FAQ - Measuring Earthquakes

Q: What are those beachball figures?

A: In addition to determining the location and magnitude of earthquakes, seismologists are now routinely determining the "fault plane" solutions or "focal mechanisms" of events. A fault plane solution illustrates the direction of slip and the orientation of the fault during the earthquake. These solutions, which are displayed in lower-hemisphere projections frequently described as "beachballs", can be determined from the first-motion of P-waves and from the inversion of seismic waveforms. These figures help identify the type of earthquake rupture: strike-slip, normal>/a>, or thrust.

Strike-slip earthquakes are typical of the San Andreas fault zone, which forms part of the boundary between the North American and Pacific plates. Normal earthquakes are associated with extension, particularly with formation of plates at mid-ocean ridges. Thrust or reverse earthquakes are associated with compression, particularly with the subduction of one plate under another as in Japan. (UC Berkeley)

For further information see:
Focal Mechanisms

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