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Eruption-induced tsunami unlikely, experts say

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The threat of a deadly tsunami — to residents of Hawaii island and the rest of the state — appears to be the least of the concerns amid eruptions, spewing lava and all of the seismic activity rocking Lower Puna.

Even fears of molten lava mixing with the water table to create a steam-driven Kilauea explosion big enough to send car-size boulders flying through the air isn’t raising alarms of a deadly tsunami threat at the Pacific Tsunami Warning Center in Ewa Beach.

Tsunami experts worry most about damaging waves being triggered by an earthquake of magnitude 7 — or higher — occurring in “deep, old, cold rock” off of the southwest flank of Mauna Loa, said Cindi Preller, geologist, outreach and duty scientist at the Pacific Tsunami Warning Center.

The 6.9-magnitude quake that struck May 4 — and a 5.0-magnitude quake that hit the day before — instead occurred on the southeast flank of Kilauea.

And the earthquakes were much different from the ones that would raise alarms at the Pacific Tsunami Warning Center.

Instead of the earth suddenly shifting in dramatic fashion, potentially triggering a tsunami, slow-moving lava is causing earthquakes by pushing tons of sediment out of its way like a snowplow.

Preller said it’s the difference between trying to break cold, uncooked pasta — then trying to do the same with cooked pasta.

“In a traditional earthquake you have deep, old, cold rock breaking against other deep, old, cold rock — like trying to break uncooked pasta,” she said. “It will snap. This (seismic activity) is like cooked pasta. The rock is gooey. It doesn’t break.

“The shelf we’re talking about in the Puna/Kalapana region is not going to do a sudden collapse because it has that ridge of sentiment out in front stabilizing it,” Preller said. “It’s super low-risk, super unlikely. No one in scientific circles is even conjecturing that that collapse is a possibility. We’re not even entertaining it.”

The 6.9-magnitude earthquake did cause wave action in the waters around Hilo but reached a height of only 20 centimeters, or about 8 inches, said Richard Rapoza, spokesman for the Hawaii Emergency Management Agency.

For context, Preller said, “we’ve never witnessed a damaging tsunami smaller than 50 centimeters.”

Even though Kilauea could blow at any moment, the eruption would occur in the wrong location to worry about triggering a tsunami.

“When Kilauea explodes and throws car- and refrigerator- sized rocks in the air, there will be an ash hazard,” Preller said. “We’re not expecting a tsunami hazard from that.”

Preller was working on the day of the 6.9-magnitude earthquake.

Even though the quake was originally registered at a magnitude of 6.0, the epicenter on the southeast flank of Kilauea, below Kilauea’s so-called East Rift Zone, meant there was little risk of a tsunami.

“We knew it was definitely not the doomsday scenario,” Preller said.

It’s up to the Pacific Tsunami Warning Center to alert the public of a tsunami, said Rapoza of HI-EMA.

Following procedure, Preller was getting ready to notify HI-EMA of the quake when a call came in from Hawaii County Civil Defense “and said, ‘Pacific Tsunami Warning Center, what’s going on?’” Preller remembered.

But, even as the magnitude of the quake increased twice — eventually reaching 6.9 — “we knew it wasn’t a threat,” Preller said.

For those who want to worry how the next tsunami — or even a mega-tsunami — could be triggered from a Hawaii island earthquake, Preller said there is a formula.

“You need a lot of energy,” she said. “In very, very general terms, you need a quake larger than a 7, maybe even larger than a 7.5, that occurs in the ocean near land,” less than 100 kilometers deep.

“Those are the most dangerous anywhere in the world,” Preller said. “We don’t worry if it’s deeper than 100 kilometers (62 miles). For the big ones, we worry they would occur off of the southwest flank of Mauna Loa causing the mega-tsunami.”

For now, however, there is little risk.

“At the moment,” Preller said, “we’ve got instrumentation that’s not showing any instability on Mauna Loa, so that’s good.”

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