Earthquakes are scary natual disasters. If you have questions, we have answers.
A: No. Neither the USGS nor any other scientists have ever predicted a major earthquake. We do not know how, and we do not expect to know how any time in the foreseeable future. USGS scientists can only calculate the probability that a significant earthquake will occur in a specific area within a certain number of years.
A: The earliest reference we have to unusual animal behavior prior to a significant earthquake is from Greece in 373 BC. Rats, weasels, snakes, and centipedes reportedly left their homes and headed for safety several days before a destructive earthquake.
A: A temporary increase or decrease in seismicity is part of the normal fluctuation of earthquake rates. Neither an increase nor decrease worldwide is a positive indication that a large earthquake is imminent. The ComCat earthquake catalog contains an increasing number of earthquakes in recent years--not because there are more earthquakes, but because there are more seismic instruments and they are able to record more earthquakes.
A: In the 4th Century B.C., Aristotle proposed that earthquakes were caused by winds trapped in subterranean caves. Small tremors were thought to have been caused by air pushing on the cavern roofs, and large ones by the air breaking the surface.
A: An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.
A: "Foreshock" and "aftershock" are relative terms.
Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.
Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes. (modified from Univ. of Washington)
A: It isn't that simple. There is not one magnitude above which damage will occur. It depends on other variables, such as the distance from the earthquake, what type of soil you are on, etc. That being said, damage does not usually occur until the earthquake magnitude reaches somewhere above 4 or 5.
A: Sometimes.
Earthquakes, particularly large ones, can trigger other earthquakes in more distant locations though a process known as dynamic stress transfer/triggering. This means that the energy from the seismic wave passing through can cause a new earthquake, usually in vulnerable locations prone to frequent earthquakes (e.g., volcanic regions). Examples of large events that triggered distant seismicity include the 1992 M7.3 Landers earthquake, 2002 M7.9 Denali earthquake, and the 2004 M 9.1 Sumatra earthquake that ruptured an area ~1300x200 square km, and triggered aftershocks from northern Sumatra to just south of Myanmar.
If a triggered earthquake is within a distance of about 2-3 fault lengths of the fault rupture associated with a mainshock, the earthquake is considered to be an aftershock, not a triggered event.
A: Although earthquake magnitude is one factor that affects tsunami generation, there are other important factors to consider. The earthquake must be a shallow marine event that displaces the seafloor. Thrust earthquakes (as opposed to strike slip) are far more likely to generate tsunamis, but small tsunamis have occurred in a few cases from large (i.e., > M8) strike-slip earthquakes.
