Gravity Anomaly on the Mashu Caldera

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Observation of a Bouguer gravity anomaly over a frozen lake in Mashu caldera, north Japan, revealed a funnel-profiled gravity low for this caldera. Water density beneath the lake surface was replaced with rock density in the processes of gravity corrections, because terrain correction is usually based on DEM which do not include water depth data. However, this gravity low vanishes after the terrain correction processing the effect of water gravitation in a caldera lake. This means that the gravity low for Mashu caldera originates from lake-water assumed to be rock. The caldera fill is neither pyroclastic flows nor fallback deposits, but is simply the water of the caldera lake. The lake bottom almost exactly represents the original caldera floor. Hence, Mashu caldera is an 'empty' caldera. This caldera cannot be classified as a funnel caldera, because the lake has a flat bottom.Numerical simulations show that in general the mass difference between water and rock in a disk-like lake should deform the true gravity anomalies of calderas, except in very shallow lakes. The gravity gradient generated by the gravitational effect of the mass difference along the lake coast would draw a ghost image of a funnel-type gravity low by extrapolation toward the middle of the lake. Water mass correction is indispensable in the case of lake-filled calderas.