Pacific Northwest May Be at Most Risk for the 'Big One' Because of Seafloor Sediments

The next big earthquake is due in the Pacific Northwest—but now scientists have pinpointed where along the coast a large earthquake is most likely to happen, according to a study published Monday.

"We observed very compact sediments offshore of Washington and northern Oregon that could support earthquake rupture over a long distance and close to the trench, which increases both earthquake and tsunami hazards," lead author of the study Shuoshuo Han, a postdoctoral fellow at the University of Texas Institute for Geophysics, said in a statement.

Offshore of Washington and northern Oregon, ocean floor sediments are much more compact than in central Oregon—indicating that sediments are less porous and have less water between the grains. That all adds up to a disaster about to happen, according to the study, which was led by the University of Texas at Austin and published in Nature Geoscience.

The compact sediments in an area called the Cascadia subduction zone offshore the Pacific Northwest increase the likelihood of major earthquakes and tsunamis.

Subduction zones are where a tectonic plate dives underneath another plate. Stress builds up between the two plates over time. At Cascadia, wedges made from sediments nearly two miles thick pile up at the front of the overlapping plates. This thick of a build up does not happen at many subduction zones, Han told Newsweek in an email.

Earthquakes typically don't reach shallower depths along the megathrust fault—which is the type of fault that forms at the plate interface of subduction zones, she explained. The wedge sitting atop the megathrust fault moves more easily when compact, increasing the likelihood that a rupture would occur shallower beneath the seafloor. The shallower rupture causes the movement of the water above, as in, tsunamis.

For the study, researchers collected data from a seismic survey of up to four miles of sediment layers on top of the subduction zone using five-mile-long seismic streamers—a tool that uses soundwaves to map out the seafloor. These tools are the best available to efficiently collect data on these zones, according to researchers.

"These kinds of long-streamer marine seismic studies provide the best tools available to the science community to efficiently probe subduction zones in high resolution," co-author Suzanne Carbotte, a research professor of of marine geology and geophysics at Columbia University, said in a statement.

The last large earthquake from Cascadia occurred in 1700. Typically, this zone has a large earthquake every 200 to 530 years. Scientists expect another one to occur, but when exactly it will happen is impossible to determine, according to the researchers.

The San Andreas fault—notoriously known as the fault that is expected to cause the next so-called "big one" in California—in some ways has less dangers than Cascadia. Cascadia could cause a tsunami, and did so during the magnitude 9 earthquake in 1700 that damaged coastal villages in Japan and left behind scars of hollowed out, dead spruce tree trunks in California, as reported by CNN. Cascadia isn't visible, since it is deep underwater, unlike the San Andreas, which is a visible line in some parts of California.

Many Bay Area residents may remember the 1989 Loma Prieta as the "big one," but the U.S. Geological Survey says that earthquake was far less devastating than previous ruptures. Loma Prieta was a magnitude of 6.9. Robert Page from the USGS said in a statement that the earthquake was "a wake up call to prepare for the potentially even more devastating shocks that are inevitable in the future."

Earlier this month, a moderate earthquake occurred near the San Andreas Fault. Though no damage or injuries were reported, the 4.6-magnitude earthquake hit Monterey County in California and spread as far north as San Francisco.