Name : OKUBO,  Shuhei
            Position : Professor
            Division/Center : Division of Monitoring and Computational Geoscience
            Research Area : Geodesy, Global Slow Deformation, Gravity
 

       Theoretical and observational studies using space-time variation of gravity field.

       My group have been working on formulation of co-seismic gravity change based on the dislocation theory since 1980s. Okubo (1991)
       succeeded in formulating gravity change caused by a point dislocation in a homogeneous half-space. The theory developed to include
       gravity change raised by fault motion on a rectangular plane （Okubo, S. 1992). The group have presented gravity and potential
       changes caused by dislocations in a realistic spherical Earth model(Sun and Okubo, 1993; Sun and Okubo, 1998). It is surely leading
       theoretical gravity research in the world, which can be verified by the fact that one of the members (Okubo) was awarded with the
       Guy Bomford Prize of the International Association of Geodesy in 1991.

       My gravity group is quite unique in the sense that they do gravity measurements by themselves as well. They adopt an advanced
       hybrid observation scheme, doing absolute measurements at a reference point while measuring gravity difference to the reference with
       much lighter relative gravimeters(LaCoste and Romberg type). The hybrid scheme enables them to determine space-time variation of
       the {it absolute gravity field in areas of geophysical interest as discussed below.

       (a) Subduction Plate Margin: Tokai.
       Tokai earthquake has long been anticipated at the northern tip of the Sagami Trough, a subduction plate boundary. ERI gravity group
       continues the hybrid gravity measurement around Omaezaki together with the Geographical Survey Institute since 1996. They
       cooperate with each other to repeat absolute gravity measurement four to five times a year. The research will reveal the secular
       gravity change in the pre-seismic stage around the subduction plate boundary.

       (b)Earthquake swarm area: Izu peninsula.
       Izu peninsula is a quite unique place where earthquake swarms occur frequently (once a year or two) on the eastern coast. The
       problem what triggers the swarm has been debated and remains unsettled. Some postulate magma intrusion as a source while the
       others claim mechanical weakening by hydrothermal water flow as a triggering agent. My gravity group started the hybrid
       measurement in 1997 and repeated it two to three times a year. So far, they studied two swarm activities which occurred in March
       1997 and in April/May 1998. They modeled the two events in terms of dislocation models to explain both gravity and displacement
       fields as revealed by leveling, GPS and EDM. During the second event was detected gradual gravity change at the reference point
       exceeding 7 microgals in a few days, which strongly suggests motion of fluid underground (magma or hydrothermal water)(Yoshida et
       al., 1999).

Publications:

Yoshida, S., G. Seta, S. Okubo, and S. Kobayashi, Absolute gravity change associated with the March 1997 earthquake swarm in the Izu Peninsula, Earth Planets Space, 51, 1, 3--12, 1999.