Early Warning Systems for Tsunamis

Tsunami evacuation sign, California. Flickr / epugachev

Tsunami warnings can be issued by NOAA, the USA’s National Oceanographic and Atmospheric Administration. The meters work by measuring water pressure (which changes as a wave passes over) and then using past data to find out the wave height. Many are used by the NOAA in the notorious Ring of Fire in the Pacific Ocean, where vast numbers of earthquakes and tsunamis occur each year. Here is the NOAA warnings page which is constantly updated with new information on possible tsunami threats.

DART

DART (Deep Ocean Assessment and Response of Tsunamis) systems are made up of two components – a pressure meter known as a “tsunameter” which is anchored close to the sea bed and is fully submerged, and a surface buoy which can relay information from the tsunameter to satellites. A GPS unit can tell NOAA the location of the buoy, and whether or not it has been moved away from it’s mooring. Satellites can then relay the information to the ground, where organisations can give the appropriate messages to governments that may be affected.

NOAA also has multiple buoys and meters around the world, and these are mapped here. They are controlled by NDBC, the National Data Buoy Centre.

The setup of a DART system – two units, an anchored tsunameter and an anchored surface buoy work in tandem to relay data to satellites.

Example: Sumatran Earthquake April 2012

NOAA can collect the relayed data and produce graphs for selected earthquakes and tsunamis. The one below is from a Sumatran earthquake/ minor tsunami in 2012, and this meter – named DART 23227 – collected amplitude or wave height data. This one sits on the coordinate 6.255 N 88.792 E.

The waves began passing through the DART station at around 0.75 hrs after the earthquake struck. The initial large peaks are caused by the earthquake itself – when the earthquake strikes, the sea floor shakes and the water pressure changes significantly. It takes a while for the waves to propagate from the epicentre to the DART buoy.

At the same time, another meter was recording – named DART 23401, situated on the coordinate 8.905 N 88.537 E.

As in the previous example, the earthquake caused an immediate peak. The tsunami waves arrived some time later.

The difference in the two graphs is fairly clear – in DART 23227, the waves arrived earlier than they did at DART 23401. Since they are both in open water, it is likely that DART 23227 is closer to the epicentre of the earthquake than DART 23401.

Several other things can be found using the graphs, including the height of the waves. DART 23227 experienced waves of around 9cm in displacement, whereas DART 23401 experienced waves of around 7.5cm in displacement. Since waves reduce in height as they travel (because they lose energy) we come to the same conclusion; that it is likely DART 23401 is further away from the epicentre than DART 23227. It’s also clear that this tsunami wasn’t a threat to coastlines, with such a small amplitude.

So the conclusion is DART 23401 is further away from the epicentre of the earthquake which caused the tsunami than DART 23227

Using the coordinates of the two DART systems and the epicentre of the earthquake, we can see if the conclusion drawn from the graphs is correct. I used this calculator to find the distance between the two sets of co-ordinates. The epicentre of the earthquake which caused the tsunami was later found to be 2.327 N, 93.063 E.

DART 23227 is approx. 886.3km away from the epicentre.

DART 23401 is approx. 644.2km away from the epicentre.

So the hypothesis is correct; the waves arrived at DART 23401 first and with greater amplitude, therefore DART 23401 was closer to the epicentre than DART 23227.

NOAA was able to model the potential tsunami waves and was successful in both cases (red lines). They use past data and observations to give accurate short-term tsunami forecasts based on an earthquake’s magnitude.

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