45 quakes rock Loihi over 12 hours
HILO » Loihi seamount, the small underwater volcano off the southeastern coast of the Big Island, experienced a swarm of 45 small earthquakes Tuesday night and yesterday morning, the Hawaiian Volcano Observatory said.
The most likely explanation is "structural adjustment" of the earth's crust in response to the weight of the small mountain, said Steven Brantley, second in command at the observatory.
Number of quakes: 45
Time: 10 p.m. Tuesday to 9:48 a.m. yesterday
Magnitudes: 1.8 to 4.7
Breakdown: 16 quakes were 3.0 or bigger 45 quakes rock Loihi over 12 hours
The description refers to rocks breaking under stress, he said.
Loihi has not been erupting, and there is nothing to indicate that the swarm represents an eruption, he said.
The quakes started at 10 p.m. Tuesday and continued to 9:48 a.m. yesterday, according to the observatory Web site.
The biggest, with a magnitude of 4.7, was also apparently the deepest at 17 miles. Another was magnitude 4.0. A third earthquake during that time had a magnitude of 4.2, but it was on the Big Island near the lava flow area, unrelated to the Loihi swarm.
Brantley warned against relying on any of the preliminary numbers, which were posted on the Web site, since seismometers used to develop data are all on land at some distance from Loihi. The seismometers are also all on one side of the underwater mountain, so they provide no information from a different angle.
For example, the 4.7 quake was first thought to be north of Loihi's summit but was later calculated to be five miles east of the summit.
The 4.7 quake is similar in size to the 4.5 quake that originated near where lava is flowing into the sea and gave the island a short, sharp jolt on Nov. 29.
Both quakes are considered to be "light," Brantley said.
While the Nov. 29 quake originating under land was felt across much of the island, no one reported feeling Tuesday night's quake from under the sea, Brantley said. Police in the Kau district facing the Loihi area also said they received no reports.
Out of 45 quakes in the swarm, 29 were less than magnitude 3, a size that is not normally felt by people under any circumstances.
John Wiltshire, acting director of the Hawaii Undersea Research Laboratory of the University of Hawaii, who first studied Loihi in 1982, said such swarms are fairly common occurrences there.
In a 2003 description of Loihi, the Hawaiian Volcano Observatory said the underwater volcano has had swarms almost every year since 1980.
In 2003 the observatory described Loihi as a mystery for most people. But a lot of information was learned following a huge swarm of quakes in 1996 that lasted from mid-July to mid-August.
Scientists counted 4,377 quakes during that time, of which more than 100 were larger than magnitude 4.
Dives by a HURL submersible revealed that part of the summit of Loihi had collapsed, creating a crater. An area of hydrothermal or hot water vents called "Pele's Vent" had collapsed, but new hydrothermal vents formed.
The overall structure of Loihi is known to be a long ridge, thus earning the mountain its name, which means "long" in Hawaiian. Loihi also has steep sides, with slopes of 35 to 40 degrees being common, the observatory said.
The steep slopes are caused by "rampant" land-sliding, the observatory said. The overall effect is similar to the appearance of the Koolau Range, they said.
While periodic lava flows mean Loihi is growing, the little mountain is not expected to reach the surface of the sea for another 150,000 years, the observatory said.