Hiroshi Takiguchi

Crustal deformations associated with the great Sumatra-Andaman earthquake deduced from continuous GPS observation

We analyzed continuous GPS data from more than 20 sites in Asia, Australia and islands in Indian Ocean in order to detect crustal deformations associated with the Sumatra-Andaman earthquake of December 26, 2004. Coseismic steps can be recognized at sites about 3,000 km away from the epicenter such as Kunming in south China, Quezon in Philippines, and Diego Garcia Island in central Indian Ocean. The largest displacement of about 26 cm is found at Phuket in Thailand about 600 km away from the epicenter, about twice as large as that at Sampari, the nearest site in northern Sumatra. These observations suggest that as large slip as 14 m occurred beneath the Nicobar Islands. Large postseismic displacements are observed at Phuket and Sampari after the mainshock, but the former is three times larger than the latter. This suggests that the spatial distribution of afterslip is different from the coseismic slip distribution. The temporal variation of postseismic displacements can be explained by a logarithmic function derived from rate-state dependent friction law with short characteristic time. The area where coseismic displacements from the Nias earthquake of March 28, 2005 are detected is much smaller than that from the December mainshock, but displacement at Sampari is larger than that during the mainshock. These displacements suggest less than 4 m slip on a shallow dipping thrust fault and resultant moment release is smaller than that estimated from seismological data. Finally, total moment released by afterslip amounts to 3.83 × 1022 Nm which is equivalent to Mw 8.99 for about five months, including the afterslip for the Nias earthquake.

Ultra-rapid UT1 measurement by e-VLBI

The latency of UT1 measurement with Very Long Baseline Interferometry (VLBI) has been greatly reduced by using e-VLBI technology. VLBI observations on the baseline formed by the Kashima 34-m and the Onsala 20-m radio telescopes achieved ultra-rapid UT1 measurements, where the UT1 result was obtained within 30 min after the end of the observing session. A high speed network and a UDP-based data transfer protocol ‘Tsunami’ assisted the high data rate and long-distance data transfer from Onsala to Kashima. The accuracy of the UT1 value obtained from the 1-h single baseline e-VLBI experiment has been confirmed to be as the same level with the rapid combined solution of Bulletin-A. The newly developed technology is going to be transferred to the regular intensive VLBI sessions, and it is expected to contribute to the improved latency and accuracy of UT1 data.

GPS observations in Thailand for hydrological applications

We report the delineation of the onset of the Asian Monsoon based on GPS sensing of water vapor in Thailand. We conducted GPS observations at five sites in Thailand since March 1998 under the hydrological project called GAME-T. The objective of the project is to clarify the water and energy cycle system in the Asian Monsoon area. As a preliminary analysis, we used data from March to June 1998 and estimated the water vapor content in the zenith direction (PWV) every 30 minutes using GIPSY software (GPS-PWV). A comparison of the resultant PWV with those estimated from rawinsonde data (Sonde-PWV) suggested that, generally, the long term trends of both GPS-PWV and Sonde-PWV are consistent and a rapid increase of water vapor content is visible in May, which corresponds to the onset of the Monsoon. However, systematic differences between GPS-PWV and Sonde-PWV are eminent. The RMS of the difference (RMSD) between Sonde-PWV and GPS-PWV reaches about 8.7 mm. This large RMSD can be reduced to about 5 mm by removing some unreliable sonde data and making a linear correction to Sonde-PWV. In addition, a comparison of GPS-PWV with other meteorological data (temperature, humidity, and rainfall) showed that there is a strong correlation between a rapid increase of GPS-PWV and heavy rainfall in Bangkok and in Chiang Mai, which may be used to judge the onset of the Monsoon in the area accurately.

Stability improvement of an operational two-way satellite time and frequency transfer system

To keep national time accurately coherent with coordinated universal time, many national metrology institutes (NMIs) use two-way satellite time and frequency transfer (TWSTFT) to continuously measure the time difference with other NMIs over an international baseline. Some NMIs have ultra-stable clocks with stability better than 10−16. However, current operational TWSTFT can only provide frequency uncertainty of 10−15 and time uncertainty of 1 ns, which is inadequate. The uncertainty is dominated by the short-term stability and the diurnals, i.e. the measurement variation with a period of one day. The aim of this work is to improve the stability of operational TWSTFT systems without additional transmission, bandwidth or increase in signal power. A software-defined receiver (SDR) comprising a high-resolution correlator and successive interference cancellation associated with open-loop configuration as the TWSTFT receiver reduces the time deviation from 140 ps to 73 ps at averaging time of 1 h, and occasionally suppresses diurnals. To study the source of the diurnals, TWSTFT is performed using a 2 × 2 earth station (ES) array. Consequently, some ESs sensitive to temperature variation are identified, and the diurnals are significantly reduced by employing insensitive ESs. Hence, the operational TWSTFT using the proposed SDR with insensitive ESs achieves time deviation to 41 ps at 1 h, and 80 ps for averaging times from 1 h to 20 h.

