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.

On 20 March 2012, a Mw 7.5 thrust earthquake started a series of large events up to magnitude Mw 8.2 (2017) that struck central Mexico during a period of nine years. Before this event, the Mexican subduction zone did not experience subduction earthquakes (Mw > 7.0) for at least 12 years. Most of the events during this highly active period (2012-) occurred in the plate interface, resulting in a significantly larger interplate slip rate in the states of Oaxaca and Guerrero. In this study, we explore how the aseismic slip transient caused by the 2012 Mw 7.5 earthquake affected the region and whether this earthquake had a causal relationship with a nearby similar magnitude event that occurred in 2018 (Mw 7.2). To this end, we identified and analyzed characteristic repeating earthquakes along the Mexican subduction zone for assessing the plate interface slip history and found a notably increase in the aseismic slip rate inferred from repeating earthquake activity following the 2012 mainshock, which suggests a long-standing slip perturbation in Oaxaca near the trench that continued until the 2018 Mw 7.2 Pinotepa Nacional earthquake.