A geophysical study on the Hwasan caldera in the Euisung sub-basin, Korea, using gravity and magnetotelluric data

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Eom, Jooyoung۱; Park, Gyesoon۱; Yang, Junmo۲; Kwon, Byung-Doo۱; Lee, Heuisoon۳
Holding Date 04 October 2008

The Hwasan caldera, located in the Euisung sub-basin in southeastern part of the Korean peninsula, is known to have erupted in the Cenozoic era (about 54.5 Ma). Because much of the volcanic mass of the Hwasan ring complex have been collapsed and eroded away, few geological surveys have been conducted to investigate the subsurface structure of the caldera. Although gravity and magnetic surveys performed previously on this area have provided some information on the regional depth of basin basement and spatial distribution of volcanic rocks, information are not enough to reveal the detailed subsurface structure of internal and marginal part of the caldera, which are crucial for studying the volcanic activity and caldera formation mechanism.
To delineate a more detailed subsurface structure of the caldera, a more precise two-dimensional gravity survey with GPS receivers and magnetotelluric (MT) survey at intervals of 2-3 km in the E-W direction across the caldera were carried out. Results of the 3-D gravity inversion and 2-D MT inversion lead to the following conclusions. First, the depth of the basin basement inferred from the MT inversion results matches well with the result of gravity inversion, but the basement resistivity appeared to be fairly low when compared to the general values reported in many other calderas. 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 up to 4000 ohm-m are imaged around two external ring fault boundaries.
These anomalies are thought of as the response of the rhyolitic dykes intruding along the ring fault, and well correspond to relatively high density anomalies in the gravity data. Thirdly, low resistivity zone reaching down to 200 ohm-m is detected at a depth of 1 km beneath the central part of the caldera. Considering the geological evolution model of Hwasan calderas, this zone is regarded as the sedimentary layer that had been subsided during the period of forming the external ring fault system. In addition, the relatively low density anomaly observed in the central part of the caldera may also be attributed to this sedimentary layer.