A Jan. 15 undersea volcanic eruption in the South Pacific caused a powerful tsunami to hit the Kingdom of Tonga and resulted in tsunami advisories for the entire West Coast, including the San Francisco Bay Area.
A San Jose State University student, who preferred to go by Josie for privacy concerns, said she was at the Santa Cruz Beach Boardwalk that morning with her friends and initially had no idea why the waves were so high.
“I remember being bummed that we had driven all that way for a nice beach day and [the sky] looked like a storm was coming,” she said in an email.
“The waves were huge and [were] essentially crashing against the hillside and you could even hear rocks falling.”
Josie said it all made sense once she got a tsunami alert from the university and checked social media.
SJSU Interim President Steve Perez sent a campuswide email at 11a.m. the day of the volcanic eruption advising the campus community of the tsunami alert.
“Avoid areas that are prone to flooding because the arrival of surges may overlap high tide,” Perez stated in the email.
The Hunga Tonga-Hunga Ha’apai volcano erupted about 40 miles from the Tonga capital Nuku’alofa and just over 1,000 miles off the coast of New Zealand, according to a Jan. 18 The New York Times article.
Donald Reed, a retired SJSU marine geology and geophysics professor, said the volcanic eruption was identified immediately by satellites and the global tsunami system quickly detected the tsunami’s arrival to land.
“This volcano eruption occurred more than 5,000 miles away from [California] so the prediction time of [the tsunami] was well known hours in advance,” Reed said in a Zoom call. “[The tsunami] was predicted to arrive between 7:30-8:30 a.m.”
The Jan. 15 tsunami arrived when the tide had already reached the highest point of the day.
“[The tsunami] happened during a high tide, adding another foot and a half, almost two feet on top of a five foot- high tide,” Reed said.
He said the high tide allowed the water to extend further inland and the high swell added to the energy that affected the shoreline.
Swells are wind-generated waves that have traveled out of the area where they originated, according to the National Weather Service Marine Definitions webpage.
Reed said not all volcanic eruptions are the same and a subsequent tsunami will depend on the size of the volcano and where it’s located relative to the sea level.
“In this case, the volcano was very close to the sea surface so it allowed that volcanic eruption to quickly pierce the sea surface causing a great blast and generated the tsunami that propagated away in the ocean but also a pressure pulse in the atmosphere that also propagated a wave,” Reed said.
He said this tsunami was unusual because the pressure pulse in the atmosphere added to the force of the tsunami.
“[This tsunami] maintained a very similar height at a large number of places. For example, the areas of California and Hawaii all had a height of roughly 2 to 3 feet,” Reed said. “Whereas often the height of a tsunami decreases barely uniformly with distance traveled by that tsunami.”
The type of tsunami that hit California is known as a “distant tsunami” because its source was far away from the coast.
In this case, there is more time to issue and respond to warnings.
However, the tsunami was categorized as “local” for those in Tonga and the surrounding areas, allowing less time to issue warnings and evacuate, according to the National Weather Service’s Tsunami Inundation webpage.
Although tsunamis caused by volcanic eruptions are not as common as those caused by earthquakes, the tsunamis can be similar.
SJSU sedimentology assistant professor Ryan Portner said the cause of the event, whether an eruption or earthquake, doesn’t change the arrival time of a tsunami, all that matters is the source location of the event.
“Those islands around Tonga had less than 30 minutes to respond, whereas in California you had 12 hours or more,” Portner said in a Zoom call.
Reed said there are scientific instruments to measure tsunami arrival time scattered around the Pacific Ocean in the middle of the ocean basin and along the coast lines.
Ocean basin is the land surface under an ocean that includes the various topography of the water, according to the WorldLandforms webpage.
Reed said the main detection instruments are Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys, which detect the passage of a tsunami using sea and tide gauges on land that measure the height of the sea surface.
Scientists can detect a distant tsunami and determine the progression toward a location over time using the data collected through DART buoys.
“Once you’ve detected the initial event using basic physics for the speed of the tsunami combined with instruments, you can make predictions of the tsunami’s arrival,” he said. “Based on the height of the tsunami with these instruments, you can predict how high that tsunami may be.”
All of this information is how researchers at the National Oceanic and Atmospheric Administration (NOAA) generate the tsunami warnings.
The NOAA tsunami program is a federal and state partnership with a goal of saving lives and protecting property before, during and after tsunami collisions, according to its webpage.
Although it’s unlikely a tsunami would reach as far as San Jose, Kenneth Mashinchi, SJSU senior director of strategic communications and media relations, said in an email the university is prepared to alert students about natural disasters.
“In cases of emergencies, the Alert SJSU system is the best way for the campus community to stay informed,” he said.
Alert SJSU is a campus emergency notification system that disseminates emergency information to the campus community during critical incidents, according to the University’s Alert SJSU webpage.
“SJSU has protocols in place to respond to various types of emergencies, including natural disasters,” Mashinchi said.
“We also encourage the campus community to follow the social media accounts for Santa Clara County and the county in which they live to receive additional information pertinent to their location and those of their family and friends.”
Physical oceanography assistant professor Tom Connolly said when there’s a tsunami people should retreat to safe, higher ground to observe the water.
“There are safe vantage points if you’re high up but going down to the waters edge is risky and not worth [your life] because the surges can be so unpredictable and come up so fast,” Connolly said in a phone call.
“There may be a case where you’re at the beach and you don’t know there’s a warning, if you see the water receding rapidly that’s an indication that a tsunami is coming, rather than going towards [the water], get away from the beach.”
