Yuki Hatanaka

Coseismic crustal deformation from the 1994 Hokkaido‐Toho‐Oki Earthquake Monitored by a nationwide continuous GPS array in Japan

Using a brand-new nationwide continuous GPS array, we monitored coseismic displacements from the October 4, 1994 Hokkaido-Toho-Oki (Kurile islands) earthquake (MJMA=8.1). Based on 2-week time series of site coordinates of 21 GPS stations, we present a coseismic deformation field of whole Hokkaido within 1 cm precision. For example, the station at Nemuro, 170 km west of the epicenter, moved 44 cm to the east and subsided 10 cm. Even stations in southern Hokkaido, 600 km apart from the epicenter, moved a few cm horizontally toward the epicenter. We compare the GPS result with displacements calculated from two preliminary models, assuming fault planes parallel or perpendicular to the Kurile trench. Since the observed were far-field displacements on land, both models can explain the GPS result.

Coseismic and postseismic crustal deformation after the M w 8 Tokachi-oki earthquake in Japan

The permanent Global Positioning System (GPS) array in Japan detected coseismic and postseimic deformation of the 2003 Tokachi-oki earthquake (Mw 8). We estimate the time evolution of its postseismic slip, together with its coseismic slip distribution. The result shows that the postseismic slip has been occurring mainly in an area adjacent to the coseismic slips, propagating to the northeast and southwest. This suggests that, as of March 6, 2004, the postseismic slip of the strongly coupled area neighboring the coseismic rupture partly released seismic moment, equivalent to an earthquake of Mw 7.8.

Calibration of antenna-radome and monument-multipath effect of GEONET—Part 2: Evaluation of the phase map by GEONET data

Phase maps for GSI/GEONET (Geographical Survey Institute/GPS Earth Observation NETwork) monuments and antennas obtained in Hatanaka et al. (2001) are evaluated by applying them to the analysis of GEONET. We used the same strategy as the GEONET routine analysis except for the antenna phase model. The coordinate solutions change by more than 10 cm in height when we apply the new phase maps. A scale change of up to 20 ppb is also observed for one of the sub-networks. The height bias is not constant but changing daily and seasonably, which implies that seasonal variations in the GPS solution are related to mis-modeling of the phase characteristics and that other environmental or geometric factors are coupled to the phase mis-modeling. Two more checks are done by comparing tropospheric delay estimates, and by conducting elevation angle cut off tests. Both tests show dramatic improvement when the new phase maps are applied, as compared to applying the standard antenna phase maps. It is concluded from this experiment with almost 1000 GEONET sites, that monument/antenna specific phase characteristic calibrations are essential for any application of GPS to achieve the highest accuracy for Earth science applications.

Calibration of antenna-radome and monument-multipath effect of GEONET—Part 1: Measurement of phase characteristics

Severe, 10-cm level, elevation angle cutoff dependence of the height solution is detected in the analysis of sample baselines from GSI/GEONET (Geographical Survey Institute/GPS Earth Observation NETwork). It is inferred that station and monument specific differences in the radome and/or multipath environment are responsible for this effect, because it can be observed over short sample baselines for identical antennas mounted atop different monument types. Our test results show that both the radome and multipath from the metal plate at the top of the pillar affect the baseline solutions. A calibration experiment was carried out by observing short (<10 m) known baselines to obtain phase correction maps for the typical GEONET monuments. The antenna/monument phase centers of GEONET monument replicas with the same attachment and radomes but shortened in height, were determined relative to a tripod-mounted TRM29659.00 antenna. Phase maps were obtained with the BERNESE software for 6 typical combinations of 3 antenna types, 3 monument types, and 3 radome types commonly used in GEONET. We find monument/antenna specific phase differences up to 1 cm. These phase differences can result in more than 10-cm station height biases when tropospheric delay parameters are estimated, which is consistent with the height errors observed for GEONET.

Low-cost densification of permanent GPS networks for natural hazard mitigation: First tests on GSI’s GEONET network

