本文是USGS的What is an earthquake swarm?一文中部分段落的中文翻译。请点击链接来浏览英语全文。 / This is a translation of several selected paragraphs from What is an earthquake swarm? on USGS. Please refer to the link for the full English text.
译序 / Preface
经过半个晚上的提心吊胆，终于上车准备合眼。睡前在微博上看到@风云梦远提到我正经历的地震属于震群型，在求知欲和求生欲的双重作用下终于在网上找到一篇由浅入深介绍震群型地震的文章，稀里糊涂向作者申请了翻译和搬运就睡了。没想到天亮之后 是的！活着见到明天的太阳了 收到了作者的许可。于是带着重生的喜悦迅速写就了这篇小翻译，希望能和大家分享一些有关地震的知识。
中文译文 / Chinese translation
在我们给出震群型地震的定义、它区别于其他地震活动的特征以及震群型地震的形成过程1之前，让我们先退一小步。许多地震具有一种并非震群型，而是主震-余震的主余型发震规律。在最简单的情况下，这种规律意味着在地震序列中先发生最大的地震，而后发生一系列小地震，而这些小地震的震级在几周到几十年的时间内慢慢消减。一般来说，震级最大的余震要比主震在震级上小一个震级单位2。
然而并不是所有的地震序列都会表现出这种规律。有时，最严重的地震并不会成为一个地震序列中的第一次，相反，它可能会发生在序列的中间；有时，一个地震序列中可能有数次震级与最大震级相当的地震；有时，序列中地震的震级并不随着时间减弱，而是在数日、数周、或数月保持稳定甚至与时俱增。这些不符合主余型地震规律的地震序列通常被认定为震群型序列。
何时一个主余型地震序列会成为一个震群型地震序列？其实没有一个准确的定义。事实上，它们之间的区别并不是非黑即白的。各类地震序列的发震规律落在”典型主余型“和”典型非主余型“之间。当相对多的地震集中在一个相对小的区域，而这些地震并不符合主余型地震规律时，我们会用”震群型“来描述它们。
Corresponding text / 英文原文
Before we answer these questions, let’s take a small step back. Many earthquakes follow a non-swarm pattern known as a mainshock-aftershock sequence. In its simplest form, this means that the largest earthquake occurs first in this sequence, followed by a series of smaller shocks, decaying over a time period ranging from weeks to decades. On average, the magnitude of the largest aftershock tends to be about one magnitude unit smaller than the mainshock.
However, not all earthquake sequences follow this pattern. Sometimes, the largest event might not be the first event in a sequence; instead it might occur in the middle. Sometimes sequences can have many earthquakes with magnitudes similar to the largest earthquake of the sequence. Sometimes sequences don’t decay over time, but rather remain steady or even increase in their activity rates over periods of days, weeks, or even months. Sequences that don’t fit a mainshock-aftershock pattern are typically considered swarms.
There is no precise definition of when a mainshock-aftershock sequence becomes a swarm. In reality, the distinction is not sharp. Earthquake sequences follow a whole range of behaviors from “very mainshock-aftershock” to “very unlike mainshock-aftershock.” The “swarm” designation is typically applied when we observe relatively many earthquakes within a relatively small area, which just don’t fit the pattern of a mainshock-aftershock sequence.
