ASRA Earthquakes Module 2007

Data and Analysis

Over the three days we recorded data, we registered approximately 97 earthquakes. Some were nearby, some were far away, and they varied in strength. The figure below illustrates when those earthquakes occurred. Note: we are not experienced seismologists, so some quakes may have escaped our notice while we were cataloging (which could explain why there are no earthquakes recorded from 4am-8am on day 195), while others may be recorded that were not actaully earthquakes.

As you can see, they seem to be fairly randomly distributed. This is actually a good sign. The tectonic plates don't run on a clock, so random distribution helps validate that the signals we picked up are in fact earthquakes, not man or animal made vibrations. (For example, if the seismometers picked up buses traveling between 10AM and 4PM every day, there would be a spike in "earthquakes" during that time period.) Note: the instruments were picked up before they recorded data between 8pm and midnight GMT on day 196, so that is why there is no data there (Alaska Standard Time is 8 hours behind GMT).

Next we have some actual seismograms. The first one is an example of a nearby earthquake with a short S-P lag time. We can tell it is an earthquake because of the sudden onset of the shaking (the P-wave hitting), then a small slackening followed by another spike (the S-wave hitting).

The second seismogram records an earthquake with a longer S-P lag time. Again, we can tell it is an earthquake by the pronounced spikes of the P- and S-waves. This earthquake is farther away than the one above (distance can be deduced from the difference in arrival time between the P- and S-waves).

The third figure shows an earthquake registered on all three stations, ARPT, GRVL, and PATH. As you can see, PATH picked up the earthquake first. That was the case with all our quakes, and occurred because PATH's GPS didn't get the correct time and had to be calibrated manually, leading to a slight difference in the times it recorded versus the times other stations recorded. Again, you can see the P-wave spike followed two seconds later by the S-wave spike.

Fourth we have a signal that is a bus instead of an earthquake. We can tell it is a bus as opposed to an earthquake because of the slow onset on the waves (there's no sudden spike like on the other seismograms). Also, the signal only appeared on the GRVL station, while earthquakes would register on ARPT and PATH as well. The seismometer at GRVL was also located close to the road, making it likely that the station picked up bus signals.

Last we have an interesting signal we picked up at GRVL. We aren't sure what the signal represents, but it is an interesting anomaly. The amplitude is steady on the north and vertical axes, but fluctuates on the east axis. The signals are timed very regularly, which makes it unlikely that the cause was natural, but we still aren't sure what the signal represents.

This chart shows approximately how far away the earthquakes were that we recorded (we multiplied the S-P lag times by ten to get the distances).

The two areas that showed up with the most earthquakes in our data were in regions 20-60km and 120-160 km away from the sites. The map of Denali Park (the second one below) shows the earthquakes picked up by the Alaska Earthquake Information Center (AEIC) on the days our seismometers were in the park. The upper map is a larger scale one with dotted circles representing 60, 120, and 180km from the center (near our stations). The opaque colored circles are more quakes picked up by the AEIC. Our first group of earthquakes, from between 20-60 km away, come from the Kantishna cluster, South-West of out seismometers (you can see the large number of other quakes recorded there in the upper map). The other group, from between 120-160 km away, are aftershocks still occurring from the magnitude 7.9 Denali Fault earthquake in 2002. That earthquake and its aftershocks occurred to the east of our seismometers between the second and third rings on the upper map (you can see a few others recorded in this area).

Download these Google Earth files here

This graph shows the correlation between amplitude and the S-P lag time.

The quakes are represented as points and then grouped into general categories. Strong earthquakes have high amplitudes but amplitude decreases as distance from the earthquake's epicenter increases. The lag time between the P and S waves also grows larger the farther away an earthquake is. Since we know this, we can plot which earthquakes are large and close (small S-P lag, high amplitude), which are large and far away (large S-P lag, varying amplitudes depending on size), which are small and close (small S-P lag, low amplitude), and which fall somewhere in the middle. The majority of the earthquakes we detected were small and close, with amplitudes of up to 15,000 counts and S-P lag times of up to 10 seconds.

Here is an Excel file with the data about all earthquakes recorded.