Seismology reveals volcano’s past, sheds light on future dangers

Seismology
reveals volcano’s past, sheds light on future dangers

BY JADE BOYD
Rice News staff

The Hawaiian
Islands are home to the largest documented shoreline collapse
in history, an ancient seaward landslide that sent rocks
from the island of Oahu to sites more than 100 miles offshore.
The avalanche of debris from the northeast shore of Oahu
probably occurred between 1.5 million and 3 million years
ago, and it undoubtedly created one of the largest tsunamis
in Earth’s history, a wave large enough to inundate
every coastline of the northern Pacific Ocean.

Today, geologists
are studying whether seismic and tectonic forces are creating
the potential for a similar disaster on the southeast shore
of the big island of Hawaii, near Kilauea volcano. The world’s
most active volcano, Kilauea is continually growing larger.
At the same time, its seaward flank is moving toward the
Pacific, currently at the rate of about 10 centimeters per
year. Kilauea’s movement takes several forms. Layers
of lava and sediment atop the mountain are pulled down by
the force of gravity. The entire mountain itself also moves
slowly out to sea as magma derived from deep within Earth’s
mantle intrudes into the core of the volcano.

“From previous
studies, we know that Kilauea is the site of an active landslide,
the Hilina slump, which has moved in historic times,”
said Julia Morgan, assistant professor of Earth science
at Rice University. “We now recognize that Kilauea
also experienced a catastrophic landslide in the past, possibly
within 25,000-50,000 years, which is quite recent in geologic
terms.”

The 10-by-15-mile
Hilina slump is partially detached from the seaward flank
of Kilauea and is thought to be a candidate for catastrophic
collapse. At the fall meeting of the American Geophysical
Union in San Francisco, Morgan presented new findings that
the debris left over from the last catastrophic landslide
on Kilauea is forming a buffer that stabilizes the Hilina
slump. Morgan and her colleagues, Gregory Moore at the University
of Hawaii and David Clague at Monterey Bay Aquarium Research
Institute (MBARI), reached this conclusion after a comprehensive
analysis of two offshore seismic and seafloor mapping surveys
conducted in 1998 by the Lamont-Doherty Earth Observatory
and MBARI.

They found that
the most recent collapse on Kilauea involved a detached
piece of the mountain similar in size to the Hilina slump
and located immediately to its northeast. When this section
of the volcano slid away, it settled beneath the ocean at
the base of Kilauea. As the entire volcano grew and slid
oceanward, this debris piled up, much like snow piles up
in front of a snowplow. The result is a broad, bench-like,
submarine structure that sits at the foot of the mountain,
about 15-20 miles off the coast. The downslope edge of the
Hilina slump now impinges on the outer bench.

“Based on
what we’ve seen, we believe that the outer bench is
still growing, and we expect that the buttressing effect
it exerts on the Hilina slump will increase accordingly,”
Morgan said. “This interaction reduces the likelihood
of catastrophic detachment of the Hilina slump under present
conditions.”

However, because
the outer bench contains a good deal of loose sediment and
debris, it is also subject to catastrophic failure. For
instance, the bench is riddled with small-scale faults and
fractures. A massive volcanic eruption or a large earthquake
like the 7.2-magnitude temblor that hit Hawaii in 1975 could
shake the outer bench to pieces. Morgan said there is geologic
evidence that something similar occurred on nearby Mauna
Loa about 100,000 years ago.

The research
was funded by the National Science Foundation, with additional
support from Landmark Graphics Corp.