Physics of Tsunami

Earthquake at Sea 1st possibility:

Earthquake at Sea 2nd possibility:

Tsunami Tsunami (tsoo – nah – mee) Tsu (harbor) & nami (wave) “A natural phenomenon consisting of a series of waves generated when water in a lake or the sea is rapidly displaced on a massive scale” (www.wikipedia.org)

Theory of Waves Traveling Wave Wave moves energy, not matter! Energy  E ~ f ~ v/  T = 1/f

Ocean Waves Wind blows  grab water molecules Water’s surface stretches  ripples (capillary waves) Ripples moves  more molecules collected Size and momentum increase  reach the beach

Tsunami Waves

Basic Tsunami Mechanism An earthquake causes a vertical movement of the seafloor, which displaces the sea water. These images show how the vertical displacement of the ocean floor starts the generation of tsunami waves Large waves then radiate from the epicenter in all directions.

Tsunami Explained A tsunami is series of traveling ocean waves of extremely long length generated primarily by earthquakes occurring below or near the ocean floor. Tsunami waves propagate across the deep ocean with a speed exceeding 800 km/h ( 500 mph) and a wave height of only a few tens of centimeters or less. As they reach the shallow waters of the coast, the waves slow down and their height increases up to tens of meters (30 ft) or more. Underwater volcanic eruptions and landslides can also generate tsunamis. Tsunami waves are distinguished from ordinary ocean waves by their great length between wave crests, often exceeding a 100 km (60 mi) or more in the deep ocean and by the time between these crests, ranging from 10 minutes to an hour. The effects can be amplified where a bay, harbor or lagoon funnels the wave as it moves inland. Tsunami waves cannot be seen nor felt on ships in the deep ocean. The first wave may not be the largest in the series of waves. Source: NOAA

Water Recession: A Precursor Wave Generation Draw Down Effect From: Nature Publishing Group From: Nature Publishing Group Kalutara Beach, Sri Lanka The first visible indication of an approaching tsunami may be recession of water (drawdown). When a tsunami is generated, a crest and trough make up the shape of the wave. Sometimes the trough reaches land first; sometimes the crest does. If the trough reaches land before the crest, observers will see a recession of the ocean water at the shore. This concept is shown in the two drawings. In this tsunami, the areas to the east of the earthquake epicenter experienced recession. Reports from witnesses in Thailand say that the ocean receded just before the first tsunami waves hit, exposing several hundred meters of beach and seabed. The satellite image shows the extent to which the ocean receded between tsunami waves at Kalutara Beach in Sri Lanka. It gives a sense of phenomenon that many observed before the tsunami waves reached the shore. From: Digital Globe

Tsunami Wave Appearance Source: www.waveofdestruction.org A tsunami wave crest has three general appearances from shore: Fast-rising tide Cresting wave A step-like change in the water level that advances rapidly (called a bore) A bore on the Qian Tang Jiang River, China Series of waves Most tsunamis come in a series of waves that may last for several hours The outflow of water back to the sea between waves can cause more damage than the original incoming wave fronts The first wave is rarely the largest There are several ways that a tsunami wave crest may appear to observers on the shore. The first is like a very quick rising of the tide. The second is a large cresting wave. The most often described appearance is called a bore. A bore appears like a wall of approaching water that is significantly higher than the existing water level. The image shows an example of a tidal bore in China. Although the bore shown is not a tsunami bore, it still demonstrates the appearance and characteristics of a bore that could be generated by a tsunami. Tsunamis generate a wave series that may continue for many hours. The first wave is rarely the largest in the series. Much damage can be caused by the outflow of water back to the ocean between waves. In some cases this reversed flow can cause more damage than the incoming waves.

d = water depth Tsunami At Sea Unlike a normal wave, energy of a tsunami moves through the water, not on top of it. Velocity: d = water depth Moves fast in deep waters  400 km/hour! No more than 1 meter high  can not be seen.

