Seismic reflections from the upper mantle discontinuities beneath the Mid‐Atlantic Ridge observed by a seismic array in Hokkaido Region, Japan

Following a recent big deep event (mb=6.8) in the Fiji islands region, a seismic array in Hokkaido region, Japan recorded clear impulsive precursors to the P′P′ phases (P′ is another symbol for PKP and P′P′ is the surface reflection of P′) in the epicentral distance range of 70°‐72.4°. Reflection points of the phases are located 1.5°‐3.5° west of the Mid‐Atlantic ridge (13°S, 15°W). The precursors were also recorded at two WWSSN stations in Japan. Travel times and slowness measured by the array are best explained as the precursors being underside reflections from a depth of 660 km. The amplitude ratios of P′660P′/P′P′ are about 0.1, a value similar to previous observations. The observation suggests that there exists a sharp discontinuity at a depth of 660 km beneath the area close to the Mid‐Atlantic ridge.