Wave warning
Tsunami risk on San Diego coast could be higher than previously
thought
By Robert Krier
STAFF WRITER
July 6, 2005
* Experts keep a nervous eye on the Indian Ocean
Despite last month's tsunami warning, Southern California will
never be another Banda Aceh, Indonesia.
The faults off our coast are not capable of a cataclysmic 9-plus
earthquake like the one that struck near the island of Sumatra late
last year. The quake, the second largest temblor ever recorded by a
seismograph, caused a massive uplift underwater. That earth
movement generated the tsunami that washed over thousands of
miles of coastline on two continents, killing more than 250,000
people.
Wave warning (PDF)
But while Southern California's geology differs from Indonesia's,
geophysicists who have studied local offshore faults and geology
say the coastal region is far from seismically benign, and it isn't free
from tsunami risks.
A question some of the experts are asking is: Could Southern
California be another Papua New Guinea? The answer to that
question could have major consequences.
Landslide danger
In 1998, a 7.1 temblor, minuscule compared with the Sumatra
quake, struck Papua New Guinea. The quake triggered a large
underwater landslide, which in turn generated a tsunami.
Submarine landslides disturb and set in motion the overlying water
column as sediment and rock slump downslope and are
redistributed across the sea floor.
The wave that hit the coast of the South Pacific nation a few
minutes after the quake was nothing like the monster in the Indian
Ocean. Major damage in Papua New Guinea was confined to an area
only about three miles wide. But still, the wave exceeded 30 feet
and killed 2,200 people.
Jose Borrero, a tsunami expert at the University of Southern
California's Tsunami Research Center, has traveled to Papua New
Guinea, Banda Aceh and around the world to study tsunami
aftermaths. He says the geology off our coast could create
something similar to the New Guinea tsunami.
Associated Press
Villagers from Sissano in Papua New Guinea searched through
debris left behind after a tsunami struck in 1998. Some researchers
believe the Southern California coast could produce an equally
devastating tidal wave.
"The main thing is getting a consistent message out to the public,"
Borrero said. "It's not going to be another Aceh, but it can be
potentially bad."
Some of our offshore faults could produce 7.0 or larger quakes, and
those quakes, like in Papua New Guinea, could trigger landslides,
Borrero believes. The resulting tsunami would likely be narrow in
width, but in a worst-case scenario, as high as 30 feet in places.
If such a wave were to strike in heavily populated and urbanized San
Diego County at the wrong time and wrong place, tsunami modelers
believe there would be very little warning, the death toll could be
greater than 2,200 and damages could be in the billions of dollars.
Much of our coastline would be protected by high bluffs, but heavily
trafficked, low-lying coastal areas – such as Lindbergh Field,
SeaWorld and the Del Mar Racetrack – could be inundated.
Such events are what tsunami experts call "low probability but high
impact." The problem for Borrero and other experts is getting hardto-obtain data on the offshore topography and using the scant
records of past events to determine just how high that probability is
in Southern California.
"We absolutely just don't know how to quantify the risk," said
Graham Kent, an associate research geophysicist at the Scripps
Institution of Oceanography who also studies tsunamis. "But we
know in our guts that there is a risk."
Danger overstated?
However, other geologists say that while tsunamis can happen in
Southern California, the waves pose much less of a danger than
quakes, fires, floods and onshore landslides.
Lucy Jones, scientist-in-charge at the U.S. Geological Survey (USGS)
office in Pasadena, said the Indian Ocean event has raised local
tsunami concern, perhaps beyond what is warranted.
"People respond to the disaster du jour," she said. "Nobody's
completely safe, that's for sure. If you live on the coast, it's
something that you have to be aware of. But tsunamis are the least
likely (natural disaster) to kill you.
"If you were asking me in Seattle or Humboldt Bay, I'd say there's a
significant (tsunami) risk, and wake up. But San Diego is definitely
less likely to be affected by a tsunami than the Northwest."
Jones said that coastal residents in the Northwest face a greater
tsunami risk due to their proximity to the Cascadia Subduction
Zone offshore. The seismic potential in the Cascadia zone rivals
that of the area off Sumatra.
Subduction zones, where one tectonic plate is being pushed
beneath another, create very long fault lines capable of far larger
quakes than the comparatively small faults off our coast. A quake
along a subduction-zone fault can thrust up a huge chunk of land
hundreds of miles long, which can set in motion a corresponding
column of water above in the form of a tsunami.
