GPS data used to model effects of tidal loads on Earth's surface

Up until now, the best way to explore Earth's internal structures -- to measure geological properties such as density and elasticity -- has been through seismology and laboratory experiments. "At its most fundamental level, seismology is sensitive to specific combinations of these properties, which control the speed of seismic waves," says Mark Simons, professor of geophysics at Caltech's Seismological Laboratory, part of the Division of Geological and Planetary Sciences. "However, it is difficult using seismology alone to separate the effects that variations in density have from those associated with variations in elastic properties."

Now Simons and Takeo Ito, visiting associate at the Seismological Laboratory and assistant professor of earth and planetary dynamics at Nagoya University in Japan, are using data from GPS satellite systems in an entirely new way: to measure the solid earth's response to the movements of ocean tides -- which place a large stress on Earth's surface -- and to estimate separately the effects of Earth's density and the properties controlling response when a force is applied to it (known as elastic moduli).

Their work was published in Science Express.

By using measurements of Earth's movement taken from high-precision, continuously recording permanent GPS receivers installed across the western United States by the Plate Boundary Observatory (PBO), the researchers were able to observe tide-induced displacements -- or movements of Earth's surface -- of as little as one millimeter. PBO is a component of EarthScope, a program that seeks to understand the processes controlling earthquakes and volcanoes by exploring the structure and evolution of the North American continent.

The team focused on understanding the properties of the asthenosphere, a layer of weak and viscous upper mantle that lies below Earth's crust, and used those measurements to build one-dimensional models of Earth's response to the diurnal tides in the western United States.

"The asthenosphere plays an important role in plate tectonics, as it lies directly under the plates," explains Ito. "The results of our study give us a better understanding of the asthenosphere, which in turn can help us understand how the plates move."

The models provided a look at the variations in density from Earth's surface down to a depth of about 400 kilometers. The researchers found that the density of the asthenosphere under the western United States and the eastern Pacific Ocean is abnormally low relative to the global average.

"Variations in density can either be caused by variations in the chemical makeup of the material, the presence of melt, or due to the effects of thermal expansion, whereby a given material will decrease in density as its temperature increases," explains Simons. "In this study, we interpret the observed density anomaly to be due to the effects of elevated temperatures in the asthenosphere below the western United States and neighboring offshore areas. The required peak temperature anomaly would be about 300 degrees Celsius higher than the global average at those depths."

This type of data provides keys to understanding the chemical and mechanical dynamics of the planet, such as how heat flows through the mantle and how tectonic plates on Earth's surface are evolving.

"It is amazing that by measuring the twice-a-day centimeter-scale cyclic movement of Earth's surface with a GPS receiver, we can infer the variation of density 220 kilometers below the surface," says Simons.

Now that the researchers know it is possible to use GPS to derive measurements of internal Earth structures, they anticipate several new directions for this research.

"We hope to extend the observations to be global in scope, which may require temporary deployments of GPS in important areas that are typically tectonically bland -- in other words, devoid of significant earthquakes and volcanoes -- and thus do not have existing dense continuous GPS arrays already in place," says Simons. Next steps may also include going beyond the current one-dimensional depth-dependent models to build 3-D models, and combining the GPS approach with more conventional seismic approaches.

"The method we developed for gathering data from GPS devices has significant potential for improving 3-D images of Earth's internal structure," says Ito.

This research was supported by the National Science Foundation; Grants-in-Aid for Scientific Research, part of Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT); JSPS Postdoctoral Fellowships for Research Abroad; and the Gordon and Betty Moore Foundation.