At today’s NSF session, Dr. Graham Kent, who is quoted in this piece, briefly described a presentation he will be giving on this topic–the potential for a major seismic disaster resulting from an earthquake in the 9.0 range. He will talk to the NSF soon on the likelihood and impact of such a shake–what I call the “Everything west of I-5 is toast” scenario.
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Tahoe earthquake ‘domino effect’ studied
Jeff DeLong, [email protected]9:15 a.m. PDT September 9, 2015
With the frightening potential of a massive earthquake and tsunami striking the Pacific Northwest under increasing discussion, scientists are also focusing attention on possible connections between that area and dangerous faults at Lake Tahoe.
There, like along the coastlines of Washington, Oregon and Northern California, a big quake could someday trigger a deadly tsunami, with some preliminary research suggesting seismic connections between the areas.
“If everything in this region is synchronized, all of a sudden there could be a domino effect” after a big quake in the northwest, said Graham Kent, director of the Nevada Seismological Laboratory at University of Nevada, Reno.
“We know these things have a long arm,” Kent said.
Largely due to a July article in The New Yorker, “The Really Big One” by Kathryn Schulz, much public attention is being directed toward the risk of a major earthquake and tsunami that could pulverize coastal communities and the urban centers of Seattle and Portland in a disaster of unprecedented scale.
The danger is associated with the so-called Cascadia subduction zone that stretches hundreds of miles from Vancouver Island south through Oregon and Washington to end near Mendocino, Calif.
Little understood until relatively recently, the Cascadia zone — a place where one of the Earth’s tectonic plates is colliding with and plunging beneath another — is believed to have been the source of some massive earthquakes in the past. The last was a magnitude 9 monster that hit in January 1700, causing a huge chunk of shuddering land to shift and splash up a tsunami that inundated the Pacific Northwest coastline and hammered the coast of Japan hours later.
Scientists estimate there is roughly a one-in-three chance of another big quake hitting the Cascadia subduction zone within the next 50 years. The New Yorker article — which rattled nerves across the region — examined the consequences of a disastrous temblor of a magnitude between 8.7 and 9.2 along with a tsunami, a scenario experts said could kill nearly 13,000 and leave coastal areas as “toast.”
Alarmist? Perhaps, but also possible, said UNR’s Kent, who helped survey the Cascadia subduction zone aboard a research vessel in 2012.
“That place can definitely have magnitude 9 earthquakes, and has, bunches of them,” Kent said. “You could have a magnitude 9.2 there tomorrow and no one would be surprised.
“Is everything west of I-5 toast? It depends on your definition of toast,” Kent said. “It’s going to be a bad day.”
Could it also be a bad day for the Reno-Tahoe area? It’s possible under one line of thought Kent wants to research further.
That’s because near where the Cascadia subduction zone terminates in Northern California, it meets a series a faults that continue south through the Mohawk Valley — stretching from Lake Almanor to Truckee — which in turn abut faults that run beneath Lake Tahoe.
That major quake in the northwest some researchers say is likely in the coming decades might set off a chain reaction, triggering quakes south all the way to Tahoe, Kent said.
That didn’t happen when that last big quake hit Cascadia in 1700, perhaps because faults to the south weren’t poised to let go at the time, Kent said.
But they may have done just that during another event that occurred 7,800 years ago, when evidence suggests major earthquakes hit the Cascadia subduction zone, the Mohawk Valley and the Lake Tahoe Basin at about the same time.
Submerged debris flows scientists found off the northwest coast point to such an event, as do some Kent has studied on Tahoe’s bottom that offer a “tell-tale sign of extreme ground shaking” 7,800 years ago, Kent said. Trenches dug by scientists and another debris flow on the bottom of Donner Lake suggest the same thing may have happened in the Mohawk Valley, he said.
“It would be a hell of a coincidence that Cascadia went and then all these other ones just happened to go,” Kent said. “The question is can you ripple down from Cascadia to Mohawk Valley to Tahoe,” Kent said.
The debris flows in question sit directly atop an ash layer associated with the massive eruption of Mount Mazama, a volcano that “blew its brains out” about the same time, creating present-day Crater Lake, Ore., Kent said. He doubts the eruption triggered subsequent quakes in the three areas but said the proximity of the debris flows to the ash layer is important because it provides scientists with a particularly specific snapshot in geologic time as to when the quakes occurred.
“It basically marks a precise point in time,” Kent said. “When all of the debris flows sit right on top of that ash, we know it happened right after that event, probably within decades.”
Kent is hoping to soon survey the bottom of other Sierra lakes, Independence and Gold lakes among them, to shed further light on what happened in the Mohawk Valley 7,800 years ago.
The West Tahoe Fault, Lake Tahoe’s largest and one of two believed to have ruptured during the event Kent is scrutinizing, likely triggered a quake of magnitude 7 or greater, he said. That fault last ruptured about 4,500 years ago and is about due to do so again, Kent said.
Were a magnitude 7 quake to hit Tahoe today, it would likely cause a tsunami — perhaps as high as 30 feet — that would wipe out the lake’s marinas and inundate many lakeside neighborhoods with the potential of significant loss of life, Kent said.
The Tahoe quake 7,800 years ago “almost certainly created a tsunami,” Kent said. “It would be very hard not to.”
A potential link between earthquakes at the Cascadia subduction zone all the way south to Tahoe is not just of academic importance. Emergency managers plan and prepare for disasters hitting specific places or regions but big earthquakes hitting across much of the West at one time could be overwhelming.
“The rubber hits the road with your emergency response,” Kent said. “Our ability to have emergency response is somewhat predicated on not having too many things happening at the same time. It becomes much more difficult to handle.
Reprinted with permission from the Reno Gazette Journal.