During the summer of 2021, the magnitude 8.2 Chignik earthquake struck near the Alaskan coast, providing scientists, such as a team led by Parameswaran of University of Alaska Fairbanks, with a test case for a future early warning system for the region. The shaking from this event was recorded on local GPS and seismic stations, and the data shows that they could be used together.
Alaska is situated on the North American plate, with the Pacific plate subducting beneath it. This subduction zone forms the Aleutian Megathrust, which has been the site of many historically large earthquakes. Alaska is the most seismically active state in the United States, averaging around 1,000 earthquakes each month; the second largest earthquake ever recorded, referred to as the “Great Alaska Earthquake,” occurred in 1964 at a magnitude of 9.2. Alaska is particularly prone to landslides following earthquakes, often causing extensive damage with little to no warning. Because of these hazards, researchers are studying options for a potential earthquake early warning system in the area.
Earthquake early warning systems—which can quickly detect earthquakes and broadcast alerts before shaking arrives at most locations—have traditionally been built on seismometers, but GPS instruments can also be used. The two systems actually complement one another: seismometers are more sensitive and can detect tiny earthquakes, whereas GPS stations are better at quickly identifying the largest earthquakes. Seismometers measure the velocity of ground motion as they are disturbed by shaking, while GPS stations record changes in their position. So for an apples-to-apples comparison, the researchers used the moment-to-moment differences in the GPS station’s position calculations to produce a record of its velocity.
Since the team behind this study used pairs of GPS and seismic stations that are very close to each other, they were able to directly compare the corresponding data sets from the Chignik earthquake. This earthquake was ideal for comparing GPS and seismic data because it was large enough to stand out clearly from any background noise.
The data collected during this earthquake shows strong similarities between GPS and seismic stations. The earthquake magnitude estimates within the first few minutes matched up between them. And the map of measured ground motion was also quite similar. One exception is that the GPS station nearest the earthquake recorded a slightly weaker motion than its paired seismometer. This is probably because some of the shaking there was occurring at a higher frequency than the station’s once-per-second measurement rate, the researchers say. Seismometers generally record 50-100 data points per second, allowing them to capture higher frequency shaking.
The researchers state that the similarities “demonstrate the potential to use them as interchangeable datasets or in combination for ground-motion estimation”. This means that a number of GPS and seismic stations could contribute to a robust earthquake early warning system in Alaska—with some accounting for slight differences. Since Alaska is such a large and seismically active state, it can only help to have more options.