Tidal strains observed at the Chu-Chie station, Taiwan, had the largest deviation in the NW-SE direction from the predicted tidal strains. When the elasticity of the surrounding crust was examined by using the observed tidal strains, the minimum and maximum Young's modulus were calculated to be 13 GPa in the N52W-S52E direction and 48 GPa in the N38ES38W direction, respectively. It was considered that the anisotropic elasticity might be caused by local disturbances such as topographical and geological effects. At first, we exam ined a topographical effect with FEM. In the stress-strain relation modified by the topog raphical effect, the minimum and maximum Young's modulus were obtained to be 28 GPa in the N22W-522E direction and 36 GPa in the N68E-S68W, respectively. The topographical ef fect was insufficient to explain the anisotropic elasticity estimated from the tidal strains. Secondary, we considered the influence of fractures in the crust. The Chu-Chie station is located in the fracture zone which strikes NE-SW. Stress along to the fracture zone causes less strain change at the station than the perpendicular stress, because stress along to the fracture zone is sustained by the surrounding crust with few fractures. It is considered that the Young's modulus 13 GPa and 48 GPa estimated from tidal strains indicate the striped structure of the crust.

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A new observation system was developed for GPS/Acoustic seafloor positioning. The newly developed acoustic system for medium water depth (about 3, 000 m) simultaneously measures ranges from three precision acoustic transponders (PXPs) on the seafloor. Precise ranging is achieved with cross-correlation analysis between transmitted and received acoustic signals. Another important feature of the observation system is a 1.5 m square towed buoy used for the positioning. The buoy is fairly stable, and three GPS antennas on the buoy can measure the position and attitude of the buoy. The buoy towed from a survey vessel with a rope and cables of 100 m is efficient to reduce the underwater acoustic noises generated from the thrusters of the vessel. We deployed and located 3 PXPs off Miyagi Prefecture in August 2003, and carried out the positioning of the PXP array center twice in August 2004, each time for about one day. Although estimated observation error is large, the results obtained twice agree within 2 cm.

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We developed a new data acquisition system optimum for the unattended continuous obser vation of crustal movements. This system built in an A/D converter of 18 bits quantity, large capacity memory, GPS clock, communication functions using a public communication circuit and a self recovery function. A special function of this system is the self recovery one. Thisfunction will substantially save necessary labor for observation. In practice, our system is operating smoothly without maintenance for about 2 years.

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Conspicuous periodic variations often appear in GPS site coordinate time series. These variations could be influenced by various loads, such as atmospheric, non-tidal ocean, continental water, and snow loads. To eliminate the load influences from the GPS site coordinate time series, we estimated the influences at the GEONET (GPS Earth Observation Network) sites by using meteorological and other loading data sets. Because the GEONET coordinates (F2 solution) suffer scale errors due to a software bug and groundwater variations at the International GNSS Service (IGS) station in Tsukuba, we first corrected these errors. Then, the corrections of loading influences were evaluated for several combinations of the loads. The results show that a combination of atmospheric, non-tidal ocean, continental water, and snow loads can eliminate about 20% of the annual signal in the coordinate time series for both the horizontal and vertical components. They also show that the influence of the non-tidal oceanload is the largest of all the loads. We applied the loading correction to the data of the 1997 Bungo channel slow slip event and showed that the correction can benefit the analysis of sucha non-periodic event.

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