August 2, 1996
A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.
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.
Here is a small version of the map showing earthquakes from 11 to 25
July, 1996. Select the image for a larger image (190 kbyte gif).
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.
Kilauea Eruption/Earthquake Update
The Kilauea 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.
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