地震：大地為什么會顫抖

一、地震從何而來

地球的外殼并不是一整塊，而是由若干塊巨大的巖石板塊拼接而成。這些板塊每年以幾厘米的速度緩慢移動——大約和指甲生長的速度差不多。當兩塊巖層在邊界處相互擠壓、拉伸或錯動時,巖石內部會不斷積累彈性應變能。

一旦應力超過巖石的強度極限，巖層就會沿著薄弱面(斷層)驟然破裂、錯動，積累了幾十甚至上百年的能量在幾秒到幾十秒內釋放出來，以地震波的形式向四周傳播——這就是地震。全球約 90% 的地震都發生在板塊邊界附近，環太平洋地震帶和歐亞地震帶是兩條最主要的"高發地帶"。

破裂開始的那個點叫震源，它在地表的垂直投影點叫震中。震源深度小于 70 公里的叫淺源地震，雖然釋放的總能量未必最大，卻往往破壞最嚴重，因為能量傳到地面時衰減得少。

二、震級和烈度不是一回事

很多人把這兩個概念混為一談，其實它們描述的是完全不同的東西。

震級衡量的是這次地震本身釋放了多少能量，一次地震只有一個震級數值。它是對數標度：震級每增加 1 級，能量約增大 32 倍；增加 2 級，能量就相差約 1000 倍。所以 7 級地震絕不是 6 級的"稍強一點"，而是強了三十多倍。現代地震學多使用矩震級(Mw)，比傳統的里氏震級更適合描述大地震。

烈度衡量的是某個具體地點受到的影響和破壞程度。同一次地震，震中附近烈度高，越遠烈度越低；地基松軟的地方烈度會被放大，建筑抗震設計好的地方破壞則明顯更輕。所以一次地震有一個震級，卻有無數個烈度值。

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地震波主要分兩類：跑得快的 P 波(縱波)最先到達，造成上下顛簸；跑得慢的 S 波(橫波)隨后到來,水平晃動更劇烈,破壞力更強。地震預警系統正是利用了兩者的時間差——在 S 波到達前的幾秒到幾十秒發出警報。

三、震時怎么做

地震發生時的國際通行原則是三個動作：伏地(Drop)、遮擋(Cover)、手抓牢(Hold On)。

• 室內：立刻蹲下，躲到堅固的桌子下并抓住桌腿，用手臂護住頭頸。遠離窗戶、玻璃隔斷、懸掛物和高大家具。不要跑向樓梯或電梯，晃動中奔跑極易摔傷，被墜物砸中的風險也更高。
• 室外：向空曠處移動，遠離建筑外墻、玻璃幕墻、廣告牌、電線桿和圍墻。
• 車內：靠邊停車，避開橋梁、立交和隧道，留在車內等待晃動結束。

主震結束后，余震可能持續數天甚至更久，且強度不可預測。撤離時走樓梯不乘電梯，注意關閉燃氣和電源。如果被困，優先用敲擊管道、墻壁等方式發出規律聲響求救，比長時間呼喊更省力、也更容易被搜救隊探測到。

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結語

以目前的科學水平，人類仍無法準確預報地震發生的時間、地點和強度。但這并不意味著我們束手無策——建筑抗震設防、地震預警系統、應急演練和科學的避險知識，每一項都能實實在在地減少傷亡。減災的關鍵不在于預測，而在于準備。