地表垂直向地震仪记录的P波初动方向有向上的,记为正号,也有向下的,记为负号。正号P波又称压缩波,因为这种波初动到达台站时,台站受到来自地下的一个突然挤压,介质体积发生一微量缩小。负号波又称膨胀波,初动使台站受到来自地下的一个突然拉伸,介质体积发生一微量膨胀。 The vertical direction of the P wave recorded by the seismograph perpendicular to the surface of the earth has a positive sign, a positive sign and a negative sign. The positive P-wave is also called compressional wave, because when this wave first arrives at the station, the station is subjected to a sudden squeezing from the ground and the volume of the medium is reduced by a small amount. Negative wave, also known as the expansion wave, the first move to the station by a sudden stretch from the ground, the media volume occurred a slight expansion.