Earthquakes are caused by tectonic plates sliding past each other, and predicting them was impossible before the 1970s. In 1975, scientists predicted an earthquake in China, saving thousands of lives. Long-term forecasts have limited value, and predicting large earthquakes is difficult. Injecting waste liquids to release pressure has been discussed but not implemented. Predicting earthquakes disrupting nuclear power plants is a concern.
Earthquakes occur when tectonic plates beneath continents slide violently past each other, causing strong vibrations. Unlike eclipses, earthquake prediction was entirely impossible before the 1970s. However, some progress has been made in recent decades.
In February 1975, scientists predicted an earthquake five hours in advance. The earthquake occurred in Haicheng in northeastern China and was the first time an earthquake prediction has come true. Millions of people had time to evacuate their homes and factories before the earthquake, saving tens of thousands of lives. Although many cities were completely destroyed, only a few hundred people died.
Another Chinese earthquake took place near T’ang-shan in August 1976. Although the earthquake was predicted some years in advance, about 700,000 people still died from the earthquake. Long-term forecasts appear to be of limited value.
Earthquakes can be predicted scientifically by observing tiny cracks in rocks and how they widen when the rock is under stress. Other changes are also seen when a rock begins to compress, including changes in electrical resistance and the speed at which sound waves travel through the rock. The swelling of cracks in a rock before it breaks is called expansion. It begins when the force on the rock is about half the force needed to break it.
In Russia and the United States, small earthquakes have been predicted up to five years in advance, but it can be difficult to predict large earthquakes or earthquakes around certain fault lines. The certainty rate must be high for a warning to be issued. Earthquake prediction is still a very imperfect science.
In 1966 in Denver, Colorado, waste liquids were injected under high pressure into a well. This loosened the friction between the rocks in a fault, causing small earthquakes. Using this technique to release pressure in places like the San Andreas Fault has been discussed, but not yet implemented. Clearly, smaller controlled earthquakes are preferable to a huge tectonic stress release. Earthquakes disrupting the operation of nuclear power plants are a particular concern prompting research into predicting their occurrence.
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