"It's a horizontal motion of two pieces of the Earth's crust on either side of a vertical crack in the Earth,"

VOA: standard.2010.01.13

For example, suppose you use vertical motion and you use y instead of x; and a would be g or -g; that's a particle falling down under the affect of gravity.

例如,假设在竖直运动中,你用 y 来代替 x,那么 a 就是 g 或者 -g,这就是一个受重力作用下落的质点

耶鲁公开课 - 基础物理课程节选

According to Teruyuki Kato, the project's leader, this arrangement can measure a buoy's vertical motion with an accuracy of a few centimetres, which will pick up dangerous tsunamis in the open ocean where they are mere ripples on the surface (they rear up into killer waves only when they reach the shallows).

ECONOMIST: New ways of tracking these killer waves may help save lives