A quake on the scale of the Dec. 26 event in the Indian Ocean is not
only possible off the Northwest coast, it is fairly likely within the
next 200 years. Quakes in the 9.0 range happen in that area on the
average of every 500 years; the last occurred in 1700. Much of the
coasts of Washington, Oregon and Northern California could be
devastated by the resulting tsunami.
Even geologists who say our local tsunami threat has been
underplayed acknowledge that while a deadly wave might hit
tomorrow, it also might not strike the region for thousands of
years.
"Right now, it's difficult to know where we are in the cycle," Kent
said.
Bends in faults
Mark Legg, a geophysicist and president of Legg Geophysical in
Huntington Beach, has studied Southern California's offshore faults
for decades. He and Borrero have created tsunami models and
helped assemble the state Office of Emergency Services' coastal
inundation maps for Southern California. Legg has also advised
state and local emergency services officials on tsunami mitigation.
He believes many people, including some in the field, have
incorrectly dismissed the local tsunami danger.
"I've heard people say there's no tsunami danger in San Diego,"
Legg said. "That's B.S."
Most of the faults off our coast are strike-slip in nature, which
means the dominant movement in the fault zone is horizontal, with
one tectonic plate moving north and the other south.
The West Coast and Alaska Tsunami Warning Center's Web site
states that "Earthquakes which produce horizontal sea floor motion
will generate little to no tsunami." But Legg says it's not that black
and white, because strike-slip faults often have zones with
movement that resembles those in thrust faults like those in the
Northwest.
For many years, Legg said, he's been battling the perception that
you don't get a tsunami with a strike-slip fault. "That's wrong –
dead wrong," he said. "There's lots of evidence that strike-slip
faults have generated tsunamis, but it's been overlooked."
Legg said strike-slip faults often have bends or irregularities –
points where earthquake stresses build up. At restraining bends,
the seafloor is squeezed up; at releasing bends, it is pulled apart
and drops down. Both create vertical movement like that in thrust
faults.
In a 2003 report funded by the Federal Emergency Management
Agency, Legg, Borrero and USC tsunami expert Costas Synolokis
noted that there are major restraining bends in several faults off
Southern California.
Legg, who has examined offshore topography up close in small
submersibles, said he has seen underwater fault scarps, or areas of
uplift caused by slips in faults, that were 3 to 10 feet high – which
rivals the surface movement seen after the 7.3 quake in Landers in
1992. He maintains that the upthrust at those scarps could have
generated tsunamis that matched the scarps in height.
In 1994, a 7.1 quake on a strike-slip fault on Mindoro Island in the
Philippines generated an estimated 21-foot tsunami that killed 41
people. And Legg said there are many other examples of strike-slip
faultscreating tsunamis, including an 8.1 quake around Macquarie
Island near Antarctica on Dec. 23, 2004, just three days before the
Sumatra quake. The 1906 San Francisco quake also generated a
tsunami, although in both of the latter instances, the tsunamis were
small and little or no damage was reported.
Legg said the local offshore faults alone could generate quakes that
could propagate tsunamis of around 16 feet. If a landslide similar to
that in Papua New Guinea also occurred with the quake, the tsunami
potential locally would be about 30 feet, he said.
But Eric Geist, a geophysicist with the USGS office in Menlo Park
who has done tsunami modeling and studied the Papua New Guinea
event, said there is still much to be learned about landslidegenerated tsunamis. Also, he said, an offshore landslide will not
automatically create a tsunami.
"It depends on the speed of the slide," he said. "It has to be fastmoving to be tsunamigenic."
Telltale deposits?
There are no historical records of a large tsunami before 1850 in
San Diego County, but Legg believes deposits in North County's
Batiquitos Lagoon suggest a 25-to 29-foot wave may have struck
before that date. Attempts to date the deposit have been hampered
by contamination.
In 1862, a tide-gauge engineer named Andrew Cassidy observed a
small tsunami near what is now the Scripps Institution in La Jolla.
The wave occurred in conjunction with a magnitude 6 quake that
caused onshore landslides and may also have generated offshore
slides. The wave caused a beach run-up of about 4 feet, according
to Cassidy's descriptions, but it's unclear if he meant the water
merely went 4 feet farther inland, or the wave was 4 feet high.
Locally generated tsunamis of any size should occur on average
about once every 100 years in Southern California, Legg estimates,
if recent past history is a guide. A 10-to 13-foot tsunami was
recorded north of Santa Barbara after a 7.7 quake in 1812, and a
nearly 6-foot wave was seen at Port San Luis after a 7.3 quake at
Point Arguello in 1927.
