We compare 30 yr of background seismicity of the Kenai Peninsula–Cook Inlet region (extending from eastern Prince William Sound–Kayak Island to west of the Cook Inlet volcanic arc) to potential field (gravity, magnetics), tomographic, Global Positioning System–geodesy, and geologic information to better determine factors controlling seismicity. We also compare the occurrence of Mw>5 events over the past 80–100 yr to background seismicity in an effort to determine how reflective background seismicity is of the regional seismic hazard. Our results indicate that contrasts in upper plate rheology leading to changes in density, magnetic susceptibility, and velocity of the upper plate serve to concentrate seismicity. The southwestern edge of the subducting Yakutat microplate has a strong influence on both upper and lower plate seismicity. Density and velocity contrasts also indicate changes in lower plate rheology that influence the location of deeper (>30 km) seismicity and the occurrence of a double seismic zone within the subducting Pacific plate.