Analysis of P wave velocities of an ocean-bottom cable (OBC) data set demonstrates the existence of gas hydrates and free gas on the northern flank of the Storegga Slide. The distribution and concentration of gas hydrates and free gas show significant variation over the length of the OBC. Average gas hydrate saturation of pore space is 5%, when modeled by effective-medium theory with hydrates as a sediment-frame component. Average gas saturation is 0.45% assuming homogeneous distribution. The bottom-simulating reflector (BSR) is generally identified as termination of enhanced reflection but in some places appears as a reflection proper. Amplitude anomalies related to the BSR are primarily caused by the gas and not the hydrates. The gas hydrates at the Storegga Slide develop from gas-rich fluids migrating into the gas-hydrate stability zone (GHSZ) from the sediments below. Development of polygonal faults and the related expulsion of formation water might drive the fluid flow in the area. The physical and geological properties of the sediments control the distribution and concentration of gas, which migrates predominantly along strata rather than along the base of the GHSZ. This migration mechanism controls the flux of gas-rich fluids into the GHSZ and hence the distribution of gas hydrates and the variation in gas hydrate concentration.