Unusual high-heat flux on the surface in the southeastern Korean Peninsula and inferring its origin from P-wave travel-time tomography and a tectonic process associated with the back-arc opening of the East Sea (Sea of Japan) in the Cenozoic era

The southeastern Korean Peninsula (SeKP), which is located along the eastern margin of the Eurasian, has experienced various tectonic events manifested as intra-continental earthquakes from small to moderate magnitudes, high-heat flux on the surface, mantle-driven gas and hydrothermal alterations, affected by back-arc extension and opening of the East Sea (Sea of Japan) in the Cenozoic era. Several regional faults including the NNE-trending Yangsan Fault runs through the SeKP where nuclear power plants of thousands of GWh and nationally fundamental industrial facilities are in operation, and thus many issues related to seismic hazards and geoscientific processes are concerned. However, due to a lack of a high resolution seismic investigation, the detailed crustal and upper mantle structures have not been revealed. To understand the Cenozoic tectonic evolution of the SeKP with respect to crust and upper mantle structures, we conducted teleseismic P-wave travel-time tomography using a dataset from a dense seismic network of 200 broadband seismographs with average inter-station distance of ~3.5 km for over 2 years. We applied the adaptive stacking method to accurately calculate relative residuals of teleseismic P waves using carefully selected high quality waveform dataset based on inter-station waveform coherency. Based on a set of the selected residuals, we conducted tomographic inversion using a fast-marching tomography method with an initial velocity model based on a 1-D crustal velocity model of this area. We found three distinct low-velocity anomalies (Anomaly 1-3) at different depths that were resolvable according to a series of resolution tests: (1) Anomaly 1; the east-west trending anomaly with sharp lateral velocity contrast with surrounding areas at depths of 0-10 km, which is associated with ~9 km-thick consolidated basin, (2) Anomaly 2; the north-south trending central low velocity anomaly at depths of 20-40 km, which could be a thermally elevated structure in subcontinental lithospheric mantle as a source for high-heat flux and mantle driven gas, (3) Anomaly 3; the northcentral low-velocity anomaly in east-west direction at depths of 40-70 km, which could be the thermally enhanced upper mantle structure formed by the intense extensional deformation producing partial melt in late Early to Middle Miocene age. We tested our model by conducting a series of geodynamic simulations that reflects major tectonic processes during the Cenozoic, and together with previous geological, geophysical, geochemical observations, we suggest that a modified lithospheric uppermost mantle drives a hydrothermal system that is possibly related to the enhanced local seismicity and distinct surficial geological features at the continental margin of Northeast Asia.