Early Saturday morning, an undersea volcano erupted near the island of Tonga in the Southwest Pacific Ocean, generating a very rare Pacific-wide tsunami. According to Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington, “the event was the first Pacific-wide tsunami warning from a volcanic eruption rather than from an earthquake. The only other volcanic eruption to trigger such a wide-ranging tsunami was the 1883 eruption on the Indonesian Island of Krakatua.”
Weather satellite imagery captured the undersea volcanic eruption. The eruption was more explosive as water hit the rising lava. The explosive underwater eruption displaced the entire volume of water above the volcano, resulting in the tsunami.
Most tsunamis are generated by strong earthquakes involving vertical displacement of the ocean bottom. A tsunami is a series of waves generated not only by major earthquakes, but also volcanic eruptions or large landslides. The series of waves can last 12 hours or more.
The undersea eruption generated a tsunami that swept across the entire Pacific Ocean. A tsunami advisory was issued by the National Tsunami Warning Center in Palmer, Alaska, that included all Washington coastal waters. A tsunami advisory is for events involving waves of 3 feet or less, along with strong eddies and currents in harbors, bays and estuaries. Outer Washington coastal tide gauges measured tsunami waves of 1 foot or less around 8:30 AM Saturday morning. Measurements in the Puget Sound region were a matter of a few inches.
Tobin added, “Events like this really help us wake up and consider the possibility of a more locally generated earthquake event, the big one.” The west coast of North America is part of the Pacific ring of fire – earthquake and tsunami territory. Director Tobin noted the big one, being the Cascadia Subduction Zone just off the coast from Vancouver Island south to Northern California.
The last time the Cascadia Subduction Zone ruptured was on January 26, 1700, and was estimated to be a 9.0 magnitude quake. That earthquake unleased a Pacific-wide tsunami that struck not only the west coast of North America including the North Sound, but also all the way to eastern Asia with recorded damages in Japan called the Orphan Tsunami.
Speaking of Japan, March 11th will mark 10 years since the big Japan earthquake and massive tsunami. Again, this was a Pacific-wide tsunami with waves as high as 3 feet along the outer Washington coast.
A tsunami is not an angry Hollywood looking wave, but rather a surge of water with tremendous power that picks up anything in its path. Here is tsunami video from the March 2011 Japan event that illustrates what a tsunami looks like. On December 26, 2004, a 9.1 magnitude earthquake in the Eastern Indian Ocean generated a massive tsunami. Here is some video from that event. If a tsunami is anticipated, head to higher ground.
The Cascadia Subduction Zone is expected to rupture again someday. Historical records show it does so about every 300 to 500 years. The earthquake will be felt throughout the Pacific Northwest with violent shaking for up to 5 minutes or so. That is what happened in Tokyo hundreds of miles away from the March 2011 event. Cascadia tsunami waves would rush onto the outer Washington coast within 30 minutes with tsunami action moving into the Puget Sound region not long after that.
Locally generated tsunamis can also be generated from local earthquakes along faults like the South Whidbey Island fault, the Seattle Fault, and the Tacoma fault. You can be prepared for the next big earthquake with resources from Washington State Emergency Management and Snohomish, Island or Skagit County Emergency Management.