Multichannel seismic reflection data recorded between Arauco Gulf (37A degrees S) and Valdivia (40A degrees S), on the Chilean continental margin, were processed and modeled to obtain seismic images and sub-surface models, in order to characterize the variability of the bottom-simulating reflector (BSR), which is a geophysical marker for the presence of gas hydrates. The BSR is discontinuous and interrupted by submarine valleys, canyons, as well as by faults or fractures. The BSR occurrence is more common south of Mocha Island due to moderate slopes and greater organic matter contribution by rivers in that area. Tectonic uplift and structural instability change the stability gas hydrate zone and consequently the BSR position, creating in some cases missing or double BSRs. Our modeling supports the presence of gas hydrate above the BSR and free gas below it. Higher BSR amplitudes support higher hydrate or free gas concentrations. In the study area, gas hydrate concentration is low (an average of 3.5%) suggesting disseminated gas hydrate distribution within the sediments. Also higher BSR amplitudes are associated with thrust faults in the accretionary prism, which serve as conduits for gas flow from deeper levels. This extra gas supply produces a wider thickness of gas hydrates or free gas.We thank the support of the Chilean FONDEF project D04I1111 (Hidratos de gas submarinos, analisis de los escenarios de exploracion y produccion como contribucion a la matriz energetica nacional), Centro de Investigacion Cientifica y de Educacion Superior de Ensenada of Mexico (CICESE), Instituto Antartico Chileno (INACH), and the Hydrographic and Oceanographic Service of the Chilean Navy (SHOA). We also thank the Federal Institute for Geosciences and Natural Resources of Germany (BGR) for releasing the SPOC seismic data. Constructive observations and valuable opinions from Nina Kukowski and Peter Clift are appreciated, and also the comments from an anonymous referee.5FONDEFcrodrigo@inach.clChilean FONDEF [D04I1111]; Centro de Investigacion Cientifica y de Educacion Superior de Ensenada of Mexico (CICESE); Instituto Antartico Chileno (INACH); Hydrographic and Oceanographic Service of the Chilean Navy (SHOA)1FONDEF30