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The tremors that an earthquake produces are merely a small portion of the total energy that a quake expels. A quake can additionally invoke a rush of warmth, accompanied by a domino-like fracture of subterranean rocks. However, determining the exact amount of energy allocated to each of these three processes is exceedingly challenging, if not unattainable, to assess in real-world settings.
Now, geologists at MIT have tracked the energy released by “laboratory quakes” — miniature versioins of natural earthquakes that are meticulously triggered in a regulated lab environment. For the first instance, they have quantified the entire energy allocation of such quakes, in terms of the share of energy diverted into heat, shaking, and fracturing.
Their findings indicate that only close to 10 percent of a lab quake’s energy results in physical shaking. An even tinier portion — under 1 percent — is responsible for fragmenting rock and generating new surfaces. The predominant share of a quake’s energy — on average 80 percent — is devoted to raising the temperature of the area surrounding a quake’s epicenter. Indeed, the researchers noted that a lab quake can generate a temperature surge sufficient to liquefy adjacent materials, transforming them briefly into molten form.
The geologists also discovered that a quake’s energy distribution is influenced by a region’s deformation history — the extent to which rocks have been relocated and disturbed by former tectonic activities. The ratios of quake energy that create heat, shaking, and rock fracturing can vary based on the region’s prior experiences.
“The deformation history — essentially what the rock recalls — greatly impacts how devastating an earthquake could be,” states Daniel Ortega-Arroyo, a graduate student from MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “This history affects a multitude of material characteristics within the rock, influencing how it will ultimately slip.”
The team’s laboratory quakes serve as a simplified analogy for the events that unfold during a natural earthquake. In the future, their findings could assist seismologists in forecasting the potential for earthquakes in areas prone to seismic activity. For example, if researchers are aware of the level of shaking a quake caused previously, they might estimate how much the quake’s energy impacted deep underground rocks by melting or shattering them. This, in turn, could illuminate the region’s vulnerability to future quakes.
“We could never replicate the intricacy of the Earth, so we need to isolate the physics of what occurs in these lab quakes,” explains Matěj Peč, associate professor of geophysics at MIT. “We aim to comprehend these processes and then attempt to extrapolate them to real-world scenarios.”
Peč (pronounced “Peck”) and Ortega-Arroyo disclosed their findings on Aug. 28 in the journal AGU Advances. Their co-authors from MIT include Hoagy O’Ghaffari and Camilla Cattania, alongside Zheng Gong and Roger Fu from Harvard University and Markus Ohl and Oliver Plümper from Utrecht University in the Netherlands.
Below the surface
Earthquakes are propelled by energy accumulated in rocks over millions of years. As tectonic plates gradually grind against one another, stress builds throughout the crust. When rocks surpass their material strength, they can abruptly slip along a narrow zone, resulting in a geological fault. As rocks slip on either side of the fault, they produce seismic waves that ripple outward and upward.
We sense an earthquake’s energy primarily in the form of ground shaking, which can be assessed with seismometers and other terrestrial instruments. Nonetheless, the other two major forms of a quake’s energy — heat and subterranean fracturing — remain largely unreachable with existing technologies.
“Unlike the weather, where we can observe daily patterns and measure a variety of relevant variables, understanding what happens far beneath the Earth is very challenging,” Ortega-Arroyo mentions. “We are unaware of the conditions affecting the rocks themselves, and the timescales over which earthquakes recur within a fault zone span centuries to millennia, complicating any actionable forecasting.”
To gain insight into how an earthquake’s energy is distributed, and the implications of that energy budget on a region’s seismic risk, he and Peč ventured into the lab. Over the past seven years, Peč’s team at MIT has developed techniques and instruments to replicate seismic events on a microscale, in an effort to comprehend how earthquakes at a macroscale might manifest.
“Our focus is on what occurs at a truly small scale, where we can control numerous elements of failure and seek understanding before scaling it to natural phenomena,” Ortega-Arroyo remarks.
Microscale quakes
For their latest investigation, the team produced miniature lab quakes that imitate the seismic slipping of rocks along a fault zone. They utilized small samples of granite, which represent rocks within the seismogenic layer — the geological region in the continental crust where earthquakes generally initiate. They ground the granite into a fine powder and blended the crushed granite with an even finer powder of magnetic particles, which served as an internal temperature sensor. (A magnetic particle’s field strength will alter in response to temperature variations.)
The researchers positioned samples of the powdered granite — each roughly 10 square millimeters and 1 millimeter thick — between two small pistons and encased the setup in a gold jacket. They then applied a strong magnetic field to align the powder’s magnetic particles in the same initial direction and to the same field intensity. They inferred that any change in the particles’ orientation and field strength afterwards would indicate how much warmth that area experienced as a result of any seismic event.
Once the samples were ready, the team introduced them sequentially into a custom-built apparatus that they calibrated to apply progressively increasing pressure, mimicking the stresses that rocks endure in the Earth’s seismogenic layer, about 10 to 20 kilometers beneath the surface. They utilized unique piezoelectric sensors, developed by co-author O’Ghaffari, which they affixed to either end of a sample to detect any shaking that occurred as stress on the sample was increased.
They observed that at certain stress levels, some samples slipped, generating a microscale seismic event akin to an earthquake. By analyzing the magnetic particles in the samples afterward, they obtained an estimate of how much each sample was momentarily heated — a method developed in collaboration with Roger Fu’s lab at Harvard University. They also gauged the amount of shaking each sample experienced using data from the piezoelectric sensor and numerical models. The researchers examined each sample under the microscope at various magnifications to assess how the size of the granite grains changed — whether and how many grains fractured into smaller fragments, for example.
From all these evaluations, the team could estimate each lab quake’s energy expenditure. On average, they determined that roughly 80 percent of a quake’s energy is directed toward heat, whereas 10 percent results in shaking, and less than 1 percent contributes to rock fracturing or the formation of new, smaller particle surfaces.
“In some cases, we observed that, in close proximity to the fault, the sample increased from room temperature to 1,200 degrees Celsius in just micoseconds, and then rapidly cooled once the movement ceased,” Ortega-Arroyo notes. “In one instance, we observed the fault move approximately 100 microns, indicating slip velocities essentially around 10 meters per second. It happens very rapidly, although it does not persist for long.”
The researchers suspect that analogous processes occur in real, kilometer-scale quakes.
“Our experiments offer a comprehensive approach that provides one of the most thorough views of the physics of earthquake-like ruptures in rocks observed to date,” Peč states. “This will shed light on how to refine our current earthquake models and natural hazard mitigation strategies.”
This investigation was partially funded by the National Science Foundation.
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