Locating a minor earthquake Many around the world are monitoring the story of missing Malaysia Air flight MH370. On Friday, geology entered the story, and I give credit to the US Geological Survey for carrying the day and motivating me to write about earthquake location About 2 hours after the last known contact with the plane, there was a small, magnitude 2.7 earthquake recorded by seismic stations on the Malay Peninsula – stations shown by blue triangles in the left image. The last contact with the plane is shown by the black star and on Friday a group from the University of Science and Technology of China suggested that the red star was the location of this earthquake and could mark somehow the plane crashing and exploding in shallow water. As you can see from the seafloor topography, there is not much in that area which would drive earthquakes (no mountains or anything), so an earthquake at that location could well be difficult to explain. To understand what went wrong however, we need to look at how earthquakes are located. Seismic waves travel through the crust at speeds that are known, a property that allows earthquakes to be located if they are detected by more than 1 station. Seismic waves will take different amounts of time to reach each seismic station, so each station will see the quake signal at a different time. The difference in the time each station records the quake can be turned into a distance on the Earth’s surface. In the image at the left, you can see this step from the two pink circles; each one represents how far seismic waves could have traveled before reaching that station. Once 2 different stations detect the same quake, the difference in time narrows the earthquake location down to a couple points. But you also can see that there are 2 intersections between those circles; the highlighted one and another just off the coast of Sumatra. Based only on 2 stations, either of those locations could have produced the earthquake; to locate the quake in 3 dimensions at least 3 stations are needed. Enter the US Geological Survey, who obtained data from 2 additional seismic stations offshore of Sumatra, shown in the image at the right. Each of these stations recorded the earthquake and the time it arrived, giving 4 points and allowing confirmation of the quake’s location off the coast of Sumatra. Sumatra’s coast is very seismically active; it’s the same area that produced two incredibly large earthquakes in 2004 and 2005, so a small quake in this area is not surprising at all. It’s difficult to explain how a crashing plane could set off a signal that looks like an earthquake, but explosions do leave seismic signatures, so an earthquake in an unusual area might well have been worth investigating, but credit to the USGS for verifying that this quake was a perfectly normal event. This discussion should also show the benefits of having detailed seismic coverage; the more seismic stations available, the easier it is to locate an earthquake to a specific fault or location; data which can sometimes be vital in unexpected ways. -JBB Image credit and reports: scmp/news/asia/article/1448353/us-suggests-search-missing-flight-expand-indian-ocean-mh370-satellite earthquake.usgs.gov/earthquakes/eventpage/usc000nb9b#summary
Posted on: Sat, 15 Mar 2014 03:15:01 +0000
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