Focal Mechanisms
Seismologists refer to the direction of slip in an earthquake and the
orientation of the fault on which it occurs as the focal mechanism.
They use information from seismograms to calculate the focal mechanism
and typically display it on maps as a "beach ball" symbol. This symbol
is the projection on a horizontal plane of the lower half of an
imaginary, spherical shell (focal sphere) surrounding the earthquake
source (A). A line is scribed where the fault plane intersects the
shell. The stress-field orientation at the time of rupture governs the
direction of slip on the fault plane, and the beach ball also depicts
this stress orientation. In this schematic, the gray quadrants contain the
tension axis (T), which reflects the minimum compressive stress
direction, and the white quadrants contain the pressure axis (P), which
reflects the maximum compressive stress direction. The computed
focal mechanisms show only the P and T axes and do not use shading.
These focal mechanisms are computed using a method that attempts to
find the best fit to the direction of P-first motions observed at each
station. For a double-couple source mechanism (or only shear motion on
the fault plane), the compression first-motions should lie only in the
quadrant containing the tension axis, and the dilatation first-motions
should lie only in the quadrant containing the pressure axis. However,
first-motion observations will frequently be in the wrong quadrant.
This occurs because a) the algorithm assigned an incorrect first-motion
direction because the signal was not impulsive, b) the earthquake
velocity model, and hence, the earthquake location is incorrect, so
that the computed position of the first-motion observation on the focal
sphere (or ray azimuth and angle of incidence with respect to vertical)
is incorrect, or c) the seismometer is mis-wired, so that "up" is
"down". The latter explanation is not a common occurrence. For
mechanisms computed using only first-motion directions, these incorrect
first-motion observations may greatly affect the computed focal
mechanism parameters. Depending on the distribution and quality of
first-motion data, more than one focal mechanism solution may fit the
data equally well.
For mechanisms calculated from first-motion directions as well as some methods
that model waveforms, there is an ambiguity in identifying the fault plane on
which slip occurred from the orthogonal, mathematically equivalent, auxiliary
plane. We illustrate this ambiguity with four examples (B). The block diagrams
adjacent to each focal mechanism illustrate the two possible types of fault
motion that the focal mechanism could represent. Note that the view angle is
30-degrees to the left of and above each diagram. The ambiguity may sometimes
be resolved by comparing the two fault-plane orientations to the alignment of
small earthquakes and aftershocks. The first three examples describe fault motion
that is purely horizontal (strike slip) or vertical (normal or reverse). The
oblique-reverse mechanism illustrates that slip may also have components of
horizontal and vertical motion.
Please send comments or suggestions regarding this page to Dave
Oppenheimer.
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