Project Abstract
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We analyze observations of stress field orientation provided by earthquake focal mechanisms and borehole breakouts to quantify relative consistency between these two sets of observations and characterize the implications for crustal stress heterogeneity. We identify 6 geographic subsets of high quality earthquake focal mechanisms that are near borehole observations, each with two possible depth criteria and three possible focal mechanism quality criteria. We then invert each subset for local stress field orientation using established methods and compare SHmax from these to SHmax observations from boreholes to assess the compatibility of the stress fields indicated by the two methods. In particular, we test the hypotheses that discrepancies are caused either by (1) the smoothing of the focal mechanism inversion process, or (2) the difference in crustal depth sampled by each method. We find that the actual improvement in fit between SHmax indicated by boreholes and local inversions of focal mechanisms over regional smoothed inversions of focal mechanisms is moderate at best. This suggests that while over-smoothing may be a factor, it is not principal cause of the discrepancy. Furthermore, we find that in most sub-regions, inverting only shallow focal mechanisms makes very little difference relative to either the regional smoothed focal mechanism inversion or the local inversion of focal mechanisms of all depths. These two factors are the ones most often cited by stress researchers when discussing the merits of one stress state indicator over another, but our analysis demonstrates they are insufficient mechanisms for explaining observed differences. |
