What are those beachball figures?

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 projects 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, 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.

A Web page developed by the IRIS Education and Outreach Program illustrates the different types of fault motion with a Java animation. California Geology has published a teacher feature on California faults, which includes nice figures on the different types of faults.

Charles Ammon, now at Penn State, gives an illustrated explanation of focal mechanisms and fault plane solutions on a page about faults and faulting that he created for his introductory earthquake class at St. Louis University.

Examples of fault plane solutions for Northern California, using data from the Berkeley Digital Seismic Network, can be found on the regional moment tensor page. Examples for global earthquakes can be found in the Havard Centroid Moment Tensor (CMT) catalog.

Figure courtesy of David Oppenheimer of the USGS.

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