Raehee Han

Fast slip with inhibited temperature rise due to mineral dehydration: Evidence from experiments on gypsum

Anomalously low heat flow around active faults has been a recurrent subject of debate over past decades. We present a series of high-velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large-scale endothermic dehydration reaction. The tests were performed at 1 MPa normal stress and a velocity of 1.3 m s −1 , while measuring the temperature close to the sliding surface and the relative humidity around the sample. The temperature close to the sliding surface is remarkably stable at ∼100 °C during the dehydration reaction of gypsum. Microstructural and X-ray diffraction investigations show that dehydration occurs at the very beginning of the test, and progresses into the bulk as slip increases. In the hottest parts of the sample, anhydrite crystal growth is observed. The half-thickness of the dehydrated layer ranges from 160 μm at 2 m slip to 5 mm at 68 m slip. Thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10% to 50% of the total mechanical work input. The temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam. We suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake. This is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature.

The landslide stage of the Hsiaolin catastrophe: Simulation and validation

[1]xa0Typhoon Morakot struck southern Taiwan in the summer of 2009, causing the most severe flooding since the 1950s. In the early morning of August 9, rainfall triggered the Hsiaolin landslide, and the resulting debris avalanche covered the township of Hsiaolin Village, Kaohsiung. Around five hundred people were buried alive. Reconstruction of the runout of the debris avalanche would increase understanding of the large-scale avalanches for future hazard mitigation purposes. Simulation of the debris avalanche runout can provide valuable information for this purpose. A new continuum shallow-water model is applied to flow over general topography. The Coulomb friction law is adopted; the friction coefficient is initially determined by high pressure rotary-shearing tests and subsequently fine-tuned by an iterative procedure to minimize the difference between the simulation and the measurement. The friction coefficients measured by laboratory tests are found to be in reasonable agreement with the best-fit result of the simulation. In addition, Voellmy rheology is applied, but it is found that the role of the fluid viscous drag is insignificant. The simulation result in the village area is further corroborated by near-surface magnetic surveys. These indicate that the northern part of the village is dislocated, while the artifact structures of the southern part are buried near their original locations. By comparing the landslide front and the flow direction of the simulation, we are able to confirm, as also described by survivors, that the landslide swept the northern part of the village into the Cishan River, while the southern part was flooded subsequently by the debris from a dam breach about 20 min after the landslide.