How can we determine fault orientation & motion from the seismograms? Or what are those little beach balls on all the maps?
A focal mechanism is a representation of the geographic orientation of the fault that caused an earthquake, based on information from the seismograms. Dip, strike & rake conveniently indicated on a diagram of the focal sphere, which is an imaginary sphere surrounding the hypocenter of the quake. There are two basic types of focal mechanisms available. In a first-motion solution, the direction of the first motion on the vertical seismogram is indicated on the focal sphere, at the location where the ray to that particular station left the hypocenter. “Up” or “compression” first motion indicates that the rock on that side of the hypocenter moved away from the hypocenter first. “Down” or “dilatation” first motion indicates the opposite. There are two possible fault planes, at right angles to each other, for each first motion solution. The preferred one is usually selected based on knowledge of the geology. Examples for each type of fault can be found at http://quake.wr.usgs.gov/recenteqs/b
