Volcano Watch — A massive earthquake swarm at Lo`ihi Seamount

It was only 42 years ago that Lo`ihi and four other seamounts were discovered during a bathymetric survey of the area south and southeast of the Big Island by the U.S.S. Patapsco. Hawaiian language authorities Mary Pukui and Martha Hohu provided the Hawaiian names for the five seamounts - Papa`u, Lo`ihi, Wini, `Apu`upu`u, and Hohonu. The names were based upon the physical characteristics of the seamounts, and Lo`ihi, as the name implies, is an elongated volcano.

Lo`ihi has a summit caldera and two rift zones which radiate to the north and to the south. There are two pit craters within the summit caldera, which is about 3,200 feet below the surface of the ocean. The base of Lo`ihi is difficult to determine because of submarine landslides and the dip of the sea floor down to the Hawaiian Deep. The height of Lo`ihi is between 10,000 and 13,000 feet, depending upon where the base is placed.

The first earthquake activity definitely attributed to Lo`ihi occurred in 1971, but a portion of a large, offshore swarm in 1952 may have originated there. Lo`ihi experienced more earthquakes in 1972 and 1975, and in 1978, the first dredge samples and photographs of Lo`ihi were obtained by a team of geologists from the U.S. Geological Survey. The fresh, glassy crusts of the rocks indicated the youthful age of the lava and furnished definitive proof that Lo`ihi was an active volcano. Surveys by University of Hawaii scientists in 1980 and 1981 acquired additional dredge samples and photographs that showed the presence of fresh lava flows in the south summit region.

The present swarm of earthquakes from Lo`ihi started on July 16. From then to the morning of August 2, the U.S. Geological Survey's Hawaiian Volcano Observatory seismographic network has recorded 4,229 earthquakes. Ninety five of the temblors had magnitudes of 4.0 or larger. Three of the largest quakes have been reported felt by residents of Ka`u. A magnitude 4.9 shaker was felt on July 23 at 3:25 a.m., and another magnitude 4.9 earthquake shook residents at 7:38 a.m. on July 24. The largest felt earthquake of the swarm registered a magnitude 5.0 on July 27 at 11:30 p.m. 

Researchers at the University of Hawaii are responding to the current seismic activity by scheduling a survey of the area this week. Detailed SeaBeam sonar and sonobouy data will be gathered, together with water temperature and chemistry measurements. If conditions allow, the submersible Pisces will make three dives to Lo`ihi. By the next article, we hope to know the cause of the earthquakes.

Volcano Activity Update

The Kīlauea eruption continues unabated, and flows enter the ocean in the Lae`apuki region. The lava pond within Pu`u `O`o was sluggish during the past week, and the level was about 295 feet below the lowest part of the rim. A glow from the pond reflecting off the fume cloud over the cone often can be seen at night.

The three large Lo`ihi earthquakes mentioned above are the only ones reported felt during the past two weeks.

If you feel an earthquake, we would be interested in receiving that information. Please call HVO at 967-7328 and tell us when and where you felt the Earth move.