We report on the Sr isotopic composition of pore fluids recovered from Sites U1480 and U1481 drilled during International Ocean Discovery Program Expedition 362, which sampled the incoming sedimentary section of North Sumatra to investigate the causes of shallow seismogenesis in the Sumatra-Andaman margin. Sr isotope data are valuable in identifying diagenetic alteration of the incoming sequence, which can alter mechanical properties of the sedimentary wedge and subsequently affect its seismogenic behavior. Site U1480 recovered input sediment to ~1420 meters below seafloor (mbsf), and sediment was sampled from 1150 to 1500 mbsf at Site U1481. To determine the Sr isotopic composition, acidified pore fluid samples recovered at sea were loaded directly onto columns containing EICHROM Sr-Spec resin and followed by analyses using a NU multicollector inductively coupled plasma–mass spectrometer (MC-ICPMS). We observed a marked increase in 87Sr/86Sr ratios to 0.71376 in the Sr contribution from alteration of terrigenous material from the Bengal-Nicobar Fan. This trend is reversed in the deeper sequence, where 87Sr/86Sr ratios decrease to 0.70820 in the deepest sample analyzed (1300 mbsf). Only the deepest sediment was recovered at Site U1481, and the pore fluids also show a decrease in 87Sr/86Sr ratios from 0.71296 at 1172 mbsf to 0.70913 at 1495 mbsf.