Abstract
Kaimondake volcano is located at the southern tip of the Satsuma peninsula in Kyushu, Japan. We conducted self potential (SP) surveys on the volcano along the traverse to the summit from the northwestern coast and obtained a characteristic SP profile. A topographic effect of -3 mV/m was clearly indicated on the stratovolcano area below 400m above sea level (ASL), while the effect was unclear on the central cone of the volcano occupied above 400m ASL. Several local SP anomalies were observed around the boundary between the central cone and the 885 lava flow. In order to clarify the origin of the SP profile, we implemented VLF-MT measurements and zeta potential experiment of the sampled volcanic rocks. Although apparent resistivities of the volcano showed a small variation of 100 to 600 Ωm along the SP survey line, zeta potentials of the rock samples were measured to be -1.3 to -20.4 mV, which can be categorized into three groups corresponding to the geological units (stratovolcano area, ∼-10mV; the central cone area, ∼-1mV; 885 lava, ∼-20mV). We conducted numerical simulations of groundwater flow and SP of the electrokinetic origin within the volcano based on the results of VLF-MT survey and zeta potential experiment. The simulation results indicated (a) topographic effect (-3mV/m) on the stratovolcano area below 400m ASL and (b) unclear topographic effect on the central cone above 400m ASL. Additional simulation of the SP caused by an anomalous zeta potential distribution qualitatively explained (c) local anomalies around the boundary between the central cone and the 885 lava. These results suggest that the SP profile on Kaimondake volcano is mainly controlled by the zeta potential distribution reflecting the distribution of three geological units of the volcano.
