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 eyewitness reports , who experienced the tsunami on December 26, 2004 in the class IV impact region, near Khao Lak, Thailand. These data include (1) paths by which eyewitnesses 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. These data are broadly consistent with (1) tidal gauge measurements, (2) measured runup heights (Tsuji et al., 2006; Choi et al., 2006) and (3) numerical simulations (Ioulalen et al., 2007). Based on these data we make the following tentative interpretations:
1. The tsunami impacted in an east-northeasterly to northeasterly direction.
2. The sea began retreating rapidly at approximately 10.00 a.m. local time.
3. The first wave front impacted at 10.30 a.m. local time.
4. The wave period was 35-50 minutes. Shorter wave periods (21-27 and 16-30 minutes) were estimated from eyewitness data. These probably reflect the complex geometry of the wave maxima with multiple (2-4) crests, which may have been experienced as separate waves.
6. The wavelength was between 15-20 and 6-8 km. Given the wave period (35-50 minutes), we note that this could reflect attenuation of the incoming wave from a velocity of 18-34 km/hour to a velocity of 7-14 km/hour.
7. Eye witness reports suggest that the wave height was 5-12 m. These estimates are broadly consistent with tidal gauge measurements (7-8 m) and runup heights (8-11 m).
8. Wave refraction off the peninsula, west of Taph Lamu, might have been responsible for a smaller and weaker “pre-wave” which was experienced by survivors in the central part of the Khao Lak area and evident both from tidal gauge measurements and the model simulations.
Based on the broad consistency between eyewitness reports, tidal gauge and runup measurements and model simulations, we conclude that eyewitness reports can provide a robust source of both qualitative and quantitative data, which can be used to constrain numerical models of tsunami propagation.