The McLaskey Research Group, led by Prof. Gregory McLaskey, civil and environmental engineering, has made a breakthrough in the study of earthquakes by experimentally showing that the fracture energy and breakdown energy of earthquakes are not equivalent. A laboratory experiment, followed by computer modeling, led them to this conclusion.
Earthquakes are a complex phenomenon, affecting ecosystems all across the world. They take place every day, along fault lines — fracture zones between two blocks of rock, and on a larger scale, between the earth’s tectonic plates — but most of them are not powerful enough to be felt on the surface. However, the few whose tremors are felt are capable of wreaking widespread damage.
This so-called “power” of an earthquake is related to its energy budget — a quantity that indicates the balance between the strain energy that causes the earthquake, the dissipated energy that spreads out in the form of heat and fracture energy and the radiated energy.
“Understanding this energy budget of an earthquake is important for trying to understand them and model them…[I]t essentially tells you how much shaking you will have,” McLaskey said.
The group’s research focused on fracture energy and breakdown energy — two quantities, which for the past two decades, have been considered equivalent.
“Fracture energy is a material property whereas breakdown energy is what people try to determine after an earthquake happens,” McLaskey said. “Breakdown energy is a parameter you get by analyzing ground motions.”
According to their new paper, “Earthquake Breakdown Energy Scaling Despite Constant Fracture Energy,” recently published in Nature magazine, McLaskey’s group demonstrates that while earthquake fracture energy is dependent on the energy dissipated through the earth, breakdown energy — a seismological measure related to the slip weakening process along fault lines — is not necessarily a proxy for the same amount of energy.
The McLaskey-led group found that breakdown energy seems to be related to the pattern in which the earthquake rupture propagates — crack-style or pulse-style — and how the tremors are eventually brought to an end.
To reach this conclusion, the group initially performed an experiment using a pair of two-ton granite blocks in Cornell’s Bovay Lab. Each block represented one side of a fault and they were pressed together to simulate an earthquake’s behavior.
Inspired by their observations from the granite block experiment, the group performed a series of numerical simulations in which the breakdown energy varied despite the fracture energy being constant. This led them to the idea that the breakdown energy was less related to the fracture energy and more to the style of rupture.
Based on these findings, the group theorized that calculating the breakdown energy could help deduce the pattern of the earthquake and consequently its rupture style. This means that once the breakdown energy is distinguished from the fracture energy, it can be used to predict how the earthquake will propagate and how it will eventually end.
This information could play a vital role in the future study of earthquakes. “If we understood the energy budget [of the earthquake] better, we could predict earthquakes better,” McLaskey said.
This breakthrough also has implications for other areas of exploration in seismology. According to McLaskey, one of the areas the group is exploring is the interaction of multiple earthquakes since research relies heavily on understanding rupture patterns and the initial amount of stress present in the fault — factors that are impacted by the fracture as well as breakdown energies.
Further, the group is also investigating the phenomenon of triggering earthquakes via fluid injection. In this type of earthquake, continuous injection of fluid into a small section of the fault has the ability to trigger an earthquake that propagates outward along the rest of the fault.
The “size” of the earthquake is once again dependent on its initial stress state, determined by the energy budget.
This idea of fluid injection is especially important for Cornell, due to the University’s Earth Source Heat project. The system is a part of Cornell’s plan to become carbon-neutral by 2030, aimed at building a geothermal system that will use Earth’s internal heat to warm the Ithaca campus.
ESH would, in theory, use fluid injection to pump water into the ground. However, McLaskey explained that this has the potential to trigger earthquakes, so the study of the energy budget and fracture energy would once again be vital in ascertaining the feasibility of such a project.
The breakthrough made by the group could majorly alter the study of earthquakes. By creating a distinction between the fracture and breakdown energies, quantities which were assumed to be the same in previous research, the group has successfully enabled a more comprehensive overview of the way in which earthquakes propagate and terminate.
“[The prediction of earthquakes] is the ultimate goal,” McLaskey said. “Better understanding of these earthquakes should eventually lead to predicting them better.”