The main activities of this Division are development of a special instrument, theoretical and observational studies on gravity field, study of Earthquake Ruptures, and strong ground motion simulation using dynamic rupture model.

Development of Brand New Geo-monitoring Instruments

A new instrument with high performance or high reliability often opens up new fields of geophysics. With this belief, we developed new instruments.
(1) Laser Interferometers
(2) Novel Instrumentation with Ultra-Precision Machining
(3) High-Tc Superconductive Accelerometer
Researches on floating pendulum control are being carried out. The pendulum mass, consisting of a permanent magnet, is levitated by interaction with a high Tc superconductor, to simulate the test mass of the accelerometer used on spacecrafts for future space mission to map Earth's gravity field. The technique will be quite useful for other applications such as super conductive gravimeter which is substantially compact and easy to use, still keeping comparable sensitivity to the present low temperature models.

Left: A permanent magnet levitated by a high Tc superconductive bulk. Right: Position and orientation of a levitated object, with the permanent magnet located inside, will be detected by optical sensors, and fed back to electrostatic actuators.

Theoretical and Observational Studies on Temporal and Spatial Variations of the Gravity Field

Minute gravity chages occur when crustal deformation and/or transport of underground materials occurs: ascent of magma, co-seismic uplift/subsidence, pre-seismic groundwater migration. We try retrieving information on the subsurface deformation from observed surface gravity.

(1) Gravity field monitoring at tectonically active regions. The most remarkable result is that continuous absolute gravity record around Mt. Asama enables us to estimate the height of the magma head.

(2) Co-seismic deformations and gravity changes due to earhquake in a spherical viscoelastic earth model

(a) Absolute gravity around Mt. Asama volcano. Eruption occurred every time gravity enters decreasing phase. (b) Magma head inferred from gravity vatiation. Numbers (1)-(5) correspond to those in the upper panel (a).

Crustal deformation measurement by interferometric synthetic aperture radar (InSAR)

Theoretical Study of Earthquake Ruptures

(1) Effects of fluid flow on earthquake rupture
(2) Development of geometrical complexity of fault system
The Earth's crust is considerably heterogeneous and contains many small-scale fault segments. We numerically study the dynamic growth of two preexisting interactive faults, faults 1 and 2, as a fundamental problem to understand how such fault segments evolve into a large fault system. A fault slip is assumed to have already occurred only on fault 1, dynamic rupture is assumed to be nucleated on fault 2. A snapshot of fault evolution is shown in a figure. Our calculations show that fault 2 either coalesces with or is repelled from fault 1 according to their initial configuration. There exists a critical value for the step over width across which pattern is switched. The above findings imply the evolution of fault system due to repeated coalescence of nearby fault segments.

A snapshot of fault evolution

Computer Simulation of Earthquake Rupture Process and strong ground motion

Reconstruction of the dynamic rupture process of the earthquakes and the study on near-field strong ground motion simulation using dynamic rupture model.

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