Moho-offset beneath the western margin of Lake Biwa, Japan, deduced from gravity data and short-period receiver function imaging: Who is pulling down the crust beneath the lake?

We discuss subsurface density structures and some problems regarding isostasy in and around Lake Biwa in the Kinki district, Japan. Lake Biwa is the most largest lake in Japan. The Paleo Lake Biwa can be traced back to almost 4 million years ago. The lake region is characterized by strong negative Bouguer anomalies. Especially, a steep horizontal gradient zone of gravity anomaly field is distributed along the western margin of the Lake. The large amount of the gravity anomaly depression (>50mgals) can not be explained only by low-density sediments beneath the lake. This large gravity depression indicates that a down-warping structure extends to the Moho depth. This conjecture has been strongly supported by short-period receiver function imaging. Moho discontinuities derived from the imaging shows the clear offset of about 8km under the steep gravity gradient zone. A question arises as what is the driving force to create such large down-warping structure against isostatic buoyancy? We consider that the subduction of shallow slab under the region (Philippine Sea Slab) may affect to the crustal deformation through the high-viscous fluid between them. In order to confirm this speculation, we simulated the induced mantle flow and the pressure distribution due to the subduction of the Philippine Sea Slab. This numerical experiment has provided a result that the induced flow makes a negative pressure zone under the lake region, when the slab has an offset beneath one side of the region. This offset of the slab coincides with some plate models deduced from hypocentral distributions and/or Sp phases of the deep-focus earthquakes.