Interpreting Data from DART®

Because the bottom pressure sensors are also sensitive to the seismic signals from the earthquake, care must be taken when reading water height data from DART® buoys.  The newest generation of 4G DART® tsunameter being deployed are designed to automatically filter out any seismic shaking that is taking place during the arrival of tsunami waves. But earlier instruments are still in operation, and occasionally the 4G filtering is not activated.

• Earthquake seismic waves travel at a speed of 6-10 kilometers per second, so will arrive very quickly at nearby DART stations

• Seismic waves are higher frequency (signal looks spikey) and at stations near the earthquake may be quite large in amplitude (meters rather than centimeters)

• Tsunami waves travel at a speeds up to 800 kilometers per hour in the deep ocean – less than 1 km per second –  fast, but well behind seismic waves.

• Tsunami waves are much lower frequency (longer wavelength) so they appear smoother than seismic waves.

• Both types of waves may be present at the same time, making it hard to distinguish one from the other visually.

As an example, DART 46403 was recently upgraded with this capability, but the filtering did not activate during the 29 July 2021 M 8.2 earthquake and tsunami off the Aleutian Peninsula in Alaska.

Located near the earthquake epicenter, the data shows a strong seismic signal before the arrival of the tsunami.  The seismic waves arrive very quickly, are high frequency (spiky), and in this case are extremely high amplitude (13 meters!) Compare with the filtered signal in the record of the next closest buoy, DART 46414, where the tsunami arrived 4 minutes after DART 46403. Some earthquake waves can still be seen, but there is much less contamination of the tsunami signal.

        

Additional Information
• Tsunameters and DART®
• DART® System Components
• DART® Resources