The Iran and China earthquakes as captured by the UNH seismometer

This past week has been has been a tense one for sure. Not only was there the Boston Marathon bombings and the fertilizer plant explosion in Texas, but there were also major earthquakes in both Iran and China.

The 7.8 magnitude event in Iran occurred on April 16th, near the Iran/Pakistan border. Pakistan bore the brunt of the quake, with at least 35 dead and an entire town essentially destroyed. You can view all the scientific/technical details of the event on its USGS page. We can see the earthquake quite clearly on the UNH seismometer. The event hits our station at 10:56 UTC (06:56 am EST). The shaking intensity when the surface wave hits is strong enough that the seismic wave looks clipped in the display (in green).

The magnitude 6.6 earthquake in China struck on April 20th. Although smaller than the Iran earthquake, this earthquake was very shallow and hit a more densely populated area. The latest estimates put the death toll at 188, with over 11,000 people injured and many are still missing. The USGS page for the event can be seen here.

It takes about 13 minutes for the initial P-wave from the earthquake to reach our station, and you can see when it hits if you look at the top black line. Just after the 30-minute mark on the same line, you can see the S-wave hit. The much larger amplitude surface waves can be seen in red, starting about 21:03 EST.

These earthquakes have devastated the villages they have hit, particularly the one in China. Construction in these small towns is often very old and not up to code. Landslides bury entire structures and block roadways, making it hard for aid to get to those who need it the most. 

My thoughts go out to everyone affected by this past's weeks events, both here in the US and abroad.

The UNH seismometer is part of the larger New England Seismic Network. You can visit their page here.

Detecting Earthquakes at UNH

I've decided a great way to get back into blogging would be to show some of the great seismograms the UNH seismometer has been recording since we set it up in 2011.

While most of the signals we record on the station are from passing trains, we have also recorded some pretty impressive earthquakes.

Check out this seismogram from an earthquake swarm near the Solomon Islands on Feb. 8, 2013. In a little over 40 minutes, there were 3 large earthquakes. A magnitude 8.0 struck at 1:12 (UTC), followed by a 7.1 event 11 minutes later, and a 7.0 event 30 minutes after that. In addition, there were smaller magnitude 5's and 6's both preceding and succeeding these larger events. When the surface waves from the 7.0 event hit our station, surface waves from the preceding 7.1 and 8.0 event were already shaking it. In the image below, the signal is so strong that it actually goes off the page!

Remember the Colorado and Virginia earthquakes that occurred back in August of 2011? The UNH
seismometer captured those earthquakes as well:

While we are looking at more local events, let's not forget the small Maine event (magnitude 4.0) that occurred on Oct. 16 of last year. See that impuse response in green at the bottom of the chart in the image below? Well, that's it. The earthquake occurred very close to our station, so the P, S, and surface waves hit at nearly the same time. The response that we see in the long-period signal, therefore, is very sharp. I have to admit, I was quite excited when this earthquake occurred. This was the first earthquake that I really felt and realized what it was while it was occurring -- well, truth be told I first thought my furnace was about to explode, but only for the first second or two. In fact, I not only felt the earthquake, but I heard it. I actually heard the earth rumble.

Finally, let's look at a seismogram from a magnitude 6.9 earthquake that occurred on Feb. 02, 2013 in Japan. This is a great seismogram because we can clearly see when the P, S, and surface waves arrive at the station.

Pretty cool, eh? If you want to check out some seismograms for yourself, you can do so at the New England Seismic Network webpage. To view the UNH stations, click on current seismograms and select DUNH as the station.

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.