Jakarta is the capital city of Indonesia with a population of about 9.6 million people, inhabiting an area of about 660 square-km. In the last three decades, urban development of Jakarta has grown very rapidly in the sectors of industry, trade, transportation, real estate, and many others. This exponentially increased urban development introduces several environmental problems. Land subsidence is one of them. The resulted land subsidence will also then affect the urban development plan and process. It has been reported for many years that several places in Jakarta are subsiding at different rates. The leveling surveys, GPS survey methods, and InSAR measurements have been used to study land subsidence in Jakarta, over the period of 1982-2010. In general, it was found that the land subsidence exhibits spatial and temporal variations, with the rates of about 1-15 cm/ year. A few locations can have the subsidence rates up to about 20-28 cm/year. There are four different types of land subsidence that can be expected to occur in the Jakarta basin, namely: subsidence due to groundwater extraction, subsidence induced by the load of constructions (i.e., settlement of high compressibility soil), subsidence caused by natural consolidation of alluvial soil, and tectonic subsidence. It was found that the spatial and temporal variations of land subsidence depend on the corresponding variations of groundwater extraction, coupled with the characteristics of sedimentary layers and building loads above it. In general, there is strong relation between land subsidence and urban development activities in Jakarta.

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S U M M A R YWe have analysed the Ny-Ålesund very long baseline interferometry (VLBI) data over the period 1994 August to 2004 May, and we obtain secular displacement rates relative to a NNR-NUVEL-1A reference frame of 0.2 ± 0.5 mm yr −1 , −1.7 ± 0.5 mm yr −1 and 4.8 ± 1.1 mm yr −1 for the north, east and vertical directions, respectively. The corresponding global positioning system (GPS) station displacement rates relative to the same reference frame for the north, east, and vertical directions are 0.2 ± 0.6 mm yr −1 , −2.3 ± 0.6 mm yr −1 , and 6.4 ± 1.5 mm yr −1 at NYA1 and -−0.1 ± 0.5 mm yr −1 , −1.6 ± 0.5 mm yr −1 , and 6.9 ± 0.9 mm yr −1 at NALL, where these GPS rates were derived from the ITRF2000 velocity solution of Heflin. From the comparison at 25 globally distributed collocated sites, we found that the difference in uplift rate between VLBI and GPS at Ny-Ålesund is mainly due to a GPS reference frame scale rate error corresponding to 1.6 mm yr −1 in the GPS vertical rates. The uplift rate was estimated to be 5.2 ± 0.3 mm yr −1 from the analysis of the tide gauge data at Ny-Ålesund. Hence the uplift rates obtained from three different kinds of data are very consistent each other. The absolute gravity (AG) measurements at Ny-Ålesund, which were carried out four times (period: 1998-2002) by three different FG5 absolute gravimeters, lead to a decreasing secular rate of −2.5 ± 0.9 μGal yr −1 (1 μGal = 10 −8 m s −2 ). In this analysis, the actual data obtained from a superconducting gravimeter at Ny-Ålesund were used in the corrections for the gravity tide (including the ocean tide effect) and for the air pressure effect. We have estimated three geophysical contributions to examine the observed rates: (1) the effect of the sea-level (SL) change on a timescale of a few decades, (2) the effect of the present-day ice melting (PDIM) in Svalbard and (3) the sensitivity of the computed post-glacial rebound (PGR) effects to different choices of the models of past ice history and Earth's viscosity parameters. Our analysis indicates that the effect of SL change can be neglected as the main source of the discrepancy. On the other hand, the effect of PDIM cannot be ignored in explaining the mutual relation between the observed horizontal and vertical rates and the predicted ones. A large melting rate of the order of −75 cm yr 11 (i.e. roughly 1.6 times larger than the mean rate derived from glaciology over Svalbard) would explain the observed uplift but only half of the gravity changes. Our comparison results clearly point out the importance of both the estimation accuracy of the elastic deformations and better observation accuracy to constrain the size of PGR effects in the northwestern Svalbard more tightly.

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