Spatial variation of in situ stress at shallow depth in South Korea

We investigate the characteristics of in situ stress at shallow depths (< 1 km) measured from borehole hydraulic fracturing tests in South Korea by comparing them with the deep tectonic stress field. We compile a large number of hydraulic fracturing test data (stress orientations and magnitudes) from 207 boreholes over the country. A hydraulic fracturing test yields a point measure at a borehole, which differs from the regional value estimated by the inversion of earthquake focal mechanical solution in various aspects including measurement technique, depth and covering area. We use a smoothing technique to derive stress trends from the individual hydraulic fracturing stress data and find some distinct patterns in shallow stress fields that are both consistent and inconsistent with the deep tectonic stress direction. Two main regions, the northeast and southeast, show differences between the shallow and deep stress orientations. In the northeast, the maximum horizontal principal stress is rotated by ∼60° from that of the deep-seated stress to the direction subparallel to the topography of the region’s mountain ridges. The southeast has heterogeneous stress orientations, so no representative stress orientations are resolved. This stress heterogeneity in southeast may have arisen owing to several possible factors including lithology variation, coastal lines, and numerous faults in this region. The magnitudes of the horizontal principal stress, despite their severe scattering, tend to be affected by lithology as a primary factor. Horizontal stresses measured in sedimentary rocks are shown to be slightly lower than those in crystalline rocks, indicating differentiated stress transfer depending on rigidity of rocks. The depth-wise variation of horizontal stresses with respect to vertical stress shows that the prevailing stress regime changes with depth from reverse-faulting (0–200 m depth), via transitional (200–500 m), to strike-slip faulting (below 500 m). Our study shows that the point-measurements of stress often capture locally perturbed stress due to the heterogeneity of geologic media, in contrast to the average tectonic stress field. Our study also suggests that depth-wise stress characterization may be necessary in addition to spatial characterization.