Eye-witness reports of the Indian ocean tsunami of December 26, 2004, from the Khao Lak area, Thailand

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Skelton, Alasdair۱; Sanden, Michael۲; Asavanant, Jack۳; Jakobsson, Martin۱; Ioualalen, Mansour۴
Holding Date 07 October 2008

On December 26, 2004, an MW 9.1 earthquake which occurred off the western coast of Sumatra triggered tsunami waves which propagated across the Indian Ocean. We present data collected from 20 eye-witness reports providing both onshore and offshore descriptions of the impact of these tsunami waves in the Khao Lak area of Thailand. These data include (1) paths by which eye-witnesses were carried by the tsunami wave(s), (2) number of waves experienced, (3) relative strength and height of the wave(s), (4) the geometry of the first impacting wave front and (5) the time interval between the arrivals of subsequent waves. Eye-witness carried offshore by the tsunami offshore experienced 7-15 waves with an average period of 16-30 minutes, whereas eye-witnesses carried onshore experienced fewer waves. Tidal gauge measurements agree with the reports of fewer waves and suggest an average period of 35-50 minutes.
These measurements suggest that the larger number of waves which were experienced by several eye-witnesses who were carried offshore relate to the complex geometry of these waves, with multiple crests. Based on eye-witness reports we interpret a wavelength of 6-8 km for the first wave as it impacted the shoreline at Khao Lak. This is shorter than the wavelength of 15-20 km estimated from numerical simulations of the tsunamis propagation and might relate to the model failing to fully accommodate for the tsunamis final deceleration. Eye-witness reports confirm that the first wave impacted at 10.30 and that this was preceded by the sea receding at 10.00. Reports of a short-lived "pre-wave" followed by a "change of direction" and affecting only the central part of the Khao Lak area are consistent with wave rebound off the peninsula, opposite Taph Lamu and subsequent wave interference, which is predicted by numerical simulations.
Eye-witness data further confirm an increase of wave height towards the northern part of the Khao Lak area wehich is suggested by tidal gauge and runup measurements. Based on the general consistence of three data sets (eye-witness reports, tidal gauge measurements, runup measurements) and numerical simulations we conclude that eye-witness reports can provide a robust source of data for testing numerical models of tsunami propagation.