Geodynamic modeling for stress and seismicity in the southern Korean Peninsula driven by lateral variations of lithospheric thickness and plate kinematics

Abstract:

The southern Korean Peninsula (SKP), located in the diffuse boundary between the Eurasian and Amurian plates, which is a component of their continental margins, continuously experiences earthquakes and complex tectonics. Although previous studies have proposed that the structural gradients inherent in this continental margin region are the cause of seismicity, the origin of the stress responsible for the seismicity remains unclear, especially near the diffuse boundary. In this study, we assess the contributions to the region’s stress field from the upper mantle heterogeneities imaged by seismic tomography which is an unprecedented level of detail based on the teleseismic relative arrival time data from dense local seismic arrays (Song et al., 2020). We firstly calculate upper mantle temperature using a modified method in a relative way (Lee et al., 2020) originally suggested by Cammarano et al. (2003). The resultant of converted temperature is sequentially used for an input file of a geodynamic model, ASPECT, (Heister et al., 2017; Kronbichler et al., 2012) and then numerical models are constructed with and without GPS-derived East Asian plate kinematics (Becker et al., 2015) and performed to provide velocity and pressure fields that are in a static equilibrium with the given gravitational force and boundary conditions. We show that only the stress field generated with both plate motions and upper mantle heterogeneities is consistent with the SKP seismicity. The stress field shows a wide area where the favored faulting type is strike-slip, which is previous studies already reported as the prevailing type of focal mechanisms in the SKP. The extensional plate kinematics applied on the western boundary is necessary for normal faulting events off the western coast of the SKP. The locations of the reverse faulting events coincide with the thick lithosphere suggesting the contribution of negative buoyancy to those events. Localized asthenospheric upwelling lowers the differential stress in the northeastern SKP, where seismicity is relatively low. The results underline the importance of high-resolution seismic tomography and high-accuracy geodetic observations for slow-deforming and diffuse seismic zones such as the SKP.