The essential feature is differential movement parallel to the plane of the fracture. Let's get closer and use the compass to measure the attitude of that inclined plane.
Suppose you decide to make a trek to the North Pole. You can't catch a plane there. So instead, you pull out your compass, watch the needle swing northward and plot a path, right?
假设你决定长途跋涉去北极，又不能坐飞机去，那么只好拿出指南针，根据指向北方的指针绘制出路径，对么？
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Rick: I know. It's OK. With this kind of compass, we'll be able to make our way back to the plane if we get lost inside this big old ship and can't find our way out.