A teletsunami (also called an ocean-wide tsunami, distant tsunami, distant-source tsunami, far-field tsunami, or trans-ocean tsunami) is a tsunami that originates from a distant source, defined as more than 1,000 km (620 mi) away or three hours' travel from the area of interest, sometimes travelling across an ocean. All known teletsunamis have been generated by major earthquakes such as the 1755 Lisbon earthquake, 1960 Valdivia earthquake, 1964 Alaska earthquake, 2004 Indian Ocean earthquake, 2011 Tohoku earthquake, and the 2021 South Sandwich Islands earthquakes.
Overview
Teletsunamis can be generated several different ways, the most common being earthquakes with magnitudes higher than 7.5. Vertical displacement on a thrust fault is more likely to produce a teletsunami than lateral displacement from strike-slip fault. Because of this, subduction zones, which occur when dense oceanic crust burrows underneath less-dense continental crust, are at greater risk of producing teletsunamis. The Pacific coast of North America is an example of a subduction zone: it includes the Cascadia subduction zone, which lies off the coasts of British Columbia, Washington, Oregon, and Northern California. The regions around the Aleutian Islands and Gulf of Alaska are also capable of producing large offshore earthquakes and thus large tsunamis. Natural precursors that may indicate the approach of a teletsunami include a drawback, when the ocean water recedes well below low tide. While drawbacks may not always occur, their presence are considered a sign of impending danger.
Characteristics The general characteristics of teletsunamis are similar to those of local tsunamis. The interval between waves can range from 5 to 60 minutes, although it usually falls between 10 and 30 minutes. The speed at which the teletsunami travels is dependent on the depth of the water, decreasing as the water becomes more shallow. On average, tsunamis in the Pacific Ocean travel at about 773 km/h (480 mph); however, due to the depth of the ocean, the height may only be a few feet. The low amplitude, along with the broad wavelength, which spans approximately 80 to 240 kilometres (50 to 149 mi), makes vessels in open water unaware of the passing tsunami. In shallow water, scuba divers caught in the 2004 Indian Ocean tsunami were reportedly tossed around underwater, yet boats floating above were unaffected and failed to notice the wave as it passed by. Teletsunamis generally consist of a series of waves rather than a single wave. The number of waves can vary, but data have shown that there are usually between two and ten. The first wave is typically not the largest one. During the 2004 Indian Ocean tsunami, the second wave was the largest, and in the 1964 Alaska tsunami, it was the fourth. The retreat of the first wave may falsely imply that the tsunami has "finished", which can lead people to return to the beach out of curiosity, only to be swept away by the next incoming wave. Coastal water usually recedes before the first tsunami strikes, and many witnesses have reported that the approaching teletsunami waves create a loud roaring sound similar to that of a train or a jet.
Tsunami watches and warnings
Although teletsunamis are usually generated by a large earthquake, many of the areas affected by the tsunami are too far from the earthquake's epicenter to feel it (hence the prefix tele-, or "distant", in "teletsunami"). Teletsunamis are also virtually undetectable to the human eye until they approach the shoreline. Several scientific organisations have been developed to establish tsunami warning system, which are to provide sufficient forewarning of an approaching teletsunami to initiate emergency preparations and evacuations. The Pacific Tsunami Warning Center (PTWC) in Hawaii provides warnings for Pacific-based teletsunamis to almost every country around the Pacific, including island states. The National Tsunami Warning Center (NTWC) in Palmer, Alaska, watches for teletsunamis approaching the West Coast of the United States and Canada. In order to prevent confusion, the PTWC does not issue watches or warnings for the west coast unless the NTWC fails to do so. There are several guidelines set by the NTWC for issuing watches and warnings:
A watch is first issued if the arrival time for a potential teletsunami is more than 2 hours from the time of the warning. In this case, there would be enough time to verify the existence or nonexistence of a tsunami by way of NOAA buoys before a warning or cancellation message was issued. Either a watch or warning is immediately issued if a magnitude 7.5 or greater earthquake occurs anywhere in the Pacific Rim, depending on circumstances. A warning is issued if the potentially resulting tsunami would arrive onshore within 2 hours of the warning. For example, if an 8.0 earthquake occurs in the Aleutian Islands, West Coast states such as Washington, Oregon, and California would receive a tsunami watch first; if the tsunami is verified, a warning would follow. If no wave is observed, a cancellation message would follow the watch instead. If an 8.0 earthquake were to occur off of the West Coast of the United States, a warning without verification of a tsunami would be issued, as there would not be enough time to first verify a wave and then conduct an evacuation of vulnerable areas.
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