, sand, soil, etc. The chart below gives an estimate of travel time for various seismic waves versus angular distance from the earthquake:

source: http://geophysics.eas.gatech.edu/classes/Geophysics/misc/Seismology.html

If you wish to check out the seismograms that are being generated by the UNH seismometer, or any of the New England Seismic Network (NESN) stations, check them out here. You can select DUNH from the station drop-down list and check out the seismograms for any day since the seismometer was successfully installed. If you click on station list, you can see all the stations in the NESN. If you want to hunt for teleseisms, check the period to long-period. If you think you've found one, you can go to the USGS Earthquake page (http://earthquake.usgs.gov/earthquakes/recenteqsww/) and see if you can find a matching event. Large earthquakes greater than Mw 5.5 usually generate teleseisms that most modern seismometers anywhere in the world can see. You can also use the above chart to figure out how long after the event you should see seismic waves arriving at the station.

UPDATE:  Kurt Schwehr just blogged about the UNH seismometer as well. He talks about network setup and how we initially configure the seismometer with a Palm Pilot. He also shows the results of a jump test.