Researchers from The University of New South Wales (UNSW), Australia, and from the Geographical Survey Institute (GSI), Japan, have commenced a joint project to develop, deploy and test an innovative hardware/software system design for an automatic, continuously-operated ground deformation monitoring system based on low-cost GPS receiver technology. Conventional continuously-operated GPS (CGPS) networks, such as the one established in Japan by GSI to precisely measure earth surface movement, are very expensive. The high cost being primarily due to the fact that dual-frequency receivers are used. Japan’s nationwide GEONET network is the world’s largest, numbering nearly 1000 receiver stations, with an average station spacing of the order of 30 km. In order to densify such CGPS networks (important when high spatial resolution for the monitoring of the deformation phenomenon is required), and to promote the use of the CGPS technique in lesser developed countries, a significantly cheaper system architecture is needed. The proposed design is an integrated, dual-mode network consisting of low-cost, single-frequency GPS receivers across the area of interest, surrounded by a sparser network of dual-frequency GPS receivers. Initial tests of data collected at selected stations in the GEONET network have already shown that through enhanced data processing algorithms a CGPS network containing both single-frequency and dual-frequency receivers would be able to deliver better than centimetre level accuracies, at considerably lower cost than present systems based exclusively on dual-frequency instrumentation. This paper reports the results of the first field test of this new CGPS system design, in the Tsukuba area of Japan, in August 1999. The test network consisted of: (a) several stations of the GEONET network surrounding (b) an inner network of four single-frequency Canadian Marconi GPS receivers installed by UNSW researchers. The data from both the GEONET and the UNSW receivers were processed using a specially modified version of the Bernese GPS Software Package. The software first processes the GEONET GPS station data in order to generate empirical corrections which are then applied to the double-differenced data of the GPS baselines located within the test area enclosed by the dual-frequency CGPS stations. These corrections have the effect of improving baseline solution accuracy by up to an order of magnitude, even for baselines ranging up to 100 km in length. The baselines connecting the inner network to the surrounding GEONET stations are processed in a number of modes, including 24 hr files (as is the standard practice for geodynamic applications) and hourly data files (as in volcano deformation monitoring applications). The results indicate that single-frequency-with-correction processing can achieve accuracies of better than 5 mm in the horizontal components and 3 cm in height, while the dual-frequency results can achieve accuracies better than 2 mm in the horizontal components and 6 mm in height. In the authors’ opinion, for certain geodynamic applications there are no significant differences between the single-frequency-with-correction results and the dual-frequency results, especially for the horizontal components.

Transient crustal deformation in Tokai region, central Japan, until May 2004

An analysis of Global Positioning System (GPS) data reveals a continuation of the transient crustal deformation that was first reported by Ozawa et al. (2002) in the western Tokai region. Kalman filtering following the time-dependent inversion shows a slow-thrust slip occurring on the plate boundary in the western Tokai region, with its center around Lake Hamana, which is adjacent to the anticipated Tokai earthquake source area. The moment release from the Tokai silent earthquake was observed to be continuous surpassing Mw 7.0 in May 2004, with relative slip increases to the northeast and north of Lake Hamana from 2002. The moment release rate decreased in 2002, and then increased in 2003, reaching the rate observed in 2001.

An impact of estimating tropospheric delay gradients on tropospheric delay estimations in the summer using the Japanese nationwide GPS array

[1] We used various analysis strategies to evaluate zenith tropospheric delays (ZTDs) retrieved from the Japanese nationwide Global Positioning System (GPS) array in the summer of 1996. We compared results from the network solution obtained by daily routine data analyses and two-point positioning analyses with and without a tropospheric delay gradient model. We investigated two 14-day periods in summer 1996, when water vapor distributions were highly azimuthally asymmetric. ZTD differences up to 4 mm, dependent on clusters used in the network analysis, were found between the network and the point analyses. We also found that differences in the estimated ZTD between the twopoint positioning analyses were correlated with the north components of the estimated tropospheric delay gradients. This is consistent with our simulation studies based on the north-south asymmetry in the satellite coverage. The estimated tropospheric delay gradient vectors averaged over the periods pointed southward, which matches with the general meteorological condition in summer over the Japanese Islands. The temporal and spatial variations in the gradient estimates matched well with the moisture field determined by ZTD, in particular, during the passage of a weather front. Thus, tropospheric delay gradients obtained by GPS are expected to contain real horizontally anisotropic distribution of water vapor. INDEX TERMS: 0394 Atmospheric Composition and Structure: Instruments and techniques; 1294 Geodesy and Gravity: Instruments and techniques; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 6904 Radio Science: Atmospheric; KEYWORDS: GPS, tropospheric delay, tropospheric delay gradient, GEONET

Groundwater-induced vertical movements in Tsukuba revisited: installation of a new GPS station

A new continuous GPS station, 06S061, was installed in Tsukuba, Japan, which is directly anchored to the soil at a depth of 190 m, so that it is less affected by seasonal poroelastic deformations of aquifers induced by groundwater extraction for irrigation. A comparison of GPS-derived vertical movements of surrounding GPS stations relative to 06S061 with the independent observations revealed that 06S061 has an observation quality comparable to a standard GPS station in terms of coordinate repeatability. The poroelastic deformations of aquifers below 190 m recorded at 06S061 were estimated to have peak-to-peak values of about 1 cm, which is half of the total poroelastic deformations of aquifers observed at surrounding GPS stations.