|MadSci Network: Earth Sciences|
The broad question you are asking is an excellent one! Are there more earthquakes where lots of people live? In other words, are earthquakes caused by highly populated places? It often seems like this might be the case, but it actually turns out to be a big coincidence. Be sure to check out some of the webpage links that Iíve attached below that show global maps of earthquake locations, magnitude and depths. One of the things you can immediately notice from these maps (especially the first link) is that earthquakes donít appear randomly around the world. Instead, there seem to be definite patterns of where earthquakes happen. For example, look at all the earthquakes that happen around the edge of the Pacific Ocean. Also, there are long strings of earthquakes that happen in lines through the middle of the oceans. In general, earthquakes outline the edges of tectonic plates. Tectonic plates are large pieces of the Earthís crust that are moving very slowly across the globe. You can think of these plates as floating on top of a more fluid, hotter and deeper layer of the Earth, called the mantle. At the center of the oceans, the tectonic plates are spreading apart and hot mantle material is flowing up, cooling and creating more oceanic crust. At these regions, called mid-ocean ridges, there are small, shallow earthquakes (the lines of earthquakes in the centers of the oceans happen along the mid-ocean ridges). At other places on the Earth, like around the edge of the Pacific Ocean, these tectonic plates are crashing into each other (very slowly) and the oceanic crust plunges beneath the continent. In this case, you can get very large earthquakes that occur deep inside the Earth as the plate moving downward pushes against the surrounding mantle material. These regions are called subduction zones and the edge of the Pacific Ocean is surrounded by subduction zones, so there are lots of earthquakes that happen at the edges of continents next to the Pacific Ocean. The third type of boundary between tectonic plates is called a strike-slip zone, where the plates are sliding past each other. The most famous strike-slip zone in the world happens to be all along the coast of California, through Los Angeles and San Francisco. At strike-slip zones, you can get very dangerous earthquakes happening at shallow depths. OK, that was a lot of explanation just to describe the earthquake patterns that you see on the world maps below. Back to your question about people and earthquakes. Again, although it may initially appear like places with more people have more earthquakes, that isnít actually the case at all. Rather, it is a complete coincidence. First, there are many earthquakes that happen where people donít live at all, like in the middle of the oceans. Second, if you just look at the United States, while there are lots of people who live on the west coast, there are even more people who live on the east coast. But, there are many more earthquakes that happen along the west coast of the United States. This is because the west coast of the United States is actually a subduction zone (in Washington and Oregon) and a strike-slip zone (in California). Thus, you can get very intense earthquakes all along the west coast. Plus, most of the worldís population lives at the edges of continents, where there is a beautiful ocean, warm temperatures and wonderful sights. This is dangerous, especially if the plate boundary near by can have large earthquakes along it (which is often the case). In fact, many geologists worry about the huge cities that are built in dangerous earthquake regions, especially if the people who build the cities are not careful and build buildings that can be destroyed easily. One particular geologist who is very worried about population growth at tectonic plate boundaries is named Roger Bilham and he is a geologist at Oxford University in England. Iíve referenced two of his papers that might be interesting to you at the bottom of the list of web pages. Just to quote from his letter in the journal Nature (1999), "About one-third of the worldís supercities Ė those with populations of more than two million Ė are located near tectonic plate boundaries, where damaging earthquakes have occurred and will recur". Third, I think one of the main reasons that there seems to be a connection between large numbers of people and earthquakes is because of the news that we get from around the world. Earthquakes (especially small, not dangerous ones) are extremely common around the Earth. Iíve listed a link below which will show you the total number of earthquakes in the past 30 days, and itís amazing how many earthquakes there are every day! Luckily, often these earthquakes are so small that you canít even feel them. However, when a big earthquake strikes, weíre more likely to hear about it in the news if it struck a place with lots of people, like the earthquakes that happen in California and recently in Turkey. The truth is, we tend to hear more news if the earthquake was dangerous and damaged a lot of property and possibly hurt a number of people. So, unless we dig deeper than the evening news shows, we often only hear about large earthquakes in populated cities. For all of these reasons, geologists definitely agree that it is not high numbers of people that are causing earthquakes, but rather that people tend to like living at the edges of continents and thatís where many earthquakes happen. So it is a complete coincidence. But you also had another more subtle question and that is about the affect of water on earthquakes. First, there is no evidence at all that earthquakes follow any kind of yearly pattern. That is, there arenít any more earthquakes during the summer than the winter, or during dry seasons more than wet seasons. Earthquakes happen at all times of the year, under all sorts of different conditions. Second, if water somehow affected earthquakes, it would only affect shallow, relatively low intensity earthquakes in the upper few miles of the Earthís crust. Again, there is no evidence that this is common. Third, and this is a bit strange, there isnít any evidence to suggest that pumping water out of the ground causes earthquakes, but there is a little bit of evidence that suggests if you pump, or inject, water into the ground, small shallow earthquakes might happen. Geologists studied this effect for a while as an idea for triggering small earthquakes (so small that you wouldnít be able to feel them) so that larger earthquakes could be avoided. In the end, they decided that even when pumping water into the ground, they could not easily predict the size and location of the small earthquakes they were creating. Also, it is just a bad idea in general to try and make earthquakes. Instead, geologists are trying very hard to learn how to predict when a large earthquake might occur. Also, engineers and geologists are working together to design and build buildings that will be able to withstand the forces of earthquakes, thereby reducing the damage to property and human life that occurs with large earthquakes. Itís a fascinating topic, and this has been a very long answer. I hope this helps you out. Map of the world with earthquake locations on it: http://atlas.geo.cornell.edu/education/instructor/earthquakes/worldseis.html Last thirty days of earthquake activity in the world: http:// wwwneic.cr.usgs.gov/neis/qed/qed.html All sorts of earthquake data can be found at the United States Geological Surveyís earthquake homepage and their National Earthquake Information Center: http://earthquake.usgs.gov/ http://neic.usgs.gov/ Two articles by Roger Bilham about population and earthquakes: Bilham, R. (1999) "Millions at risk as big cities grow apace in earthquake zones", Nature, vol 401, pg. 738. Bilham, R. (1988) "Earthquakes and urban growth", Nature, vol 336, pp. 625- 626.
Try the links in the MadSci Library for more information on Earth Sciences.