zmap a tool for analyses of seismicity patterns typical applications ...

Using Gephart’s code
An alternative code to compute stress tensor was given by Gephart [Gephart, 1990a;
Gephart, 1990b; Gephart and Forsyth, 1984]. His code performs a complete grid search
of the parameter space. The ZMAP6 version of the code is essentially unchanged from
the ZMAP5 version, with the exception that now a precompiled PC version is also
available. The data input format is identical to the one for Michael’s inversion. Note that
significant differences between the two methods have been observed in special cases.
For UNIX or LINUX version, you need to precompiled a few files, that are located in the
external/src_unix directory. Check the INFO file in this directory for information on
compiling.
To initiate a stress tensor inversion, select the "Invert for stress tensor" option from the
Tools pull down menu of the Cumulative Number window. The dataset currently selected
in this window will be used for the inversion. The actual inversion is performed using a
Fortran code based on Gephart and Forsyth [1984] algorithm, and modified by Zhong
Lu.
The actual program is described and discussed by Gephart and Forsyth [1984], Gephart
(1990), [Lu and Wyss, 1996; Lu et al., 1997] and [Gillard et al., 1995]. Two main
assumptions need are made: 1) the stress tensor is uniform in the crustal volume
investigated; 2) on each fault plane slip occurs in the direction of the resolved shear
stress. In order to invert the focal mechanism data successfully for the direction of
principal stresses, one must have a crustal volume with faults representing zones of
weakness with different orientations in a homogeneous stress field. If only one type of
focal mechanism is observed, then the direction of the principal stresses would be poorly
constraint (modified from Gillard and Wyss, 1995)
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