GPS & Earthquakes

We are reminded of how strong earthquakes are when we see photographs showing Earth's crust torn apart or deformed (Figure 1). GPS is a powerful tool for studying earthquakes because it can very accurately tell scientists how much the ground moved during the earthquake. How does GPS do this? Geoscientists use the data collected by continuously-operating GPS networks. Figure 2 shows a GPS time series for a station in Japan collected before and after the great Tokachi-Oki earthquake (September 25, 2003). You can see that the ground moved approximately 80 cm to the east and 35 cm to the south. This is sometimes called the "co-seismic offset." GPS can also measure changes in the vertical component. During this earthquake, this station moved ~20 cm downwards.


Figure 1.Earthquakes can rupture the surface of the Earth. Credits: Akihiko Ito, Wes Wallace, geologycafe.com, Dick Proctor.

Figure 2.The horizontal (east and north component) GPS time series for station 0532 are shown. To compute the total horizontal motion, you combine the east (magenta) and north (green) motions. Since the ground moved in a negative north direction, this means the ground moved south. These large ground displacements occurred during the M8 Tokachi-Oki earthquake, the largest earthquake in 2003.


At the time of the Tokachi-Oki earthquake, the Japanese GPS network (operated by the Geographical Survey Institute) had more than 1000 instruments. The Tokachi-Oki earthquake was a thrust (compressional) earthquake, and so the GPS stations on Hokkaido moved to the southeast and downwards. The stations closest to the epicenter moved the greatest distances. The earthquake displacement vectors shown in Figure 3 are used by scientists to model the rupture. Shown to the right in Figure 3 are the "normal" displacements experienced in Japan due to plate tectonics. Credit: Kristine Larson


Figure 3.Left: The Tokachi-Oki earthquake produced large ground displacements throughout the Island of Hokkaido. Here they are separated into their horizontal and vertical components. The result for GPS station 0532 is highlighted in red; Right: "Interseismic" deformation that occurs due to plate tectonics. The plate boundaries surrounding Japan are shown in gray.


GPS Seismology

Historically, GPS scientists have used all the data from one day to calculate a single position. But if the GPS receiver is recording data every second, it is possible to calculate position every second. This means that a GPS receiver can act like a seismometer. Shown in Figures 4 and 5 are examples of GPS seismology from the great Maule earthquake in Chile and the M7.2 earthquake in Baja California, Mexico.

For a GPS receiver to provide useful information about seismic waves, the ground motions must be fairly large. This means GPS is best used for earthquakes bigger than magnitude 6. If the GPS data are made available in real-time (that is, almost immediately), these data can be used for earthquake and tsunami early-warning systems. The goal of these systems is to warn people before the seismic tsunami waves reach them.



Figure 4.Motion of station CONZ during the 2010 Maule Earthquake. Motion of this site was calculated once per second. Credit: Kristine Larson

Figure 5.Motion of station P496 during the 2010 El Mayor - Cucapah earthquake. Motion of this site was calculated five times per second. Credit: Kristine Larson


 

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