During large quakes, rocks can jump or shift from their initial locations. How far they traveled tells scientists how much the ground moved during rupture.
By Alka Tripathy-Lang, Ph. D., science writer (@DrAlkaTrip)
Citation: Tripathy-Lang, A., 2020, Large quakes make rocks jump, Temblor, http://doi.org/10.32858/temblor.129
If a large earthquake ruptures Earth’s surface, rocks sitting nearby can jump into the air, landing nearby, somewhat askew. New research shows that earthquake scientists can identify just how far the rocks shifted during the quake by examining the scarred ground.
“Immediately after the 2019 Ridgecrest earthquake sequence, teams of scientists surveyed the area looking for damage,” said Western Washington University Master’s student Mindy Zuckerman during her presentation at the Geological Society of America’s annual meeting this week. These teams noticed that some rocks had clearly either jumped or otherwise shifted from their initial position, leaving behind a groove in the ground. By tracing the rocks back to their “sockets left in the desert floor,” said Zuckerman, she and her colleagues determined how far and which way the rocks moved, with tools as simple as a measuring tape and compass.
This information, collected from rocks spaced 330 to 650 feet (100 to 200 meters) away from one another along transects 0.3 miles (0.5 kilometers) apart, can enhance ground motion data from accelerometers spaced kilometers apart, she says. The journeys these rocks take can tell scientists how strongly the ground moved near the fault during an earthquake, which can be used to model earth-shaking during future quakes.
“We’re interested in seeing if rock motion decreased with increased distance from the fault,” said Zuckerman. Preliminary data do not show a strong relationship between rock motion and distance from the fault. Clast size and distance traveled also failed to show an obvious correlation, she said. However, Zuckerman’s next steps include adjusting for variables like the slope, substrate and fault complexity. For example, the steeper the slope, the further a rock may move during a quake.
Once she makes these adjustments, Zuckerman and her colleagues can estimate ground acceleration from rock displacement data by thinking back to calculus and physics — the first derivative of displacement is velocity, and the second derivative of displacement yields acceleration. They will then compare their calculations directly to data from nearby accelerometers.
The Mojave Desert’s aridity helped preserve these clues to the how the ground roiled as it broke during the Ridgecrest earthquake. “This method would not work well in a more vegetated region,” Zuckerman said. “Our method does not require expensive equipment or long data processing times, but part of working with fragile geological features is that they’re ephemeral so we must act quickly.”