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 × 10 22 Nm which is equivalent to Mw 8.99 for about five months, including the afterslip for the Nias earthquake.

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 openloop 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.

The Indochina peninsula has been hypothesized to be in the Sunda block, a rigid block in SE Asia. However, its northern boundary is not obvious. To elucidate the boundary and the deformation on the peninsula, six permanent GPS sites were established in Thailand, and observations have been conducted since March 1998. Estimated velocities suggest that most of the Indochina peninsula is rigid and can be considered in the Sunda block. However, close examination suggest small but significant strain in the order of 10−8 exists in Thailand. Statistical test suggests that there is no significant differential motion between the Sunda block and the South China block. On the other hand, acute clockwise rotation of east Himalayan syntax indicates left lateral shear in the north of Thailand, which may be partially accommodated by left lateral motion along Mae Chan fault.

No abstract

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.