Plain Language Summary The interplate zone beneath Mejillones Peninsula (MP), northern Chile, presents an anomalous seismogenic behavior with aseismic pulses, low coupling values, and acting as a seismic barrier for earthquakes occurred in adjacent areas. We believe that this anomalous behavior is due to the presence of fluids in the interplate zone under the MP. To corroborate this, we study the seismicity recorded by a local seismological network and constructed a tomographic velocity model. Our results show that within the oceanic crust and in the lower continental crust exist the presence of fluids that concentrates to the north and center of the MP, which correlate with the presence of the Mejillones fracture zone and with more fractured and permeable lithologies of the continental crust. This situation changes to the south of the MP where fluid concentration is lower. This is the first detailed attempt to characterize the role of fluids in the rheology of barriers within subduction zones and to distinguish geological controls on the fluid distribution. We believe that this type of study is fundamental for facilitating future prospective analysis of earthquake distributions and conducting hazard assessments.