How big does a Hawaii quake have to be to trigger a dangerous tsunami? It depends

VOLCANO, HAWAII (HawaiiNewsNow) - In the last 30 days, there have been at least 56 quakes on the Big Island with a magnitude 4.0 or greater.

After each one, authorities must issue a statement letting people know if there's a tsunami risk.

But Brian Shiro, seismic network manager at the Hawaiian Volcano Observatory, says it takes more than a 4.0 to cause serious concern about a tsunami.

"Generally speaking, local tsunamis can be generated by shallow near- or off-shore earthquakes above about magnitude 6.8," he said.

A 6.7-magnitude earthquake in 2006, which was centered on the Big Island, did not generate a tsunami.

But a 6.9 earthquake last month that was centered near Kilauea's summit did generate small tsunami waves.

Shiro said the tsunami information statements released following quakes smaller than magnitude 6.8 are aimed at reassuring people there is no threat.

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Experts added, though, that while a 6.9 magnitude quake is technically considered grounds for a tsunami warning, it does not mean a tsunami will pose a large threat.

Rhett Butler, geophysicist at the University of Hawaii Manoa, says last month's 6.9 event was small.

"You could detect it but it was not large enough to be a concern," he said.

That quake was something called a thrust event, which is especially typical for creating tsunamis, Butler said. Thrust events deform the seafloor after one tectonic plate is forced under another. These events produce some of the most powerful earthquakes in the world.

"If it had been a magnitude 7.9 event, it would have had a much more serious event," Butler said.

Such an event is not completely out of the question, he added.

"It is conceivable that you could get a large earthquake in the area," he said, adding that a bigger seismic event would "not be unexpected."

The U.S. Geological Survey says the most recent and destructive tsunami in Hawaii was a magnitude 7.7 earthquake off Kilauea's south flank in 1975.

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