Útdráttur:
Small volcanic explosions generate small ground displacements that can be recorded with seismometers and tiltmeters. Studying these records helps us understand the physical processes of explosions. Very long period (VLP) signals are often observed on volcanoes during explosions, and ultra long period (ULP) signals are observed between explosions. A focal mechanism is classically determined at a source point for explosions. However, ground displacement data show that the source is not punctual, so a finite source must be considered, as with earthquakes recorded in the near field. We present a simple model of a pressurized magma conduit below the crater to explain the source time function of explosions at the Stromboli volcano. We treat each point on the seismogram as GPS data. We demonstrate that the source time function corresponds to the pressure increase prior to the explosion, peaking at the moment of the explosion, followed by a pressure decrease afterward. A polarization study shows outward ground motion before the explosion and inward ground motion after the explosion, with a total duration of about 20 seconds. This is very similar to what is observed in GPS data from other volcanoes with large explosions, where there is outward ground displacement before explosions and inward displacement after explosions, but on a completely different timescale of years.
Öll velkomin!