Tsunami !
• A tsunami (sue-naa-me) is scientifically described as a
series of very long wavelength ocean waves caused by the
sudden displacement of water by earthquakes, landslides,
or submarine slumps and are mostly caused by
earthquakes of magnitude 7.5 or greater. Undersea
landslides and volcanic eruptions can also cause tsunamis.
They travel in the open ocean at speeds of 600-800
kilometers per hour, but with wave heights of just a few
centimeters. As they approach shallow water they begin to
slow down and their wave heights may increase by several
meters. Though devastating, this kind of a tsunami is very
rare. Much more common are tsunamis that strike in the
form of a steadily rising high tide. There are no towering
wave and the water rises rapidly, flooding low-lying areas,
then receding. Often wrongly referred to as "Tidal Waves"
they have nothing to do with tides!
• The largest know tsunami in
recent times occurred in the
Pacific Ocean in 1960 and
was spawned by an MW9.5
earthquake in Southern
Chile. It caused damage and
casualties in many countries
along the Pacific Rim, as far
as Japan. The eruption of
Krakatau in 1883 caused a
great tsunami in the Indian
Ocean which killed 35,000
people. It affected harbors
at Chennai and Colombo
and as far as the Cape of
Good Hope.
Aftermath of 1960 Chilean Tsunami in Hilo,
HI where 61 deaths and $23 million in
damage occurred. Parking meters were bent
by the force of the debris-filled waves.
Aerial View of Chilean Coast after 1960 Tsunami, showing tsunami
damage and wave extent. Two hundred deaths were reported and
total property damage from the combined effects of the earthquake
and tsunami in Chile was $417 million
small amplitude in open ocean
displaced
water
offset seafloor
earthquake
large amplitude in shallow water
• In the Alaskan Earthquake
of 1964 a section of
seafloor more than four
hundred miles long and one
hundred miles across
suddenly arched upward,
forcing the overlying water
upward and outward as if
the seafloor were a giant
paddle. Although this
happened almost
instantaneously, it was not
the speed of the uplift but
the sheer volume of water
displaced that produced the
powerful tsunami.
Formation of a tsunami by sudden offset of the seafloor.
Wave has low amplitude as it travels in the deep sea, and
grows as tsunami enters shallow water and approaches
the land
• A tsunami generated by a
sudden change in the deep
ocean floor is a wave that
extends from the sea
surface to the bottom, miles
below. The wave travels at
great speed, 500 miles an
hour or faster, depending on
the depth of water. The
waves traveling down the
coast are slower than those
in the open sea, so that the
wave front makes a
sweeping turn to the left
and attacks the coast almost
head on.
Tsunami Magnitude (Mt)
The tsunami magnitude, or Mt, is a number used to
compare sizes of tsunamis generated by different
earthquakes and calculated from the logarithm of
the maximum amplitude of the tsunami wave
measured by a tide gauge distant from the tsunami
source.
Damage Resulting from 1964
Tsunami in Alaska (March 27,
1964). View of the north end of
Resurrection Bay at Seward,
Alaska, about 75 km from the
epicenter.
Extent of tsunami
Aerial view of Valdez, Alaska, showing the extent of inundation
along coastline. During the earthquake, the shaking caused failure
of the unstable, water-saturated material, and a slice, approximately
1,220 m long and 183 m wide, slid into the sea and carried the dock
area and portions of the town with it. The slide generated a wave
which slammed into the waterfront within two to three minutes of
the onset of the earthquake. This wave demolished what was left of
the waterfront facilities, caused the loss of the fishing fleet, and
penetrated about two blocks into the town. Property damage of $15
million was incurred at Valdez and there were 30 fatalities.
Surge Wave Produced by 1964 Alaska Earthquake
Surge wave left a 2 x 12 in. plank in a truck tire at
Whittier, Alaska. Whittier incurred $10 million in
property damage (1964 dollars). One of the waves,
probably the same one that caused the major damage
in Whittier, reached a height of 31.7 m above low
tide.
