The Andaman Islands Earthquake and Tsunami - 26 Dec 2004
An analysis from Ottawa Canada - The author is not a seismologist!!

The earthquake was caused by a major inter-plate shift running the entire length of the Andaman and Nicobar Island chain, with three separate, noticeable epicentres. This caused "differential propagation" of the tsunami wave - strong to the east and west, but much weaker to the north and south. A simplistic analysis follows - the International Co-ordination Group for the Tsunami Warning System in the Pacific have published a web page showing the propagation of the tsunami every 10 minutes, (based on informed modelling done in Japan).

A detailed analysis, from a seismological perspective, has been produced (30 Dec 2004) by Roger Bilham, University of Colorado.

Hugh Reekie

Analysis of progress of the tsunami shock wave of 26 Dec 2004

As observed by Hugh Reekie from web page: - http://ioc.unesco.org/itsu/   The animation, entitled 2004 Sumatra Earthquake, shows the situation every 10 minutes up to 180 minutes. Red is (perhaps) an indication of a rise in water level, and blue a decrease. Another web page cites this animation as by Kenji Satake of Japan. If this animation is correct, the author makes the following observations, analysis and evaluation. A similar modelling and animation has been produced by NOAA, with less coastal detail display in the imaging (web page taken down mid January); this is not analyzed here. Another simplistic NOAA image is available; I think this image is misleading.

1. The quake was along the entire length of the Andaman and Nicobar Islands - almost 1000 miles, but with three areas of significant additional intensity - which I will call epicentres; no doubt one of the locations was first to move - then the others (and places in between to a lesser extent) in quick succession.

2. Although the shock created a surge (as shown in the animation) that was essentially a quasi-plane wave, the image appears to show 3 almost equi-distant epicentres, each about 500 miles apart: on closer inspection, 300 miles north to middle, 500 miles middle to south (three epicentres at approx 14°N 92°E, 9°N 92°E and 4°N 94°E).

3. Travelling to the west across the Indian Ocean, the wave became distorted by the presence of the 3 separate epicentres. In particular, the force wave became focused in 2 areas, the northern one hitting the Indian coast north of Chennai (Madras) near Machilipatnam; the southern one clearing to the south of Sri Lanka.

4. This more southerly Indian Ocean shock wave veered to the right, or north, by diffraction as it passed the Indian subcontinent, but remained a strong quasi-linear wave. It headed for Africa and the Somali coast; the energy to the south and north of this main thrust is both noticeably diminished and dispersed.

5. The wave to the east, which passed through the island chain without difficulty, was concentrated, as the three epicentres create a focussing effect: the Andaman and Nicobar Islands, which follow the fault line, are in a curve. This "focusing" appears to have had two consequences: 1. There is a large "lower than normal sea level" initial propagation (shown in blue on the web page animation) - which is not present at all on the western-travelling wave. Perhaps either the shallower sea depth, or the presence of the island chain and very shallow inshore water, had someting to do with this; or perhaps it was the nature of the 'quake. 2. The shock front from each of the three epicentres became focussed - near Phuket, Thailand and surrounding area.

6. Australia, whose north-western coast is exposed to the quake, has had little reported damage. The direction of tsunami travel to this coast is almost at right angles to the three epicentres, so significantly diminishing the force of the shock wave. Three separate, weak, shock waves can be seen heading SSW. The energy heading south and SSE diminishes so as to be indiscernible before the end of Sumatra is passed - so Java is hardly affected, for example.

7. Local coastal effects, of a complicated nature, changed the tsunami waves reaching the Indian shore south of Madras, and the Thai western coast, increasing the intensity in the final minutes before landfall. There is a particularly intense and lingering red area, possibly caused by coastal reflection, at 8°N. Also, coastal reflection created a similar intensity near Bandar Akeh in NW Sumatra.

8. Another NOAA Page - 10 Jan 2005 - analyzes satellite images in detail, and relates this to their modelling programs. It shows, amongst many other details, that other plate junctions around the Indian Ocean, many miles away fro the quake zone, suffered significant movement, as observed by immediate sea level changes. NOAA work by John Leslie and others.

- Hugh Reekie, Ottawa 2 Jan 2005 This is web page
http://members.allstream.net/~max-com/tsunami.26dec04.html   -    vert 75%