Otsuki, Kenshiro （Material seismology and Structural geology）
Research Interests:
Plate tectonics of island arcs, fractal
of faults and seismic faulting
Theme-1: Deciphering physical and chemical processes during seismic frictional slip events from fault rocks.
Fault rocks are studied using EPMA, SEM, XRD, XRF and XGT as well as the fundamental knowledge for the physics of granular materials, tribology and fractal geometry. We found the evolution law of grain size distribution of fault gouges and the related slip-weakening law, the method for identifying fluidized gouge and its frictional properties. The frictional properties of frictionally melted materials (pseudotachylyte) also were intensively investigated. The analytical result for the fault rocks from the seismic Nojima fault of the 1995 Kobe earthquake and those from the seismic Shelung-pu fault of the 1999 Chi-Chi, Taiwan earthquake wwas published in J. Geophys. Res. in 2003 and was submitted to The Island Arc in 2003, respectively.
Theme-2: Studies for the mechanisms of earthquake generation using a high-pressure and high-temperature rock deformation apparatus.
We converted our pre-existing tri-axial apparatus with a pressure medium
of oil to that with a pressure medium of Argon gas, enabling the maximum
confining pressure and temperature of 200 MPa and 800 oC. The data acquisition
system also was upgraded to the continuous data sampling in sync with all
8 channels at the rate of MHz. We succeeded at producing frictionally melted
layers during stick-slip events, as well as detecting tribo-electric signals.
The experimental results demonstrated that the formation of melt layers
arrest slips, and a part of them is now submitted to Geophys. Res. Lett.
in 2004.
Theme-3: Experimental study on the state equation of the fluids in the earth crust.
It is well-known that the crustal fluids are in composition of H2O, NaCl
and CO2, and that they affect strongly the generation of earthquakes and
the plastic deformation of crustal rocks. However, the state equation of
these fluids is hardly established by now. We developed a new apparatus
for this experiment, a small piston-cylinder set in the pressure vessel
of the tri-axial apparatus and a CO2 injection system. We are now conducting
the experiments, and a impedance analyzer also will be attached to this
system in the near future.
Theme-4: Fractal geometry of fault populations and fault zones.
We found that both the fractal size frequencies and spatial distribution
of fault populations evolve being governed by only one parameter, input
energy density measured by dissipative energy density. Fractal dimensions
of the size distribution and spatial distribution were unified into one
universal law representing their co-evolution. This result was published
in Geophys. Res. Lett. in 2003. We demonstrated that experimentally formed
fault zones are composed of fault segments and fault jogs, and that they
show a hierarchically self-similar geometry. From this geometry, we could
successfully derive the two well-known empirical laws; the Gutenberg-Richter's
law and the law that seismic moment is proportional to the seismic nucleation
size to the 3 power. This result is now submitted to J. Geophys. Res. The
relationships between dynamic parameters and the internal structures of
fault zones also are investigated using a sandwich-type rock friction apparatus.
Theme-5: Study of earthquake prediction.
We started to study for predicting the impending large earthquakes off-Miyagi
Prefecture and on the active Nagamachi-Rifu fault, as a special project
of COE (Center of Excellence) cooperating with Pros. H. Fujimaki, J. Igarashi,
A. Hasegawa and Y. Tsuchiya. Using a deep borehole penetrating the Nagamachi-Rifu
fault zone at the depth of 820 m, the underground water temperature, concentrations
of radon and chlorine ions are now continuously and precisely measured.
We intend to increase the observation stations and to consolidate them
into a observation network in the future.
Selected papers:
● Otsuki, K., Monzawa, N., and Nagase, T., 2003, Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms. Jour. Geophys. Res., 108, No.B4, 2192, doi:10.1029/2001JB001711.
● Monzawa, N. and Otsuki, K., 2003, Comminution and fluidization of granular fault materials: Implications for fault slip behavior. Tectonophysics, 367, 127-143.
● Sakuma, H., Tsuchiya, T, Kawamura, K., and Otsuki, K., 2003, Large self-diffusion of water on brucite surface by ab initio potential energy surface and molecular dynamics simulations. Surface Science, 536, L396-L402.
● Goto, K. and Otsuki, K., 2004, Size and spatial distributions of fault populations: Empirically synthesized evolution laws for the fractal geometries. Geophys. Res. Lett., 31, L05601, doi:10.1029/2003GL018868.
● Otsuki, K., Uduki, T., Monzawa, N. and Tanaka, H., 2003, Contrastive fault rocks from two boreholes penetrating Chelungpu Fault, Taiwan: Suggestions for the slip behavior during 1999 Chi-Chi earthquake. The Island Arcs, submitted.
● Otsuki, K., Uduki, T., Monzawa, N., and Tanaka, H., 2003, Fractal size and spatial distributions of fault zones: An investigation into the seismic Chelungpu fault, Taiwan. The Island Arcs, submitted.
● Koizumi, Y., Otsuki, K., Takeuchi, H., and Nagahama, H., 2004, Frictional melting terminates seismic slip: Experimental results of stick-slips. Geophys. Res. Lett., submitted.
● Otsuki, K., and Dilov, T., 2004, Evolution of hierarchical self-similar geometry of experimental fault zones: Implications for seismic nucleation and earthquake size. J. Geophys. Res., submitted.