|Location of today's earthquake|
I'm not sure what all of the official criteria are for a tsunami, but here are a few factors that are important:
Not all earthquakes generate tsunamis, a problem that causes nightmares to tsunami forecasters. When should a warning be issued? When should one not be issued? To forecast whether or not a tsunami will form and how big it might be, geophysicists and oceanographers work together with complicated data analysis and modeling software on huge computers that continuously monitor seismic signals coming through the earth. It takes about 25 minutes for seismic waves from an earthquake to reach all stations of the global seismic arrays and another ten minutes for computers to crunch the data through models. (I recently discovered that the USGS sometimes learns about the occurrence of an earthquake via text messages even before the seismic waves arrive at stations! How cool is that?!!)
Officials can now issue a preliminary estimate of tsunami danger in less than an hour after a quake. During this hour after an earthquake, scientists are searching for four pieces of information that determine the likelihood of a tsunami: the geometry of the fault motion, the energy and depth of the earthquake, and the depth of water over the fault rupture zone.
Some earthquakes produce tsunamis, others do not. Earthquakes in which the dominant motion is lateral, so-called “strike-slip” like that on much of the San Andreas fault in California, do not generate tsunamis. They form, rather, during quakes in which rocks have some vertical motion along a fault, causing a bulge or depression to form in the water. Such quakes, known as “tsunamigenic” earthquakes, occur in the subduction zones of plate tectonics—primarily those bordering the Pacific Rim, where two thirds of all tsunamis occur, but also in the Indian Ocean, the Caribbean, and the Mediterranean. During the Tohoku earthquake, an area of nearly six thousand square miles--the size of Connecticut--thrust up as much fifteen feet, disturbing an enormous volume of water. Oceanographers need to know the depth of the ocean over the rupture zone because the more water above the displaced crust, the more water that moves out into the tsunami.
The energy of an earthquake and the process of transmitting the energy to the water are both related to the area of the displaced ground and how far it moves during the quake, its displacement. Both of these quantities are captured by the seismic moment, a quantity that is related to the Richter magnitude. This relation of magnitude to area and displacement of the crust is the reason that estimates of the magnitude of the earthquake are so important in prediction of the size of a tsunami. Boxers understand this piston physics well—a big fist thrown in a long punch is more powerful than a small fist with a short reach.
The potential for devastation by a tsunami depends on the fault geometry in one another way. Waves from a fault rupture are bigger in some directions than in others. Waves spreading away from the long sides of a rupture--that is, spreading perpendicular to the fault plane--tend to be much bigger than those that spread away from the tips of the fault, parallel to the fault plane. If two coastlines are at equal distances from a fault, the one perpendicular to it is more likely to experience damage than the one parallel to it. Because of this geometric effect, Bangladesh—though lying nearly at sea level—was spared much devastation from the Sumatra earthquake.
The depth of an earthquake matters because a tsunami is only produced if the sea floor moves into the overlying ocean. If the earthquake is buried too deep in the crust, the fault that caused it may never break through to the sea floor or, if it does, may be moving too sluggishly to give the water a good punch. Shallow earthquakes with substantial vertical displacements and high moment magnitudes generate the largest tsunamis. The depth of water over the displaced crust matters because it, along with the area of the displaced crust, determines the initial volume of water displaced and able to move into the tsunami.
**This is an excerpt from my forthcoming book "The Dynamics of Disaster", due out summer 2013 from Norton Press. It can be preordered from Amazon.com here.