A geophysical study on the Hwasan caldera in the Euisung Sub-basin, Korea, using magnetotelluric survey

Abstract

Among the calderas of the southeastern region of the Korean Peninsula, the Hwasan caldera has a comparatively large spatial size and its ring fault system accompanied by forming caldera structure has been preserved well until now. Several previous gravity and magnetic surveys on this area have provided regional geophysical knowledge such as depth of basin basement and spatial distribution of volcanic rocks, but it was difficult to reveal the detailed subsurface structure of internal and marginal part of the caldera. To extend our detailed knowledge for the Hwasan caldera, we carried out magnetotelluric (MT) survey, which is pretty sensitive to electrical property variation in both horizontal and vertical direction of subsurface, across the Hwasan caldera with the direction of EW. The 2-D inversion results of observed MT data lead to following conclusions. Firstly, the depth of the basin basement inferred by the MT inversion results matches well with that suggested by previous potential studies, but the basement resistivity seems fairly low when compared to that of general case. This feature might be related with the large-scaled, highly conductive layer beneath the Euisung Sub-basin suggested by Lee (2006). Secondly, the high resistivity zones reaching to 4000 ohm-m are imaged around two external ring fault boundaries. These zones are thought of as the response of the rhyolitic dykes intruding along the ring fault, and in the previous gravity data correspond to relatively high density anomalies. Thirdly, low resistivity zone reaching to 200 ohm-m is detected around a depth of 1km beneath the central part of the caldera, which has been not yet reported in korean geophysical literatures. If we take account of the evolution model of the Hwasan caldera, this zone is regarded as the past sedimentary layer that subsided during the period of forming external ring fault system. In addition, the relatively low density anomaly observed in the central part of the caldera may b