Raehee Han’s research while affiliated with Gyeongsang National University and other places

Fault rocks exhibit structures resulting from different styles of shear deformation (either distributed or localized) during fault displacement. However, how the fault rock structures affect fault slip behavior remains poorly understood. We conducted shear experiments on thick and thin gouge layers of various mineral compositions, which simulate the predominant development of velocity-strengthening distributed deformation zones and velocity-weakening shear localized zones, respectively. Here, we show that deformation zones with contrasting structures and frictional properties can be developed together in a single fault. If the rheological stiffness of shear localization zones in the fault exceeds the elastic stiffness of neighboring wall rocks, stick-slip can intermittently occur along the localization zones while stable slip persists in the distributed deformation zones. These findings suggest that contrasting rheological stiffnesses resulting from fault rock structures may induce the simultaneous operation of multiple slip modes in a fault and explain the occurrence of earthquakes in creeping faults.

Rapid slip, at rates in the order of 1 m/s or more, may induce frictional melting in rocks during earthquakes. The short-lived melting has been thought to be a disequilibrium process, for decades. We conducted frictional melting experiments on acidic, basic, and ultrabasic silicate rocks at a slip rate of 1.3 m/s. The experiments and microstructural observations reveal that all minerals in the rocks are melted at temperatures below their known melting temperatures (Tm); e.g., quartz is melted at ~ 1000–1200 °C, not ~ 1720 °C, while olivine at ~ 1300 °C, rather than ~ 1700 °C. The low-temperature melting is incompatible with the conventional disequilibrium melting, and may be caused predominantly by grain size reduction and phase boundary reactions during the early and later stages of slip, respectively. The newly estimated Tm and the melting mechanisms should be considered for understanding the mechanics of earthquakes, landslides, and caldera collapses.

Earthquake fault slip accompanied by surface ruptures may occur not only along main fault cores but also along subsidiary faults in damage zones of major (or mature) faults. Nevertheless, most previous studies of fault and earthquake geology have focused on geological observations of main core zones rather than subsidiary faults. We conducted microstructural and mineralogical analyses of fault rock materials from two subsidiary faults (F1 and F2) of the NNE-SSW-striking Yangsan Fault, which is a major strike-slip fault in southeastern Korea (F1 at Pohang Bogyeongsa and F2 at Ulsan Eonyang-Bangok), to understand their possible slip zone processes and slip behaviors. The fault cores of the subsidiary faults are up to 20 cm thick and are composed of clay-rich gouge bands measuring a few millimeters in thickness and enclosed fractured lenses. Microscopic observations reveal that linear, and narrow micro-scale principal slip zones (micro-PSZs; < 20 µm thick), which are characterized by strong preferred orientation of clay minerals, occur not only at the boundaries between the gouge band and adjacent fault rocks but also in the central part of the gouge band. Along the micro-PSZs, microstructures such as clasts truncated by rapid slip localization and gouge injections by thermal pressurization of wet gouge materials during rapid slip are observed. Thus, the structures together may indicate the occurrence of seismic slip on the subsidiary faults. Mineralogical analyses reveal that the total clay fractions (consisting mainly of illite, chlorite, and kaolin) of the gouge materials of F1 and F2 are 60.1 and 59.7 wt%, respectively. The gouge band of F2 is enriched with kaolin (59.7 wt%), which is regarded as a gouge material that can trigger dynamic weakening by dehydration-induced thermal pressurization during seismic slip. Therefore, these results imply that the kaolin-rich gouge band in F2 may be dynamically weakened when seismic reactivation occurs along F2. This study shows that a comprehensive investigation of slip behaviors of subsidiary faults as well as main fault cores is necessary to improve our understanding of the seismic faulting mechanisms of major tectonic fault zones.

We conducted outcrop‐to‐nanoscale structural observations on a principal slip zone (PSZ) of the Yangsan fault, a major strike‐slip fault in southeast Korea, and some high‐velocity rotary shear tests on the PSZ gouges to understand shear localization and physico‐chemical processes in the PSZ during earthquakes. At the fault outcrop, the PSZ appears 1–2 cm thick. At smaller scales, the PSZ is subdivided into Units 1 to 4 based on their microstructures. Microscopic observations confirmed the shear localization in Units 1 (20–500 μm in thickness) and 3 (0.8–3 mm in thickness). Further localization was observed in several μm‐thick shear bands. Various structural and mineralogical features that recorded physico‐chemical processes in slip zones were found in the PSZ, including gouge fluidization structures relevant to thermal pressurization, solidified frictional melt of clay‐rich gouge, and mineralogical changes by frictional heating‐induced illitization. The gouge melting and mineralogical changes were reproduced in our experiments at 1.3 m s⁻¹. The gouge fluidization has been observed in previous tests at seismic slip rates. Thus, the observations from the natural and experimental faults indicate that the shear localization in the PSZ was coseismic. In a cataclasite zone next to the PSZ, altered veins of pseudotachylyte were found. All these observations indicate temporal changes of the PSZ materials (e.g., formation of solidified melt, alteration of the melt into clay minerals, friction‐induced mineralogical changes). Associated with such changes, coseismic slip zone processes and dominant weakening mechanisms may also vary, even at the same location on a fault.

Due to brittle deformation, fractures and deformation bands may form in non-porous rocks and porous rocks, respectively, and some of them may evolve into faults. We conducted structural observation and material analysis on deformation bands in the semi-consolidated, porous sediments of the Eoil Basin, Gyeongju, SE Korea, to understand their structural evolution into faults. According to the outcrop-scale observation, the deformation bands are shear bands in terms of kinematic classification. Most of the identified shear bands are several millimeters wide, and the apparent shear displacement along them is less than 10 cm. Microstructural observation of the typical shear band confirms that it has a protocataclasis structure with a high clast vs. matrix ratio despite grain size reduction due to mechanical crushing. At the outcrop, the shear band's width appears to increase as the apparent displacement increases, and a small number of the shear bands change to faulted deformation bands (or faults) which are as wide as up to ~3 cm and have slickensides (or smooth slip surfaces). The increase in the band's width with the displacement increase may be due to the strain-hardening behavior associated with the protocataclasis structure. In the faulted deformation bands, principal slip zones (PSZs) with a width of several tens of µm, where shear strain is localized, are developed. The PSZs are composed of ultrafine grains of quartz, feldspar, and clay minerals (mostly smectite). The observation implies that the formation of ultrafine particles by ultracataclasis or the formation of clay minerals with low frictional strength may contribute to the strain-softening behavior and the shear localization in the PSZs (or the evolution of the shear bands into faults), whereas the development of the protocataclasis structure in the shear band of small displacement may cause the strain-hardening behavior and increase in the band thickness.