Source process of small repetitive explosions at Stromboli volcano
inferred from a temporary very-near-field broadband seismic observation
Abstract
We analyze seismic waves that are repeatedly excited by small explosions
at the summit of Stromboli volcano, Italy, to understand the source
process of volcanic explosions. At the end of September 2016, we
deployed five broadband seismometers at locations only 100-300 m away
from the active craters. Conducting moment tensor inversion to the
entire seismic signals at 0.05-0.2 Hz band, we locate the source
centroids at a depth of 170 m and 150-200 m west/southwest of the crater
where acoustic waves are excited. On the contrary, the source centroids
of seismic waves at 0.2-0.5 Hz and 0.5-1.0 Hz bands, which are excited
almost at the same time of explosion onset, are located close to the
crater. We further conduct semblance analyses to the seismic signals
from about 30 s before to the end of explosion. The results show that a
small preceding phase about 10-20 s before the onset of each explosion,
which is first detected by our very-close observation, is radiated from
a depth of 170 m beneath the west of the craters. Then, the source moves
about 50 m toward the active crater area just before an explosion.
During the explosion, however, the source moves back to the location
where the small preceding phase is excited. These spatio-temporal
changes estimated by the two source location analyses reflect the
lateral migration of magma and gas bubbles, which may be a large slug as
presented in many previous studies, in the shallow magma reservoir
beneath the crater area.