We collected information on non-magmatic unrest events reported mainly of Japanese volcanoes that have been monitored for extended time periods, with the aim of conducting a statistical evaluation of these phenomena, which can sometimes lead to an eruption. We assessed the geomagnetic field changes and ground deformation, which indicate demagnetization and inflation, respectively. We then compiled the source depth, intensity, and the rate (i.e., magnetic moment rate / deformation rate), based on the single magnetic dipole model or the Mogi model, since these models are the most frequently cited in the literature. We found clear positive correlations between the source depth and its intensity and rate, exhibiting a linear trend on the log-log scatter plots. We confirmed that all the magnetic events and some of the deformation events were plotted markedly above the cutoff line related to the detection limit. We suspected that the geometry of the monitoring network and/or the simplistic point-source modeling could contribute to these positive correlations; otherwise some physical requirements such as subsurface structure might be the cause.
Although the number of unrest events collected was small, we investigated the validity of the evaluation of these unrest events from different viewpoints. Upward deviation from the linear trend on the scatter plot was found to be a potentially useful criterion, whereas the source depth had no definitive relationship with the subsequent occurrence of eruptions.
We assert that observable non-magmatic unrest events should be plotted within a diagonal area between the lower-right and upper-left domains in a scatter plot of source intensity (or rate) and depth. This is because weak sources at great depths cannot be detected, while overly strong sources close to the ground surface are physically impossible. In the intermediate zone, “unerupted” and “erupted” unrest events both occur. Based on this idea, we propose a statistical evaluation of the severity of unrest events, by measuring the deviation from the “unerupted unrests”, although this hypothesis must be tested with a sufficient number of unrest events in the future. Meanwhile, care must be taken in interpreting the data collected here, since they are based on the simplified point-source models that assume a uniform half-space, and they may not accurately recreate the physical entities.
Further studies that can relate the physical conditions and their time evolution in the subsurface to geophysical monitoring records at the ground surface will be necessary to evaluate how such unrest events are related to eruptions.
