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.
- vibration [vaiˈbreiʃən] n.振动
- transmit [trænzˈmit] n. 传送
- primary [ˈpraiməri] adj. 初级的
- secondary [ˈsekəndəri] adj. 第二等级的
- primary wave [ˈpraiməri - weiv] n. 纵波
- secondary wave [ˈsekəndəri - weiv] n. 横波
- radiate [ˈreidieit] v. 散发
- compression [kəmˈpreʃən] n. 压缩
- back and forth [bæk - ænd - fɔ:θ] adv. 来回地
- alternate [ɔ:lˈtə:nit] adj. 交替的
- seismic wave [ˈsaɪzmɪk - weiv] n. 地震波
- shear [ʃiə] v. 剪切
- twist [twist] n. 扭曲
- side to side [said - tu - said] adv. 从一边到另一边
- up and down [ʌp - ænd - daun] adv.上上下下
- perpendicular [ˌpə:pənˈdikjulə] adj. 垂直的
- epicenter [ˈepisentə] n.震中
