[S-IT19] Coupling of deep Earth and surface processes

[SIT19-P06] In-depth seismic crustal structure of the southeastern Korean Peninsula from P receiver function analysis: Implications for crustal deformation since the backarc opening

*Minkyung Kim¹, Hobin Lim², YoungHee Kim¹, Junkee Rhie¹, Tae-Seob Kang³, Kwang-Hee Kim⁴, Jin-Han Ree⁵ (1.Seoul National University, 2.Korea Institute of Geoscience and Mineral Resources, 3.Pukyong National University, 4.Pusan National University, 5.Korea University)

Keywords:Back-arc opening, Continental margin, Receiver function, Seismic crustal structure

To the west of the Pacific Plate subduction zone, the southeastern region of the Korean Peninsula (KP) located at the easternmost passive margin of the Eurasian Plate has undergone significant tectonic activity since the Early Jurassic. This area thus serves as an excellent natural laboratory for studying crustal dynamics in the passive continental margin associated with major tectonic processes such as backarc spreading and crustal extension. Furthermore, the highest seismicity is observed to date in the southeastern KP following the 2016 M_L 5.8 Gyeongju earthquake. A total of 200 broadband seismic stations were densely installed to monitor seismic activity over four years in the mainshock zone covering an area of 60 km by 60 km. The array encompasses two major fault systems, namely the Yangsan Fault (YSF) to the west and the Ulsan Fault (USF) to the east, trending in NNE–SSW and NNW–SSE, respectively. This dense array data offers a unique opportunity to detail the crustal seismic structure in terms of crustal thickness and the ratio of P- to S-wave velocities (V_P/V_S) using the teleseismic receiver functions (RFs). The azimuthal variations of the RFs further provide constraints on the seismic anisotropy in terms of fast symmetric axis (FSA) and delay time. Our primary focus in modeling seismic anisotropy is to characterize rock fabric inherited from past and recent tectonic activities as well as to provide new insights into the tectonic processes that have influenced the evolution of the passive margin.
Our RF results show that the Moho is deepest in the west (33.5 ± 0.35 km) and shallowest in the east (25.6 ± 0.42 km) across a horizontal distance of 60 km. Vertical Moho offsets of about 6 km and 2 km are observed beneath the YSF and USF, respectively. The FSA exhibits varying trends across three domains bounded by the two faults; it trends NNW–SSE in the west of the YSF, N–S in the center, and NNE–SSW in the east of the USF. The FSAs found in the west and center might exhibit fossil anisotropy inherited from the backarc opening, as they align perpendicular to the current horizontal compressional stress direction (ENE–WSW). While the FSAs in the east are generally parallel to the current stress state, a high degree of scatter in the FSAs may be attributed to complex local structures. On average, the V_P/V_S across the study area is 1.82, slightly above the global average for continental crust (1.77; Christensen, 1996), suggesting a more mafic crustal composition. Our crustal seismic properties reflect complex deformations and enhanced crust-mantle interactions, possibly in response to tectonic extension and magmatic underplating since the backarc opening.