Mechanisms of Tsunami Formation
most common event that generates tsunamis is submarine faulting
which causes part of the ocean floor to be thrust upwards. Because
an earthquake and tsunami result from the same fault movement, tsunamis
are often referred to as "seismic sea waves", and the earthquakes
as "tsunamigenic earthquakes".
abrupt submarine faulting and vertical displacement of the sea floor
causes rapid displacement of the overlying water column, and forces
it into a wave-like turbulence. This turbulence is expressed as
a series of seismic sea waves (Fig. 1).
the deep ocean, these waves are barely detectable and are difficult
to distinguish from normal ocean swells. They have low wave heights
(the vertical distance between trough and crest is generally around
a metre), and long wavelengths (the horizontal distance from crest
to crest can be up to 100 km). The waves travel at speeds of up
to 1000 km/hr. A tsunami may consist of 10 or more waves, forming
a "tsunami wave train". A popular misconception is that there is
only one giant wave in a tsunami; this misconception has resulted
in many lives lost. The waves in a tsunami train may arrive at the
shoreline several minutes or hours apart. The first wave to arrive
is often the smallest.
seismic activity beneath the oceans is concentrated in the narrow
fault zones adjacent to the great oceanic trench systems (subduction
regions). Hence, most tsunamigenic earthquakes originate in areas
adjacent to the ocean trenches, e.g. the Aleutian Trench (south
of the Aleutian islands) and the Atacama Deep, west of Chile.
a small number of submarine earthquakes result in tsunamis. Most
have a small spatial effect or are too low in magnitude to cause
vertical displacement of the ocean floor. Generally, only those
earthquakes with a Richter magnitude of 6.5 or greater, and shallow
focal depths (< 50 km), are likely to be accompanied by tsunamis.
The largest recorded tsunamis were generated by submarine earthquakes
of magnitude 8.5 that had large spatial effects. Very large areas
of ocean floor (> 100 km2 ) were displaced by several metres in
largest tsunamis are those associated with elliptically-shaped ground
displacements, i.e. the sea floor uplifted is in the approximate
shape of an elongated ellipse. In both the 1960 Chilean and 1964
Alaskan events (discussed in section 5), the length of permanent
ground displacement parallel to the fault line was 800-950 km and
the width, perpendicular to the fault, was around 150 km. In plan
view, the perimeters of the uplifted areas can be approximated by
an ellipse (e.g. Fig. 2).
is radiated from the centre of the ellipse in a direction perpendicular
to its long axis, with the long axis almost always parallel to the
nearby ocean trench. Hence, most tsunami waves travel in a direction
perpendicular to the trench systems.
Volcanic eruptions and landslides
accounts of eruptions at many of the world's coastal volcanoes attribute
much destruction and loss of life to tsunamis generated during the
volcanic eruptions. Of those killed during volcanic eruptions, nearly
25% have died as a result of tsunamis.
historical tsunamis were once thought to have been generated by
volcanic eruptions. It is now considered that submarine tectonic
earthquakes generated most of these, although earthquakes may also
generate eruptions. A good example is the 1930 eruption of Stromboli.
An earthquake, accompanied by a sharp 1 m uplift and rebound of
the island, took place an instant before the climactic explosions.
The resulting tsunami reached 2 m in height.
vast majority of tsunamis of true volcanic origin are attributed
to debris avalanches or landslides entering coastal waters. Other
important mechanisms of generating volcanic tsunamis include: pyroclastic
flows impacting on water, submarine explosions, and caldera collapse
or subsidence. Less important causes include lava flows and lahars
entering the sea.
largest of the volcanic tsunamis are generally due to destructive
processes such as subsidence and disruption of lava domes and emplacement
of pyroclastic flows, rather than constructive processes (e.g. submarine
cone building eruptions and lava flow extrusion). In only a few
instances have tsunamis been attributed to the flow of lava into
of the most famous accounts of tsunami generation associated with
a volcanic eruption is that of the 1883 eruption of Krakatoa. The
Krakatoa eruption was responsible for one of the worst natural disasters
in history. Tsunamis were generated that swept the low-lying coasts
of Sumatra and Java, drowning at least 36 000 people. Waves estimated
at 35 m high arrived at the shores. Most of the lives lost during
the eruption resulted from the tsunamis and not from erupting ash
and pyroclastic avalanches. Tsunamis of similar magnitude can be
expected in any future cataclysmic eruption of an island volcano.
origins of the Krakatoa tsunamis have long been debated, but were
probably generated by multiple origins including: subsidence or
collapse of the main island during caldera-forming explosions, submarine
explosions, and pyroclastic flows and avalanches entering the sea.
Many small sea waves recorded are thought to be air/sea coupled
waves produced by explosions, and are not true tsunamis.
associated with large slope-failure events can extend the impact
of an eruption well beyond the volcano itself. A 0.5 km2 volcanic
debris avalanche, generated on the slopes of Mt Unzen (Kyushu, Japan)
during eruptions in 1792, swept through the town of Shimabara and
into the almost enclosed waters of the Ariake Sea. This initiated
a 10 m high tsunami which affected the low-lying parts of Shimabara.
Most of the 10 000 casualties in the city resulted from the tsunami.
The tsunami also swept opposite coasts, killing a further 4300 people.
The tsunami travelled at an estimated 40 km/hr, which would enable
only a 30 minute warning to the communities on the far side of the
caused by debris avalanches entering the sea have also affected
coastal communities on the Japanese main islands. Dangerous tsunamis
are expected to be generated following any future large debris avalanches
from the slopes of Sakurajima volcano. Such avalanches are expected
to enter Kagoshima Bay (southern Kyushu) and generate tsunamis.
A tsunami hazard also exists where lakes occur in close proximity
to volcanoes. Wave runup at Spirit Lake from the 1980 Mt. St. Helens
debris avalanche reached a height of 200 m.
of the world's damaging tsunamis are known to have been caused by
submarine and subaerial avalanches displacing water in relatively
confined coastal bays or harbours. Submarine landslides were common
in Alaskan harbours during the 1964 Alaskan earthquake (section
5.3) and were the cause of local tsunamis along the Alaskan coastline.
has recently been postulated that tsunamis may be generated in areas
of the sea floor where gas expulsion or seepage pits occur. These
structures, up to 20 m wide and 1-2 m deep, emit water and natural
gas derived from underlying intrusions. The subaerial equivalents
of these structures are mud volcanoes and mud pools which are found
along faults and in areas of diapiric upwelling (i.e. where there
is an upward injection of material through the surrounding strata).
Mud volcanoes and pits generally have a similar form and dimensions
to those on the sea floor, and also emit gas and saline water.
sea floor pits occur in the offshore Gisborne-Poverty Bay region,
and are thought to mark sites of potential tsunami generation. Two
New Zealand tsunamis (25 March 1947, and 17 May 1947) were generated
in an area of sea floor off Gisborne. This area is penetrated and
fractured by diapiric intrusions that have occasionally exploded
mud and breccia originating at a depth of several kilometres.
March 1947 tsunami was probably caused by diapiric intrusion and
gas blowout on the sea floor. An earthquake accompanied the tsunami,
but due to its low magnitude (<6), it was not considered to be a
tsunamigenic earthquake. During the earthquake, a Wainui resident
reported seeing the sea froth. Foaming of the sea is consistent
with disturbance of the sea bottom and shelf sediments with emission
of gas during explosive mud volcanism.