Based on the observed slip rate(the rate at which two sides of a
fault move against each other) of 5 millimeters a year on the San
Clemente fault (along the west side of San Clemente Island, and
north and south of the island), Legg says that a large quake along
that fault should hit every few hundred to few thousand years.
When the quake potential of all local offshore faults is considered,
Legg said, the frequency of a large, damaging tsunami in Southern
California could be as often as once every 500 years.
Geophysicists know that there are steep slopes off the coast in La
Jolla Canyon, Coronado Canyon and a canyon off the Carlsbad
coast. Those canyons might produce an underwater slide after a
quake, Legg said. And there is already evidence of large landslides
at the 30-mile and 40-mile banks off our coast, he said, although
it's difficult to determine the age of those slides.
"If that happened again, it could generate a big tsunami, but the
return period (frequency) could be in the tens of thousands of
years," he said. The potential for an offshore slide triggered by a
quake could be greater after a very wet year like the one just
passed, he said, because runoff and flooding build up large
amounts of unstable sediments at the heads of the submarine
canyons. And the longer the time between strong offshore quakes,
the more sediment that can build up.
But Geist said the Southern California coast does not appear to pose
an unusually high risk of landslide-generated tsunamis.
"You could argue for almost every coastline that you might get a
landslide," Geist said. "I don't think there's any place on Earth that is
free from tsunami risk."
Newfoundland tsunami
There have been other episodes of landslide-generated tsunamis,
besides the event off Papua New Guinea in 1998. In 1929, a similar
scenario unfolded in the North Atlantic. John Orcutt, deputy director
for research at the Scripps Institution of Oceanography, testified
about tsunami preparedness before the U.S. House of
Representatives Jan. 26 this year.
Orcutt said a 7.2 earthquake triggered a massive underwater
landslide in the Grand Banks, off the coast of Newfoundland,
Canada. The landslide broke trans-Atlantic cables and generated a
tsunami that raised the sea level more than 20 feet in some areas
and as much as 80 feet at the head of narrow bays on
Newfoundland's Burin Peninsula. Twenty-seven people were killed
by the waves.
In both the Papua New Guinea and Grand Banks tsunamis, the
waters offshore were deeper than they are off our coast, Borrero
said, and those deeper waters may have helped magnify wave
height.
Distant-source tsunamis generally pose much less of a risk in
Southern California. Borrero said Southern California is largely
protected from huge tsunamis generated far away because of how
our area is oriented toward likely quake sources in Asia, South
America and Alaska. However, the 1960 Chilean quake did create a
tsunami that caused an estimated $1 million damage in Los Angeles
and Long Beach harbors, and lesser damage in San Diego.
"We're not completely protected, but we're not looking into the
barrel of a gun, either," Borrero said.
Lack of information
Legg and Borrero said there has not been enough study of local
offshore faults and landslide potential to come up with more solid
answers about the risks, particularly off the San Diego County
coast.
"For tsunami modeling, we're really pretty far behind," Legg said.
"We know a great deal about the surface of Mars, but we know very
little about our offshore topography."
Offshore faults don't get as much attention as onshore faults,
Borrero said, because they tend to be less active than onshore
faults, and they are far more difficult to get to and more expensive
to research. It's comparatively easy to dig a trench that would
quickly yield centuries of slip data for a fault onshore.
Geist said scientists are making strides in understanding events
after they happen, and in creating models that show worst-case
tsunami scenarios.
"But when you get into howlikely it is," Geist said, "that's a much
more difficult and time-consuming effort."
Kent, the Scripps geophysicist, said scientists still have a long way
to go to clarify the dangers.
"We have to develop techniques and equipment that don't exist
today," he said."As a society, we have to find a better way to
quantify the risk.
"We really don't know. Some people think there is a low (tsunami)
risk, but I don't think that's the attitude to take."
WAVE WARNING
Locally generated tsunamis
Although a tsunami like the one in Indonesia last
year will probably never happen in Southern
California, some researchers think our coastal
geography could create a smaller-scale tsunami,
with potentially devastating consequences.
Tsunami risk on San Diego coast
could be higher than previously thought
LOS ANGELES
COUNTY
Restraining bends
MALIBU
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Pacific Ocean
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Del Mar
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SAN
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Mission
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SeaWorld
San Diego County
KEY
Fault movement
MAJOR FAULT
Populated area
Restraining
bend
Suspected
restraining bend
Although most of
the San Diego
area is protected
by high bluffs,
heavily trafficked,
low-lying coastal
areas, such as
Lindbergh Field,
SeaWorld and the
Del Mar Race
Track, could be
swamped.
