Tsunami damage in Hilo, Hawaii.
Scientists have talked for awhile about the tsunami threat to California coming from an underwater earthquake in the Aleutian Islands of Alaska. That's where the big waves have come from before, and where they will likely originate again. A new study by USGS researchers published in the online journal Geochemistry, Geophysics, Geosystems won't help you sleep any easier, if you are prone to worry about these things. The scientists looked at a segment of the subduction zone where the Pacific plate is diving under the North American plate, and where the directional orientation means that a major tsunami would be pointed right at California. The Semidi segment is too quiet, seismically speaking, suggesting a build-up of tremendous tension.
From an NPR story on Tuesday:
Miller says this segment of the trench, called the Semidi, poses a special risk. A tsunami created by its rupture would travel outward at a 90-degree angle from the segment. "[A] perpendicular [line] to that section of the trench," he says, "aims right at California." He says that means a big quake could produce a tsunami that would score a direct hit on California's coastline from San Francisco to Los Angeles.
In the past hundred years, other tsunamis have come from other parts of the Aleutians, he points out; since the trench is shaped in an arc, each segment of it "points" in a slightly different direction in terms of the waves created when it ruptures. Consequently, tsunamis emanating from that region of the seafloor in the recent past have mostly missed population centers, or struck only glancing blows. The Semidi, in contrast, points directly at Central California.
Miller and his team have found evidence that the Semidi segment ruptures about once every 180 to 270 years. The last time it erupted was 1788.
Miller says a tsunami from the Semidi could be as big as the one that struck Japan in 2011. "I think the public just needs to be aware that tsunamis of this magnitude can occur, and they can cause a lot of damage," he says.
A separate study of a nearby segment of the Aleution subduction zone found evidence of a similar threat that has hit Hawaii before. The 1959 tsunami that destroyed part of Hilo came from Alaska. The best thing about these tsunami threats is that people in coastal California and Hawaii would have several hours of advance notice, unlike the east side of Japan in 2011.