Both the cross-sectional (view from the side) and map-view (view from above) show different projections of the 3D fault orientation. This is what gives the alternating pattern for the strike-slip mechanisms (that looks like a ‘BMW’ symbol).įocal Mechnisms and Faulting A) The focal mechanism is in fact a hemisphere describing the orientation of a fault in 3D. The azimuths that it is observed to be up are shaded, whereas the azimuths that it is down are left unshaded. These diagrams are obtained from the analysis of seismic waveform data, the most basic type being whether the P-wave first motion is up or down on the vertical instrument. This figure, prepared by the US Geological Survey is a sort of “Rosetta Stone” of focal mechanisms relating the geologic block motions to the focal mechanism diagrams. A curved line close to the edge of the circular diagram illustrates a near horizontal dip. These circular diagrams shown in the following figure are able to illustrate the strike of a plane with respect to North and its dip, where the straighter the line the more vertical the dip. These diagrams are called focal mechanism diagrams, or sometimes “beach-ball” diagrams. When geologists and seismologists try to examine the orientation of faults in map view to either assess the relationship between a given earthquake and nearby fault, or to investigate the nature of the aftershocks are generated, they use a diagram to illustrate the three-dimensional orientation of a fault in the two-dimensions of a map.
Types of Faulting Figure showing the block motions of normal, strike-slip and reverse faults. Strike-slip faults have the two blocks sliding horizontally past one another. Normal faults have the hanging block moving downward whereas the reverse faults have the hanging block thrust upward. Normal and reverse faults are dip-slip faults where one block moves vertically with respect to the other. The following figure illustrates the three fundamental fault types, namely normal, strike-slip and reverse faults. Geologists measure the orientation in terms of the strike (angle that a fault plane makes with the North direction), the dip (angle from the horizontal describing how a fault slices into the earth) and the slip (direction that two blocks move with respect to each other).
A seismic moment tensor is a mathematical description of the forces that drive earthquake rupture, and they can be interpreted in terms of the nature of the faulting. The Berkeley Seismological Laboratory has been routinely estimating the seismic moment tensors of earthquakes in northern California since 1992. Berkeley Moment Tensor Project About Moment Tensors and "Beach Balls"