英语时差：震波决定地震的方向和速度（下）

And the direction the crystals are pointing makes a difference to the speed seismic waves travel through the rock: a wave traveling parallel to the crystals–in other words, along the length of the toothpick–moves faster than a wave traveling in the other direction, across the width of the crystals. So if seismic waves going through one part of the earth always travel faster in one direction than they do in another, there’s a good chance that the rock there is moving in that direction. Based on this kind of information geologists can construct models of what’s going on in parts of the earth no one has ever seen–models that show us the slow movement of molten rock, welling up from the earth’s core, escaping here and there through volcanoes, and slowly pushing the continents around on the earth’s surface.