Researchers determined that Cascadia is a standout among similar seismic zones worldwide because it produces fewer aftershocks than its global counterparts.
By Lauren Milideo, Ph.D., science writer (@lwritesscience)
Citation: Milideo, L., 2021, Earthquakes in Cascadia produce relatively few aftershocks, Temblor, http://doi.org/10.32858/temblor.176
In regions that sit atop a tectonic plate boundary, researchers and hazard planners expect earthquakes. The subduction zone of the Pacific Northwest is one such place. Relatively large earthquakes generally trigger a slew of aftershocks. Yet, not many aftershocks seem to occur in Cascadia — the region extending along the Pacific Coast from northern California, through Oregon and Washington, into British Columbia. The apparent paucity of aftershocks here, compared to similar regions around the globe, has significant implications for hazard assessment, and the authors of a new study wanted to know: when it comes to aftershocks, is Cascadia really different?
Knowing what to expect in terms of aftershock productivity following an earthquake is an important aspect of hazard planning. Seismologists estimate this in part based on observations from other locations with similar geology, says the study’s lead author Joan Gomberg, a seismologist with the U.S. Geological Survey. Gomberg says that she and coauthor Paul Bodin of the University of Washington’s Department of Earth and Space Sciences wanted to know if these globally-derived observations apply to Cascadia. The region has only experienced three recent damaging earthquakes — in 1949, 1965 and 2001 — notes Gomberg, and these came with few aftershocks. Recent quakes in other places, however, had much greater aftershock production, “so the question was, did we just happen to have the three really weird earthquakes,” or should researchers actually expect these quakes not to produce many aftershocks?
33 years of earthquakes
To get at the question of whether Cascadia has truly unusual aftershock productivity, the researchers assembled a catalog of 37,475 local earthquakes from 1985 to 2018, drawn from the Geological Survey of Canada and the Pacific Northwest Seismic Network. The team focused on mainshocks of magnitude-4.5 and greater — a “sweet spot,” Gomberg says, that encompassed larger quakes while retaining small aftershocks in the catalog. The catalog contained both deep and shallower earthquakes.
The team looked at the number of earthquakes over time, identifying mainshocks and aftershocks, and determined the productivity of aftershock occurrences. They compared this productivity to similar locations in other parts of the world. The work was challenging due to Cascadia’s relatively low rates of seismicity, says Gomberg. Unlike in Southern California, or Japan, or other seismically active places, Cascadia does not have many earthquakes. “If you have a very complicated approach and model … many parameters, you need a lot of observations,” notes Gomberg. “And we don’t have a lot of observations.”
The solution, Gomberg said, was to “[make] some very big simplifications.” The team relied on empirical relations describing how the productivity of aftershocks slows down and disappears over time, derived from data that seismologists have observed globally. She notes that these relations yield a simple means of estimating how many aftershocks would be expected following a certain magnitude earthquake, in order to calculate a quake’s aftershock productivity. For the Cascadia quake catalog, the team compared aftershock productivity in the upper plate (shallower events) to intraslab (deeper quakes). This, too, was challenging as the exact depths of the plate boundary in this region are not well-constrained, Gomberg says.
Unusually low aftershock productivity
What about aftershock productivity in Cascadia? “It’s unusual,” Gomberg says. In fact, the researchers found, productivity in Cascadia is less than half of what would be expected based on tectonically similar regions worldwide. The exception was quakes that occurred in the upper plate, which had productivities more similar to what would be expected based on global observations. The authors note this may show that knowing the depth of the mainshock can help with predicting aftershocks as well.
Although researchers are not yet sure why Cascadia is so different, says Gomberg, the reason may have to do with the age of the subduction zone: it’s young, geologically speaking, and the subducting plate is hotter than subducting plates in similar areas globally, which could potentially decrease seismicity.
Many aspects of Cascadia’s geology and faults remain unclear, which creates challenges for researchers. “I think they did a very good job of thinking carefully about the uncertainties in their study and being as careful and upfront as possible about the limitations,” notes seismologist Maureen Long of Yale University’s Department of Earth and Planetary Sciences.
The likelihood of aftershocks
The research also provides useful insight into aftershocks overall, Long notes. “One of the things that [the researchers] were able to do using their catalog of earthquakes in Cascadia is to test some hypotheses about what aftershock behavior looks like,” she says. “All of those kinds of studies move us closer to the goal of having a better understanding of the underlying physical processes.”
Understanding these processes has tremendous practical implications. Several years ago, Gomberg and a colleague from the Washington State Emergency Management Department performed a survey of emergency managers that asked, “What could we provide you that would be helpful after an earthquake?” She says, “Universally they said, ‘we want to know what the likelihood of aftershocks is,’” because emergency managers want to avoid having personnel enter unstable or unsafe buildings if aftershocks are on their way. This study can help to inform aftershock forecasts in such situations.
In the future, Gomberg says, researchers may use improved instrumentation to capture small earthquakes that are not currently detectable, which will provide a more complete picture of seismicity in Cascadia. She says that another potential area of promise is the use of new offshore instrumentation to obtain needed data.
Gomberg, J. and P. Bodin (2021). The Productivity of Cascadia Aftershock Sequences. Bulletin of the Seismological Society of America. https://doi.org/10.1785/0120200344
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