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| General Questions and Answers on Earthquakes |
| Q: What is the
biggest earthquake that has ever happened? A: An earthquake in Chile in 1960 broke a fault over one thousand miles long, and had a magnitude (Mw) of 9.5. The Richter scale is not unlimited; a San-Andreas type earthquake rupture that circled the entire Earth would have a magnitude of about 10. (USGS list of the ten largest earthquakes) Q: What is a foreshock? What is an aftershock? A: All of these are earthquakes. Aftershocks are smaller earthquakes occurring after a main shock, and they keep the hazard relatively high for a certain period after any large earthquake. Foreshocks are smaller earthquakes occurring before a main shock. Only with hindsight can we distinguish what are foreshocks and aftershocks. Q: What is the Richter scale? A: It gives us a measure of the total energy released by an earthquake. It is exponential; an increase of one magnitude means thirty times as much energy. Ideally you should get the same magnitude for an earthquake no matter where you measure it. Seismologists also use an intensity scale to quantify the effects of an earthquake at particular places. Q: What's the difference between an M 4 and an M 5, or an M 4 and an M 6 earthquake? A: Each step of one in magnitude is an increase of thirty times in the total energy of the earthquake. That is the same as an increase of ten times in amplitude, or the size of typical ground motion. So an M 6 quake has 900 times as much energy as an M 4 quake. Also, as the magnitude increases, the shaking gets stronger, and for a magnitude 6 or more earthquake it lasts a lot longer. Q: Is any place safe from earthquakes? A: No place is completely safe from natural hazards. Ask yourself what kinds of hazards you are willing to live with, and to prepare for. Regions of the U.S. that have almost no earthquake hazard have instead far greater hazards from floods, tornadoes, or hurricanes. It is possible to protect and insure yourself and your family quite well from earthquake hazards. Q: Could bridges collapse in an earthquake? A: Yes. Several collapsed in the Northridge earthquake in January 1994. Also in the ChiChi, Taiwan earthquake in September, 1999, a large number of bridges collapsed. The Nevada Department of Transportation should be consulted if you are interested in any specific bridge, since we don't have any expertise in bridge design. In the past few years, they have done a lot of work to strengthen bridges for earthquake resistance. |
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