How well constrained are well-constrained T, B, and P axes in moment tensor catalogs?

The T, B and P axes of earthquake moment tensors (MT) are often used to eva luate regional stress directions and other tectonic parameters; we here und ertake three comparisons to assess the uncertainty in the orientations of t hese axes. These are (1) a direct comparison of common MT in the Harvard, U .S. Geological Survey, or Earthquake Research Institute (ERI) catalogs; (2) a comparison of MT slip vectors and plate motion vectors in several tecton ically straightforward regions; and (3) an analysis of the axial variabilit y in the Harvard and ERI catalogs implied by the reported uncertainties in individual MT components. All three comparisons indicate that there is cons iderable variability within the catalog concerning the axial orientation of MT, but all suggest that axis orientations of the majority of Harvard MT h ave uncertainties of 15 degrees or less. For compensated linear vector dipo le (CLVD) components among the three catalogs, the correlation is very low. For the Harvard catalog, three statistics are especially useful for select ing better constrained MT; these are (1) the relative error E-rel, which is the ratio of the scalar moments of the reported error tensor and of the MT itself; (2) f(CLVD), a measure of the strength of the CLVD component; and (3) n(free), the number of MT elements not fixed at zero in the inversion. For selecting better constrained MT, the appropriate statistical cutoffs ch osen depend on the problem of interest, the data available, and personal pr eference. However, for analysis of shallow earthquakes we have used E-rel l ess than or equal to 0.15, f(CLVD) less than or equal to 0.20, and n(free) = 6. While this eliminates 53% of the catalog, our calculations suggest tha t nearly all the remaining events have T, B, and P axes with azimuth and in clination angle uncertainties of 5 degrees-10 degrees or less.