3-D Uppermost Mantle Structures from Pn and Sn Tomography across the Southern Korean Peninsula

Abstract:

The uppermost mantle is a primary part that features in it depict the evolution process of continental crust by interactions with underlying upper mantle. The crust beneath the southern Korean Peninsula has been affected significantly by tectonic processes at continental margins since its formation in the Precambrian, including multiple massive intrusions during the Mesozoic and the early Cenozoic. In this study, the Pn and Sn arrivals from the regional earthquakes were exploited to provide high-resolution tomographic images for P- and S-wave structure of the uppermost mantle. We manually measured arrival times of 3,586 Pn and 2,130 Sn from 59 earthquakes with magnitude greater than 2.9 from 2005 to 2020. Measured data are reliable with a relatively consistent range of cross-over distances (~150 km) and average Pn and Sn velocities of 7.86 km/sec and 4.31 km/sec, respectively. Locations of earthquakes were relocated by using a non-linear algorithm (NonLinLoc). We used a tomography method with a 3-D fast marching ray-tracing algorithm (FMTOMO) to account more realistic propagation effects of refracted crustal phases. Velocity structures derived from Pn and Sn arrivals generally consistent with each other. However, the result for Sn presented a striking NE-SW to ENE-WSW high velocity anomalies along the southern and eastern margins of the southern Korean Peninsula, resulting in low Vp/Vs ratios. This not only confirms the previously reported magma underplating, but also shows, from the spatial extent of the anomaly, more detailed interactions of the modification process with pre-existing continental structures at their margins.