The largest earthquakes in the world occur at subduction zones, where two tectonic plates converge and one plate descends beneath the other, Dr. Herman explained. However, some sections of these subduction zones are thought to be incapable of having such large events. The new study shows that – perhaps paradoxically – the inability for these regions to have typical large earthquakes can cause them to have other kinds of unexpected large earthquakes.
“The potential for unusual earthquakes in these regions makes sense from our computational models,” Dr. Herman said. “But it is still pretty counterintuitive that making the expected kind of earthquakes less likely actually makes other types of big earthquakes more likely.”
The duo’s study focuses on the Alaska-Aleutian subduction zone, where the Pacific tectonic plate descends northward under Alaska and the Aleutian Islands. Along most of this subduction zone, the two tectonic plates are stuck to each other along their boundary.
“As the plates slowly converge, they build stress for decades or centuries before the boundary finally breaks in a megathrust earthquake, such as the second largest event ever recorded (the magnitude 9.2 1964 Prince William Sound earthquake),” Dr. Herman said. “The exception to this behavior is the area near the Shumagin Islands, which appears to be poorly coupled – in this section of the subduction zone, the plates slide past each other without sticking and therefore do not have megathrust earthquakes.”
It was surprising to Drs. Herman and Furlong, then, that on October 17, 2020, a magnitude 7.6 earthquake occurred within this “Shumagin Gap.” However, this earthquake did not break the plate boundary like most large earthquakes in the Aleutian Islands. Instead, the earthquake broke into the descending Pacific plate.
According to this new research, earthquakes like the October event are actually strongly favored if the Shumagin Gap is poorly coupled. The researchers also discovered that the more typical kind of earthquake that occurred in July in the more strongly coupled section of the subduction zone to the northeast seems to have set off the October earthquake.
“The seismic hazard near poorly coupled sections of other subduction zones and other plate boundaries like the San Andreas fault could be higher than we previously thought,” Dr. Herman said. “These unusual events could be triggered by earthquakes we have come to expect.”
The groundwork for this study was laid through collaborations over the past decade between Dr. Herman and Dr. Furlong, his graduate school advisor at Penn State. On the day the October earthquake occurred, they had their usual virtual chat about the event and its plate tectonic context. In that conversation, they came up with the concept for this study, and within 24 hours they had demonstrated the basic mechanism with their models. After a busy month of cleaning up the model results, generating figures, and writing text, the initial manuscript was submitted to Science Advances in November.
“I think this study points to the importance of letting data help guide the research, and not simply trying to fit observations into a standard model for earthquakes in a region,” Dr. Furlong said. “Although unexpected, the earthquake behavior we found in this study does point out an additional concern for hazard studies along major plate boundaries.”
The full study is available to read here. Science Advances is an open-access journal, so the article is free to read and download.
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