Tetsuro Hirono

Investigation of the records of earthquake slip in carbonaceous materials from the Taiwan Chelungpu fault by means of infrared and Raman spectroscopies

This research was supported by a Grant-in-Aid for Young Scientists (A) from the Japan Ministry of Education, Science, Sports, and Culture (23684039, 2013).

Effect of density-driven flow on the through-diffusion experiment

Diffusion is one of the main mechanisms of solute transport in pore water of geologic media. The effective diffusion coefficient of a solute in a rock is usually measured by the through-diffusion experiment. However, in this experiment, the effect of advection, induced by density difference between dense aqueous solution and pure water, has not been considered. To evaluate the effect of density-driven flow, a through-diffusion experiment using Fontainebleau sandstone was conducted for KCl and KI aqueous solutions with various densities. The measured effective diffusion coefficients were positively correlated with the density difference; the effective diffusion coefficient of a 1 M KI solution (density difference, 0.119 g/cm(3)) was one order of magnitude larger than that of a 0.1 M KCl solution (density difference, 0.005 g/cm(3)). The result is explained by a theoretical model using a diffusion-advection equation including Darcys law. Based on the theory, a diagram to evaluate the condition at which the measured effective diffusion coefficient does not include the effect of advection is presented.

Near-trench slip potential of megaquakes evaluated from fault properties and conditions

Near-trench slip during large megathrust earthquakes (megaquakes) is an important factor in the generation of destructive tsunamis. We proposed a new approach to assessing the near-trench slip potential quantitatively by integrating laboratory-derived properties of fault materials and simulations of fault weakening and rupture propagation. Although the permeability of the sandy Nankai Trough materials are higher than that of the clayey materials from the Japan Trench, dynamic weakening by thermally pressurized fluid is greater at the Nankai Trough owing to higher friction, although initially overpressured fluid at the Nankai Trough restrains the fault weakening. Dynamic rupture simulations reproduced the large slip near the trench observed in the 2011 Tohoku-oki earthquake and predicted the possibility of a large slip of over 30 m for the impending megaquake at the Nankai Trough. Our integrative approach is applicable globally to subduction zones as a novel tool for the prediction of extreme tsunami-producing near-trench slip.

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

We used a field analysis of rock deformation microstructures and mesostructures to reconstruct the long-term orientation of stresses around two major active fault systems in Japan, the Median Tectonic Line and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, active since the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching in the Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinity during each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more fault normal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting an extremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of this long-term stress perturbation include a nearly total release of shear stress during earthquakes, a low static friction coefficient, or lowelastic properties of the fault zones comparedwith the country rock. Independently of the preferred interpretation, the nearly fault normal orientation of the direction of maximum compression suggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluid pressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than in preearthquake fluid overpressures.

Trace element anomaly in fault rock induced by coseismic hydrothermal reactions reproduced in laboratory friction experiments

Friction experiments at high velocities (0.1 to 0.4 m/s) at room temperature under high pore fluid pressures (2 or 5 MPa) and trace element analyses of the run products were carried out on simulated fault gouge derived from the Chelungpu fault, Taiwan. The friction coefficient decreased to ~0.1 during sliding, and the slip surface temperature reached 300°C by 6 m of slip displacement. The slip caused a small, but distinct decrease of Li in the deformed gouge when the slip surface temperature exceeded 300°C. The observed Li depletion agreed with calculated results, indicating that it resulted from a hydrothermal reaction at a high temperature produced by short-duration (up to 40 s) frictional heating. The geochemical data suggest a small fluid/rock ratio and low reaction temperature in the simulated fault, which is explained by higher normal stress or repeated earthquakes which elevated fluid-rock ratios in the natural fault.

Correction to “A chemical kinetic approach to estimate dynamic shear stress during the 1999 Taiwan Chi-Chi earthquake”

[1] In the paper ‘‘A chemical kinetic approach to estimate dynamic shear stress during the 1999 Taiwan Chi-Chi earthquake’’ by T. Hirono et al. (Geophysical Research Letters, 34, L19308, doi:10.1029/2007GL030743), the equation for estimation of dynamic shear stress, the calculations provided for stresses normal to the fault, and the resultant shear stresses and frictional coefficients were incorrect. [2] The fourth sentence of paragraph 1 should read as follows. [3] The dynamic shear stress during the Chi-Chi earthquake was deduced to be 1.37 MPa, and the frictional coefficient to be 0.03–0.08. [4] The second sentence of paragraph 11 including equation (9) should read as follows. [5] From equations (2), (4), (8), and f(a) = (1 a), a in the center of the fault zone (x = 0) is expressed as

Cathodoluminescence and fluid inclusion analyses of mineral veins within major thrusts in the Shimanto accretionary complex: evidence of hydraulic fracturing during thrusting

To elucidate fluid-rock interaction in a seismogenic zone along a plate-subduction boundary, we investigated the occurrence of mineral veins within the major thrusts in the Shimanto accretionary complex and examined their microstructures using a cathodoluminescence technique. We found discriminative structures, for example, a jigsaw-puzzle structure, within the quartz veins in the thrusts, which could indicate that hydraulic fracturing occurred by abnormal pore-fluid pressure during thrusting. Pore pressure values, estimated quantitatively by fluid inclusion analyses, were 3–27 MPa higher than the surrounding parts, which may be direct evidence of abnormal pore-fluid pressure. High pore-fluid pressures and subsequent hydraulic fracturing may play an important role within major thrusts along a plate-subduction boundary.

Mechanochemical Effect on Maturation of Carbonaceous Material: Implications for Thermal Maturity as a Proxy for Temperature in Estimation of Coseismic Slip Parameters

Because shear stress in a fault is directly related to the frictional heat generated during slip, the thermal maturity of carbonaceous material has been used as a proxy for fault-rock temperature. We used infrared and Raman spectroscopic analyses and friction and heating experiments to investigate enhancement of the maturation of lignite by shear damage (the mechanochemical effect) and explored the resultant implications for the use of the thermal maturity of lignite as a proxy for temperature. We showed that higher shear stress applied to lignite samples resulted in greater progression of the thermal destruction of aliphatic C─H chains and the survival of the stronger C─C bonds. Thus, we demonstrated that shear damage during earthquake slip causes mechanochemical enhancement of organochemical reactions related to the aromatization of lignite. Our results suggest that shear stress estimated from Raman spectroscopic analyses in previous studies might have been overestimated.

Preservation of amorphous ultrafine material: A proposed proxy for slip during recent earthquakes on active faults.

The criteria for designating an “Active Fault” not only are important for understanding regional tectonics, but also are a paramount issue for assessing the earthquake risk of faults that are near important structures such as nuclear power plants. Here we propose a proxy, based on the preservation of amorphous ultrafine particles, to assess fault activity within the last millennium. X-ray diffraction data and electron microscope observations of samples from an active fault demonstrated the preservation of large amounts of amorphous ultrafine particles in two slip zones that last ruptured in 1596 and 1999, respectively. A chemical kinetic evaluation of the dissolution process indicated that such particles could survive for centuries, which is consistent with the observations. Thus, preservation of amorphous ultrafine particles in a fault may be valuable for assessing the fault’s latest activity, aiding efforts to evaluate faults that may damage critical facilities in tectonically active zones.