First Motions

Another discovery linking the geological and seismological views of an earthquake, made possible with the expansion of seismic networks, deals with first motions. In the early 20th Century, Japanese seismologists began investigating the sense of motion that accompanied the very first seismic wave arrival at a seismometer. They found that these first motions could be anaseismic, meaning upward or away from the source, or kataseismic, meaning downward or toward the source. (You should have seen examples of these two types of first arrivals in Activity #2 of this section.) Though initially thought to be the result of different types of earthquakes, it was soon discovered that a single earthquake could produce both types of motion.

This knowledge, combined with the developing theories of fault rupture, led to the suggestion that these first wave motions were indicators of slipping motion from the actual fault rupture producing the seismic waves. Imagine an east-west oriented, right-lateral strike-slip fault, like the one shown at left. Now imagine standing on the south side of that fault, facing east, when an earthquake begins to rupture the stretch of fault in front (east) of you. What would you feel?

The first motion you would experience would be anaseismic, pushing you away from the source of the earthquake, as your side of the fault experiences compressional force from the motion of the fault rupture. Play the animation at left to help you envision this. The areas of the block diagram that turn red experience compression when the fault moves; those areas that fade to white experience the opposite kind of motion, dilatation, as they are initially pulled toward the source with kataseismic motion.

Though "anaseismic" and "kataseismic" are somewhat outdated terms, replaced by "compressional" and "dilatational", the study of first motions has become a staple of modern seismology. Analysis of records made from the seismic instruments surrounding the source of an earthquake allow seismologists to determine the sense of slip of that earthquake, even if the rupture does not reach the surface. This is done by creating a model of the initial rupture motion, called a focal mechanism.