At 7:16 p.m. on September
1, 1992, an earthquake with
a magnitude of 7.0
generated a tsunami with
waves between eight and
fifteen meters high that
struck twenty-six towns
along 250 km of Nicaragua's
Pacific coast. More than
40,000 people were affected
by the loss of their homes or
means of income. The
waves left 116 dead, 63
missing, and another 489
injured. Tsunami inundation
of 1,000 meters was
reported at Masachapa,
where at least 15 people
were killed.
The tsunami damage at El Tranisto, (population 1,000), the area most
devastated by the tsunami in Nicaragua. Sixteen people were killed (14
children and two elderly men) and 151 were injured. More than two
hundred houses (nearly all the houses in El Tranisto) were destroyed by
waves that reached more than nine meters at this site.
Structures at El
Popoyo, Nicaragua
Structures at El
Popoyo, where fifteen
people lost their lives.
Waves at this location
reached a height of 5.6
m. One wall and a
foundation are all that
remain of a house that
was entirely removed
by the tsunami at El
Popoyo, Nicaragua.
On December 12, 1992 at 05:29 UT a 7.8
surface wave magnitude earthquake occurred in
the Flores region of Indonesia (8 degrees 31' S,
121 degrees 54' E). Flores Island is located
about 1,800 km east of Jakarta. The death toll
as a result of the combined earthquake and
tsunami effects was more than two thousand.
This includes 1,490 at Maumere and 700 on
Babi Island. (About one-half of the deaths were
attributed to the tsunami.) More than 500
people were seriously injured and 90,000 left
homeless. In addition, 28,118 houses, 785
school buildings, 307 mosques, and 493 store
and office buildings were destroyed or
damaged. Nineteen people were killed and 130
houses were destroyed on Kalaotoa. Severe
damage occurred at Maumere with 90 percent
of the buildings destroyed by the earthquake
and tsunami. Damage also occurred on Sumba
and Alor. Tsunami inundation of 300 meters
with wave heights of 25 meters were reported
on Flores Island along with landslides and
ground cracks at several locations around the
island.
A sandy beach is all that remains after the waves removed all trace of
Riangkroko. An extremely large tsunami runup (26 m) was measured
at this small rural village on Flores Island, and 137 people lost their
lives to the earthquake and the tsunamis. The inundation distance
from the shoreline along the river is approximately 600 m.
A view of the complete devastation of Pagaraman, on Babi Island.
Due to strong earth shaking and tsunami waves, about 700 people
were reported killed and more than 100 were reported missing.
This area had the largest death toll of any location for this event.
Eyewitnesses described gruesome scenes including human
remains suspended from tree branches.
Tsunami Generated by Earthquake of April 1, 1946,
Aleutian Islands, Alaska. Tsunami breaking over Pier
No. 1 in Hilo Harbor, Hawaii. The man in the
foreground became one of the 159 fatalities in the
Hawaiian Islands from the tsunami. The photograph
was taken from the Brigham Victory which was in the
harbor at the time of the event. The ship was caught by
the waves and tossed about but was able to use its own
power to avoid the reefs and get past the breakwater to
the open sea.
• The Seismic Sea Wave
Warning System was set up
in the Pacific after the
devastating Aleutian
tsunami of April 1, 1946, in
order to reduce the danger
from Pacific tsunamis. The
system is international with
observatories in Honolulu,
Japan, Taiwan, the
Philippines, Fiji, Chile,
Hong Kong, New Zealand,
Samoa, and elsewhere.
A view of tsunami and related fire damage on southeast Okushiri Island
in the community of Aonae. Numerous fires broke out following the
tsunami, adding to the property loss and misery. More than 120 people
were killed in Japan (Okushiri and Hokkaido Islands) by the tsunami.
Fishing boat (configured for catching squid) is beached high and dry
near a damaged fire truck.
• The Japanese Islands with
their dense populations are
particularly prone to
devastating tsunamis generated
very near to the shore.
Consequently much effort has
been made to reduce the
hazard. The system was
urgently upgraded in 1994
following public criticism of
the disaster on the west coast
of Okushiri Island and
Hokkaido on July 12, 1993.
