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01 February 2005

Tsunami testing refined

On 26 December 2004, the most powerful earthquake in more than 40 years struck deep under the Indian Ocean off the west coast of Sumatra, Indonesia, triggering a massive tsunami.

The Indian Ocean area to the east of Indonesia and all the way west to Africa has no tsunami sensing and warning system because it is a poor area and also due to India's fierce independence from the cooperative agreements that are necessary to institute such a system.

DART mooring system

The Pacific region, the Americas, Oceania, and Europe do have such a warning system. Wiring in South Asia is in the future. Here's how it worked that day and the sensing technology it leverages:

At 12:59 am GMT on Boxing Day, a massive 9.0 earthquake occurs in the Indian Ocean off Sumatra, Indonesia. The quake is so large people feel it in the neighboring countries of Thailand, Malaysia, and Singapore.

Huge buildings in the Thai capital of Bangkok shake under the force of the earthquake and its aftershocks. Bangkok is 1,242 miles (nearly 2,000 km) from where the earthquake took place.

A mere eight minutes later, stations in Australia alert the NOAA Pacific Tsunami Warning Center of the earthquake and the potential tsunami threat. There are widely conflicting reports from different sources about the size of the quake, with magnitudes varying between 6.6 and 8.9.

At the same time, an Indonesian radio station reports the death of nine villagers as the result of a tidal wave. While Indonesia didn't have much of a chance since it was nearly at the epicenter of the quake and tsunami, any warning is worth it, and certainly those thousands who died on Sri Lanka and in other surrounding areas would have welcomed the hour-and-a-half heads up that was possible.

Kalutara, Sri Lanka—the immediate aftermath of the tsunami and receding waves.

Scientists have made great strides in monitoring and predicting the ongoing threat of tsunamis. One center continuously monitoring seismic events and changes in the tide level is the Pacific Tsunami Warning Center in Hawaii.

Tsunamis are detected by open-ocean buoys and coastal tide gauges, which report information to stations within the region. Tide stations measure minute changes in sea level, and seismograph stations record earthquake activity.

A tsunami watch goes into effect if a center detects an earthquake at 7.5 or higher on the Richter scale. If a threatening tsunami passes through and registers to the gauge station, a tsunami warning goes out to all potentially affected areas and evacuation procedures begin.

The Deep-Ocean Assessment and Reporting of Tsunamis (DART) uses unique pressure recorders that sit on the ocean bottom—bottom pressure recorders (BPR). These recorders detect slight changes in the overlying water pressure. The DART system is capable of detecting a tsunami as small as a centimeter high above the sea level.

Tsunami sensor

The tsunami BPR uses a pressure transducer that includes a very thin quartz crystal beam, electrically induced to vibrate at its lowest resonant mode. This oscillator attaches to a Bourdon tube that is open on one end to the ocean environment.

As a tsunami wave crest passes over the instrument, the increased pressure causes the Bourdon tube to uncurl, stretching the quartz crystal and increasing the vibrational frequency.

Conversely, the passage of a tsunami trough reduces the pressure, allowing the Bourdon tube to curl more tightly, thereby compressing the quartz crystal and lowering the vibrational frequency.

The electronics can precisely translate these quartz crystal vibrational frequency changes into the corresponding changes in tsunami height. For periods greater than a minute or so, and for deployments at depths of 5000 meters, the transducer is sensitive to changes in wave height of less than a millimeter.

Nicholas Sheble (nsheble@isa.org) edits the Sensors department.