Report of GPS-99 Session 9
Modeling of the crustal process based on GPS measurements

This session was held in the Thursday afternoon and well attended. Conveners are proud that there were no cancellations of oral presentations.

Topics presented in this session can be categorized into two themes: modeling of regional tectonics and spatio-temporal variation in the vicinity of faults or volcanoes.

Thatcher discussed continental deformations in western United States in relation to gravitational potential energy (GPE). He pointed out that GPE does not contribute to deformation in the Basin and Range.

Zhao and Hashimoto presented a kinematic model for the deformation field obtained by GEONET with slip deficit and block motion model. Zhao's model introduced deficit in tensile motion of faults. Both of them concluded that most deformations in the central Japan are accommodated not along the Itoigawa-Shizuoka Tectonic Line but along the Shinanogawa Seismic Zone and Atotsugawa fault. Their conclusion is consistent with that by Hirahara, who studied displacements of local traverse GPS network across the Atotsugawa fault. Hashimoto also pointed out that slip along boundaries of their modeled blocks are lager than the previous estimate using 100 year old triangulation and trilateration data.

Papers on recent progress in the development of time-dependent inversion were one of the most attractive topics as well. Segall showed the basic scheme of network inversion filter and its power to resolve temporal variations in slip and opening associated with earthquakes and volcanic eruptions. As one of its successful application, Aoki showed dike intrusion history at the 1997 swarm off Izu peninsula, Japan. Ozawa also showed slip history associated with the 1998 slow deformation in the vicinity of Bungo channel, southwest Japan.

One of the most excitng and controversial results from GEONET was presented by Murakami. Murakami extracted a coherent seasonal component over the whole Japanese islands from three year GPS data. He discussed spatial correlation between the amplitude of this component and plate motion. Many questions arose from the audience concerning the possibilities of non-tectonic origin such as meteorological effects, antenna stability, and so on. Anyway Murakami has ignited a new controversy on short-term fluctuation in crustal deformations, whose nature and source must be examined from the several viewpoints in the future.

If there was something missing in the session, that would be mechanical or dynamical aspects of the crustal deformation. There are only a few papers that dealt with mechanical modeling, and most papers stayed within kinematic description of interpolated deformation field. Researches on mechanical or dynamical modeling will be highly encouraged till the next time.

Manabu Hashimoto

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