6.7 quake's effects felt strongest in North Kohala
New data indicate that is where the ground shook most
HILO » The Oct. 15 Kiholo Bay earthquake was intense. But how intense?
"When you're going through fear, it seems like an eternity," said North Kohala resident Marilyn Bernhardt.
By now, almost everyone knows that the 7:07 a.m. quake was a magnitude 6.7, and that was followed by a magnitude 6.0.
But intensity is another matter, measuring the quake's effects, subject to personal feelings and local conditions. Scientists from the U.S. Geological Survey are still trying to get a handle on the varying intensities felt on the Big Island.
One of the oddities of the quake is that it took place at Kiholo Bay, 15 miles north of Kailua-Kona, but its greatest intensity appears to have been in North Kohala, 27 miles farther north.
USGS scientists know that because they asked people to send them e-mail reports describing what they felt and saw. Then they matched those reports with the Mercalli scale, a way of assessing intensity, less known than magnitude.
The Mercalli scale is divided into Roman numerals rather than regular numbers. According to the Federal Emergency Management Agency version of the Modified Mercalli Intensity Scale, in an intensity of VIII, "Houses that are not bolted down might shift off their foundations. Tall structures such as towers and chimneys might twist and fall."
In fact, an old smokestack once attached to a long-gone sugar mill in Kapaau collapsed.
Based on 2,947 responses, sorted by the zip code of the e-mail writer, the USGS determined that in North Kohala the quake had an intensity of VIII. Waimea (Kamuela) was also VIII.
Kailua-Kona, where the zip code extends to Kiholo, was slightly less at VII, described as "Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings."
Kailua-Kona is the site of Hulihee place, where a lot of plaster was knocked free, and some stones were loosened but did not fall.
Waimea and North Kohala probably got hit hard because they have a lot of deep soil, which magnifies the effects of an earthquake. Kailua-Kona sits on rocky ground, which dampens earthquake effects.
Scientists want a better way of measuring the intensity than public comment linked to zip code.
At the Geological Survey's Hawaiian Volcano Observatory, Scientist-in-Charge Jim Kauahi-kaua said most of the observatory's instruments were too sensitive to give an accurate reading of big quakes. But there are about 20 "strong motion" instruments on the island that can withstand such shocks.
The trouble is that only a few of those instruments send information directly by telephone to USGS offices in Menlo Park, Calif., for interpretation. Several, including one covering North Kohala, work on a photographic system in which the film has to be collected by hand and then shipped to Menlo Park, Kauahikaua said.
The result is that an objective map of the intensity on the Big Island is still incomplete.
As for how long did the earthquake last? Like magnitude, that's pretty exact. Rocks were breaking underground for about 15 seconds, said Gerard Fryer at the Pacific Tsunami Warning Center.
But how people experienced it is completely different.
"It just seemed like it went on forever," said Bernhardt in North Kohala. She guessed that the reality might have been 30 to 60 seconds.
Kauahikaua said the maximum shown by instruments so far has been 15 to 20 seconds, depending on location.
But Fryer said with energy bouncing around inside the earth, it could have seemed longer. "Everybody's been telling us it's a lot longer than 15 seconds," he said. "Everybody's right, depending on what you're describing," he said.
