Gravity and magnetic field interpretation to detect deep buried paleobasinal fault lines contributing to intraplate earthquakes: a case study from Pohang Basin, SE Korea

Abstract

SUMMARY Potential field interpretation is a powerful method to locate deep buried tectonic fault lines that contribute to intraplate earthquakes. A magnitude 5.4 earthquake (2017, M5.4_PO) occurred in the Middle-Miocene Pohang Basin (PB), SE of the Korean Peninsula on 15 November 2017, in the area where no fault lines appear on geological and tectonic maps. To constrain fault locations, we calculate the gravity effect of the current basin fill with a gravity stripping method and used curvature analysis to identify former geological and tectonic structures, assumed formed in the Early-Miocene. The Early-Miocene PB is divided into two subregions (northern- and southern sub-basins) by a modelled NW–SE fault line, similar to the other Early-Miocene basins (e.g. Eoil basin). Fault line trends are NE–SW in the northern sub-basin, and NNE–SSW in the southern sub-basin. 2017M5.4_PO arose from a tectonic movement along the eastern boundary of the northern sub-basin, the cross-over area from the isolated high-magnetic/low-gravity region to low-magnetic/high-gravity region. The largest aftershock of the 2017M5.4_PO occurred along the NW–SE fault line bordering the northern- and southern sub-basin.

Tectono‐sedimentary development of early synrift half‐graben subbasins in the Miocene Pohang Basin, southeastern Korea

“…We studied the fan‐delta to marine sequences that fill the half‐grabens to trace the development of bounding faults, with the aim of understanding the early opening of the subbasins in the context of the Pohang Basin evolution. The results of this study, combined with recent geophysical work in the area (Choi et al, 2020; Kang et al, 2020; Ree et al, 2021), help us to better understand the early development of the Pohang Basin and the Miocene rift system of southeastern Korea, including: (1) the factors that influenced the development of depositional sequences, (2) a structural model that best explains the depositional pattern of the half‐grabens, and (3) the link between regional tectonism and crustal deformation in the north‐central Pohang Basin.…”

“…In contrast, the early synrift system in the north-central area of the Pohang Basin demonstrates that the Chilpo and Sinheung subbasins formed together with transrotation of detached crustal blocks under a regime of NW-SE extension. These findings, combined with geophysical data sets (Figure 13a,b;Choi et al, 2020;Kang et al, 2020;Ree et al, 2021), indicate that (1) the complex development and interaction amongst smaller-scale block bounding faults during the basin formation could have governed the differential subsidence of the Pohang Basin and that (2) the NW-and NNW-trending strikeslip faults adjusted the differential extension in the basin, suggesting a wedge-shaped transtensional basin model (Figure 13d). This interpretation conflicts with the previous pull-apart model that the subsidence of the Pohang Basin was simply controlled by large-scale normal faults (i.e., the Heunghae and Gokgang faults; Figures 1d and 13a,b; Song et al, 2015).…”

“…I G U R E 1 3 (a) Isopach map of the Pohang Basin, estimated from the horizontal to vertical spectral ratio-derived resonance frequency (afterKang et al, 2020). (b) Distribution of internal faults inferred from dip-curvature analysis of the gravity field (afterChoi et al, 2020). The locations of the fault-bounded Chilpo and Sinheung subbasins are shown in both modified figures.…”

Geodynamics

“…As it is mentioned in [Mikhailov, 2002], "in contrast to the surface structures, deep faults form powerful zones with a width of kilometers to tens of kilometers." Such zones are called zones of "dynamic impact of faults" [Sherman et al, 1983]; in such zones the distribution of cracks has a wide range of sizes and directions and their formation depends on depth, rock properties, hydrological and tectonic conditions (compression, tension), size of accumulated tensions that exceed the strength of rocks in the vicinity of the fault [Guiju, et al, 2015;Castro, et al, 2012;Nurwidyanto et al, 2019;Sungchan et al, 2020].…”

Expected Segmentation of the Chugaryung Fault System Estimated by the Gravity Field Interpretation

“…9719/EEG.2021.54.6.743 추가령단층대의 중력장 데이터 해석 (Kim et al, 2017) 이 경상분지의 평균 밀도(2.67 g/cm 3 )에 비해서 밀도가 낮은 고립된 화강암(2.60 g/cm 3 )에 응력이 작용하여 발생 한 것으로 보고되었다 (Choi et al, 2019). 2017년 11월에 발생한 포항지진의 원인에 대해서는 다양한 이견이 있지 만, 대체로 경주지진에 의해서 활성화된 포항 분지의 알 려지지 않은 단층에서 일어난 것으로 알려져 있다 (Kim et al, 2018;Choi et al, 2020). 이처럼, 경주지진과 포항 지진과 같은 큰 규모의 지진들은 양산단층(Fig.…”