Summary of Kikai Submarine Caldera Researches Conducted by KOBEC (kobe Ocean-Bottom Exploration Center)

activities are identified by five months of seismicity using five (OBSs): 1100 volcanic events with its fundamental frequency of 2 high frequency seismic events with obvious P and S arrival, and monochromatic variation events are detected beneath and/or around the caldera (Seto et al., this meeting). Imaging current structures are provided by multi-beam echosounder mapping with backscatter imaging, gravity analysis, and marine electromagnetic experiment. All these structures outside of the caldera wallshows asymmetric features between the trench side and the back-arc side. The backscatter images show rough morphological features in the back-arc side, suggesting recent local eruptions, in contrast with smooth morphology in the trench side. The gravity analysis indicates that thinner upper crust in the back-arc side than that in the trench side (Morita, 2019). The electromagnetic field variation data obtained by 10 ocean-bottom electro-magnetometers (OBEMs) observation allowed us to image a 3-D electrical resistivity structure, which also shows asymmetric feature at <5-10 km depths (Matsuno et al., this meeting). These asymmetric features between the trench side and the back-arc side suggest that Kikai submarine caldera formation is basically controlled by the subducting slab.