新托福词汇分类突破 – UNIT 145

The sudden movement of rocks along a fault causes vibrations that transmit energy through the Earth in the form of waves, called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km per second. P waves are followed by S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists to locate the focus and epicenter of an earthquake.