HILO, Hawai'i — Scientists at the Pacific Tsunami Warning Center are rethinking the guidelines they use to decide which Hawai'i earthquakes trigger a local tsunami warning. They tentatively plan to lower the threshold so an alert will be issued for temblors as small as magnitude 6.7 if the earthquakes are shallow.

Now, the warning criteria call for issuing a local tsunami alert for Hawai'i quakes that are magnitude 6.9 or larger, or the size that historically has generated potentially damaging local tsunamis.

The magnitude 6.7 and 6.5 earthquakes on the Big Island on Oct. 15 delayed the proposed changes in the alert criteria because those earthquakes were in an unexpected area, prompting scientists to further refine the tsunami warning guidelines, said Gerard Fryer, geophysicist at the center.

The primary threat of a local tsunami is from shallow earthquakes near the base of Hawai'i's volcanoes, such as the 7.2 earthquake in 1975 and the estimated 7.9 earthquake of 1868. Both generated severe tsunamis, with the 1975 tsunami killing two campers at Halape on the coast south of Kilauea volcano.

Those shallow earthquakes are more likely to generate local tsunamis because they cause greater disturbance to the ocean floor, and it is abrupt movement of the ocean bottom that can generate the damaging waves, Fryer said.

Earthquakes are considered shallow if they are less than 20 kilometers deep, or less than about 12.5 miles down.

Present guideline calls for issuing a warning in the county where the earthquake is centered and the neighboring county — such as the Big Island and Maui, in the event of a Big Island earthquake — if the quake is 6.9 or larger, Fryer said.

That standard was adopted, in large part, based on observations of the magnitude 6.9 Kona earthquake of Aug. 21, 1951, which generated a tsunami about 3 feet high.

That size tsunami is considered the threshold beyond which there could be significant damage, so the 1951 earthquake led to the present practice of issuing a tsunami warning for earthquakes of magnitude 6.9 or larger.

The concern has been that a 6.9 magnitude earthquake that could generate a tsunami might be initially estimated to be smaller. That could prompt scientists to announce there was no tsunami threat when, in fact, a potentially damaging tsunami might have been generated.

To avoid underestimating an earthquake and its potential to produce a tsunami, scientists planned to lower the warning threshold to 6.7.

OCT. 15 CONCERNS

Fryer said state and county civil defense officials were open to the idea, but new concerns emerged when the Oct. 15 earthquakes shook the Islands.

Had the new standard been in effect then, the 6.7 temblor centered at Kiholo Bay would have met the trigger for a tsunami alert, when in fact there was no tsunami generated.

Fryer said scientists were confident, based on the location of the earthquake, that it had been deep, and was therefore highly unlikely to trigger a tsunami. The seismic record for that area near the northern slopes of Hualalai and the western slopes of Kohala suggests there were no shallow earthquakes there, he said.

That helped clinch the decision to announce there was no tsunami hazard, a judgment that was vindicated when scientists verified the quake was more than 18 miles deep, and no damaging wave had been produced.

But it also raised questions about the proposed new warning trigger, Fryer said. "The last thing we wanted with Kiholo Bay was to complicate the issue with a (false) tsunami warning," he said.

FALSE-ALARM GUARD

To guard against a false alarm, the new standard has been refined to take into account not only the magnitude of an earthquake but also its location and estimated depth.

Location is important because some areas, such as the west coast of the Big Island from South Point to Keahole Point, have such an extensive history of shallow earthquakes that a local tsunami warning should be issued immediately for any large earthquake there, Fryer said.

In the area around Kawaihae, the situation is very different because there "there's essentially no risk at all of a tsunami from an earthquake, and we want to build that understanding into the criteria," he said.

Fryer said the proposed changes in the warning criteria have now been debated for more than a year, and Fryer presented a paper on the subject at the American Geophysical Union fall meeting in San Francisco last month to solicit feedback on the latest changes from representatives of the U.S. Geological Survey.

The issue is now being studied by civil defense officials, but "we will not adopt new criteria until we know that they are happy with them," Fryer said.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.