I think it is time to learn something about earthquakes, given the terrible tragedy in Haiti and more recent event in Chile.
The United States Geological Service hosts an excellent web site at http://earthquake.usgs.gov/. There are quasi-interactive maps allowing you to see where recent earthquakes are occurring. Here is a map of South America showing many after-shocks in Chile within the past 24 hours:
From the brief summary of the Chile earthquake, one learns that Chile has a lot of earthquakes – 13 events of magnitude 7.0 or greater since 1973! No wonder President Michelle Bachelet is able to handle such a terrible event with calm determination. In 1960 there was an earthquake of magnitude 9.5 – the worst one in 200 years – which spawned a tsunami that engulfed the entire Pacific Ocean. In 2007 there was an earthquake of magnitude 7.7, in 2005 one of magnitude 7.8, and in 1995 and 1985 two of magnitude 8.0.
Chile has an “extravagant” geology, fringed by the Pacific Ocean and the Andes. It rests on the Nazca Plate which moves eastward about 10 cm per year, and forces itself under the continental plate of South America proper. One consequence of this movement is the formation of the Peru-Chile Trench, which is 150 km wide and 5 km deep. Basically, Chile rests on top of a giant precipice, covered from view by the ocean. Another consequence is the formation of the Andes mountains, including many volcanoes. The movements of the Nazca Plate is responsible for the high incidence of earthquakes. (Source: country studies)
Scientists are able to fit the data to a model of the movement of the plates. They need some knowledge of the geometry and geology of the region, which they refine each time earthquakes are recorded. Fits to the data indicate a depth of 35 km and a “strike-slip” fracture. The USGS web site has a wonderful glossary with an animation. Here is the rate of energy released as a function of time – the numbers are astronomical:
Here is the predicted travel times around the globe, in minutes:
From what I can gather, a fault generates an acoustic wave which is transmitted in a channel defined by the surface of the earth and a deeper level with higher density in which the wave can propagate fast. Refraction plays a major role. See some nice animations at Jeffrey Barker’s web pages (SUNY Binghamton). Another source of animations and elementary explanations is the web site for the Incorporated Research Institutions for Seismology.
Despite the experience Chileans have with earthquakes, the situation there is very bad. According to an article in the New York Times, two million people are displaced, with several hundred killed. This earthquake is about 1000 times stronger than the one in Haiti, but because earthquakes are far more common in Chile, buildings and infrastructure are much better designed, and emergency services are much better prepared.
Update: Apparently the earth’s rotation has been measurably changed by this earthquake, shortening the day by a bit more than a microsecond. For more information, see The Reference Frame.
Entry filed under: Education.