Crustal Stress Information Revealed by the Distribution of P and T Axes of Earthquake Focal Mechanisms

ABSTRACT Earthquake focal mechanisms provide essential constraints on the stress state of the Earth’s crust. Conventional stress tensor inversion relies on fitting slip directions on multiple faults, but because double-couple focal mechanisms can also be represented by the orientations of P and T axes, such inversions effectively fit the spatial distribution of these axes. This study examines whether the distribution patterns of P and T axes alone can directly constrain the crustal stress tensor. We perform modeling experiments that simulate fault instability and slip under a range of stress states and fault failure criteria. The results show that systematic changes in stress conditions produce distinct and predictable transitions in P and T axes distributions. The geometric centers of clustered axes correspond to the orientations of the principal stress axes. A clustered P axis distribution combined with a circular T axis pattern indicates comparable magnitudes of σ2 and σ3, whereas a clustered T axis distribution with a circular P axis pattern does not require and to be similar in magnitude. Randomly intermingled P and T axes distributions do not occur in the simulations, indicating that such patterns observed in real data likely reflect stress heterogeneity or inaccuracies in focal mechanism solutions. These conclusions are validated using real fault-slip data and earthquake focal mechanisms. The results provide a practical framework for diagnosing stress heterogeneity, evaluating data quality, and anticipating the reliability of stress inversion outcomes.