What are the differences between P and S waves?
Scientists have learned about Earth’s internal structure by studying how these waves travel through the Earth. The technique is straightforward — it involves measuring the time it takes for both types of waves to reach seismic stations from the epicenter of an earthquake. Since P waves travel faster than S waves, they’re always detected first. The farther away from the epicenter, the larger the time interval between the arrival of P and S waves — and if the Earth were built of a uniform substance, that would be the only variation measured. Scientists, however, noticed variations that could not be accounted for based simply on the distance traveled from the epicenter. For instance, they noticed places in the Earth through which S waves didn’t travel. Geologists inferred that these sections of the Earth were liquid, through which S waves (which, remember, are shear waves) cannot travel. You may not know it, but you are probably already familiar with this phenomenon. In a bathtub, if you
