Earthquakes deep within the Juan de Fuca plate produce few aftershocks

Environment Search | Science | UW News Blog

April 13, 2021

In the Cascadia Subduction Zone, medium and large “intraslab” earthquakes, in which slip within the oceanic plate and under the continental plate, are likely to produce only a few aftershocks that can be detected, according to a new study from the University of Washington and the US Geological Survey.

The results, published April 13 in the Bulletin of the American Seismological Society, could help seismologists better predict aftershocks in the Pacific Northwest.

This image shows Highway 302 after the 2001 Niskwale earthquake, which had few aftershocks.

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

Joanne Jomberg, associate professor of earth and space sciences and researcher at the US Geological Survey in Seattle, and Paul Bowden, professor of Earth and space science research at the University of Washington, decided to study this phenomenon after recent earthquakes in Mexico and Alaska. Dozens of aftershocks, including some large tremors.

“This was amazing, because the knowledge in Cascadia was that internal earthquakes had weak sequences,” said Jomberg, who led the study. The Cascadia region experienced three earthquakes measuring 6.5 to 6.8 on the Richter scale in 1949, 1965 and 2001 that produced little or no aftershocks.

“In addition, the US Geological Survey has begun issuing quantified earthquake aftershocks forecasts based on global patterns initially,” said Gomberg. “Given these contrasting regional experiences, it appears that the time has come to generate some objective numbers on which to base the Cascadia projections.”

The researchers analyzed earthquake catalogs between 1985 and 2018 from the UW Pacific Northwest Seismic Network and Geological Survey of Canada. They found that earthquakes in the upper plate caused the largest number of aftershocks. The rates of aftershocks were the lowest for intra-plate earthquakes in the Puget Lowlands portion of the subduction zone, which contains the Seattle metropolitan area, while the rates of aftershocks varied at the northern end of the area, near Vancouver Island, and at the southern edge, near Cape Mendocino. In Northern California.

The researchers said that the tectonic environment at each end of the subduction zone could help explain why the production of tributaries is so high at the edges. They noted that the boundaries of multiple tectonic plates meet in these regions, which can “concentrate pressure, so there are more errors and are closer to failure than other regions.”

It remains unclear why Cascadia produces so few aftershocks, Bowden said, but “one strong possibility appears to be that the temperature of deep earthquakes is a dominant control factor.” In Cascadia, “the hot little Juan de Fuca plate wedged under North America.”

The deeper the earthquake, the higher the temperatures, and the researchers found that the aftershock activity decreases with depth. “However, this is not much different from southern Mexico, where, as we have indicated, recent major tremors inside the plant have supported the sequence of strong aftershocks,” Bowden said.

Analysis was limited by low earthquake rates in Cascadia, and sparse data to determine the location and depth of most earthquakes in the region. In the paper, the authors argue that methods that help researchers detect and locate small earthquakes can provide a better sense of overall aftershock rates and the physical processes that control them.

For more information, contact Bodin at [email protected] or Gomberg at [email protected].

Adapted from a press release from the American Seismological Society.

Mark (marks): College of the Environment • Department of Earth and Space Sciences • Seismology and Seismology • Paul Bowden

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