What are the most common dangers on the coast

Tsunamis - the deadly danger from the ocean

by Prof. Dr. Heidrun Kopp, GEOMAR

The Japanese word TSUNAMI means "big wave in the harbor". The Japanese origin of the word already suggests that this natural phenomenon occurs most frequently in the Pacific. The name is said to have originated because Japanese fishermen, who were traveling far out on a seemingly calm sea, found completely destroyed villages on their return home. In fact, TSUNAMIS are barely or not at all noticeable in the open sea, while they have catastrophic effects on the coast. How does this happen?

Tsunamis are up to several 100 kilometers long, advancing waves in the sea that are generated by submarine earthquakes (magnitude greater than 7, depth less than 10,000 meters), submarine volcanic eruptions or landslides on the coast or in the sea. When wind or currents create waves, only the surface of the water is moved. In contrast, during a tsunami the entire water column is in motion.

The very long wavelengths combined with the relatively low wave heights of around one meter ensure that tsunamis are not perceived by ships in the open sea. The waves spread in a ring from the center of the event (epicenter) at high speeds of 700-800 km / h. You can therefore traverse large ocean areas in a short time, e.g. the entire Pacific from South America to South Asia in 24 hours. If the tsunami wave reaches shallow coastal waters, it is strongly slowed down. At the same time, it is getting higher and higher and can thus have an enormous destructive effect on the coast.

Depending on how exactly the tsunami originated (earthquake, landslide, volcanic eruption) or how the seabed and coast are shaped, the water level in one region can initially fall, followed by several wave crests, while other regions are first reached by a tidal wave before the water level falls. The depth effect of tsunami waves can be seen in the fact that deep-sea fish from water depths of around 1,000 meters appear on the surface of the water.

Tsunami early warning systems

After a tsunami with great damage, work began on setting up the world's first tsunami early warning system in Hawaii as early as 1946. The Pacific Tsunami Warning Center (PTWC) in Honolulu / Hawaii has been monitoring the development of tsunamis since 1965, calculating their duration and issuing Pacific-wide warnings. The PTWC has its own seismic and level measurement data as well as satellite-supported data connections to other regional networks and data centers. It alerts you to earthquakes with a magnitude greater than 6.5, calculates their position and exact magnitude within 10 to 15 minutes, estimates the tsunami probability, and checks the levels of 75 stations across the Pacific for possible signs of tsunami-related sea level fluctuations , calculates the transit time of the tsunami wave front through the Pacific and issues appropriate warnings when it occurs.

However, 99 percent of damaging tsunamis occur less than 400 km or 30 minutes away from the nearest coast. Only regional tsunami centers can give effective advance warning in this case, provided they have seismic and level sensors with satellite or telemetric data transmission in almost real time, automatic alarm systems and computer programs for event evaluation and magnitude calculation as well as direct communication links to disaster managers and centers. Today, warnings can be issued within 20 to 30 minutes. Today there are regional tsunami warning centers in Alaska, Sakhalin, Hong Kong, Seoul, Manila, Papua New Guinea, Tahiti, Mexico, Chile, but also in southern Italy. With a tsunami running time of less than ten minutes, however, there is little chance of a warning.

In the catastrophic tsunamui of December 26, 2004, the epicenter was about 350 kilometers off the coast of North Sumatra. The tsunami reached the coast in about 30 minutes. After an hour the wave reached Thailand, after two hours Sri Lanka, after three hours the Maldives and after about seven hours the East African coast.

How did the tsunami earthquake on December 26th, 2004 come about?

On December 26, 2004, a severe seaquake of magnitude nine occurred off the Indonesian island of Sumatra and generated several tidal waves. The tsunami caused great damage in India, Thailand, Malaysia, Indonesia, Sri Lanka, the Maldives and in East Africa (Somalia, Kenya). Most affected were western Thailand, northern Sumatra and Sri Lanka. According to current estimates, more than 200,000 people were killed.

The detailed echo sounder measurements in the world ocean revolutionized the understanding of the earth system in the 1960s. The result was the model of plate tectonics: plates about a hundred kilometers thick float - moved by convection currents - on the viscous mantle of the earth. This leads to collisions between individual plates. If a plate closes in such a collision
belongs to one continent (continental plate) and the other to the sea floor (oceanic plate), the higher density of the oceanic plate causes it to slide under the less dense continental plate. Oceanic crust is "swallowed", that is, fed into the earth's mantle. This process is called subduction. The subduction zones are located in the deep-sea trenches that reach depths of up to 11,000 meters.

The earthquake in the Indian Ocean on December 26, 2004 occurred in the Sunda Trench, about 250 kilometers SSE off Banda Aceh on Sumatra. There the oceanic Indo-Australian plate dips under the Sunda plate and the Burma microplate at a speed of about five centimeters per year. If the plates get caught in one another during this process, enormous tension builds up over the years, which discharges in the form of earthquakes when the plates finally jerk against each other. The quake spread along the fault zone at a speed of 3.5 kilometers per second. The main quake caused a break in a south-north direction of more than 1,000 kilometers in length. The earthquake data were evaluated in the German Regional Seismological Network, about 9,000 kilometers from the epicenter.

 

Prof. Dr. Heidrun Kopp works in the "Geodynamics" research unit at the GEOMAR Helmholtz Center for Ocean Research Kiel. Her main research interests are natural hazards, especially earthquakes and tsunamis, plate tectonics and marine geodynamics. Since 2010 she has been a member of the board of the German Geophysical Society.

You can find more information

at the Geoforschungzentrum Potsdam (GFZ): http://www.gfz-potsdam.de
on the Intermargins website (source of information about tsunamis; in English)
at the International Coordination Center for Tsunami Warnings in the Pacific (in English)
at the Pacific Tsunami Warning Center (NOAA, USA): http://www.prh.noaa.gov/ptwc/
(English)