Tide gauge records are the best and usually the only instrumental data of tsunamis. Tide gauges provide the valuable information on the tsunami arrival times and the changes in sea level due to tsunamis. The sea-level measurements from the tide gauges can be de-tided (removing of the tidal effect) to get the changes in water level due to tsunamis alone.
The great tsunami of 26 December 2004 was recorded on all the tide gauges located in the Indian ocean. We analysed the data from the tide gauges along the coast of India (maintained by Survey of India) and from elsewhere in the Indian ocean that are located to the west of the rupture zone. At all these stations, the water level first rose, then fell. Available data from Indonesia show that the stations to the east of the rupture zone first saw a drop in water level (Merrifield et al., Geophy. Res. Lett., 32, 2005).
The tsunami struck the Indian east coast around 0330 UTC (0900 IST). The amplitutde was 2m above the tide at Chennai, Paradip and Colombo. The east coast of India (and of Sri Lanka) was hit shortly after high tide; Tuticorin and Colombo, however, were hit shortly after low tide. The tide gauge at Nagapattinam (the worst affected place among all the Indian east coast stations) was destroyed by tsunami waves and hence the data from there are not available. The tsunami wave propagated northward along the Indian west coast.
The prominent periodicities associated with the tsunami waves are observed to be 35-45 min and about 20 min. Along the Indian east coast, there is another prominent peak between 1-2 hours. One possible cause of these high period (hence the low-frequency) oscillations is coastally trapped edge waves triggered by the reflection of the tsunami waves. Another possibility for this "anisotropy" in the spectrum is the lower frequencies triggered by rupture propagated more to the north-northwest towards the Indian coast. Available geophysical evidence suggests that such an anisotropy existed in the forcing: the rupture propagation speed and rate of slip were not uniform along the rupture arc (Bilham, Science, 308, 2005). For more details, see: Nagarajan et al., Earth Planet. Space: 58; 2006; 211-215.
Please acknowledge Survey of India and National Institute of Oceanography for these data, and cite the following paper: Nagarajan et al., Earth Planet. Space: 58; 2006; 211-215.