HR: 0800h
AN: T21B-0588    [Abstracts]
TI: The State of Stress in a Seismogenically Quiet Region: a Case Study in the Southeastern Korean Peninsula
AU: * Chang, C
EM: cchang@cnu.ac.kr
AF: Dept of Geology, Chungnam Natl Univ, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, Korea, Republic of
AU: Lee, J
EM: ljb3mths@cnu.ac.kr
AF: Dept of Geology, Chungnam Natl Univ, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, Korea, Republic of
AU: Kang, T
EM: tskang@kigam.re.kr
AF: Korea Institute of Geoscience and Mineral Resources, 92 Gwahak-ro, Yuseong-gu, Daejeon, 305-350, Korea, Republic of
AB: We collected a number of in situ stress data (mostly from hydraulic fracturing stress measurements) and contemporary earthquake focal mechanism data to analyze the current state of stress in a tectonically young region of the southeastern Korean peninsula. The direction of the maximum horizontal principal stress estimated from 84 borehole measurements (maximum depth of 315 m) is N66°±31°E, close to the best-fit stress inversion using the focal mechanism results, N62°E. From the hydrofracturing test results, the vertical stress is the least principal stress, indicating a thrust faulting stress regime. We classify the study area into three divisions based on the magnitude of stress ratio K (\equivhorizontal stress/vertical stress). In the east and south divisions (respectively region I and II), the K value is relatively high, ranging between 2.5 and 3.2, while it is relatively low in the diagonally central division (region III), ranging between 0.8 and 1.4. It is interesting that the region of low K coincides with that of relatively high fracture (or lineation) density, due to the major fault zones including the Ulsan and Yangsan faults. This tacitly implies that the high-density fracture network might contribute to the stress release in the low K region. This is also corroborated by the fact that the moderate contemporary earthquakes (M≥3) are predominated in the proximity of eastern part of the study area. We extended our study to the analysis of mechanical stability of quaternary faults in this area. In some high K regions, faults oriented optimally with respect to the crustal stress field can be critically stressed based on the Byerlee frictional coefficient larger than 0.6. However, all known 23 quaternary faults are situated such that they are in the low K region. This fact again suggests a possible stress release due to the quaternary faults.
DE: 7215 Earthquake source observations (1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8123 Dynamics: seismotectonics
DE: 8164 Stresses: crust and lithosphere
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting