THIS PROPOSAL WILL INVESTIGATE A NUMBER OF IMPORTANT SCIENTIFIC ISSUES RELATED TO SUBDUCTION ZONE DYNAMICS BY FULLY UTILIZING A RANGE OF GEODETIC DATA TYPES INCLUDING OBSERVATIONS FROM THE GRAVITY RECOVERY AND CLIMATE EXPERIMENT (GRACE) AND GRACE FOLLOW-ON (FO) MISSIONS THE GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) AND AVAILABLE SEAFLOOR GEODETIC MEASUREMENTS. THE PROPOSED RESEARCH IS DRIVEN BY A NOVEL YET CONTENTIOUS SCIENTIFIC IDEA THAT SOME GREAT MEGATHRUST EARTHQUAKES COULD BE TRIGGERED BY ACCELERATED DEEP SLAB DEFORMATION PROCESSES AS PROPOSED BY BOUCHON ET AL. (2016) PANET ET AL. (2018) AND BEDFORD ET AL. (2020) BASED ON REANALYSIS OF SEISMICITY GRACE GRAVITY AND GNSS TIME SERIES SPANNING THE 2010 MW9 TOHOKU-OKI EARTHQUAKE RESPECTIVELY. IF VALID THESE FINDINGS COULD REPRESENT A SIGNIFICANT SCIENTIFIC BREAKTHROUGH SINCE THEY WOULD NOT ONLY REVOLUTIONIZE OUR UNDERSTANDING OF THE DYNAMICS OF THE SUBDUCTION PROCESS BUT COULD ALSO PROVIDE IMPORTANT INSIGHTS FOR IMPROVED SHORT-TERM SEISMIC HAZARD FORECASTING. IT IS THUS CRITICAL TO RIGOROUSLY TEST AND VALIDATE THIS HYPOTHESIS THROUGH THE INTEGRATED ANALYSIS AND SELF-CONSISTENT MODELING OF BEST AVAILABLE OBSERVATIONS INCLUDING BOTH GRAVITY CHANGES AND SURFACE DISPLACEMENTS.