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地震发生时纵波先达横波后至,而后者是造成灾害的主要原因.地震的危害,不仅仅在短短那十几秒到几分钟的主震,还有其后多次的余震.了解地震危害,做好避震抗震工作,我们都在努力。

April 18,1906 ,Carnifornia San Andreas Fault  snaps, shaking San Francisco  for nearly  60 terrifying seconds .When the trembling stops,the disaster is only begnning .

Gas lines rupture  ,setting off massive fires .Some 700 people die .Most of the city is reduced to ruins .This trembling of the ground causes when massive of rocks suddenly shift below the earth surface is called an earthquake. Hundreds of little earthquakes shake the planet every day ,but most pass unnoticed .They usually occur along the boundaries of the thin plates that cover the earth like an egg shell ,driven by the heat deep within the earth core. The plates grind each other along lines called folds .when a plate’s motion is blocked,stress builds up .Finally the fold gives away.The released energy races through the earth in the form of seismic waves .Scientists record these waves on a device called a seismograph .These zigzag lines show the strength of various seismic waves. Using the line, scientists grade earthquakes on the Richter Scale .For a quake to measure one number higher of Richter Scale ,it must release about 30 times as much energy as the number below it .

Every year about 100 000 earthquakes ramble through the ground hard enough for people feel them .Of these ,only about 1000 are strong enough to damage property .But a powerful earthquake leaves devastating an average of about 10000 people die each year as a result of earthquakes .The greatest recorded earthquake  ever hit North America measured a massive 9.2 .The tremor struck Alaska on March 28 1964.A camera onboard a ship docting Boutdeses recorded the draining of the entire harbor as a cabin opened up on the seafloor .There is no stopping the surface of the earth from changing and moving, so engineers are focusing on ways to create better buildings ,highways and bridges .Structures that will remain safe and stay in one piece the next time the earth begins to shake .