Stress Model of California: Fault‐Stress Interactions Across a Complex Plate Boundary System From Focal Mechanisms of Small Earthquakes

California, located along the transform boundary between the Pacific and North American plates, hosts a complex fault system, a long history of damaging earthquakes, and frequent small earthquakes. While earthquakes arise from the buildup and release of elastic stress, detailed knowledge of principal stress orientations, absolute stress magnitudes, and fault instability remains limited. Understanding stress distribution and its interaction with faults is key to assessing tectonic evolution and seismic hazards. Here we obtain 810,562 high‐quality focal mechanisms in California by combining machine‐learning‐based phase pickers and REFOC algorithm, representing 51.5% of all relocated earthquakes from 1981 to 2021. These mechanisms enable us to invert for 2D and 3D stress models of California's crust, including principal stress orientations, faulting style, R‐ratio, and fault instability (defined as how close each fault's orientation is to optimal failure) for 350 major faults. The results reveal a heterogeneous stress field: transpressional regimes dominate the northern and central San Andreas Fault (SAF) system, strike‐slip regimes dominate the southern SAF system, and transtensional regimes prevail in the Walker Lane and Eastern California Shear Zone. Stress rotations over ∼50 km are closely related to fault geometry, interactions, and strain partitioning. Most faults with low instability are either less optimally oriented to fail under the background stress field or located in areas with recent major ruptures. These findings underscore the tight coupling between stress and fault systems in California and the value of continuous stress monitoring and improved modeling for time‐dependent seismic hazard assessments and understanding of ongoing tectonic processes.