Central Washington State isn’t known for being very seismically active, especially compared with the western part of the state, where earthquakes are fairly common. But the town of Entiat, about 3 hours east of Seattle, is an exception: The area recorded hundreds of earthquakes over the past century. A group of scientists investigating this unusual and long-lasting activity recently reported that the quakes may actually be aftershocks of a larger earthquake, one that occurred 145 years ago.
Tom Brocher, a research geophysicist at the U.S. Geological Survey’s Earthquake Science Center in Menlo Park, Calif., and lead author of the study published in the October issue of the Bulletin of the Seismological Society of America, said he was surprised by the results. “I went into the study thinking, ‘you know, it’s been 145 years; they’re probably not aftershocks,’” he said. “I had more of a regional interest in trying to figure out what’s causing these earthquakes in central Washington.”
Long-lived aftershocks have been documented before. For example, seismologists have deemed some earthquakes in Japan to be aftershocks of the 1891 Nobi earthquake although they occurred a century later. However, the phenomenon of long-lived aftershocks was not fully described until relatively recently, in a 2000 paper.
Scientists have found few examples to study, perhaps 10–20 suspected cases of long-lived aftershocks worldwide, said Seth Stein, a seismologist at Northwestern University in Evanston, Ill. He coauthored a 2009 paper on long aftershock sequences.
The new paper on Entiat is “another interesting case,” according to Stein. This prolonged seismicity is “a phenomenon we understand in general, but we don’t understand the details yet, so every time you add a data point, you learn more,” he said.
At 145 years, the Entiat quakes are among the longest-lived aftershock sequences yet documented. In his 2009 paper, Stein argued that earthquakes felt in the seismic zone around New Madrid, Mo., are aftershocks from a series of four large quakes in 1811 and 1812, more than 2 centuries ago. But Stein noted that the criteria for the duration of aftershock sequences are not yet firmly established, so although a good argument can be made for New Madrid aftershocks, it hasn’t been proven unambiguously. “People can make arguments either way,” he said.
Researchers may debate whether earthquakes in specific locations are really aftershocks or conventional earthquakes caused by deformation, but the notion of long-lasting aftershocks is generally accepted, Stein said.
When a large earthquake occurs, it releases stress on the fault, but it can also shift pressure and create new stresses on surrounding faults, as well as on the main fault. Aftershocks occur as the region’s crust adjusts and gradually releases those stresses, Brocher explained. On a big, fast-moving fault like the San Andreas, another earthquake usually comes along a few years later and rearranges things again, resetting the aftershock sequence. But in areas of infrequent seismic activity, like central Washington, that period of settling and readjustment can continue without interruption.
To distinguish aftershocks from other earthquakes, scientists look at patterns. An area experiencing aftershocks will see more quakes than would be normal for the region. If those quakes are smaller than the “big one” and are gradually decreasing in intensity and frequency over time, they’re probably aftershocks, Brocher said.
Solving a Mystery
Now estimated to have a magnitude of between 6.5 and 7, the 1872 quake was the largest ever recorded in the region. “It was felt all through southern British Columbia, and even part of Alberta, as well as in Idaho, Montana, northern Oregon, and all of Washington,” Brocher said.
Although the event was well documented at the time, the quake’s location remained a mystery. That’s partly because it predated seismographs and occurred in a sparsely populated area but also because of the region’s low seismic activity. “The faults aren’t moving that quickly, so other processes like erosion and deposition are even faster than the faulting, and they tend to cover up the faults and obscure them,” Brocher said.
A 2002 paper reported a study of historical accounts of damage from the quake, which narrowed down the probable location to a large circle in central Washington. But it was the 2015 discovery of a fault scarp linked to the 1872 quake that focused attention on Entiat. That discovery made Brocher wonder whether the historic quake was linked to the high number of earthquakes in Entiat. “This cluster, if you look at a map of earthquakes in Washington, sticks out like a sore thumb,” he said. “There really are a lot of earthquakes there.”
To investigate, the scientists first looked at historical accounts of aftershocks documented by a man who lived within 20 kilometers of the Entiat fault scarp. He reported feeling 64 aftershocks in the first 7 hours after the 1872 quake, 8 of which were strong. “I calculated the number of aftershocks you would predict for the first 7 hours after the main shock,” Brocher said. “His number was consistent with the aftershocks you would suspect.” No data were available for the next 28 years, but Brocher was able to find historical accounts of earthquakes in the region after 1900.
Again, these accounts matched the aftershock model. Finally, he studied recorded earthquakes since 1976. “That’s when we got a good seismic network in that location,” he said. Once again, the accounts were in line with a predicted aftershock sequence for a large 1872 quake.
Other faults in Washington are moving at the same rate or faster than the Entiat fault, but they don’t have anywhere near Entiat’s seismic activity, he notes. “It’s hard to think that these earthquakes are being caused by ongoing tectonic deformation because we’re just not seeing that elsewhere,” he said.
Besides adding evidence for near-record-breaking, prolonged aftershock sequences and possibly solving the mystery of where the 1872 quake took place, the findings published on 25 September have practical value, Brocher noted.
With the origin of the historic quake unknown, a large area of central Washington was considered to be at an elevated earthquake hazard, with aging bridges and dams considered at risk. “Knowing exactly where that earthquake happened is important to retrofitting or, in some cases, rebuilding this infrastructure,” Brocher said, “in addition to being important for understanding the tectonics of this area.”
—Ilima Loomis (email: [email protected]), Freelance Journalist