# How much time is there between an underwater earthquake and the moment a tsunami hits the coast?

Tsunamis can be caused by underwater earthquakes, like the 2004 tsunami in the Indian Ocean. The precise value will likely depend on where the earthquake is relative to the coast. But what is the time range between the moment the earthquake is detected and the moment a tsunami hits the coast? Is it minutes? Hours?

• There are so many variables which would be in play here. Seismograph data availability, where the epicentre is (magnitude, depth, distance from coasts), how capable are the locations able to be contacted.... Sep 14, 2020 at 15:27
• I don't see how whether the locations are able to be contacted comes into play. I'm not asking how long it would take for someone with a sismograph to warn someone on the coast. Just how long between the initial earthquake and the subsequent tsunami. Sep 14, 2020 at 16:13
• @usernumber you can take a look here en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami for information about the speed and timing of the tsunami. Sep 14, 2020 at 17:49
• The question sounds pretty clear to me, vote to leave open. Sep 15, 2020 at 6:42

As a general guideline, the time between the actual earthquake and the time the tsunami arrives is on the order of minutes to hours.

Tsunamis out in the deep ocean travel very fast -- around 500 mph -- which means that they cross ocean basins in about as much time as it takes a plane to fly across. So an earthquake in Japan gives Hawaii and the US West Coast many many hours of warning.

Close to shore, tsunamis are much slower, but of course the distances are also shorter. For a place where earthquakes happen close to shore because of a subduction zone, say in Japan, this only gives you a few or a few tens or minutes of advance warning. If you take the 2011 tsunami in Japan, warnings were sounded quite quickly, and that allowed a large number of people to get to higher ground and will have saved tens of thousands of lives. But at the same time, it was not enough time to actually go door to door, put people onto buses, and get them out of harm's way if either they did not hear the warning, or refused for whatever reason to immediately evacuate.

• (+ the seismic wave created by the earthquake propagates at, say, 2 miles per second(!); so, about 15 times faster than the tsunami. If you, furthermore, have a few seismic stations close to the earthquake you can in principle warn the rest of the world about the risk pretty much instantaneously)
– Erik
Sep 15, 2020 at 7:21
• A tsunami is a shallow wave, meaning the wave reaches all the way to the bottom of the ocean, even at its deepest. Shallow waves propagate with a speed of about $\sqrt{gd}$ where $g$ is the acceleration due to gravity and $d$ is the depth. Sep 15, 2020 at 17:34
• Here's a link to a brief explanation on how to calculate the speed: weather.gov/jetstream/tsu_prop As noted, about 500 mph/800 km/hr in the deep ocean. Sep 18, 2020 at 2:57