by Sarah Derouin Friday, August 3, 2018
As tectonic plates collide and sink in subduction zones, huge megathrust earthquakes can produce devastation above. Yet, there are many unknown factors that control how much energy is released in each earthquake. Now, a team of scientists has come up with a new model to help crack the complexity and nature of megathrust earthquakes using global historical records.
Earthquakes are currently measured on the moment magnitude scale, which describes the size of earthquakes based on the amount of energy they release: Large earthquakes are labeled with bigger numbers. But two earthquakes with the same moment magnitude can have very different styles of rupture. For example, the 2011 magnitude-9.1 Tohoku megathrust quake and the 2004 magnitude-9.2 Sumatran megathrust quake were similar in magnitude. However, during the Tohoku quake, the fault slipped primarily along one area for three minutes, whereas in the Sumatra quake, the fault ruptured in multiple areas at once and shook for eight long minutes.
The way a megathrust ruptures — either continuously or sporadically, with stops and starts — can contribute to the release of different amounts of energy, says Lingling Ye, a seismologist at Sun Yat-sen University in China and lead author of the study in Science Advances. “However, we do not have a parameter, a number, to [distinguish] complexities [of rupture],” Ye says.
Now, she and her colleagues have developed a different way to measure energy release, called the Radiated Energy Enhancement Factor, or REEF, which can provide scientists with a more nuanced picture of variations among megathrust earthquakes.
REEF is the ratio of an earthquake’s radiated energy (measured by seismic instruments) to the theoretical minimum possible energy that an event of equal seismic moment and duration of rupture could produce. The result gives researchers an idea of the complexity of a rupture. High REEF values mean high complexity — and high energy release. For example, the Sumatra earthquake had a REEF value of 2.0, meaning the energy released in the complex rupture was much bigger than the potential minimum amount of energy released in a magnitude-9.2 earthquake.
Ye uses an analogy to explain how earthquake energy varies: Consider driving a car from point A to point B. The efficient option is to gradually accelerate once and then decelerate once. The second option is to accelerate and decelerate many times throughout the journey. The second option will use more gas than the first because it is so irregular. Similarly, Ye says, earthquakes that have jerky, more complex ruptures will release more energy.
The team’s analysis of 119 large historical quakes (ranging from magnitude 7 to greater than 9) from around the world, found a clear geographical pattern in the complexity of rupturing. For example, all earthquakes along the north and central parts of the South American plate boundary were complex, leading to high REEF values and a higher likelihood of triggering nearby faults to fail. Earthquakes off the coast of Central America had smoother, simpler rupture patterns that were less likely to trigger adjacent faults.
“We were a little surprised to see that,” Ye says. Megathrust ruptures are controlled by geologic factors like frictional properties, fluids, sediments and the age of plates. “What exactly the factor is [for geographic grouping] is unclear — we are working on that.”
“The complexity of the rupture is not necessarily captured in the current moment magnitude scale,” and using REEF may help capture these differences, says Helen Janiszewski, a postdoctoral seismologist fellow at the Carnegie Institution for Science in Washington, D.C., who was not involved in the study.
“At least at some of the subduction zones around the globe, we seem to see characteristic behavior over multiple different types of earthquakes,” Janiszewski says. She adds that this finding becomes important because it implies that structure, geometry and properties of a particular subduction zone can cause a certain type of rupture — which in turn produces a particular type of energy release. This discovery, Janiszewski says, may help researchers predict what kind of earthquakes might happen in certain areas.
Ye says REEF measurements can help future hazards estimates. “If an earthquake is more complex, it will generate more energy and create more damage,” Ye says. For example, it is helpful to know if the first main megathrust slip is jerky and very energetic, as it can release enough energy to trigger slip on another nearby fault, like dominoes.
Janiszewski notes that the sample size in the study is small, especially considering that many of the quakes occurred before modern instrumentation and better coverage of seismic monitoring were available. It would help, she says, to broaden the dataset by looking at earthquakes of a smaller magnitude to see if the relationships observed in this study still stand.
Ye agrees and says that the team is looking to expand to smaller earthquakes as well as looking beyond megathrust earthquakes. She notes that REEF could also be used to quantify strike-slip earthquake hazards in places like California.
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