By 10 July the eruption had lost considerable vigor. This article provides a summary of those Japanese-language reports as well as excerpts from a formal statement discussing Usu's behavior through 10 July. Satellite imagery also provided ashfall data. In addition to these background studies, as Usu entered an eruptive phase on 31 March a comprehensive suite of monitoring instruments were in place. The schematic illustrates the inferred zone of phreatic eruptions, estimated at 200-1000 m in depth.
Arrows show the idealized paths of groundwater moving through the rock.
A day or two later, these perturbations escalated rapidly. These dramatic offsets were inferred to have been driven by the influx of magma. Also, water temperatures increased at hot springs. The level of the groundwater surface in the instrumented wells peaked near the time the eruption started. For the wells with post-eruptive data on figure 2, the groundwater surface began a comparatively gradual steady decrease soon after the eruption started.