Source: AGU Advances
Earthquakes that cause conspicuous surface shaking and infrastructure damage are the result of fast ruptures and slippage along faults. But they aren’t the only types of movements occurring in Earth’s crust.
Fast earthquakes last just a few seconds or minutes and emit large, easily observed seismic waves. In contrast, slow earthquakes, mostly aseismic, can last days to years and generate tiny waves called tremors. These tremors are observed primarily through highly sensitive networks of seismic sensors. Although slow earthquakes, whose waves proceed 3 to 4 orders of magnitude more slowly than those of fast rupturing quakes, occur often and relieve stresses in Earth, the mechanisms that cause them remain largely unknown.
Yuan et al. developed a laboratory experiment to observe drawn-out fractures and better understand how slow earthquakes form. The authors 3D printed a small cylinder (about the size of a bagel) out of a stiff and transparent plastic called polymethylmethacrylate and then injected the model with pressurized viscous fluid to create a fracture. Using high-speed photography and acoustic sensors, the authors watched and listened to the breaks and cracks caused by the expansion of the fluid. The so-called hydrofracturing experiment replicated tectonic tremors and aseismic slips in the ground.
The results showed slow cracks radiating from the initial fracture in a bursty, or intermittent, pattern and that their spread was influenced by the viscosity and pressure of the injected fluid. The fractures matched observed patterns of slow earthquakes and tremors in Cascadia, a tectonically active region stretching from Northern California to British Columbia.
The results offer insights into the generation of the slow slips and tectonic tremors that comprise slow earthquakes. Additionally, the study provides clear experimental evidence that fluids and hydrofracturing contribute to these events and that radiated seismic energy can serve as a measure of slow earthquake rupture area or size. (AGU Advances, https://doi.org/10.1029/2023AV001002, 2024)
—Aaron Sidder, Science Writer