Name : KASAHARA,  Junzo
            Position : Professor
            Division/Center : Earthquake Prediction Research Center
            Research Area : Marine Seismology
            homepage : http://wwweprc.eri.u-tokyo.ac.jp/~kasa2/
 
 
 

RESEARCH

There are several research topics in our group: Crustal structure study, Scientific use of submarine cables, Development of ocean
       bottom tilt meter, Refraction tomography, Seismic activity study, and Seismic nature of hydrothermal zones.

       Crustal structure studies have been jointly done by other universities at the Nankai Trough, the Japan Trench, east of Japan Sea, and
       the Izu-Bonin Trench. In the Nankai Trough, there are distinct seismic segmentations. One of seismic segment boundaries is located
       at the Shiono Misaki canyon. The crustal structure shows heterogeneous structure beneath the canyon. The location also coincides
       with the segment boundary for the Tonankai-Nanaki earthquakes. In the Japan Trench, distinct difference of waveforms from reflection
       arrivals were observed. Those seem reflected at subduction plate boundary.

       Using the GeO-TOC decommissioned submarine cable, the first cable ocean bottom seismometer station (IZU) was installed on the
       forearc slope of the Izu-Bonin Trench at 2,800m deep. The data are combined with University Seismic Network and the data
       conversion efforts for IRIS have been carried out. During Mw6.4 event, which occurred at 80 km deep just beneath the station showed
       co-seismic and post-seismic change at accelerometers. This observation suggests high possibility of detecting precursory crustal
       deformation associated with large earthquakes, if precursory changes more than 1 micro-radian exist. For the GOGC cable between
       Guam and Okinawa, a broadband seismometer is ready to be deployed. A new type ocean bottom tilt-meter has been developed
       using bubble sensor. In the refraction tomography, it is necessary to compute ray paths and travel times for 2 or 3 D heterogeneous
       structures. We developed pseudo-bending and wave-front methods to calculate ray paths and travel times for the inhomogeneous
       structure and applied those for refraction tomography at crustal studies. We also applied the method for reflection arrivals. In the
       application of this tomogarphy to Nankai Trough, theoretical wave-forms using finite difference method show quite good agreement
       with observations. By cooperative research with JMA, we did seismicity observation at off Akita, Kozu-Zenisu, off Sanriku and off
       Kikai Is. We obtained precise hypocenter distribution for these areas. Observations at hydrothermal zone such as Mariana Trough,
       TAG area and Okinawa Trough showed low seismic activity and long period (1-2 min.)pressure change with 12 hours activity cycle.
       This observation suggests tide induced hydrothermal circulation at vent zone.
 

PUBLICATIONS

 Sato, T., G. Fujie, S.Koresawa, J. Kasahara, T. Tanaka, S.Honda,T. Yokota, K.Nakamura, G. Naito, Y. Ishikawa, and T. Hirasawa , Seismic observation at ta seismic gap in the eastern margin of the Japan Sea using ocean bottom seismometers,, Tectonophysics, 302, 1--7, 1999.

Kasahara, ., Y. Shirasaki, and H. Momma, Multidisciplinary geophysical measurements on the ocean floor using decommissioned submarine cable:VENUS project, IEEE Journal of Oceanic Engineering, 25, 1, 111--120, 2000.

Gou Fujie, J. Kasahara, T. Sato, and K. Mochizuki, Travel time and raypath computation:A new method in a heterogeneous medium, Geophysical Exploration(Butsuri-Tansa), 53, 1, 1--11, 2000.

Ito, S., Hino, R., S. Matsumoto, Shibara, H, Shimamura, H., Kanazawa, T., Sato, T., Kasahara, J., and Hasegawa, A., Deep seismic strucuture of the sesimogenic palte boudary in the off-Sanriku region, northeastern Japan, Tectonophysics, 319, 261--271, 2000.

Hino, R., S. Ito, H. Shiobara, H. Shimamura, T. Sato, T. Kanazawa, J. Kasahara, and A. Hasegawa, Aftershock distribution of the 1994 Sanriku-oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation, J. Geophys. Res., 105, B9, 21,697--21,710, 2000.

G. P. Glasby, and J. Kasahara, Influence of tidal effects on earthquake activity and its periodicity in diverse geological settings with particular emphasis on subamirine hydrotheml systems, Earth Science Review, 2000.