Seismic Source Migration During Strombolian Eruptions Inferred by Very‐Near‐Field Broadband Seismic Network

We analyze seismic waves excited by small Strombolian explosions to understand the source process of volcanic explosions. We deployed five broadband seismometers at only 100–300 m away from the active craters of Stromboli volcano, Italy. Moment tensor inversion of the entire seismic signals in the 0.05–0.2 Hz band locates the source at a depth of 170 and 150–200 m west/southwest of the crater where acoustic waves are excited. Contrary, the sources of seismic waves in the 0.2–0.5 and 0.5–1.0 Hz bands are excited almost at the explosion onset and are located close to the crater. We show for the first time that explosions are preceded of about 10–20 s by a small amplitude seismic phase. Semblance analysis shows that this phase is radiated from a depth of 170 m beneath the western part of the crater area. Our analysis indicates that the source moves about 50 m toward the active crater 10–20 s before the explosion occurs at the surface. At the explosion onset, the source moves back to the same location of the small preceding phase. These lateral migrations of the seismic source are estimated by moment tensor inversion and semblance analysis. We suggest that migration reflects the bending of the shallow feeding system toward northeast. Seismic waves are thus reflecting the history pressure generated by the rising of a gas‐rich pocket in the very shallow portion of a magma mush and by the following restoring force occurring after the explosion. We analyze seismic waves repeatedly excited before, during and after small explosions at the summit crater of Stromboli volcano, Italy, to understand the magma motions in the shallow conduit and reservoir system. To improve the spatio‐temporal resolution and detect slight movement of magma, we deployed broad‐band seismometers very close to the active craters with a distance range of 100–300 m. As a result, we succeeded in clarifying detailed horizontal spatio‐temporal change of seismic sources for about 1 min before and after each explosion. First seismic source, which is for the first time detected in seismogram as a very small amplitude phase, appears at a depth of about 170 m beneath the western part of the active craters. About 20 s later, the seismic source moves about 50 m toward the craters to generate an explosion. Just after the explosion, the seismic source moves back to the west where the initial small seismic source is located. Such detailed horizontal spatio‐temporal change reflects the migration and/or the resonance of gas‐rich magma batch in the very shallow reservoir. Migration of seismic source in a set of explosion at Stromboli is estimated by moment tensor inversion and semblance analysesSource laterally migrates from the west to the crater just before an explosion, and moves back to the initial location during the explosionSmall seismic signal in the 0.05–0.2 Hz frequency band is detected 10–20 s before the explosive onset Migration of seismic source in a set of explosion at Stromboli is estimated by moment tensor inversion and semblance analyses Source laterally migrates from the west to the crater just before an explosion, and moves back to the initial location during the explosion Small seismic signal in the 0.05–0.2 Hz frequency band is detected 10–20 s before the explosive onset