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Obviously, over time,
显然 随着时间的推移
the subtle signals for any specific earthquake
任何一次地震留下的细微痕迹
disappears very quickly,
都会很快消失
and this has virtually disappeared in less than six months,
事实上 它不到6个月就消失了
so as we're scrabbling around the hillsides,
只要我们在山坡周围寻找
we see signals every now and then,
就可以不时地发现一些痕迹
but the data is very sparse,
但是这些数据很分散
very difficult to put together.
很难把它们收集起来
Geologists like Mike hope
麦克等地质学家们希望
that by mapping the past,
通过测绘已发生的地震遗址
they'll come closer to predicting the future.
能让他们更进一步地预测未来的地震
But even if you know where to measure,
但即便你知道测量从何处下手
every earthquake is complicated by a significant factor,
仍有一个关键因素使地震趋于复杂
how big it's going to be.
即地震的规模
We don't know what makes an earthquake start today
我们不清楚地震为何发生在今日
instead of yesterday.
而不是昨天
We also don't know what makes it stop
我们也不知道它为何停止
and that's what controls the size of the earthquake.
而这决定着地震的规模
A magnitude three starts at a point,
比如 3级的地震由一点开始爆发
you start to slip at a point
地壳从这一点开始滑动
and you have a rupture front that travels out
并产生了一个延伸的破裂前沿
and causes more of the fault to slip,
造成了断层的更多滑动
and in a magnitude three, you travel out this far.
3级地震会延伸这么远
In a magnitude five, it travels out for a kilometre or two.
5级地震的时候 延伸1至2千米
In a magnitude eight, it travels out for 500 kilometres,
8级地震的时候 延伸500千米
so when we're trying to predict what the earthquake will be
所以当我们去预测地震大小时
we're saying, it starts here,
我们会说 它从这里开始
but does it stop after one kilometre,
但它是在1千米后就停止呢
or does it stop after, um, 100 kilometres?
还是在100千米后才停止
