Seismic Waves

Since the Earth or any other planetary body can be considered to be an elastic object, it will support the propagation of traveling waves. A disturbance like an earthquake at any point on the Earth will produce energetic waves called seismic waves.

由于地球或任何其他行星体都可以被视为弹性体，因此它会支持行波的传播。地球上的任何一点发生扰动，如地震，都会产生称为地震波的高能波。

The Earth's crust as a solid object will support waves through the crust called body waves and on the surface (surface waves). In a solid material these waves can be either longitudinal waves or transverse waves. For seismic waves through the bulk material the longitudinal or compressional waves are called P waves (for "primary" waves) whereas the transverse waves are callled S waves ("secondary" waves). Since any material, solid or liquid (fluid) is subject to compression, the P waves can travel through any kind of material. However, S waves depend upon a resistance to transverse or "shear" force which does not exist in a liquid or gas medium, so they can only travel in the solid parts of the Earth (see geologic example).

地球的固体外壳会支持通过外壳的波，称为体波，以及在表面（表面波）。在固体材料中，这些波可以是纵波或横波。对于通过整体材料传播的地震波，纵波或压缩波称为P波（“初级”波），而横波称为S波（“次级”波）。由于任何材料，无论是固体还是液体（流体），都可能被压缩，因此P波可以在任何类型的材料中传播。然而，S波依赖于横向或“剪切”力的阻力，而这种阻力在液体或气体介质中不存在，因此S波只能在地球的固体部分传播（见地质示例）。

P waves in air are simply sound waves and the speed of sound is around 340 m/s for ordinary temperatures. Water can support P waves but not S waves, and the speed of these P waves (speed of sound) in water is about 1450 m/s. P waves depend upon the bulk modulus of elasticity for the material as well as its density, and the wave speed in a solid material like granite can be about 5000 m/s. The P waves from an earthquake arrive first, but because of their small amplitudes don't do as much damage as the S waves and surface waves which follow.

空气中传播的P波其实就是声波，其声速大约为340米每秒（在普通温度下）。水可以支持P波，但不能支持S波，这些P波（即声波）在水中的速度约为1450米每秒。P波的传播速度取决于材料的体积弹性模量和密度，例如在花岗岩这样的固体中，P波的速度可以达到约5000米每秒。地震中，P波是首先到达的，但由于其振幅较小，造成的破坏不如随后到达的S波和表面波严重。

S waves are transverse waves which involve movement of the ground perpendicular to the velocity of propagation. They travel only through solids, and the absence of detected S waves at large distances from earthquakes was the first indication that the Earth has a liquid core. S waves travel typically 60% of the speed of P waves. They are typically more damaging than the P waves because they are several times higher in amplitude.

S波是纵波，其运动方向与波的传播速度垂直。它们只能通过固体传播，而地震发生时在远距离处未检测到S波是首次表明地球有液态核心的迹象。S波通常以P波速度的60%传播。它们通常比P波更具破坏性，因为振幅高出数倍。

Earthquakes also produce surface waves which may cause motion perpendicular to the surface or parallel to the surface. The waves which move the surface up and down are called Rayleigh waves and are sometimes described as "ground roll". Waves whose amplitude of moton is parallel to the surface are called Love waves (named after the mathematician A. E. H. Love who modeled them - definitely not an indication that anyone is fond of them).

Rayleigh waves or ground roll waves cause the surface of the ground to move up and down. They travel at roughly 90% of the speed of the S waves. Love waves involve the motion of the ground side-to-side, perpendicular to the propagation velocity. They usually travel slightly faster than the Rayleigh waves.

瑞利波或地面滚波使地面上下移动。它们的传播速度大约是剪切波速度的90%。洛夫波涉及地面侧向运动，与传播速度方向垂直。它们通常比瑞利波传播得稍快。

地震也会产生表面波，这些波可能引起垂直于表面或平行于表面的运动。使表面上下移动的波称为雷利波，有时也被称为'地面滚动'。那些振幅平行于表面的波称为洛夫波（以数学家A. E. H. Love命名，他建立了这些波的模型——这显然不表明任何人喜欢这些波）。