DART® System Components

The main components of a station in the DART® system are the tsunameter and a surface mooring.

The tsunameter is the device that measures the water pressure at the sea floor. The water pressure is directly related to sea level height and is thus a proxy for wave height. Water pressure data is recorded and stored in the tsunameter every 15 minutes. In normal conditions (no tsunami) 24 values of 15-second observations are transmitted every 6 hours. (2) In a DART® system, the tsunameter is not physically connected to the surface buoy but communicates with it by two-way acoustic telemetry.

The surface mooring uses a 2.5 m diameter buoy with a displacement of 4000 kg. The mooring line is 19 millimeter eight-strand plaited nylon line with a rated breaking strength of 7100 kg, held by a 3100 kg anchor. It is deployed to maintain a tight watch circle, keeping the buoy positioned within the cone of the acoustic transmission. The mooring components include dual acoustic transducers for tsunameter to buoy communication, a computer, modem, GPS receiver, Iridium transceiver, and RF antennas. (2)

Activation of the DART® system occurs when the tsunameter detects a change in water height greater than a set threshold. When event mode is triggered, four minutes of 15-second observations are reported, followed by data in 1-minute averages. Event mode can also be initiated manually from tsunami warning centers.

Figure below shows the raw data from a time when event mode was triggered. The first waves on the left are from the earthquake itself. Earthquake waves travel significantly faster than tsunami waves, and frequently trip the tsunameter into ‘Event Mode’ before the tsunami arrives. The vertical shifting of the seafloor from the earthquake acts to lift or compress the water column above, showing an increase in pressure as the seafloor rises, or decrease in pressure as the seafloor falls. The tsunami wave appears later, with a longer wavelength.

Filtering the data is essential to isolate the tsunami wave signal from the earthquake signal (high-frequency spikes) as well as from the tide signal (the larger wave upon which these are superimposed). The newest 4G DART® measurements enable the earthquake waveforms to be filtered out prior to transmission and allows the instruments to be placed closer to subduction zones than ever before. (4)

When a tsunami event occurs, the first information available about the source of the tsunami is based only on the available seismic information for the earthquake event. As the tsunami wave propagates across the ocean and successively reaches the DART® systems, these systems report sea level information measurements back to the Tsunami Warning Centers, where the information is processed to produce a new and more refined estimate of the tsunami source. (3).

The NOAA Center for Tsunami Research developed a forecast database pre-computed with the Method of Splitting Tsunamis (MOST) model (5, 6). Thus to forecast inundation from early tsunami waves, the seismic data and sea level measurements are used to sort through a generation/propagation. The database consists of model results from simulating the tsunamis that would be generated by earthquakes at discrete 100-km by 50-km sections of subduction zones around the globe. The SIFT (Short-term Inundation Forecasting for Tsunamis) system selects a combination of source sections that most closely matches the observational data (7). The result gives estimates of basin-wide tsunami magnitude and speed in deep water which can then be used as initial conditions for a site-specific (non-linear) inundation model.

Additional Information
• Tsunameters and DART®
• Interpreting Data from DART®
• DART® Resources