Seismic pulses suggesting an implosive source at the 1989 Ito‐Oki Submarine Eruption, central Japan

Four unusual seismic pulses of magnitude about 2 were observed at the 1989 Ito‐oki submarine eruption on July 13, 1989. Their seismograms show two distinct characteristics; ineffective shear wave excitation and downward motions of P onsets for all azimuths. Source mechanisms are studied by using data obtained on the local seismic network. Hypocenters were estimated to be in a very shallow part of the upper crust beneath a new crater found after the eruption. The downward first motion and ineffective shear wave excitation are difficult to explain simultaneously by a double couple type source mechanism, suggesting an implosive type source.