ISOSCELES TRIANGULAR UPLIFT OF SE. KOREA

Strong earthquakes rarely have occurred on the Korean peninsula and only a few events have been recorded since 27 A.D. Historical and recent earthquakes are concentrated in the southeastern part of the Korean peninsula, where more than 30 Quaternary fault exposures recently were identified and mapped. About 6-7 well-preserved wave-cut marine benches were mapped along the ~102km long southeastern coast. The study area is divided into three parts on the basis of differences in altitude of these benches. The altitude of the marine terraces in the northern and southern blocks is <1m (I), 10-12m (II), 20-22m (IIb), 33m (IIIa), and 42-44m (IV). Altitudes are greater for each marine terrace in the middle block: 4-5m (I), 19-24m (II), 33m (IIIb), 48m (IIIa), 70m (IV). Both the northern and southern parts show an uplift rate of about 0.2m/ky. However, the middle part of the coast is uplifting at ca. 0.3m/ky. The different uplift rate reveals that the coast is tectonically divided into three blocks confined by two tectonic lines. One is newly named as the GHTL (Gampo-Hyodongri tectonic line) striking N45E and has raised the middle part of the study area from the north. The southern structure is the Ulsan fault zone striking N20W. This structure has caused a higher rate of uplift between the middle and southern parts of the study area. We propose that the middle part of the field area is a fault-bounded block shaped like an isosceles triangle, herein named the Wolsung block. Many of the latest Pleistocene faults have been found within this block. Micro-earthquakes (less than MM3) have been condensed and have kept the highest frequency along GHTL. The southeastern coast of Korea has been under E-W confining pressure during late Pleistocene to Holocene time. The latest Pleistocene faults in and around the block have a top-up-to-the-west reverse sense of motion. GPS vectors of the three blocks trend 290. The lower(II) and middle(III) marine terraces have been faulted at least twice since oxygen-isotope Stage 5e. One normal fault event occurred after Stage 5c and reverse faulting occurred after Stage 5a. Most recently, Holocene uplift due to reverse faulting has occurred in the middle part of the area. Even though the Korean peninsula is located in an intraplate setting,active tectonism has occurred through late Pleistocene and Holocene time.