Monitoring Tsunamis

DART II system - click to enlarge
DART buoys (Deep-ocean Assessment and Reporting of Tsunamis) help maintain and improve the capability for the early detection and real-time reporting of tsunamis in the open ocean. The DART system consists of buoys positioned at strategic locations throughout the world's oceans.

The first notice of a possible tsunami event is based only on the available seismic information for the earthquake event. If a tsunami is generated, as the tsunami wave propagates across the ocean and reaches the DART buoys, these systems report sea level information measurements back to the Tsunami Warning Centers.

The information is processed to produce a new and more refined estimate of the tsunami source. The result is an increasingly accurate forecast of the tsunami that can be used to issue watches, warnings or evacuations.

A DART system consists primarily of a tsunameter and a surface buoy. The tsunameter sits on the ocean floor and contains a computer, acoustic modem and transducer (for communication), a pressure sensor, a tilt sensor (to determine the tilt of the tsunameter), and batteries.

The main pressure sensing element's function is to measure the weight of the water directly above the tsunameter. This weight value, measured in psia (pounds per square inch absolute) is converted to an estimated sea-surface height (height of the ocean surface above the seafloor) by using a constant 670 mm/psia.

The pressure sensor is very precise. Because, the change in water depth due to a tsunami in the open ocean can be as small as 0.4 inch (one centimeter) the pressure sensor is designed to detect water level changes of less than one millimeter at a depth of 20,000 feet (6,000 meters).

In standard operation, the tsunameter transmits water depth every 15 minutes to the surface buoy. The surface buoy relays the information to satellites which is subsequently relayed to the Tsunami Warning Centers.

Typical DART buoy output.

The image (above) is a typical display of sea-surface height from a DART buoy (Station 46409 - 240 nm Southeast of Kodiak, AK, August 2010). Covering a period of three days, this graph shows the sea-surface height changes by over 6 feet (2 meters) twice daily. If it is looks like a tide chart, it is. It is basically a graph showing the tides each day at this location.

When the tsunameter senses an earthquake, it goes into event mode and begins measuring and reporting the sea-surface height as often as every 15 seconds. The Tsunami Warning Centers can also place DART buoys in event mode to monitor for a tsunami at distant locations from the source earthquake.

DART buoy 32412 output and tsunami indication from May 27, 2010 8.8 Chile earthquake.
What does a DART buoy output look like during a tsunami? On February 27, 2010, at 0634Z (1:34 a.m. EST), a magnitude 8.8 earthquake struck off the coast of Chile. DART buoy 32412 observed the quake and went into event mode (seismic waves - #1 on image above).

Then, three hours later, the leading edge of tsunami waves passed by the buoy's location (#2) as it was spreading through the entire Pacific Ocean basin. (Note: the colors represent the reporting interval from the buoy.)

As the leading edge of the first wave moved past the buoy the sea-surface height rose about 10 inches (24 cm) in ten minutes. This change in height of sea-level would not be noticed by any ship due to the very long wavelength and low amplitude of tsunami wave. Yet this was was moving over 500 mph (800 km/h) away from the source earthquake.

Notice the daily tidal variation of sea-surface height is much greater by far compared to tsunamis induced height changes in the open ocean.