The 2004 Indian Ocean Tsunami: Inferences from the Tide-Gauge ObservationsS. NeetuNational Institute of Oceanography, Dona Paula, Goa-403004, India

Description of the Event

The tsunami struck the Indian east coast around 0330 UTC. The amplitude was 2.0 m above the tide at Chennai, Paradip and Colombo, 1.5 m at Visakhapatnam, and less than a metre at Tuticorin. The tsunami propagated into the Arabian Sea from the south and Kochi was hit at 0541 UTC. The maximum amplitude had decayed to about 80 cm at Kochi and it was less than 10 cm at Okha in Gujarat. All these tide gauges are to the west of the earthquake zone and the detided sea levels show first a rise in sea level with the arrival of the tsunami, and then a sharp decrease.

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Spectral analysis (FFT) of the residual. The period corresponds to the prominent peaks is shown by the numbers (in minutes).

How far north did the tsunami source extend?

Lay et al. (Science, 20 May 2005, p. 1127) used backward ray tracing to estimate a source region extending up to 600 km north-northwest of the epicenter. In their analysis the northern extent of the source was mainly constrained by the tsunami travel times reported from the tide gauges at Chennai, Visakhapatnam and Port Blair.

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Wavelet analysis of the residual. Note that the ordinate is plotted in log2 scale. The shading gives the wavelet power spectrum (cm2); log2 (wavelet power) is shaded and each successive shade indicates doubling of the wavelet power. Note that more power is present at the stations along the Indian east coast in the 1-2 hour period range.

The residual (cm) of the tide-gauge record. The tsunami arrival time (UTC) is given in the bottom right corner.

References:B. Nagarajan, I. Suresh, D. Sundar, R.

Sharma, A.K. Lal, S. Neetu, S.S.C. Shenoi, S.R. Shetye and D. Shankar. The Great Tsunami of 26 December 2004: A description based on tide-gauge data from the Indian subcontinent and surrounding areas. Earth Planets Space, 2006, 58 (2), 211-215

S. Neetu, I. Suresh, R. Shankar, D. Shankar, S.S.C. Shenoi, S.R. Shetye, D.Sundar and B. Nagarajan. Comment on "The Great Sumatra-Andaman Earthquake of 26 December 2004", Science, 2005, 310, 1431a

Dominant Periods of Tsunami

Spectral and wavelet analysis of the residuals show that the maximum amplitude was at a period of 35-45 min. at most stations, with another maximum around 20 min. at some stations. Along the Indian east coast, however, there is another broad peak between 1-2 hours within the first few hours after the first tsunami wave. This low-frequency peak is not prominent at other locations..

Backward wavefronts for Paradip (red), Visakhapatnam (green), and Chennai (blue) corresponding to their respective observed travel times. The asterisk marks the epicenter of the earthquake. The northern extent (~9° N) reported by Lay et al. is indicated by arrow A. The arrow B marks the new estimate (~11° N).

The asterisk marks the epicentre and the dots mark the location of the aftershocks till 10 February 2005. The locations of the tide-gauge stations operated by the Survey of India are marked by filled circles; GLOSS stations are marked by filled squares. AI: Andaman Islands; NI: Nicobar Islands.

We found that the tide gauge at Port Blair had a clock error of 46 min. and a data gap at the time of tsunami strike. Also, the backward ray tracing method works best for stations outside the source area. Hence we excluded the Port Blair data from our analysis. Adding tide-gauge data from Paradip, the northernmost of the Indian east-coast stations and therefore the most critical constraint on the northern extent of the source, we estimate that its length was greater by nearly 30%.

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