Gravity Anomaly on the Mashu Caldera

在日本北部的Mashu破火山口的一个冰冻湖上观测布格重力异常，发现这个破火山口有一个漏斗形的重力低。由于地形校正通常是基于DEM，而不包括水深数据，因此在重力校正过程中，用岩石密度代替湖面下的水密度。然而，在对火山口湖的水重力影响进行地形校正后，该重力低消失了。这意味着马舒破火山口的重力低起源于假定为岩石的湖水。破火山口填充物既不是火山碎屑流，也不是后退沉积物，而只是破火山口湖的水。湖底几乎完全代表了原始的火山口底部。因此，马舒火山口是一个“空”火山口。这个破火山口不能被归类为漏斗破火山口，因为湖有一个平底。数值模拟表明，在一般情况下，水和岩石之间的质量差异在一个盘状的湖泊应该变形破火山口的真实重力异常，除了在非常浅的湖泊。沿湖岸沿着的质量差的重力效应所产生的重力梯度将通过向湖中央的外推而绘制出漏斗型重力低的鬼像。对于充满湖泊的破火山口来说，水团校正是必不可少的。

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.