California Just Dodged a Devastating Tsunami

California may have just dodged a devastating tsunami after a 6.4 magnitude earthquake shook Humboldt County early on Monday morning.

At least two people died in the 11-mile-long earthquake, which hit off the far North Coast of California, with a dozen more injured. Power outages swept across the region and many houses were damaged.

The region is extremely tectonically active, as it is located at the southern end of the Cascadia subduction fault—where the United States sits right over the ocean floor. The fault lies at the northern tip of the San Andreas fault, the border section between two massive tectonic plates under the Earth's surface.

These plates remain relatively static, meaning they can see large pressures build up over time. And when they move, they can produce huge earthquakes. In this region, the fault is capable of producing a catastrophic sequence of natural disasters—including a magnitude 8 or 9 quake and subsequent tsunami.

Alex Simms, Professor, Department of Earth Science at the University of California Santa Barbara told Newsweek: "The earthquake that happened was not the 'big one.' This earthquake was on a 'smaller' fault in one of the two plates that converge in that region. However, that region overlies the Cascadia Subduction Zone. The Cascadia Subduction Zone is the boundary between two of Earth's big tectonic plates."

When earthquakes happen along this boundary, "they can be huge," Simms said.

The enormity of this event would be on par with the magnitude 9.1 earthquake and tsunami that hit Tōhoku, in Japan in 2011.

An earthquake and tsunami last happened in the Cascadia region on January 26, 1700. "That event happens every 300 to 500 years and so we are due for another one," Simms said.

When will a 'Big One' happen?

Scientists are able to measure signals in the fault, but not the exact time when an earthquake will occur.

"People in this region should always be prepared for a larger and more damaging earthquake and for a tsunami as well. We can forecast that there is a probability of earthquakes to come, but seismologists cannot yet predict an impending earthquake's time or location precisely, down to the year or decade, for example. We are actively researching whether there might be a telltale precursor we could detect, but we don't have one yet," Harold Tobin, a professor and Paros Endowed Chair in Seismology and Geohazards at the University of Washington told Newsweek.

"We can measure signals that show us that the stress and strain are accumulating that will someday cause the next one—a magnitude 8 or 9 with a tsunami that crosses the entire Pacific as well as washing onto local shores."

Aftershocks from the 6.4 magnitude earthquake are still rippling out across the region. The U.S. Geological Survey estimates that there is a 4 percent chance these aftershocks will be of a magnitude 5 or larger, which can happen following large earthquakes.

It is more likely, though, that eight magnitude 3 aftershocks will occur in the next week. Magnitude 3 aftershocks will not be as damaging as magnitude 5, but are big enough to be felt.

"There will certainly continue to be aftershocks to this earthquake, diminishing in number over time. Most aftershocks are smaller than the main shock, but any earthquake has about a 1 in 20 chance of having an aftershock that is larger magnitude," Tobin said.

In the meantime, seismologists are continuing to keep a close eye on the fault.

"While we can't issue advance warning of an earthquake about to happen yet, we can and do have a system to detect the earthquake as it begins and send out alert messages—this is the ShakeAlert earthquake early warning system, operated by the USGS in partnership with us at the University of Washington, as well as CalTech, and UC Berkeley," Tobin said.

"This is the cell phone alert millions of people received on Tuesday giving just enough warning to take a safety action like 'Drop, Cover, and Hold On.'"

The region where this quake occurred is the most seismically active spot in the lower 48 states; a magnitude 6.2 earthquake happened very close by exactly one year ago to the day. At Cape Mendocino in Northern California, three different tectonic plates converge and cause that activity.

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