地震：大地为什么会颤抖

一、地震从何而来

地球的外壳并不是一整块，而是由若干块巨大的岩石板块拼接而成。这些板块每年以几厘米的速度缓慢移动——大约和指甲生长的速度差不多。当两块岩层在边界处相互挤压、拉伸或错动时,岩石内部会不断积累弹性应变能。

一旦应力超过岩石的强度极限，岩层就会沿着薄弱面(断层)骤然破裂、错动，积累了几十甚至上百年的能量在几秒到几十秒内释放出来，以地震波的形式向四周传播——这就是地震。全球约 90% 的地震都发生在板块边界附近，环太平洋地震带和欧亚地震带是两条最主要的"高发地带"。

破裂开始的那个点叫震源，它在地表的垂直投影点叫震中。震源深度小于 70 公里的叫浅源地震，虽然释放的总能量未必最大，却往往破坏最严重，因为能量传到地面时衰减得少。

二、震级和烈度不是一回事

很多人把这两个概念混为一谈，其实它们描述的是完全不同的东西。

震级衡量的是这次地震本身释放了多少能量，一次地震只有一个震级数值。它是对数标度：震级每增加 1 级，能量约增大 32 倍；增加 2 级，能量就相差约 1000 倍。所以 7 级地震绝不是 6 级的"稍强一点"，而是强了三十多倍。现代地震学多使用矩震级(Mw)，比传统的里氏震级更适合描述大地震。

烈度衡量的是某个具体地点受到的影响和破坏程度。同一次地震，震中附近烈度高，越远烈度越低；地基松软的地方烈度会被放大，建筑抗震设计好的地方破坏则明显更轻。所以一次地震有一个震级，却有无数个烈度值。

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地震波主要分两类：跑得快的 P 波(纵波)最先到达，造成上下颠簸；跑得慢的 S 波(横波)随后到来,水平晃动更剧烈,破坏力更强。地震预警系统正是利用了两者的时间差——在 S 波到达前的几秒到几十秒发出警报。

三、震时怎么做

地震发生时的国际通行原则是三个动作：伏地(Drop)、遮挡(Cover)、手抓牢(Hold On)。

• 室内：立刻蹲下，躲到坚固的桌子下并抓住桌腿，用手臂护住头颈。远离窗户、玻璃隔断、悬挂物和高大家具。不要跑向楼梯或电梯，晃动中奔跑极易摔伤，被坠物砸中的风险也更高。
• 室外：向空旷处移动，远离建筑外墙、玻璃幕墙、广告牌、电线杆和围墙。
• 车内：靠边停车，避开桥梁、立交和隧道，留在车内等待晃动结束。

主震结束后，余震可能持续数天甚至更久，且强度不可预测。撤离时走楼梯不乘电梯，注意关闭燃气和电源。如果被困，优先用敲击管道、墙壁等方式发出规律声响求救，比长时间呼喊更省力、也更容易被搜救队探测到。

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结语

以目前的科学水平，人类仍无法准确预报地震发生的时间、地点和强度。但这并不意味着我们束手无策——建筑抗震设防、地震预警系统、应急演练和科学的避险知识，每一项都能实实在在地减少伤亡。减灾的关键不在于预测，而在于准备。