Lindbergh
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CORONADO
San
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San
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USA
MEXIC
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Villagers from
Sissano in
Papua New
Guinea searched
through debris
left behind
after a tsunami
struck in
1998. Some
researchers
believe the
Southern
California
coast could
produce an
equally
devastating
tidal wave.
DO B
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Area of displacement
during earthquake
ONA
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Plate
Plate
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During an earthquake, restraining
bends shift vertically. Researchers
believe the restraining bend
movement could displace enough
water to cause waves. If the quake
also generates an underwater
landslide, a narrow but damaging
tsunami could result.
SAN DIEGO
DIEG
COUNTY
CARLSBAD
Displaced
water
Restraining bend
RIVERSIDE
COUNTY
OCEANSIDE
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San
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Vertical
faults
LAGUNA
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When a strike-slip fault curves, a
restraining bend or releasing bend can
occur. Restraining bends push the
seafloor upward, a phenomenon that
created Catalina Island. Releasing
bends pull apart — the mechanism that
created San Diego Bay.
Pe
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Santa
Catalina
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Detail area
below
Releasing bend
Long Beach is the
nation’s second-busiest
port and 12th-busiest
in the world. More than
$95 billion worth of
cargo passed through
the port in 2003.
LONG
BEACH
S
Sa
San
an
Pedroo Bay
Pedro
RO
Restraining bend
D
Port of Long Beach
D
SV
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NP
SA
Most of the faults off the California
coast are strike-slip faults, where
plates move predominately
northward and southward.
PA
O
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Plate
LE
Nearly 3,000 ships
and 800,000 cruise
passengers pass
through annually; the
busiest port in the
nation and eighth
busiest in the world.
Fault line
LOS
ANGELES
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-IN
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Port of Los Angeles
STAFF WRITER
O
WP
Plate movement
By Robert Krier
NE
SANTA
San
ta
MONICA
Mo
nic
aB
ay
Large bends in fault lines might
cause tsunamis off the West Coast.
A look at how it could happen:
10 miles
Associated Press
espite last month’s
tsunami warning, Southern California will never
be another Banda Aceh, Indonesia.
The faults off our coast are
not capable of a cataclysmic 9plus earthquake like the one
that struck near the island of
Sumatra late last year. The
quake, the second largest
temblor ever recorded by a
seismograph, caused a massive uplift underwater. That
earth movement generated
the tsunami that washed over
thousands of miles of coastline on two continents, killing
more than 250,000 people.
But while Southern California’s geology differs from Indonesia’s, geophysicists who
have studied local offshore
faults and geology say the
coastal region is far from seismically benign, and it isn’t
free from tsunami risks.
A question some of the experts are asking is: Could
Southern California be another Papua New Guinea?
The answer to that question
could have major consequences.
Landslide danger
In 1998, a 7.1 temblor, minuscule compared with the
Sumatra quake, struck Papua
New Guinea. The quake triggered a large underwater
landslide, which in turn generated a tsunami. Submarine
landslides disturb and set in
motion the overlying water
column as sediment and rock
slump downslope and are redistributed across the sea
floor.
The wave that hit the coast
of the South Pacific nation a
few minutes after the quake
was nothing like the monster
in the Indian Ocean. Major
SEE
SOURCES: Legg Geophysical; U.S. Geological Survey; University of Southern California; ”Structural Geology,” Twiss, R.J. and E.M. Moores (1992) ; Port of Los Angeles; Port of Long Beach; ESRI; TeleAtlas
Tsunami, F4
MATT PERRY / Union-Tribune
.
Tsunami possible but unlikely along San Diego County coast
By: QUINN EASTMAN and JO MORELAND - Staff Writers
NORTH COUNTY ---- Although it is highly unlikely, a smaller,
locally generated version of the tsunami that devastated South
Asian countries Sunday could occur along coastal San Diego
County, geologists say.
An earthquake with a magnitude above seven in the San Clemente
fault zone 50 miles southwest of Point Loma could produce a rapid
5- to 10-foot rise in sea level, according to Mark Legg, a geologist
in Huntington Beach. The wave would take just 20 minutes to arrive
at the coast from its origin, too fast for an official warning from the
state Office of Emergency Services.
"It wouldn't be like 5-foot surf," Legg said Monday. "It's a 6-foot
surge that would sweep away cars and debris."
