The , published April 13 in the Bulletin of the Seismological Society of America, could help seismologists better forecast aftershocks in the Pacific Northwest.

Seismologists currently make aftershock forecasts based in part on data from other subduction zones around the world. But new research shows that Cascadia intraslab earthquakes, slips that occur within the subducting Juan de Fuca tectonic plate, produce fewer aftershocks compared to similar quakes in other subduction zones. The study shows that in Cascadia, the number of aftershocks for a given magnitude event are less than half the global average for this type of earthquake.

, a 91Ě˝»¨affiliate professor of Earth and space sciences and researcher at the U.S. Geological Survey in Seattle, and , a 91Ě˝»¨research professor of Earth and space sciences, decided to study the phenomenon after recent intraslab earthquakes in and produced dozens of aftershocks, including some big jolts.

“This was startling, because the lore in Cascadia was that intraslab earthquakes had puny aftershock sequences,” said Gomberg, who led the study. The Cascadia region experienced three magnitude-6.5 to magnitude-6.8 intraslab earthquakes, in , and , that produced few to no aftershocks.

“Additionally, the USGS has begun to generate quantitatively estimated aftershock forecasts based initially on global patterns,” Gomberg said. “Given these contrasting regional experiences, it seemed time to generate some objective numbers to base Cascadia’s forecasts on.”

The researchers analyzed catalogs of earthquakes between 1985 and 2018 from the UW-based Pacific Northwest Seismic Network and the Geological Survey of Canada. Earthquakes that took place in the upper plate produced the most aftershocks, they found. Aftershock rates were the lowest for intraplate earthquakes in the Puget Lowlands portion of the subduction zone, which contains the Seattle metropolitan area, while aftershock rates varied at the northern end of the zone, near Vancouver Island, and at the southern edge, near Cape Mendocino in northern California.

The tectonic environment at each end of the subduction zone could help explain why aftershock production is higher at the edges, the researchers said. Multiple tectonic plate boundaries meet in these areas, which could “concentrate stress, so more faults exist and are closer to failure than in other areas,” they noted.

Why Cascadia produces so few aftershocks is still unclear, but “one strong possibility would seem to be that temperature for the deeper slab earthquakes is a dominant controlling parameter,” Bodin said. In Cascadia, “the young, hot Juan de Fuca plate is being jammed beneath North America.”

The deeper the earthquake, the higher the temperatures, and the researchers found that aftershock activity decreases with depth. “However, this is not so different than southern Mexico, where, as we noted, recent intraslab mainshocks have supported vigorous aftershock sequences,” Bodin said.

The analysis was limited by Cascadia’s low seismicity rates, and sparse data to pinpoint the location and depth of most earthquakes in the region. Methods that help researchers detect and locate smaller earthquakes could provide a better sense of overall aftershock rates and the physical processes that control them, the authors argue in the paper.

For more information, contact Bodin at bodin@uw.edu or Gomberg at gomberg@usgs.gov.

Adapted from a by the Seismological Society of America.

, a research professor in the UW’s Department of Earth and Space Sciences, has been named the interim director of the UW-based .  PNSN is a collaboration between the 91Ě˝»¨, the University of Oregon and the U.S. Geological Survey that tracks earthquake and volcano activity throughout the two states, with the support of federal, state and private funding.

Former PNSN director John Vidale stepped down to accept a faculty position at the University of Southern California and direct the in Los Angeles. The search for his permanent replacement is expected to take about one year.

In the interim, Bodin will also serve as the Washington’s state seismologist, serving on the Washington state that makes seismic policy recommendations to the state’s Emergency Management Division and Gov. Jay Inslee, as well as answering questions from reporters and the public about earthquake and volcanoes.

Bodin is an observational seismologist whose research expertise includes studies of earthquake source physics, seismic wave propagation, and the impacts of strong ground shaking on soils.  Bodin spent the first part of his career at the University of Memphis. In Tennessee he studied earthquake processes and hazards associated with earthquakes that occur far from tectonic plate boundaries. Such earthquakes are infrequent and poorly understood, but have very large impacts when they do occur. He also performed field studies in the aftermath of large earthquakes in Mexico, California, India and Taiwan, and was part of a U.S. team monitoring underground nuclear testing in the former Soviet Union.

Bodin joined the 91Ě˝»¨faculty in 2006 to become manager of the PNSN. During more than a decade since he has overseen upgrades of the network’s technology to enable faster and more accurate detection, analysis and communication of ground shaking from a major earthquake. These network improvements have led to the inclusion of Washington and Oregon into , a West Coast-wide earthquake early warning system that will provide advance warnings for imminent large earthquakes. Bodin has also published academic papers on triggered earthquakes and tremors; seismic wave propagation and aftershocks; exploring swarm seismicity in Richland and Spokane, Washington; and on the potential for developing earthquake early warning systems in Hawaii and Chile.

Bodin earned his bachelor’s degree at the University of California, San Diego, his master’s at California’s Humboldt State University and his doctorate at the University of Colorado, Boulder.

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For more information, contact Bodin at bodin@uw.edu or 206-616-7315 or PNSN communications manager Bill Steele at 206-685-5880 or wsteele@uw.edu.