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December 26, 2004 North Sumatra earthquake and tsunami

Tsunami warning

The Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawaii has quickly (just in 15 min) determined the December 26, 2004 North Sumatra earthquake, but since its source was located well outside of the PTWC Area of Responsibility (see the map), the warning was not issued for the affected areas. Instead, the Tsunami Bulletin #1 said that "No destructive tsunami threat exists [for the Pacific - V.G.] based on historical earthquake and tsunami data". Note, that the first operational magnitude of the quake was just 8.0 that was later (in 50 min) upgraded till 8.5. The Tsunami Bulletin #2 already said that "there is the possibility of a tsunami near the epicenter". At that moment, the PTWC had no means to determine how big was a tsunami and whether it has been generated at all. It is worth to remind that just three days before (on December 23, 2004) the PTWC processed another Mw8.1 earthquake occurred 150 km south-west of New Zealand. No observable tsunami was generated by this submarine quake, only small 25-cm wave was later identified on the record of the nearest tide-gauge station in Jackson Bay, New Zealand. The value of 9.0 for Mw magnitude came about a day later, when the Harvard CMT group has collected information from the broad-band seismic stations located all over the globe.

Both Thailand and Indonesia are the members of the IOC/UNESCO-sponsored International Tsunami Coordination group for the Tsunami Warning System in the Pacific (ICG/ITSU). However, Thailand, having no experience of tsunami damage in recent history, lost its contacts with the ITSU Group long ago, so that the Thai Meteorological Department (TMD) Headquarter in Bangkok did not receive the monthly dummy tests sent out by the PTWC. During the last 20 years Thailand didn't send its representative to the biannual sessions of the ITSU Group. The person, being on duty in the TMD Seismic Monitoring Center (SMC) on this Sunday morning, was informed about the earthquake by the telephone call obtained from the Phuket Island. The local TMD office was telling about strong ground shaking being felt in the Phuket area. However, during the processing of seismic data, telemetered to the SMC from remote stations, several other calls were obtained from cities at the north telling about felt earthquake there (indeed , the NEIC catalog lists M 5.7 earthquake occurred at 01:59 GMT, i.e. just 30 min after the Sumatra shock, in the north of Thailand). These calls seriously interfered data processing creating ambiguity of the situation for the SPC personnel. Finally, in about one hour the position of the strong (Ms8.0) earthquake was determined to be some 100 km west of the Sumatra coast. Nobody in the SMC at that moment has been thinking about catastrophic tsunami that may follow this quake. Earthquake source mechanism remained unknown, its magnitude was large, but was not extraordinary, the main resort areas at the western coast of Thailand seemed to be well protected from this source by the northern tip of the Sumatra Island. When the TMA office in Bangkok started to get calls from Phuket about destructive waves, it was too late to make any action to warn other parts of the Thailand west coast.

The BMG office in Jakarta had no chance at all to warn the people on the northern Sumatra about impending hazard. The first wave hit Melabou, Calang, Banda Aceh and other nearest coastal cities before anyone in Jakarta understood the scale and potential danger of this earthquake. When the magnitude and position of the shock was determined, more than 100,000 people in three large cities and numerous small coastal villages were already killed by the first wave. The second and third waves were adding another thousands of victims to this mournful list. Despite the fact that the last dreadful tsunami in Indonesia occurred only eight years ago, nothing was done in order to warn other countries about possible tsunami. The reason is obvious - the scale of the catastrophe was not known during these first hours. Besides, never in the known history a trans-oceanic tsunami was observed in the Indian ocean. All three destructive tsunamis with human fatalities occurred in the region during the last 15 years (in 1992, 1994 and 1996) were the local events with damaged area limited no more than by 300 km of the nearest coast.

However, even in case such a warning was send to the central authorities of Sri Lanka, India, Maldives and other affected countries, there was no established communication network or organizational infrastructure to deliver the warning to the people at the coastlines. Absence of any experience with the tsunami hazard resulted in that the natural warnings - retrieval of sea, unusual tide currents or small forerunning wave - were not used by people living at the coast to protect their lives.