Abstract.The

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The 2004 catastrophic Indian Ocean tsunami has strongly emphasized the need for reliable tsunami early warning systems. Another giant tsunamigenic earthquake may occur west of Sumatra, close to the large city of Padang. We demonstrate that the presence of islands between the trench and the Sumatran coast makes earthquake‐induced tsunamis especially sensitive to slip distribution on the rupture plane as wave heights at Padang may differ by more than a factor of 5 for earthquakes having the same seismic moment (magnitude) and rupture zone geometry but different slip distribution. Hence reliable prediction of tsunami wave heights for Padang cannot be provided using traditional, earthquake‐magnitude‐based methods. We show, however, that such a prediction can be issued within 10 minutes of an earthquake by incorporating special types of near‐field GPS arrays (“GPS‐Shield”). These arrays measure both vertical and horizontal displacements and can resolve higher order features of the slip distribution on the fault than the seismic moment if placed above the rupture zone or are less than 100 km away of the rupture zone. Stations in the arrays are located as close as possible to the trench and are aligned perpendicular to the trench, i.e., parallel to the expected gradient of surface coseismic displacement. In the case of Sumatra and Java, the GPS‐Shield arrays should be placed at Mentawai Islands, located between the trench and Sumatra and directly at the Sumatra and Java western coasts. We demonstrate that the “GPS‐Shield” can also be applied to northern Chile, where giant earthquakes may also occur in the near future. Moreover, this concept may be applied globally to many other tsunamigenic active margins where the land is located above or close to seismogenic zones.

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Water vapor plays an important role in meteorological applications; GeoForschungsZentrum (GFZ) therefore developed a tomographic system to derive 3-D distributions of the tropospheric water vapor above Germany using GPS data from about 300 ground stations. Input data for the tomographic reconstructions are generated by the Earth Parameter and Orbit determination System (EPOS) software of the GFZ, which provides zenith total delay (ZTD), integrated water vapor (IWV) and slant total delay (STD) data operationally with a temporal resolution of 2.5 min (STD) and 15 min (ZTD, IWV). The water vapor distribution in the atmosphere is derived by tomographic reconstruction techniques. The quality of the solution is dependent on many factors such as the spatial coverage of the atmosphere with slant paths, the spatial distribution of their intersections and the accuracy of the input observations. Independent observations are required to validate the tomographic reconstructions and to get precise information on the accuracy of the derived 3-D water vapor fields. To determine the quality of the GPS tomography, more than 8000 vertical water vapor profiles at 13 German radiosonde stations were used for the comparison. The radiosondes were launched twice a day (at 00:00 UTC and 12:00 UTC) in 2007. In this paper, parameters of the entire profiles such as the wet refractivity, and the zenith wet delay have been compared. Before the validation the temporal and spatial distribution of the slant paths, serving as a basis for tomographic reconstruction, as well as their angular distribution were studied. The mean wet refractivity differences between tomography and radiosonde data for all points vary from −1.3 to 0.3, and the root mean square is within the range of 6.5–9. About 32% of 6803 profiles match well, 23% match badly and 45% are difficult to classify as they match only in parts