Physically informed joint inversion of absolute traveltime and differential traveltime data for accurate earthquake locations

Physical parameters such as longitude, latitude, depth, and origin time are fundamental to understanding earthquakes. Absolute traveltime has traditionally been used to determine earthquake locations, significantly advancing earthquake research and seismology. Meanwhile, differential traveltime offers enhanced sensitivity to specific physical parameters by mitigating the influence of common uncertainties. As a result, joint inversion of absolute traveltime and common-receiver differential traveltime has become a standard approach for precise earthquake location. However, conventional methods often rely on accurate ray-path determination, which is challenging in even moderately complex media. This study introduces an innovative method that jointly inverts absolute traveltime and common-source differential traveltime to address these challenges. Notably, common-source differential traveltime is independent of earthquake origin times, potentially resolving the long-standing coupling issue between origin time and depth. Instead of relying on traditional ray tracing, our method employs the Eikonal equation solved via the fast-sweeping method, ensuring accurate traveltime computation in complex media. Synthetic tests under various conditions, including noisy data and velocity model inaccuracies, demonstrate the robustness and stability of the proposed method. Finally, the method is applied to real seismic data from the Ridgecrest region of Southern California, showcasing its efficiency and accuracy in practical scenarios. This novel approach offers a promising advancement in earthquake location techniques, particularly in complex geological settings.