Abstract
A detailed processing of gravity data and a compilation of preexisting data were conducted in and around the Aso volcanic area. The density structure was then examined in detail. To remove the skin effects of surface layers (pyroclastic flows or lake deposits) and to estimate the deeper structure of the caldera, a gravity data processing technique CVUR (comparison of variance of upward-continuation residual) was proposed. The density of the surface layer in the Aso volcanic area was estimated to be 2.2-2.3 g/cm3. The Aso caldera was revealed to have a piston-cylinder type structure rather than a funnel-shaped structure (upside-down cone) model with a single low anomaly as previously believed. Newly-compiled Bouguer gravity anomalies with densely-distributed data ;.show that five local minor Bouguer gravity lows exist in the Aso caldera to produce a major gravity low zone distributed as a symbol “<” shape. The major gravity low has a steep gradient inside the caldera rim. The central areas of the minor low anomalies have relatively flat bottoms, like a pot or a bucket. Residuals of gravity anomalies corresponding to deep and shallow structure were detected by a band-pass filter designed with two upward-continuation filters. A local minimum of about -4 mGal, which was considered to be related to the deep structure appeared along the northern slope of the central cones, but not in the central cones. To explain this local minimum, a cylindrical structural model with a lower density contrast of 0.15 g/cm 3 was proposed, and the model was compared with the seismic attenuation model. In order to study the residual anomalies related to shallow structures, a three-dimensional structural analysis was performed using density-varying models in consideration of the compaction of lake deposits and pyroclastics. It was found that the gravity basement was subsided inside the caldera rim with a steep gravity gradient, and local minor depressions of the basement were found to correspond to five local flat-bottomed low anomalies. These results also suggest that the Aso caldera belongs to the “piston-cylinder type (Valles-type)” of having large depressions or collapses, rather than the “funnel-type” of caldera derived from large volcanic eruptions.
