Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Inquiry 12.3 – Locating the Epicenter of an Earthquake Since Body waves travel through the earth, scientists use the information gathered by seismographs to learn about the interior structure of the planet. Any earthquake of magnitude 5.0 or higher will be detected by any and all seismographs on earth. Because waves lose energy as they travel, the farther a seismograph is from the epicenter, the less energy it will register for the quake. Seismic waves change speed and direction as they travel through substances of different densities. Since they know how fast P and S-waves travel, scientists can get a profile of what lies beneath the crust. P-waves are deflected as they move through the earth, and S-waves are blocked by the molten outer core because they cannot travel through liquid. Seismologists also use the information from seismographs to figure out the epicenter of the quake. The epicenter is the spot on the earth’s surface where the shaking will be the most violent, because it’s closest to the focus (the point within the crust where the energy is first released). It’s important to identify the epicenter quickly so that emergency services can be sent right away. The graph below, a Time Distance Graph, shows the known rates that P and S-waves travel. Notice that both waves start at the same moment in time and at the same place on the graph – the origin or focus. As the waves travel, the amount of time between them begins to increase. This is because P-waves travel faster than S-waves. The farther the waves travel, the greater the difference in time between them. What this means is that seismographs close to the epicenter of a quake will show very little difference between P and S-wave arrival times; seismographs far away from the epicenter will show a much bigger difference in the arrival of P and S-waves. The Seismograms above all registered the energy from the same earthquake. Comparing them tells us that the epicenter of the quake was closest to Tokyo because… The P-wave reached Tokyo before any of the others. The amplitude of the waves (the squiggly lines) was highest in Tokyo, so the seismic waves had the most energy there. The arrival times of P and S-waves were the closest in Tokyo, meaning they were detected very quickly after the energy was first released at the focus. How do seismologists calculate the epicenter of a quake??? The information on a seismogram can tell scientists how far that seismograph station is from the epicenter of a quake. As seismic waves travel, the amount of time difference between the arrival of P and S-waves increases. A seismograph near the epicenter of a quake will show very little difference in time between P and S-waves; a station very far away will show a big difference between the arrival of P and S-waves. Let’s say that at 8:07 am, a seismograph station in Sitka, Alaska registers a P-Wave. Four minutes later, the S-waves are picked up. The seismogram for that station will look something like this: The seismologist at the station then goes to the time distance graph and looks to see where there is a 4-minute difference in the arrival of P and S-waves… The chart shows that his seismograph station must be 2,500 km away from the epicenter of the earthquake. This allows him to identify possible locations. Since measuring out from a single point in many directions will basically give him the circumference of a circle around the station, he uses a compass and the scale on the map to create that circle. Clearly, this information alone is not enough to identify a single point along the perimeter of the circle as the actual epicenter (by the way, the distance around a circle is called the circumference of the circle) Information is also gathered from the same quake at a station in Charlotte, North Carolina. The seismogram is shown below: Using the Time-Distance graph, the seismologist figures out that for there to be a 5 minute difference between P and Swaves, his station must be 3,500 km away from the epicenter. A third Seismograph station in Honolulu, Hawaii picks up the P-wave from the quake at 8:09, and it takes 6 minutes for the S-waves to arrive: The Time-Distance Graph shows that his station must be 4,500 km from the epicenter… When all three maps are put together, the single possibility for the location of the epicenter that satisfies all three seismograph stations is identified… This method is called Triangulation, and is what is programmed into supercomputers that calculate the information almost instantly.