Abstract
<p> </p><p>One of the differences between volcanic craters and calderas is that the latter bottoms are flatways filled with caldera deposit with lower density in comparison to country rocks. The 1883 Krakatau eruption affords us important knowledge on caldera formation even if it was not observed with modern sophisticated instruments. First, volcanic activities of the Krakatau Islands before and after the 1883 eruption are reexamined: previous suppositions involving a caldera-forming eruption of the proto-Krakatau prior to 1883 proved to be unsupported by the bathymetric topographies and gravity anomalies on and around the Krakatau Islands. Then, Anak Krakatau is interpreted as a parasitic cone of the main Krakatau volcano. As supplementary knowledge to discussion of caldera deposits, the results of drillings at several calderas in Japan and Mexico are introduced. Mass deficiency of the caldera deposit at Krakatau caldera is estimated by the gravity anomaly observed there and converted to probable volume with suitable density. For quantitative examination of the subsurface structure beneath the Krakatau complex, spatial distributions of seismic S-wave attenuation and Vp/Vs ratios have been already studied by temporary seismological observations and their results have been published. The high ratios of Vp/Vs observed approximately at Krakatau caldera may be attributable to the caldera deposit that is low density and contain much water. As additional remarks, a zone having both the characters, S-wave attenuation and zones of relatively high Vp/Vs ratio, may be a probable magma reservoir centering at a depth of about 10 km.</p><p><span style="font-size: 12.000000pt; font-family: 'TimesNewRomanPSMT';"><br /></span></p>
Highlights
The 1883 eruption of Krakatau volcano in theSunda Straits, between Java and Sumatra islands, Indonesia, and its destructive effects have been long disputed among many researchers
The present author will try to synthesize the results from recent geophysical observations on and around the Krakatau Islands, such as marine gravity surveys, measurements of S-wave attenuations and seismological tomography to determine Vp/Vs ratios, all relating to magma reservoirs or caldera deposits beneath Krakatau caldera
Synthetic images of the subsurface structure of Krakatau caldera are summarized in Figure 5 as already discussed above: Figure 5a is a horizontal section at a depth of 2 km, on which the shallow Vp/Vs ratios and all epicenters are projected but the area of S-wave attenuations are not because they are located deeper than 2 km
Summary
Sunda Straits, between Java and Sumatra islands, Indonesia (inset in Figure 1), and its destructive effects have been long disputed among many researchers. We still have unsettled problems in the field of volcanology such as identifying reliable distribution of the ejecta, their volume, modes of the explosions, origins of tsunamis, and subsurface structure left by the explosions. In the study of caldera formation, the following two facts are still the obstacles to the above unsettled dispute to be ever challenged. B) Seismometers were not available around Krakatau volcano at that time, and only a tide gauge, a pressure meter and magnetometers had been operated in Batavia located about 150 km from the volcano. 2. Volcanic activities at the Krakatau Islands before and after the 1883 eruption
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