Why are there relatively few aftershocks in some Cascadia earthquakes?

In the Cascadia subduction zone, medium and large “intraslab” earthquakes, which occur at greater depths in the Earth’s crust within the subduction plate, are likely to produce only a few aftershocks that can be detected, according to a new study.

The findings could have implications for predicting the risk of aftershocks in the Pacific Northwest, says Joanne Jomberg of the US Geological Survey and Paul Bowden of the University of Washington in Seattle in their paper published in the Bulletin of the American Seismological Society.

Researchers are now calculating forecasts of aftershocks in the region based in part on data from subduction zones around the world. But the Cascadia intraslab earthquakes produce fewer aftershocks than others in subduction zones around the world. Jomberg and Bodin concluded that rates of aftershocks in Cascadia are more than half the global average.

They also suggest that the aftershocks rates of the Cascadia earthquake generally appear consistent with the “clock advance” model, in which the master shock causes tectonically loaded fault patches to slip away earlier than would occur under the natural seismic background of the area.

Jomberg and Budden decided to study this phenomenon further after recent earthquakes in Mexico and Alaska that produced powerful aftershocks chains. “This was amazing because the knowledge in Cascadia was that the earthquakes that occurred inside the buildings had poor earthquake sequences,” Gomberg said, noting that the three Cascadia earthquakes, which measured 6.5 to 6.8 magnitude in 1949, 1965 and 2001, produced very little. Or without aftershocks.

“In addition, the US Geological Survey has begun to generate quantitatively estimated aftershocks predictions based initially on global patterns,” it added, “Given these contrasting experiences, it appears that the time has come to generate some objective numbers upon which the Cascadia predictions are based.”

The researchers analyzed earthquake catalogs produced by the Geological Survey of Canada and the Northwest Pacific Earthquake Network from January 1985 to January 2018. They found that the main shocks in the top panel produced the most aftershocks. The productivity of aftershocks was lowest for intraplate earthquakes in the Puget Lowlands portion of the subduction zone (which contains the Seattle metropolitan area), while the rates of aftershocks were variable at the northern end of the region near Vancouver Island and within the expected range of the near southern end of Cape Mendocino.

The researchers said the tectonic environment at each end of the subduction zone could help explain why the production of substrates there was so high. They pointed out that the multiple plate boundaries meet in these areas, which can “concentrate the pressure, so there are more faults and closer to collapse than in other areas.”

The reasons why the production of aftershocks from Cascadia is so low compared to global rates are still unclear, Bowden said, but “one strong possibility appears that the temperature of the deep earthquakes is a dominant control factor,” noting that “the hot youth of the Juan de Fuca plate is jammed beneath North America “in Cascadia.

Bowden explained that the greater the depth of the earthquake, the higher the temperature, and the researchers found that the productivity of aftershocks decreased with the depth. “However, this is not much different from southern Mexico, where, as we have indicated, recent major tremors inside the glades supported the strong aftershocks.”

Jomberg and Bodin said their analysis was limited by the fact that earthquake rates in the Cascadia are generally low and there is sparse data to restrict the location and depth of most earthquakes in the region. They suggested that methods that help researchers detect and locate small earthquakes could provide a better sense of the overall rates of aftershocks and the physical processes that control them.

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