Energy Loss Waves loses energy as it moves away from its source Energy loss rate ~ 1 / wavelength  The longer the wavelength, the less energy it loses Tsunami wavelength = 200 km!  Energy is almost constant far from epicenter

Normal Wave vs Tsunami Wave Ocean Wave Speed = 8-100 kph Wave Period = 5-20 seconds apart Wave Length = 100- 200 meters apart Tsunami Wave Speed = 800-1000 kph Wave Period = 10 min-2 hours apart Wave Length = 100- 500 km apart

As Tsunami Moves…

At the Beach  v &  decrease, A increase Shallow water compresses the energy Tsunami moving away the beach: fast, short height  v &  increase, A decrease Tsunami moving toward the beach: slow, tall height  v &  decrease, A increase

Some Pictures from Satellite

Tsunami 12/26 Magnitude of earthquake = 9.0 SR Slipped plate length = 1,200 km Slip over = 15 m Height of wave reach up to 30 m with speed ~ 800 km/hr

Detected Earthquake

Tsunami Animation

Tsunami in Aceh Magnitude: 9.0 on the Richter scale The Energy possessed (roughly): ~ 5 MTons of TNT = 2*1018 Joules ~ 2 x explosive energy during all WW II ~ 30% energy consumed in USA a year ~ 70 days Hurricane Isabel

Impact on Earth Some smaller island southwest Sumatera moved ~ 20 m to southwest Northern tip of Sumatera moved ~ 36m to southwest Some small islands near Aceh province are gone forever New lakes/rivers created in ex-land in Aceh, disconnecting transportation to ex-villages

Earthquake Energy Sumatra-Andaman (2004) The curves show the approximate number of earthquakes per year worldwide (in the center) according to their magnitude (on the left) and their energy release (on the right). The measurement of magnitude generally used is called the Moment Magnitude (Mw). The relation between an earthquake’s magnitude and its energy release is logarithmic. That is, an increase of 1 unit in magnitude increases the amount of seismic energy released by a factor of about 30. Source: Earthquakes by Bruce A. Bolt

MANGROVES

CASUARINA PLANTATIONS

WIDE BEACH

BEACH WITH CLIFFS

SAND DUNES

School of Earth and Atmospheric Sciences Department of Applied Geology, University of Madras

Tsunami Risk Reduction Locate and configure new development in the run-up areas to minimize future tsunami losses site planning Avoid inundation areas i.e. build on high ground Slowing water currents i.e. Conserve or replant coastal belts of forest and mangrove swamps Steering water forces i.e. angled, by-pass walls Blocking water forces i.e. Build sea walls SLOWING 3. Use site planning to avoid future losses to buildings built in the tsunami run-up zone. Four key methods can be used to help minimize loss: 1) avoiding inundation areas by placing structures in strategic locations on the site where the water is less likely to reach; 2) slowing the water currents with elements such as plants (for example. areas with natural mangrove swamps at the shoreline experienced less damage from the Indian Ocean tsunami than areas that had removed coastal belts of vegetation); 3) steering the water away from structures with well-designed walls or berms; and 4) blocking water with elements such as sea walls. STEERING Source: National Tsunami Hazard Mitigation Program (NTHMP) BLOCKING

Tsunami Risk Reduction Design and construct new buildings to minimize tsunami damage Heavy and rigid structure Raise building on stilts* Many openings on the ground floor * Orient perpendicular to the shoreline: tsunami-resistant building Elevated restaurant in Hilo, Hawaii. Lower level is designed to allow waves to pass through. Source: National Tsunami Hazard Mitigation Program 4. Design of buildings for shaking resistance often emphasizes walls solid enough to resist horizontal and lateral shaking motions. If these walls are strong enough, they may act as sea walls and reflect the water from the structure during tsunamis. In some cases, water pressures perpendicular to the walls can exceed the strength of the wall. Another approach to tsunami resistance is to allow water to flow through the structure at the lower levels. Many openings on the ground floor will allow the waves to pass through without applying a significant amount of force on the building. For this system to work in areas subject to ground shaking, the frame of the building must be carefully designed by an engineer to resist earthquake shaking. *Use caution with this design in areas with high earthquake-shaking risk.

Tsunami Risk Reduction 4. Tsunami-resistant buildings (cont.) Tsunami forces on structures Several basic principles for tsunami-resistant building: Anchor the building to the foundation. Consider piles as a foundation to avoid failure of the foundation due to buoyancy and scouring. Design for dynamic forces that come from the changing water pressure. Design for impact forces that could result from debris. Design rigid connections. Design lateral bracing for column supports to provide resistance to lateral loads. Structure designed to resist tsunami forces Source: National Tsunami Hazard Mitigation Program (NTHMP)

Base Isolation