The main earthquake that rocked the Hawaiian Islands Sunday morning was likely caused by surging lava and rock that built up a massive weight over many years, according to scientists here and on the Mainland.

The 6.7-magnitude quake was very unusual, centered deep below the ocean floor west of the Big Island, in a Pacific Plate fault zone that was previously unknown.

A second quake registered 6.0 just minutes later, and more than 80 aftershocks of magnitude-1.7 or greater continued to ripple yesterday.

The earth will likely continue to shift and settle, but it is unclear how serious a new quake could be, or exactly where it could be centered, said U.S. Geological Survey seismologist David Oppenheimer.

"Earthquakes trigger other earthquakes," he said.

But because quakes release stress beneath the earth's surface, it is unlikely that another serious temblor will strike the same fault zone soon, he said.

Seismic monitoring devices indicate that the main quake was centered nearly 40 kilometers, or 24.2 miles, beneath the ocean floor, according to the USGS.

The Big Island is a hotbed of volcanic activity, and its Kilauea Volcano has been erupting continuously since 1983, adding more than 500 acres to the island.

As lava and rock are forced to the earth's surface by volcanic activity over many years, the increased weight pushes down on the sea floor and earth's crust, causing movement farther below, said Oppenheimer, who oversees USGS seismic monitoring of northern California.

Sunday's main quake occurred so deep in the earth that it probably did not rupture the surface of the sea floor, he said.

The depth and intensity of the quake were unusual, and the fact that people felt it across the Island chain provides valuable information to scientists, he said.

"That gives us an indication of how the energy continues with distance, and that will build into our estimates of future ground-shaking," Oppenheimer said.

About 1,000 small earthquakes are detected every year around the Big Island, usually more than 13 kilometers below the earth's surface, but Sunday's initial quake was much deeper and stronger, said Cecily Wolfe, a seismologist at the University of Hawai'i-Manoa.

The Kealakekua Fault, which runs along the island's Kona Coast, is the major known fault that's closest to the new fault where Sunday's initial quake was centered, she said.

But the Kealakekua Fault ranges from 5 to 12 kilometers deep — much shallower than the new fault.

"We hadn't realized that an earthquake of this magnitude could occur in this region," Wolfe said. "But based on the past pattern of earthquakes, it's not surprising, because we don't know where all these deep fault zones are."

Sunday's second quake was much different from the first, and appears to have occurred at a much shallower depth.

The initial quake definitely occurred in the pre-existing oceanic mantel, rather than a growing volcano, and appears to be linked to stresses caused by volcanic activity known as loading and flexure, Wolfe said.

"But the mechanisms of this earthquake and its aftershocks are quite strange, and may not be quite consistent with that idea," she added. "There may be some other factors involved."

The Hawaiian Islands were created by growing volcanoes, and the Big Island is the tallest geographic feature on the planet when measured from the ocean floor, she noted. As volcanoes grow, they put more weight on the earth's oceanic lithosphere, or submerged crust.

The Big Island "is a huge feature, and the growing of this tremendous volcano in thousands of years definitely deforms, depresses and flexes the lithosphere and causes stresses," Wolfe said. "It's like putting a big weight on a beam, and the beam flexes."

Just as volcanic activity can cause earthquakes, earthquakes can cause volcanic eruptions, she said.

"It certainly has happened in Hawai'i in the past, and it's happened at other volcanoes where a large earthquake triggers an eruption," she said. "It's not always a big eruption, but it certainly is a type of interaction that has been scientifically observed before."

But if Sunday's quakes were to trigger an eruption, that would likely have already happened, she said.

The quake's direction and orientation — what scientists call the "mechanism" — was very unusual in Sunday's main quake, she and other scientists agree.

"It's a tremendously interesting earthquake," said USGS seismologist Jack Boatwright. "It's occurring in a place with a depth that's surprising, with a mechanism that we wouldn't have predicted, and finally, it produced quite strong ground motions."

In general, there are three types of earthquake faulting mechanisms: thrust, strike-slip and normal. Most Hawai'i quakes are normal faulting, meaning the strongest stress is vertical, and the weakest is horizontal. With strike-slip mechanisms, the strongest and weakest stresses are both horizontal.

The quake seems to have been an unusual combination of normal and strike-slip, and it appears to have occurred on a near-vertical fault, Boatwright said.

"It's quite hard to understand in Hawai'i," he said. "And it's particularly hard to understand deep below the earth's crust."

The main quake did not create a serious tsunami because it was so deep, and did not significantly change the shape of the sea floor, said Gerard Fryer, a geophysicist at the Pacific Tsunami Warning Center.

A small tsunami, measured at 4 inches, was detected by a tide gauge at the Big Island's Kawaihae Harbor about 10 minutes after the quake, he said.

"This thing changed the sea floor just a little bit," he said.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.