Undersea fault displacement
north of Okushiri Island
produced a magnitude 7.8
earthquakeand a tsunami with
run-ups reaching over 20
meters
High water marks are clearly evident on the
side of this partially damaged house. Runup in
this area was measured at five meters.
A large seventy-meter-long barge was moved
seventy-five meters from the harbor by the
flooding tsunami waves. The crane on the barge
is used to lift concrete dolos from the nearby
fabricating area to breakwaters and protective
bulkheads surrounding the harbor area.
SEICHES
•
Long period movement of water can also be produced in lakes and reservoirs
by large, usually distant, earthquakes, and sometimes by strong winds. In the
late nineteenth century a Swiss professor, F.A. Forel made a systematic study
of this type of a water wave, which he called a seiche (pronounce sAAsh).
Seiches are described as "a standing wave in a closed body of water such as a
lake or bay". A seiche can be characterized as the sloshing of water in the
enclosing basin. The permanent tilting of lake basins caused by nearby fault
motions has produced very energetic seiches. Seiches caused by earthquakes
are termed as seismic seiches, a term coined by Anders Kvale in 1955 to
describe oscillations of lake levels in Norway and England caused by the 1950
Arunachal Pradesh earthquake. More recently an M7.9 earthquake in Alaska in
2002, caused seiches as far as Louisiana and many other states in the central
and southern United States. The 2001 Bhuj earthquake is also believed to have
caused seiches in rivers in Bangladesh.
LANDSLIDE INDUCED
• Sometimes a landslide or an avalanche into a body of water, can also trigger
local water waves. One of the most famous wave of this kind occurred on 9th
July, 1958. A large earthquake struck a remote area of southern Alaska and
caused a landslide into a 11 kilometer long bay known as Lituya Bay. The
quake dislodged an estimated 30 million cubic meters of rock and sent it
cascading into the bay, from a height of 900 meters. A massive wave of water
was generated. It surged across the narrow bay and denuded nearly mountain
slopes up to a height of 500 meters. 5 people were drowned but others had
miraculous escapes like Bill and Vivian Swanson. They had anchored their
fishing boat, the Badger, just inside Lituya bay's opening to the sea, behind a
1.5 kilometer spit called the La Chaussee Spit. After feeling the earthquake
and watching the landslide they watched in terror as the wave raced towards
them. Their boat was washed up onto the crest of the wave, backward, and
was swept over the 140 meter wide spit. The Swansons were lucky and lived
through the experience. In 1963, a large landslide fell into the reservoir of the
Vaiont Dam in Italy. A large volume of water swept over the top of the dam
and into the Piave river, killing 3000 people.
What’s the Tsunami Hazard in
California?
• The danger of tsunami from a distant source is reasonably well
understood, based on experience from the 1960 Chile and 1964 Alaska
tsunamis that did damage in California. Warning systems now in place
should give adequate notice of this kind of tsunami.
• Response to a tsunami with a local source is not well understood or
agreed upon. Local offshore reverse faults north of Cape Mendocino,
and south of the Santa Barbara Channel potentially could generate
large tsunamis. For locally generated tsunamis, there is no time for a
tsunami warning. If the earthquake lasts twenty seconds or longer, one
should evacuate for higher ground as soon as strong shaking stops, or
to the upper stories of a building if higher ground if higher ground is
more than a few minutes away.
• Because California is a land of “extreme sports”, be
advised that tsunamis are NOT surfable, The waves do not
curl. It generally comes ashore as a rapidly rising surge of
turbulent water choked with debris, including large logs.
Three sequential photos show the arrival of a major
wave at Laie Point on the Island of Oahu, Hawaii,
about 3,600 km from the source. The 8.3 earthquake
of March 9, 1957 occurred south of the Andreanof
Islands, in the Aleutian Islands. It generated a 8-m
tsunami that did great damage on Adak Island,
especially to the fuel and oil docks. However, the
Hawaiian Islands incurred the greatest damage (about
$5,000,000 in 1957 dollars). The highest wave was
16 m at Haena, Kauai.