6-5）　Earthquake Prediction Research Center

 

The Earthquake Prediction Research Center (EPRC) was established in 1994 as a core facility to promote national or international research projects on earthquake prediction. To establish a scientific method for predicting a large earthquake, we still need basic researches for which university scientists bear responsibility. A major role of EPRC is to coordinate individual large-scale research projects on earthquake prediction so that they are carried out effectively. In 2000, the coordinating committee of the Earthquake Prediction Research Committee was established at EPRC.

                     

●Earthquake Prediction Research Committee and Coordinating Committee

 

In 1998, the Japanese Geodesy Council submitted a recommendation titled as “Promotion for the new Program of the Study and Observation for Earthquake Prediction” to the Ministry of Education, Culture, Sports, and other related ministries. This is the 5-year national program since 1999. Following the recommendation, Japanese universities established a new structure toward the earthquake forecast study (Fig.1). In September 1999, the Earthquake Prediction Research Committee was established in the Earthquake Research Institute of the University of Tokyo, which is a shared institute of Japanese universities. The EPRC promotes the earthquake prediction researches in Japanese universities to plan, conduct, and evaluate the research programs (Fig.2). The EPRC includes a coordinating committee and seven standing panels (Program Promotion Panels) for promotion of individual programs. The coordinating committee has both full-time and part-time researchers proposes basic research plans. The committee allows representatives from academic institutes outside ERI to coordinate earthquake forecast research efforts among them. The ERRC also has an external evaluation committee to evaluate the scientific merit of the university programs.

(http://www.eri.u-tokyo.ac.jp/YOTIKYO/index.htm)

 

Fig. 1  New organization of Universities for an earth-quake prediction research.

Fig. 2.  Study and observation of the processes in the earth's crust leading to earthquakes.

 

●　Individual Researches

 

1. Laboratory and numerical experiments on asperity and aseismic slip area

Recent studies of earthquake source processes and geodetic observations indicate that seismic slip event, afterslip, and episodic aseismic event occur in different areas on a plate boundary. Slip modes seem to be determined by heterogeneous frictional properties and interaction of different frictional behaviors.

Slip experiments were performed in a direct shear apparatus using large granite blocks. The length of the pre-existing fault surface is 100 cm. We insert a thin Teflon sheet along half section of the fault resulting in velocity-strengthening (a-b>0). The other half section with velocity-weakening behavior (a-b<0) acts as an asperity. Local displacements and shear stresses are measured at many points along the fault. Dynamic rupture starts from the asperity with rapid stress drop and penetrates into the velocity-strengthening area, where coseismic slip decreases with increasing distance from the asperity (Fig.3). In the velocity-strengthening area, the stress rapidly increases during the dynamic event, and then afterslip occurs with gradual stress relaxation. At a point sufficiently remote from the asperity, only afterslip occurs without coseismic slip. Next we average the displacements and the shear stresses over each of asperity and velocity-strengthening area, and compare them with the numerical simulation with the two degree-of-freedum block-spring model. The numerical simulation quantitatively reproduces the slip behavior found in the laboratory experiment.

 

Fig. 3.  Changes in shear stress (left) and displacement (right) on a large-scale simulated fault with heterogeneous frictional property. In the velocity-strengthening area (a-b>0), afterslip occurs with gradual stress relaxation.

 

2.  Deformation Process of Island Arc Crust and Active Fault Researches

 

The research on the detailed lithospheric structure using the controlled seismic sources has been performed by collaboration of Japanese universities, Japan Marine Technology Center, and University of Texas under the initiative of Earthquake Research Institute. The deep seismic profiling was carried out across central Japan in 2001 and southwestern Japan in 2002. The detailed lithospheric images beneath Shikoku, including the deep to shallow seismic profiles across the Median Tectonic Line, were obtained (Fig. 4).

 

Fig. 4. Deep seismic profile across Shikoku by low-fold reflection method.

 

3. Geoelectromagnetic observations and resistivity structure

Electromagnetic observations reveal subsurface conditions such as stress, temperature and fluid motion through piezomagnetic, thermal magnetic and electrokinetic phenomena. Resistivity is sensitive especially to existence of interstitial fluids and their connectivity. Then, while performing and promoting cooperative field experiments for electromagnetic monitoring and resistivity structure determination, we try to elucidate physical process which lies between respective fundamental processes and the observations. In Fig. 5, crustal water content is estimated from the resistivity structure beneath Tohoku backarc area (cf. 5-2, Fig.2), based on results of laboratory experiments for rock and water resistivity and crustal temperature structure estimated from surface heat flow distribution. We are also developing new techniques of field experiments, data processing and 3-D resistivity inversion.

Fig.5. Water content distribution in the crust, which is estimated from 2-D resistivity cross section beneath active Tohoku backarc area (cf. 6-2, Fig.2).Also shown are seismicity data after Umino et al.(2000) (circles), S-wave reflectors (squares) & P-wave scatterers (stars) determined by Asano(1998), seismic reflection result after Hirata et al.(1999), and seismic refraction result after Iwasaki et al.(1999).

 

 

4. GPS Researches on Crustal Deformation Process

GPS can be used for various applications. Especially real-time kinematic (RTK) application is now being developed for satisfying needs of monitoring crustal deformation more rapidly in real-time manner or estimating accurate position of floating platform. As one of such applications, we have developed a tsunami detection system using RTK-GPS (Fig. 6). Another application of RTK is called Virtual Reference Station which uses GEONET as base stations for producing virtual phase signal nearby observing site, which enable observer to locate the position without their own base station.

	Fig.6. Tsunami detecting system using RTK-GPS established onshore of Ofunato City.

 

 

5. Numerical simulation of seismic cycles

  We conduct numerical simulation studies of recurrence of large interplate earthquakes using laboratory-derived rate and state-dependent friction laws. Episodic aseismic slip events (silent earthquakes) found by geodetic observations can be simulated with the model. When the shear stress on a fault obeys the friction law, a critical fault size for the occurrence of seismic slip can be defined as a function of frictional constitutive parameters. Episodic aseismic slip events occur when the patch size with velocity-weakening frictional property is comparable with the critical fault size.