Yifang Cheng, Tongji University, Shanghai Earthquake focal mechanisms offer insights into the architecture, kinematics, and stress at depth within fault zones, providing observations that complement surface geodetic measurements and seismicity statistics. We have improved the traditional focal mechanism calculation method, HASH, through the incorporation of machine learning algorithms and relative earthquake radiation measurements (REFOC). Our improved approach has been applied to over 1.5 million catalog earthquakes in California from 1980 to 2021, yielding high-quality focal mechanisms for more than 50% of these events. In this presentation, I will elucidate how analyzing the focal mechanisms of small earthquakes advances our understanding of fault zone behaviors at varying scales, from major plate boundaries to microearthquakes. We integrate focal mechanism data with geodetic observations, and seismicity analysis to elucidate the fine-scale fault zone structure, stress field, as well as local variations of on-fault creep rate and creep direction. All observed fine-scale kinematic features can be reconciled with a simple fault coupling model, inferred to be surrounded by a narrow, mechanically weak zone. This comprehensive analysis can be applied to other partially coupled fault zones for advancing our understanding of fault zone kinematics and seismic hazard assessment. Additionally, we utilized the new focal mechanism catalog to construct a statewide stress model for California, shedding light on stress accumulation and release dynamics within this complex fault system. Our analysis suggests that local stress rotations in California are predominantly influenced by major fault geometries, slip partitioning, and inter-fault interactions. Major faults not optimally oriented for failure under the estimated stress regime are characterized by limited stress accumulation and/or recent significant stress release. Finally, I will present ongoing work that employs focal mechanisms and P-wave spectra to determine microearthquake source properties, including fault orientation, slip direction, stress drop, and 3D rupture directivity. This approach markedly improves microearthquake source characterization, thereby offering an extensive dataset for probing fine-scale fault mechanics and earthquake source physics.