Scientists aren't sure what caused two large earthquakes yesterday off the North Kona coast in an area not known for producing high-powered seismic activity.

The first earthquake, 6.6 magnitude, occurred at 7:07 a.m. 24.2 miles below Kiholo Bay on the South Kohala Coast, with a 5.8-magnitude quake occurring seven minutes later about 13 miles northwest of Kawaihae and 11.7 miles deep.

Since 1960, only 31 quakes larger than magnitude 4 have been recorded in that vicinity, said Jim Kauahikaua, head of the U.S. Geological Survey's Hawaiian Volcano Observatory on the Big Island.

The largest previous earthquake, with a 4.8 magnitude, occurred May 14, 1982, about 2.5 miles west of Kawaihae, he said.

"This area of 5 or 6 (magnitude) is new territory," Kauahikaua said.

The earthquakes and resulting aftershocks apparently did not affect the Big Island's other volcanoes, as Kauahikaua reported no change in deformation or other activity at Mauna Loa or the Hualalai volcano in West Hawai'i, and no interruption in the Kilauea eruption, now in its 23rd year.

He also said the Big Island quakes do not appear to be connected to a recent series of other seismic events around the state.

On Sept. 4, a 3.3-magnitude quake occurred off Waikoloa on the Big Island. Two other quakes, each measuring a little more than a 3 magnitude, struck Aug. 31 on the Big Island. On Aug. 28, O'ahu and Moloka'i were shaken by a 3.8-magnitude quake 26 miles east-northeast of Waimanalo. A 4.5-magnitude quake on July 27 off the Big Island was felt as far away as Kaua'i.

Hawai'i is among the top states when it comes to seismic activity, ranking behind Alaska and California, according to the USGS. Because of its active volcanoes, most are generated on the Big Island, which experiences tens of thousands of earthquakes annually.

Earthquakes commonly occur ahead of lava outbreaks at the surface, or in rift or fault zones away from the principal areas of magma movement. The ground can also be set to shaking from the periodic settling of the ever-growing mass of the Big Island.

"We do have some stress put on the Earth's crust just from all the weight of the island that needs to go somewhere occasionally," said geologist Steven Lundblad, a visiting professor at the University of Hawai'i-Hilo. "It's hard to know what caused this one. It's not a place where we expect a lot of magma. It's hard to know if there's a rift zone that goes out that way. Fault zones are defined by having motion on them, and there hasn't been much motion in that area, so there's no reason to know if there is anything there."

Kauahikaua said although it is not known if the Mauna Loa or Hualalai rift zones played a role in yesterday's earthquakes, "somehow it's related to the volcanoes; we just have to figure out how."

Fifty-five smaller aftershocks were recorded within three and a half hours of the first quake, Kauahikaua said. The largest occurred at 10:35 a.m. and measured 4.2 magnitude.

As for the likelihood of another large quake occurring soon in the same area, both Kauahikaua and Lundblad said that generally the number and severity of aftershocks go down over time.

However, unlike other natural phenomena such as tsunamis and hurricanes, scientists are unable to provide advance warning of earthquakes.