Daniel Trugman, assistant professor in the Nevada Seismological Laboratory (NSL), and Avigyan Chatterjee, doctoral student in the NSL, have published a research article in the journal Nature about how the geometry of earthquake faults might play a role in their risk of generating earthquakes.
Tectonic plates, which make up the Earth’s crust, have moved slowly over millions of years and continue to move. The motion of these immense structures can cause earthquakes along cracks in the Earth’s crust known as faults. Not all faults are the same in how they accommodate the motion.
The researchers, along with coauthors at Brown University, studied motion along faults and found that fault network geometry plays a role in how faults accommodate motion, which happens in two ways.
As tectonic plates move, two sides of the fault might get stuck on one another. Eventually, the stress in the crustal rocks will become too extreme, causing the two sides of the fault to move past each other very quickly. This event produces the abrupt jump in motion that we know as an earthquake.
The second way that faults accommodate motion is known as creep. Creep is when the two sides of the fault move past each other very slowly. The plates might move 30 millimeters in a year, but they don’t get stuck and then jump past each other, rather they grind against one another, slowly and steadily moving past each other.
Fault network complexity is a measure of how faults in a given area differ based on their directionality. For example, if the set of faults in a given location were intersecting at high angles to each other, that would be a higher fault complexity than a set of faults that were nearly parallel to one another. Increased fault complexity, the researchers argue, is what leads to tectonic plates getting stuck on one another and eventually generating an earthquake.
“It's a pretty robust connection and helps complement and expand our understanding of why some faults creep and some faults are locked and produce big earthquakes,” Trugman said.
Most of the observations were based in California, but the researchers also examined fault networks with comparably high-quality data, including the North Anatolian Fault in Turkey and the Chaman Fault in Afghanistan, to ensure that the pattern they were identifying wasn’t limited to the unique geology in California. Their findings held for the other faults, providing strong evidence that simpler faults accommodate plate motion with creep.
The Nature paper is a result of a project funded by the National Science Foundation that explores the role of fault complexity in earthquake-related processes. For a long time, researchers modeling earthquakes used simple models of faults, but those models may have missed important processes that weren’t taken into consideration, like fault complexity. In related work, Trugman and Chatterjee have shown that fault network complexity can influence other important properties of earthquakes, like the number of aftershocks or the intensity of ground shaking. This paper makes a strong argument for developing new methods to incorporate fault complexity into system-level models of earthquakes.
The data used in the analyses were largely collected by scientists from the U.S. Geological Survey using satellite data, creepmeters (which work similarly to seismographs but are aimed at measuring slow motion on nearby faults) and offset markers such as fences or roads. Trugman acknowledged how important it was to have access to public, reliable data on faults and creep.
“If we had to go out and make the measurements ourselves, this paper would never have been written,” Trugman said. “That's at least a five-year project in and of itself that we certainly wouldn't be able to do as well as the experts in those fields. I imagine that the authors of the two datasets will be happy to see that the work that they did is going to open new science scientific directions.”