VLBI measurements for frequency transfer

We carried out the intercomparison experiments between VLBI, GPS and DMTD to show the VLBI can measure the right time difference. We produced the artificial change using by line stretcher. At the artificial change part, VLBI and DMTD show good agreement, less than 10ps. The quantity and sense of VLBI results match well with DMTD results. Consequently, the geodetic VLBI technique can measure the right time difference.

Mass-redistribution-induced crustal deformation of global satellite laser ranging stations due to non-tidal ocean and land water circulation

The effect of the non-tidal ocean load (NTOL) and the continental water load (CWL) on crustal deformation are calculated for global satellite laser ranging (SLR) stations and on 4°×4° grids (only over the land). For the regions most severely affected, the peak-to-peak displacements due to the NTOL are found to be as large as 3 mm for the horizontal components and 10 mm for the vertical component. The peak-to-peak displacements due to the CWL reach 3 mm for the horizontal components and 15 mm for the vertical component. We apply the time series of NTOL and CWL to precise SLR analysis. The LAGEOS orbit analysis reveals that the Estimating the Circulation and Climate of the Ocean (ECCO) model makes the root mean square (RMS) of the range residual 0.2% smaller, and that the CWL makes it 0.8% smaller, compared with the case where loading displacement is neglected. On the other hand, with the NTOL derived from Topex/Poseidon altimetry data, the SLR orbit fit is not improved.

Ultra rapid dUT1 estimations from e-VLBI sessions

Promptness of the data processing of the international VLBI observations has been continuously improved in the past decades. In particular, e-VLBI technique has proved that it has a capability to shorten the latency by transferring the observed data to the correlator by using high speed communication networks. The method has been introduced to routine intensive VLBI sessions to monitor dUT1. To improve the promptness even further, we started an initiative to develop automated data transfer and data processing systems using Europe-Japan baselines. On February 21, 2008, we succeeded to demonstrate the effectiveness of the developed systems and estimated the dUT1 parameter 3 minutes 45 seconds after the last scan of the one hour intensive style e-VLBI session. This achievement was realized by the developments of the K5/VSSP32 data acquisition terminal, automated data processing and analysis software.

New Absolute Gravity Measurements in New Zealand

To enhance and extend the absolute gravity (AG) measurements in New Zealand, we conducted new measurements using a FG5 (#210 of Kyoto University) in January and March 2016. The measurements in the North Island were made at two existing points (the Warkworth Radio Astronomical Observatory and Wellington A) and at one newly established point at the Wairakei Research Centre, Taupo. The gravity measurements in the South Island were made at five existing AG points; Godley Head, Mt John, the University of Otago, Helipad and Bealey Hotel. At each point more than 4,000 drops were made and AG values were determined with measurement uncertainties better than 3 μGal (mostly better than 2 μGal) at 130 cm instrument height. The values are compared with those of the 2015 campaigns. Although the differences of about 10 μGal were observed at Wellington A and Godley Head, those at the other points were within 5 μGal. At points in the Southern Alps we combined AG with relative gravity measurements and achieved good agreement with the 2015 results. Definite values for long-term gravity trends at the points in Southern Alps and Christchurch could not be obtained from the survey, but the results are consistent with those of the previous studies. Further measurements are planned to accurately determine these gravity changes.

Carrier-phase two-way satellite frequency transfer between LNE-SYRTE and PTB

We performed a carrier-phase two-way satellite frequency transfer (TWCP) experiment between LNE-SYRTE and PTB for several days in September 2015. Different hardware configurations were analyzed at the LNE-SYRTE earth station and a suitable configuration was identified by placing up- and down-converters indoors. Additionally atomic fountain frequency standards were operated simultaneously at both institutes. This offered a comparison of two reference clocks at LNE-SYRTE and PTB independently by means of TWCP and via the fountain clocks. Even with a very limited and not continuous measurement time of about 8 hours for TWCP measurements, a frequency transfer uncertainty below 2×10-15 was achieved and the results by the fountain clocks and TWCP agreed well within this uncertainty.

Relativistic effect correction for Clock Transport

NICT carried out the first calibration of the GPS link between Koganei and Kobe by using GPS, TWSTFT and Clock Transport (CT). We presented the result of the calibration in EFTF 2015. The differential correction of GPS link by GPS, TWSTFT and CT were 102.5, 102.1 and 104.9 ns respectively. The CT result showed a discrepancy of 2 ns. By applying the relativistic effect correction for CT, we confirmed a good agreement of the results obtained by their three techniques.