TSUNAMI REPORT

Tsunami 26th December 2004

At 00.59 hrs GMT on 26th December 2004, a 9.3 magnitude earthquake tore apart the seafloor 160 km off the northwest coast of Sumatra, at part of the boundary (the 1,200 km Andaman-Sumatran trench) of two of the world's mighty tectonic plates. The lower plate, which carries India, (and which is part of the bigger Indo-Australian plate) has been moving slowly north/northeast, at a speed which has been compared to the growth of our fingernails (2 inches per year). It is continually being forced (subducted) beneath the upper plate (known as the Burma plate which is a tongue shaped part of the bigger Eurasian plate carrying most of South East Asia). The plates meet at oblique angles, which means the movement is not a smooth, frictionless glide but a torturous buildup of stresses that can result in dramatic perpendicular slips. Before the fault ruptured, the edge of the upper Eurasian plate was actually being dragged downward by the descending Indo-Australian plate. Released by the rupture of the fault, the edge of the upper plate sprang back up, lifting the ocean floor and setting off the tsunami. The fault slipped by as much as 20 metres in a few places. This type of earthquake is very powerful and is referred to as a megathrust. Over 100 years of accumulated stress was released by the earthquake and billions of tonnes of seawater was subsequently displaced to form the tsunami.

An exceptionally long (estimated at 1,200 km) faultline slipped typically about 10 - 15 m along the subduction zone where the India Plate dives under the Burma Plate. The slip took place in two phases over a period of several minutes. It is further estimated that 30 cubic km (7 cubic miles) of water was displaced. The tsunami waves radiated outwards along the entire 1,200 km length of the rupture, as opposed to originating from a point source.

Due to the complex rupturing of the sea floor, some tsunami waves effectively travelled with the crest first whilst others travelled with the trough first. When a trough reaches the coastline, it causes the sea to withdraw and disappear from the beaches. This is one of the classic warning signs of an approaching tsunami. Tragically many people were unaware of this sign, and were attracted by the fish left flapping on the sand. Minutes later the crest waves would arrive and the full force of the tsunami caused a thousand tonnes of water to crash down on each metre of beach.

Although not helpful to people on the beach, it has been reported that elephants seemed to sense that something was coming. They began acting strangely, stamped the ground and tugged at their chains until they broke away and headed to the hills. Elephants have special bones in their feet that enable them to sense seismic vibrations long before humans can.