The scientific party on board the R/V Thompson using the ROV JASON and the
video camera observed two eruptions that lasted only few minutes on the West
Mata volcano near Tonga Island. This was great news for the scientific community. I had always been on the look out to see visual evidence of volcanic activities
on the sea floor during all my diving cruises. I must have come close to this,
especially during our dive in the south Pacific Rise spreading center near 17°26
0 S.
Finally, in 2009, explosive eruptive events taking place at more than 1000 m depth
on a submarine volcano were able to be observed by human eyes.
Tsunami
Tsunami is a word from the Japanese language meaning ‘‘harbor waves’’. A tsunami wave travels at a speed of more than 800 km/h when crossing the great
depths of the ocean. Sometimes a tsunami is also erroneously called a ‘‘tidal
wave’’ however tsunamis have nothing to do with tides. A tsunami is generated by
underwater ground motion due to earthquakes or massive landslides. The speed of
a tsunami waves reaches about 10–100 km/h in coastal shallow water regions. But
seismic waves travel 20 times faster than tsunamis.
The upward fault thrusting of the oceanic crust generates an upward displacement of the water column producing shock waves that will keep building up
until the conclusion of an earthquake. Because of structural upheaval generated on
the sea floor, shock waves formed in the water column will propagate through the
ocean from coast to coast and, when reaching shallow depths near land, tsunami
waves will rise, just like the sea floor rises, and then they will spread out in all
directions.
When tectonic activities like upheaval or volcanic activities cause a shaking of
the Earth, this will be recorded by surface propagating seismic waves called ‘‘T’’
(tremor) or surface waves. These waves can deliver warnings about the existence
of events capable of generating tsunamis. Thus, a quick determination of the
seismic moment can help to assess the risk of tsunami. Tremor (T) waves are
obtained based on the amplitude in displacement of the surface waves over a large
period of time (50–300 s). The causes of tsunami generating waves are due to the
displacement of multiple water masses. There are volcanic tsunamis due to
underwater volcanic eruptions generating landslides or pyroclastic transported
debris and there are tectonic tsunamis that are triggered by earthquakes during
faulting and/or fracturing of the oceanic crust-lithosphere.
Tsunami waves up to 40 m above sea level were observed as reported by
Simkin and Fiske (1983) in a volume commemorating the 100th anniversary of the
Krakatau eruption. They reported in their volume that a steam boat ‘‘the Berouw’’
was thrown from its harbor mooring and was found 2 km inland between
coconut trees.
An earthquake along the south coast of Chile in May 1960 caused volcanism
and earthquakes during the subduction of a lithospheric plate underneath the
Underwater Eruptions: First Observation
139
video camera observed two eruptions that lasted only few minutes on the West
Mata volcano near Tonga Island. This was great news for the scientific community. I had always been on the look out to see visual evidence of volcanic activities
on the sea floor during all my diving cruises. I must have come close to this,
especially during our dive in the south Pacific Rise spreading center near 17°26
0 S.
Finally, in 2009, explosive eruptive events taking place at more than 1000 m depth
on a submarine volcano were able to be observed by human eyes.
Tsunami
Tsunami is a word from the Japanese language meaning ‘‘harbor waves’’. A tsunami wave travels at a speed of more than 800 km/h when crossing the great
depths of the ocean. Sometimes a tsunami is also erroneously called a ‘‘tidal
wave’’ however tsunamis have nothing to do with tides. A tsunami is generated by
underwater ground motion due to earthquakes or massive landslides. The speed of
a tsunami waves reaches about 10–100 km/h in coastal shallow water regions. But
seismic waves travel 20 times faster than tsunamis.
The upward fault thrusting of the oceanic crust generates an upward displacement of the water column producing shock waves that will keep building up
until the conclusion of an earthquake. Because of structural upheaval generated on
the sea floor, shock waves formed in the water column will propagate through the
ocean from coast to coast and, when reaching shallow depths near land, tsunami
waves will rise, just like the sea floor rises, and then they will spread out in all
directions.
When tectonic activities like upheaval or volcanic activities cause a shaking of
the Earth, this will be recorded by surface propagating seismic waves called ‘‘T’’
(tremor) or surface waves. These waves can deliver warnings about the existence
of events capable of generating tsunamis. Thus, a quick determination of the
seismic moment can help to assess the risk of tsunami. Tremor (T) waves are
obtained based on the amplitude in displacement of the surface waves over a large
period of time (50–300 s). The causes of tsunami generating waves are due to the
displacement of multiple water masses. There are volcanic tsunamis due to
underwater volcanic eruptions generating landslides or pyroclastic transported
debris and there are tectonic tsunamis that are triggered by earthquakes during
faulting and/or fracturing of the oceanic crust-lithosphere.
Tsunami waves up to 40 m above sea level were observed as reported by
Simkin and Fiske (1983) in a volume commemorating the 100th anniversary of the
Krakatau eruption. They reported in their volume that a steam boat ‘‘the Berouw’’
was thrown from its harbor mooring and was found 2 km inland between
coconut trees.
An earthquake along the south coast of Chile in May 1960 caused volcanism
and earthquakes during the subduction of a lithospheric plate underneath the
Underwater Eruptions: First Observation
139