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March 27, 1964 - The Good Friday Earthquake in Alaska had a
moment magnitude of 9.2.This earthquake also occurred along the
subduction zone, and as we saw in our study ofearthquakes, caused
deformation of the crust where huge blocks where dropped down
asmuch as 2.3 m. Because the coastline of Alaska is sparsely
populated, only 122 people diedfrom the tsunami in Alaska. With a
tsunami warning system in place in Crescent City,
California, all the townspeople moved to higher ground. After
watching four successivewaves destroy their town, many people
returned to the low lying areas to assess the damageto their
property. The fifth wave had the largest run-up of 6.3 m and killed
12 people.
September 2, 1992 - A magnitude 7 earthquake off the coast of
Nicaragua in CentralAmerica occurred along the subduction zone
below the Middle America Trench. Theearthquake was barely felt by
the residents of Nicaragua and was somewhat unusual. A 100km-long
segment of the oceanic lithosphere moved 1 m further below the over
riding plateover a period of two minutes. Much energy was released
but the ground did not shake very
much. Seawater apparently absorbed some of the energy and sent a
tsunami onto the coast.Residents had little warning, 150 people
died and 13,000 people were left homeless.
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VOLCANIC ERUPSIONS
Volcanoes that occur along coastal zones, like in Japan and
island arcs throughout the world,
can cause several effects that might generate a tsunami.
Explosive eruptions can rapidlyemplace pyroclastic flows into the
water, landslides and debris avalanches produced byeruptions can
rapidly move into water, and collapse of volcanoes to form calderas
cansuddenly displace the water.
The eruption of Krakatau in the Straights of Sunda, between Java
and Sumatra, in 1883generated at least three tsunami that killed
36,417 people. It is still uncertain exactly whatcaused the
tsunami, but it is known that several events that occurred during
the eruption couldhave caused such tsunami.
A large Plinian eruption column blasted pumice and ash up to 40
km into theatmosphere. This Plinian eruption column likely
collapsed several times to produce
pyroclastic flows, any of which could have generated a tsunami.
A loud explosive blast was heard as far away as Australia. This
blast was likely caused
by a phreatic explosion that occurred as a result of seawater
coming in contact with themagma. The explosion could have generated
at least one of the tsunami.
At some point during the eruption a caldera formed by collapse
of the volcanic island.Areas that were once more than 300 m above
sea level were found 300 m below sealevel after the eruption. The
sudden collapse of the volcano to form this caldera couldhave
caused one or more tsunami.
Earthquakes were felt throughout the eruption. Any one of these
submarineearthquakes could have caused a tsunami.
One of the tsunami had a run-up of about 40 m above normal sea
level. A large block of coralweighing about 600 tons was ripped off
the seafloor and deposited 100 m inland. One shipwas carried 2.5 km
inland and was left 24 meters above sea level, with all of its crew
swept intothe ocean.
