High-resolution <i>P</i> receiver function imaging of seismic crustal structure in the southeastern Korean Peninsula: implications for deformation associated with Miocene backarc extension

SUMMARY Located at the easternmost passive margin of the Eurasian Plate, the southeastern Korean Peninsula shows geological signatures consistent with Miocene backarc opening associated with the Pacific Plate subduction. The region comprises two contrasting crustal blocks—the Early Cretaceous Gyeongsang Basin and Miocene Yeonil Basin (YB)—and hosts multiple fault systems that record both extensional and contractional deformation, providing an ideal setting to investigate crustal evolution along a passive margin. Motivated by this complex setting, we performed high-resolution P-receiver function imaging using a dense broad-band seismic network. Our results reveal two Moho offsets: a western offset from 33 to 28 km, and an eastern offset from 28 to 26 km, coinciding with major fault zones and likely reflecting localized crustal thinning and subsequent reactivation. Crustal anisotropy, inferred from changes in fast-axis orientations, varies spatially, with Miocene fossil anisotropy in the GB and both fossil and present-day stress-induced anisotropy in the YB. Variations in P-to-S velocity ratio (VP/VS) reflect compositional heterogeneity and fault-related fracturing. Large earthquakes (M ≥ 4) occurred in low-VP/VS zones associated with relatively rigid and possibly locked crustal segments, while high-VP/VS regions coincide with zones of crustal weakening and microseismicity. Our findings suggest that extension-related deformation and inherited structural heterogeneity are preserved within the crust of this fossil backarc system, linking past tectonic processes to present-day structure and seismicity.