How exactly does your “Maps” app know exactly where you are, any time of day? The “little blue dot” on your phone tells you where you are as the result of billions of dollars invested in the Global Positioning System (GPS), a network of 27 satellites currently orbiting the Earth. Each satellite sends microwaves constantly, containing information about their location and the time the wave is sent. Your phone detects these waves with a tiny antenna, and its GPS chip calculates the amount of time it took for the waves to reach it; knowing that the waves travel at nearly the speed of light, it calculates how far away your phone is from the satellite. By doing this using four or more satellites, it displays the “little blue dot” on your screen that moves as you do, a fairly accurate estimate of your phone’s location. Pretty amazing, right? (1)
This technology was developed by the U.S. for military navigation, but has many more applications helpful for everyday life (getting directions, sharing your location with friends, or deciding where to have dinner) and scientific purposes! The location of high precision GPS instruments can be tracked over time, allowing scientists to calculate how quickly they are moving. If we know how fast they are moving, we can learn more about the processes that move them. Let’s place a GPS instrument on the side of a volcano; by measuring how much the instrument moves each day, we can determine how the volcano is behaving. If the instrument is moving up and outward away from the center of the volcano, the volcano may be “inflating” and magma might be moving closer to the surface of the Earth, warning us of an impending eruption. Professor Michael Hamburger and graduate students in the Department of Earth and Atmospheric Sciences (DEAS) at IU used this technique to track the behavior of three volcanoes in the Philippines.
GPS instruments are used to learn about how fast the Earth’s tectonic plates (large pieces of the uppermost layer of the Earth, the lithosphere) are moving in relation to one another. They can be used to measure how fast the ground moves during an earthquake, allowing scientists to almost immediately see the effects of an earthquake and to predict the onset of a tsunami. In IU’s DEAS, Professor Kaj Johnson studies how the Earth’s crust deforms and what that means for earthquakes; using these data, he studies how earthquakes occur and why they occur where and when they do. In Taiwan, he studies the speed of mountain building due to the edges of two tectonic plates colliding with one another, causing mountains in Taiwan to grow as fast as 2 centimeters per year (in contrast, Bloomington, Indiana moves downward 2 millimeters per year; 2)!
As a graduate student in the DEAS, I was involved in a project with professors Hamburger, Johnson, and the Philippine Institute of Volcanology and Seismology (PHIVOLCS) to study how fast the Earth’s lithosphere is moving in the Philippines, using GPS data. We utilized these measurements to determine the potential size of future earthquakes that may occur. As two tectonic plates collide, stress (the amount of force per unit area) accumulates; when this stress overcomes the friction on a fault, an earthquake occurs and releases the stress that has been building up over time. This can occur on faults in the upper crust, as well as at subduction zones, where one plate is being forced under another.
Using a model to calculate the build-up of stress on faults in the Philippines, we can estimate how stress is accumulating due to the movement of the tectonic plates, and in turn, calculate the size of potential future earthquakes that might occur due to the build-up of that stress. Our results suggest that there may be enough energy building up to produce a giant magnitude 9.0 earthquake every 500 years on the subduction zone nearest to Manila! Few large earthquakes have been recorded here in the past, indicating that this energy has been building up over time and is still available to produce an earthquake. With a large population and many residents located near coastlines, this could, in turn, mean a high risk for tsunamis affecting not only the Philippines, but neighboring areas of Taiwan, China, and Vietnam. All this from the same technology that produces your “little blue dot!”
- Sella, G. F., Dixon, T. H., & Mao, A. (2002). REVEL: A model for Recent plate velocities from space geodesy: REVEL-RECENT PLATE VELOCITIES FROM SPACE GEODESY. Journal of Geophysical Research: Solid Earth, 107(B4), ETG 11–1–ETG 11–30. https://doi.org/10.1029/2000JB000033