The San Francisco Chronicle
When more than
2,200 people died after a 1998 earthquake in Papua New Guinea,
it wasn't the earthquake that killed them, but a massive undersea
landslide—triggered by the quake—that within minutes
sent 50-foot waves surging overthe shoreline, drowning unwary
victims and destroying their villages.
Similar submarine
landslides down unstable coastal slopes threaten many California
coastal communities, and the slides may not even require earthquakes
to touch them off, scientists agreed yesterday at a briefing during
the American Geophysical Union meeting in Moscone Center.
A global network
of tsunami warning centers are designed to signal whenever major
earthquakes occur anywhere around the Pacific's notorious "ring
of fire," where the land and sea are marked by almost constant
quakes and volcanic eruptions.
But predicting
the onset of the threatening underwater landslides when quakes
are not involved is a science barely in its infancy, and experts
are taking the first steps toward understanding the undersea origin
of the waves.
A group of
geologists and tsunami experts described how their high- tech
maps of the offshore sea bottom along coastal areas of California
are revealing potential hazards in areas where few have feared
major dangers in the past.
Geologist
H. Gary Greene of the Moss Landing Marine Laboratories, for example,
used two remotely operated submersible vehicles from the Monterey
Bay Aquarium Research Institute to map in detail the steep underwater
slopes that dip down from the shore facing the Santa Barbara Channel.
He discovered
clear evidence that at some unknown time in the past a truly monster
slump occurred off Coal Oil Point near Goleta. It was more than
six miles wide and nine miles long, and slid in three distinct
segments, from a depth of 300 feet down to 2,000 feet.
Whether the
slide was triggered by an ancient earthquake on one of the many
faults in the region, or on the more distant San Andreas fault,
is impossible to tell, Greene said. But his submersible craft
recovered samples of chemicals and bacterial mats in the area
that indicated the undersea slopes of earth and rock might well
have been weakened enough to slide without any earthquake at all
to trigger them, he said.
While major
earthquakes on either side of the Pacific can cause tsunamis that
cross the entire ocean within 12 to 24 hours, undersea landslides
can also generate local tsunamis as the surface of the ocean first
recedes and then surges forward to crash against the shore.
Such a slump
along the Santa Barbara coast could batter the shore with waves
as high as 50 feet if it were triggered by even a modest earthquake
on one of the many known faults in the Santa Barbara Channel,
according to Costas Synolakis, an environmental engineer at the
University of Southern California.
Canadian geological
engineer Jacques Locat created another scenario affecting the
Palos Verdes Peninsula, which stands above the Los Angeles region
with its scores of beaches and dense population, based on his
calculation of what could happen if a 300-foot-thick rockfall
were to slide down the peninsula's escarpment into the sea.
Plunging to
a depth of more than 2,000 feet, the rockfall would generate a
huge, surging wall of water—a local tsunami that would batter
the coast with a wave at least 100 feet high, Locat said, and
give people on the shore at nearby Long Beach just 24 minutes
to climb to safety.
What's needed,
at least for California if not for all the world's coastal nations,
the scientists agreed, is program to map the seabed wherever offshore
slopes dip steeply enough to pose the threat of submarine landslides
and the deadly tsunamis they are capable of generating.
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