Under Makawao is more likely to be quake origin

Make Sunday's event the Makawao Earthquake.

After human analysis of seismic reports, the Hawaiian Volcano Observatory has moved the focus of the temblor from Kaupo to Makawao, and placed it much deeper in the Earth's crust.

The magnitude of the earthquake is still rated at 3.5 - minor. No damage was reported.

Janet Babb, spokeswoman for the observatory on the Big Island, said computer programs analyze automated reports from a variety of sensors and generate a preliminary estimate of the strength, location and depth of an earth movement.

This goes up at the observatory's Web site within minutes. But until the site reports that the data have been reviewed by a human analyst, the report should be regarded as preliminary.

The computer "does a pretty good job," said Babb, although it is usually better for the Big Island, where the sensor network is more extensive.

The computer initially thought the quake was shallow, about two miles down, and on the south slope of Haleakala. The human analysis places it 19 miles deep, two miles east-southeast of Makawao and four miles east of Pukalani.

The areas where the quake is felt also are tabulated automatically from submissions to the observatory's Web site.

This tends to overstate where the quakes are generally felt, because if one person notices, that whole ZIP code is marked as an area where the shaking was noticeable. For East Maui, ZIP codes cover a lot of territory.

And it depends what a person's situation is when the quake happens. The type of building construction affects whether shaking will be noticed: Houses on stilts, which are common Upcountry, quiver more than houses on slabs.

That is why the observatory welcomes as many reports as possible. And, Babb said, even reports that a quake was not felt can help the analysts interpret what the effect was.

The computer compares the times when signals arrive with sensors spaced around the world. The tiny differences in arrival times allow triangulation to determine the location, but quakes create different types of waves, which travel at different speeds. Also, the same wave changes speed depending on what kind of rock it is passing through.

Thus, it takes an experienced person to tease out the subtleties among the numerous input signals.

The observatory rated the quality of the source data as "poor," but that means there was comparatively little of it, not that the sources themselves were imprecise.

* Harry Eagar can be reached at heagar@mauinews.com.