Focal Mechanisms, BeachBalls And All That
After seeing a lot of questions and comments recently about this subject I thought that a brief explanation may not go amiss. NOTE: I don't claim to be an expert on the subject and am always open to CONSTRUCTIVE criticism.
The diagram below shows the main types of mechanisms and some of the associated tectonics. The 'beach balls' show very straight forward mechanisms ie., pure Extension, Compression, and Strike-slip and although these do occur regularly there is almost infinite number of variations of these three types. For example the Alpine Fault up the spine of the South Island of New Zealand. This fault marks the boundary between the Indo-Australian and Pacific plates and is deemed an oblique thrust fault, ie. it has a strike-slip component and a thrust component.
Fig.1 (A) shows the recorded compressional and dialational first motions as plotted on the lower hemisphere stereonet. Once a number of first motions have been collected from many seismic recording stations, a pattern forms that gives a good indication of the focal mech. of the quake. (B) shows a SW - NE trending left lateral strike-slip fault, indicating the diagonally opposite compression and dialation quadrants.


(C) here we see a typical rifting situation, eg. the East Pacific Rise or the Mid-Atlantic Ridge , where sections of spreading ridges (Extensional)are offset by fractures (strike-slip). Finally in (D) we see a cross-section through a subduction zone. This can be found in many places around the world, eg the Fiji - Tonga region, Japan, and the Aleutian Is., Alaska. A detailed study of the focal mechanisms of large subduction zone quakes indicates that they are depth dependant. Shallow and medium-depth events usually have foc. mech.'s with down-dip extension, whereas the deeper events > ~300 km have down-dip compression. The former is thought to indicate that there is a physical barrier slowing or stopping the subducting lithospheric slab. Above the downgoing slab, the continental crust is undergoing considerable compression resulting in the numerous high angle reverse (thrust) faults. For Moment Tensor Solutions and maps with the beachballs for the latest world quakes try this USGS current seismicity site scroll down the page to the reports for each month and the individual events.