Tsunami Jens Havskov and Mathilde B. Sørensen

What is a tsunami A tsunami is an abnormal large wave hitting the coast

A surf wave can be considered a small tsunami

Danger of tsunami waves

Tsunami generation by an earthquake

Tsunami generation in a subduction zone Deformation accumulates The fault slips, the sea bottom moves up and generates a wave in the water The wave starts to propagate towards the ocean and towards the coast

Tsunami generated by land slides, on land and submarine

Tsunami propagation A tsunami propagation velocity is reduced in shallow water, while the height of its waves rapidly increases. Wave height in the open ocean is less than 1 m. The periods of the tsunami waves may vary between about 6 and 150 minutes.

Tsunami run-up

Department of Earth Science (From Milburn et al., 1996) Recording tsunami signals on a tide gauge

Tsunami warning systems

Tsunami warning system, integrated system

Department of Earth Science Instrument examples

Pacific tsunami warning center (PTWC), how it works Use seismic data as its starting point. Takes into account oceanographic data when calculating tsunami threats. Tide gauges in the area of the earthquake are checked to establish if a tsunami wave has formed. The PTWC then forecasts the future of the tsunami, issuing warnings to at-risk areas all around the Pacific Basin. There are never false alarms--if the PTWC issues a tsunami warning for a particular area, the wave is already on its way and will hit. As it takes a hours for a tsunami to travel transoceanic distances, the PTWC can afford the time to make sure of its forecasts.

US Pacific tsunami warning system and zones of large shallow earthquakes

The simplest warning system

Single station warning system A single station can make a preliminary location and magnitude If M > 7, a warning isssued Problem: magnitude can normally only be determiend after some time and that may be too late

Tsunami earthquake A tsunami earthquake generates proportinally little high frequency energy Also called a slow earthquake Traditional magnitude Ms is low compared to the tsunami it generates The earthquake magnitude is in this case a poor indcator of the tsunami potential. People might not take warning since ground shaking might be less than what is usually expected for a tsunami generating earthquake. Important to include frequency analysis is the seismic tsunami warning system

Tsunami earthquakes source time functions Tsunami earthquakes rupture more slowly and radiates less energy Source time functions of normal subduction earthquakes Source time functions of tsunami subduction earthquakes

Examples

Subduction along the Sunda trench is the main source of significant earthquake activity in this region. Sumatra earthquake, Dec 26, 2004, M=9.0

From CalTech (K. Sieh) In the region, there are at least six significant historical earthquakes that occurred prior to the 2004 event. Sumatra seismic gap

Tsunami on the beach

Deadly Norwegian fjord Tsunami in Tafjord, Norway was 63 m high 1934

Future land slide in a Norwegian fjord This mountain side is now slowly sliding into the fjord. It can suddenly slide and the tsunami would destroy the village. The sliding is monitored by seismic stations and it is expected that a warning can be given. Imaginary wave caused by the coming slide at the village.

Japan 11. March 2011

Plate tectonics in Japan ISTO, Orleans, Fr Relative plate speed : ca. 8 cm/year

What was expected in Japan? Earthquakes M≥7 since 1900 From age of plate and subduction speed, a M=8.2 event was expected. USGS

What happened in Japan? A mega-earthquake: M=9.2 Lengh of breaking fault: ca 500 km Width of fault: ca 250 km Maximum displacment: m Displacment on the sea bottom: 3-5 m

Measured accelerations in Japan Max horizontal accelration: 2.7g NIED

The tsunami, up to 40 m high

Consequenses of the tsunami

Consqueces of the tsunami

Tsunami walls Ca. 40% of the coast was protected by walls but they were too small

Before and after at theTanohata-wall

Japan eathquake - conclusion Much worked well: -The warning system worked -People knew what to do -Many lives were saved But… -The maximum size of earthquake in the region had been under estimated -Fukushima…

Historical tsunamis in the Caribbean Red markers are eqarthquakes > 8, yellow upward trianges are volcanic sources and downward triangles are landslide sources. 85 tsunamis,17 with fatalties, dead since 1498.

Little monitoring of sea level

Haiti earthquake generated a small tsunami, 3 fatalities Generally low risk to tsunmis in the Caribbean compared to the Pacific Both land slides and earhquakes have generated tsunamis

Conclusion Tsunamis are mainly generated by shallow earthquakes A first warming is the earthquake, continued seismic monitorin is therfore essential. Slow eartquakes or tsunami earquakes might underestimate tsunami potential Tide gauge used to get actual tsunami height and can be used to model probable wave hights