Temporal variation of eruption rate during the 1978 activity of Usu Volcano, northern Japan, revealed by the eruptive deposits and volcanic tremor

We estimate the temporal variation of mass eruption rates during a single eruptive event occurring during the 1978 activity of Usu Volcano, northern Japan. The 1978 Usu eruption had several hundred phreatic to phreatomagmatic events. Eruptive activity started without explosion-type earthquakes and gradually increased in intensity. Events were several minutes to hours in duration. Major phreatomagmatic events can be divided into waxing and waning phases. Tephra beds corresponding to each event are characterized by alternating layers of gravelly diamicts and small, proximal pyroclastic surge deposits. We determined the mass of beds of each event in the field and compared bed mass with the temporal variation of tremor amplitude for each event as determined using a single three-component short period seismometer located 2.4 km from the vent. The number of diamict layers is consistent with the number of waxing-waning phases of tremor amplitude. Large tremor amplitudes correlate with the most voluminous diamict layers. Mass-eruption rates are calculated from the mass of each layer and the identified duration estimated from analysis of eruption tremor. We find that the eruption rate within a phase varies from one to three orders of magnitude during a phreatomagmatic event.