6-1) Division of Earth Mechanics

This division performs integrated studies on earthquakes and volcanoes with the view that earthquake and volcanic processes are flow and fracture phenomena in the Earth. Recent projects are outlined below.

 

Flow styles through partially moplten state

Migration of melt through partially molten zone controls various important factors in the Earth's evolution. Its efficiency, for example controls magnitude of the chemical differentiation. Migration as a form of homogeneous permeable flow through grain boundaries has been extensively investigated in the modelings, but its validity at high temperature for the partially moltens state is not confirmed well. To elucidate flow style of the melt, we have investigated flow properties of mixture of soft gel and viscous fluid.  When the deformability of solid phase is significant, localization of flow occurs and we have found the transition of flow style from homogeneous permeable flow in the grain boundary scale to localized channeled flow in much larger scale.

	Fig 1.  Time sequence of flow development.  When heavy viscous fluid (dyed green) is poured at the upper part of the  mixture layer (transparent, mixture of soft gel and viscous fluid. Fluid fraction 40%), style of the migration changes from homogeneous permeable flow to localized channel flow, if the deformability of solid phase is highly enough.

 

 

Mechanical properties of solid-liquid composites.

Mechanical properties of solid-liquid composites are studied both theoretically and experimentally. Especial attention is paid to the partially molten rocks and rock plus aqueous fluid systems kept at elevated temperatures. Experimental studies on the elastic, anelastic, and rheological properties of these systems were performed by using an analogue sample which partially melts near room temperature (a binary eutectic system of organic compounds). We established a practical method to derive quantitative information about porosity and geometry of the liquid pores existing in the Earth interiors from seismic tomographic images.

 

Noble gas Geo-/cosmo-chemistry

Noble gases are useful tracers of physical processes because they are little influenced by chemical processes. Isotopic compositions of noble gases in terrestrial and extra-terrestrial materials are analyzed with mass spectrometers in order to study their origins, mechanism of magmatic differentiation processes, thermal histories, and surface erosion rates. Chronological studies using K-Ar and Pu-Xe methods have also been carried out for clarifying volcanism and planetary formation.

                      

Detection of long period acoustic waves

Recently some groups showed evidence of Earth's background free oscillations even on seismically quiet days. Features of the seismic observations suggest that atmospheric disturbances may be a source for this phenomenon. There is also evidence of resonance of these seismic free oscillations with acoustic waves at periods around 230 and 270 s. The resonant amplitudes suggest the atmospheric excitation of the acoustic waves but there is no direct observation of them. For detection of the long period acoustic waves, we installed cross array of barometers in University Forest in Chiba. The observation shows existence of acoustic waves at periods from 10 to 50 s. For detection of longer period acoustic waves, we develop the observation.

 

Fig. 2 Photograph of an installed barometer

 

Fault Evolution Problem

New non-destructive, no-contact measuring method for 3D stress field in　materials is being developed. This method is intended to serve as a tool for revealing the governing mechanism　of the fault behavior. Unstable, three dimensional evolution of the fault surface can be analyzed by this method. Currently, analysis method for identification of the 3D state of stress using experimental data has been developed and experimental setup for accurate measurement is being developed.

 

Numerical Analysis Method for Failure Behavior

Due to the existence of discontinuity, failure behavior has been a difficult target for existing numerical analysis methods. New theory for easy treatment of discontinuity has been developed by introducing mass-spring model of continuum. Using this theory, failure can be regarded as breakage of springs. Instead of the existing mass-spring model with empirical spring constants, the new theory gives theoretical spring constants based on global material properties. Currently, numerical analysis method based on this theory is under development and preliminary results on evolution of complicated failure patterns are obtained.

 

Prediction of Seismic and Volcanic Activities with Crustal Deformation Survey

On the mechanical and probabilistic viewpoints, seismic and volcanic activities are analyzed to find available methods for prediction: e.g. the activity of the 2000 Izu Islands earthquakes and the frequency of the aftershocks of the 2000 western Tottori Prefecture earthquake. Crustal movements are also tried to obtain using time-differential stereoscopy.  At Usu volcano, remarkable uplift was detected in the western foot of the mountain within several days after the commencement of the eruption in 2000.Development of earthquake forecast system for swarm and aftershock activity.

 

Long-term forecast of large earthquakes

Large earthquakes of the same fault type and with almost the same magnitude have repeatedly taken place in the same focal region. Quantification of regularity and irregularity of recurrence enables us to make long-term earthquake forecasts. Estimating co-seismic slip of recurrent events of the Nankai and Tonankai earthquakes, we applied the time-predictable model to obtain the occurrence time of the next events.