Gravity Anomaly on the Mashu Caldera

对日本北部马叔火山口冰冻湖上的布格重力异常的观测，揭示了该火山口的漏斗形重力低。由于地形校正通常基于DEM，不包含水深数据，因此在重力校正过程中，湖面下的水密度被替换为岩石密度。但在火山口湖水体重力作用的地形校正处理后，这种重力低就消失了。这意味着马叔火山口的重力低源于被认为是岩石的湖水。火山口填充物既不是火山碎屑流，也不是退积物，而只是火山口湖的水。湖底几乎完全代表了原来的火山口底部。因此，马舒火山口是一个“空”火山口。这个火山口不能被归类为漏斗火山口，因为这个湖有一个平坦的底部。数值模拟表明，一般来说，圆盘状湖泊中水和岩石之间的质量差应该会变形破火山口的真实重力异常，除了非常浅的湖泊。湖岸质量差引力效应产生的重力梯度向湖心方向外推会形成漏斗型重力低的鬼影。在充满湖泊的火山口的情况下，水质量校正是必不可少的。

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.