S wave - S is

Japanese: S波 - エスは

Among seismic waves, this is one of the elastic body waves that propagate in all directions within a medium. It is also called a shear wave or torsion wave because it is a wave that transmits one medium fluctuation perpendicular to the direction of wave travel one after another. When an earthquake occurs, P waves (longitudinal waves, compressional waves) reach a seismometer at a point away from the epicenter first, and S waves are the second to arrive, so they are called S waves, taking the "secondary" S from the name. They travel at a speed of about 4 km per second in the upper layers of the earth's crust. On the other hand, P waves travel at about 8 km per second, so the distance to the epicenter can be calculated from the difference in the arrival time (in seconds) of the two waves.
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Source : Heibonsha Encyclopedia About MyPedia Information

Japanese:

地震波のうち，媒質内を四方に伝搬する弾性実体波の一つ。波の進行方向に垂直な媒質の変動が次々と伝わる波なので，横波またはねじれ波ともいう。地震が起きたとき，震源から離れた地点の地震計にはP波（縦波，疎密波）が先に到達し，S波は２番めに到達する波なのでsecondaryのSをとりS波という。地殻上層で毎秒4km程度の速度。一方P波は毎秒8km程度なので両波の到達時間（秒）の差から震源までの距離を算出できる。
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