The historical record of tsunamis hitting Southern California is
sketchy. A 1927 earthquake near Lompoc produced a 5- to 6-foot
tsunami that hit the coast near San Luis Obispo, and an earthquake
near Santa Barbara in 1812 also caused a tsunami, according to a
University of California, Santa Barbara website.
The largest sudden rise in sea level ever recorded in the San Diego
area was 3.5 feet at Scripps Institution of Oceanography pier in
1960, caused by a magnitude 9.5 earthquake in Chile, the strongest
ever recorded. Sunday's earthquake caused the sea level to fluctuate
almost a foot in San Diego and nearly 9 feet in one city on the
Mexican west coast, according to the Pacific Tsunami Warning
Center.
To protect against such surges, the National Oceanic and
Atmospheric Administration of the National Weather Service placed
buoys attached to sensors on the ocean floor in the early 1990s to
track tsunami activity off the west coasts of North and South
America.
There are three buoys off the Aleutian Islands and one each off
Oregon, Washington and South America, the areas most likely to
have tsunami activity first, said Willard Lewis, assistant director of
the San Diego County Office of Emergency Services.
The state Department of Water Resources and the Office of
Emergency Services developed dam inundation maps showing
where tsunami waters would flow as they followed local water ways.
In the event of a tsunami, residents would be alerted and warned by
the county through the Emergency Alert System on radio and
television, Lewis said. The county would staff its emergency
operations center in Clairemont Mesa to help cities affected, he
said.
But anything as massive as the Asian tsunami would have lifeguards
doing all they could to save themselves, said Chief Ray Duncan of
the Oceanside Lifeguard Service.
"We would be most vulnerable at the San Luis Rey River, Loma Alta
Creek at Buccaneer Beach, and Buena Vista Lagoon leading to the
large (Westfield Shoppingtown) mall," said Ray. "We have houses
(rather than) bluffs (along the beach). If something was big enough,
it would go right over Pacific Street (just above the beach), possibly
(up to two miles) to Interstate 5."
The lifeguards have had indoor drills aimed at dealing with the
effects of tsunamis. Coastal fire officials also said they have plans
to try to cope with tsunamis.
There have been tsunami watches along the county's coast
infrequently in the past, but nothing significant has happened in
recent memory, authorities said.
The problem in this county, officials said, is trying to convince
people to stay away from the beach when big waves are expected
because everyone wants to see them, officials said.
"We actually had people expecting the big one, and we had people
going out on surfboards and down to the beach to watch this," said
Encinitas Fire Chief Don Heiser. "We can evacuate people. We can
warn people. Bottom line is people need to be responsible for their
own safety."
The bluffs in the Encinitas area will provide some protection, but
people need to have an emergency preparedness kit with them that
will allow them to survive for 72 hours without help from
authorities, Heiser said.
"At times we have gone out into the low-lying areas and tried to get
people out of (the way of high waves)," said Carlsbad Battalion Chief
Rick Fisher. "They don't pay any attention."
Three lagoons --- Buena Vista, Agua Hedionda and Batiquitos ---would be the low-lying danger points for tsunamis along the
Carlsbad coast, Fisher said.
He said two of the city's fire stations might be threatened in the
coastal area. After warning residents, firefighters would have to get
their rigs to higher ground.
"We can't help anybody if we lose our equipment," Fisher said.
When it comes to tsunamis along the San Diego County coast, the
San Clemente and San Diego Trough fault systems are the nearest
active earthquake sources. They are strike-slip faults, where the
two plates move past each other mostly horizontally.
Geologists have classified subduction zones ---- places where one
plate dives down below another ---- as more dangerous for
generating tsunamis than strike-slip faults. The massive earthquake
Sunday came from a spot in the middle of the Indian Ocean where
the India plate plunges under the Burma plate.
"You need the up and down motion of the sea floor to generate the
tsunami," Legg said.
In a 2002 report to the United States Geological Survey, Legg
argued that bends in the faults along the Southern California coast
that restrain the plates from moving horizontally could generate
seafloor uplift. Investigation of the seafloor in the San Clemente
fault zone with the scientific submarine Alvin shows scarps, or
cliffs, that are probably less than 1,000 years old that came after
earthquakes, said Legg, an adjunct professor at San Diego State.
In a worst-case scenario, an earthquake could trigger an
underwater landslide that would amplify the tsunami generated by
the original earthquake, said Chris Goldfinger, a marine geologist at
Oregon State University.
New York Times news services contributed to this article.
Contact staff writer Quinn Eastman at (760) 740-5412 or
[email protected]. Contact staff writer Jo Moreland at (760)
740-3524 or [email protected].
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