Earthquake Prediction - To be valuable, an earthquake prediction must be accurate. A good predication would anticipate the date, location, and magnitude of the earthquake. The prediction would need to be accurate so that authorities could convince people to evacuate. An unnecessary evacuation would be very expensive and would decrease the credibility of authorities who might need to evacuate the region at a later time. Unfortunately, accurate predictions like these are not likely to be common for a long time.
The easiest thing to predict is where an earthquake will occur. Because nearly all earthquakes take place at plate boundaries, and because earthquakes tend to happen where they’ve occurred before, scientists know which locations are likely to have earthquakes. This information is useful to communities because those that are earthquakeprone can prepare for the event. For example, these communities can implement building codes to make structures earthquake safe. The added work and expense can be avoided in areas that are not at risk.
Idea by looking at the historical and geological records of earthquakes in an area. If stress on a fault builds up at the same rate over time, then earthquakes should occur at regular intervals. While this is true, there is a large margin of error in these predictions. Using this method, scientists cannot even be accurate to within a few years, and evacuation is not practical.
Seismologists have also used the seismic gap theory for long-term earthquake prediction. In this theory, scientists assume that, on average, all of the rocks on the same side of a fault move at the same rate. For example, they say that rocks on the North American plate side of the San Andreas Fault in California move at the same speed over time. While this may be true, the frequency and magnitude of earthquakes along the fault is not the same: there are more quakes in the northern and southern sections, but a relatively inactive zone in the center.
Seismologists attempted to use the seismic gap theory to predict an earthquake in a seismic gap. Around Parkfield, California earthquakes occur regularly: an earthquake of magnitude 6.0 or higher occurs about every 22 years. Using this information, seismologists predicted that a magnitude 6 or greater earthquake would strike the region in 1993. In the mid-1980s, seismologists with the United States Geological Survey set up an enormous number of instruments along the Parkfield section of the San Andreas to monitor the expected earthquake. While they were right that an earthquake was due in that segment of the fault, they were quite far off in predicting the earthquake’s timing. A magnitude 6.0 quake did not strike Parkfield until 2004, 11 years late.
Scientists have recognized some indicators that allow them to recognize that a large earthquake is likely. Large earthquakes are often preceded by small tremors, called foreshocks, that occur between a few seconds and a few weeks before a major quake. However, many earthquakes are not preceded by foreshocks and clusters of small earthquakes are not necessarily followed by a large earthquake.
Large earthquakes are also often preceded by the tilting of the ground surface, which is caused by the buildup of stress in the rocks. Seismologists measure the ground tilt and use the changes to predict an impending earthquake. While this technique has been somewhat successful, it has also been a part of predictions of earthquakes that never came and has failed to predict some that did. Water levels in wells fluctuate as water moves into or out of fractures before an earthquake. This information can also be used as a possible, but uncertain, predictor of large earthquakes.
The most successful earthquake prediction was on February 4, 1975. At the recommendation of Chinese seismologists, officials evacuated many of the residents of the Manchurian province of Liaoning. Although the region was not prone to earthquakes, the seismologists made their prediction because the area experienced about 400 small foreshocks over a few days.
The night of the evacuation an earthquake of magnitude 7.3 struck the town and only a few hundred people died. An estimated 150,000 people may have been saved. However, a little more than a year later, Chinese seismologists failed to predict the Tangshan earthquake, which killed more than 250,000 people. One month after that, Chinese officials evacuated residents of the Guandong Province for an earthquake that never came.
There is value in predicting the arrival of seismic waves from an earthquake that is already taking place. Seismometers can detect P-waves a few seconds before more damaging S-waves and surface waves arrive. Although a few seconds is not much, a coordinated computerized system can use that time to shut down gas mains and high tension electrical transmission lines, and initiate protective measures in chemical plants, nuclear power plants, mass transit systems, airports and on roadways.
Folklore tells of animals behaving erratically just before an earthquake. Mostly these anecdotes are told after the earthquake, when people remember back to the time before the shaking began. Memories are notoriously faulty. However, Chinese scientists actively study the behavior of animals before earthquakes to see if there is something to the anecdotes. One interesting tale involves the number of animals killed in the 2004 Boxing Day Tsunami, which appeared to be surprisingly low.
Reports abound suggesting that the animals had a “sixth sense” that warned them of the danger. In Sri Lanka’s Yala National Park, for example, about 60 tourists and park employees drowned but few large animals. Three elephants were seen fleeing to higher ground. On closer inspection, the elephants with tracking collars appeared to have exhibited normal movements for the day. If indeed animals sense danger from earthquakes or tsunamis, scientists do not know what it is they could be sensing, but they would like to find out.
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