Professor of Geological Sciences Karen Fischer's team has spent years analyzing layers of the Earth deep below our feet. In a study published in the journal Science Oct. 6, lead author and postdoctoral research assistant Vedran Lekic presents a new, detailed image of the lithosphere beneath Southern California.
The lithosphere, broken up into tectonic plates, is an ever-moving, ever-evolving layer. The new images present abrupt changes in depth across short distances, supporting theories about the layer's physical characteristics.
Fischer's team is mainly concerned with rifting, the process by which the tectonic plates shift and move apart. Rifting is most dramatic at active plate boundaries, such as those in Southern California where the researchers have concentrated most of their work.
Lekic could not keep himself from grinning when discussing the study. "We were trying to image the bottom of the tectonic plate," he explained.
The image was created by looking at changes in seismic waves. In her bright, ground floor office, surrounded by images of erupting volcanoes interspersed with crayon drawings, Fischer explained that her team uses seismometers like doctors use CAT scans. Seismometers set up in Southern California record raw earthquake waves, and geologists can use the changes in these waves to visualize the shape of the lithosphere.
"Fischer's lab has been pioneering this method," explained Lekic. "There are groups in Germany, France and Russia that do this, but when Fischer started, it was not an often-used technology. We had to prove that it worked."
With Fischer's body of work before him, Lekic analyzed 60,000 seismograms. Synthesizing these pictures with his knowledge of how waves change with certain textures and thicknesses of rock, he was able to create a precisely detailed picture of the lithosphere.
The graphic displayed an "abrupt transition" from deeper lithosphere to a much shallower layer. Specifically, the lithosphere beneath Los Angeles is 80 km deep, but that beneath the nearby Salton Trough and areas close to the shoreline can be a very shallow 45 km deep. In comparison, the lithosphere beneath College Hill is about 100 km deep.
"There can be localized regions that are extremely weak, while the other parts remain strong," Lekic explained. That is why only certain parts of the lithosphere are stretched and moved.
This observation allowed the scientists to infer that this deep part of the lithosphere, made of mantle rocks, must be very strong. "If the layer was warm and ductile and gushy there could not be such a sharp angle between the thicknesses," Fischer said.
The picture also allowed scientists to test previously conceived models of plate tectonics. Fischer and her team have been able to analyze which models are possible and which "classes of models do not match reality."
Finding the correct evolutionary model of plate tectonics brings scientists closer to understanding the basics of how the planet, a complicated and ever-changing system, evolves.
