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Water Vapour Bias in Measuring Interseismic Strain Accumulation with InSAR for the Altyn Tagh Fault, N. Tibet

John Elliott(1), Juliet Biggs(2), Zhenhong Li(3), Barry Parsons(1) and Tim Wright(4)

(1) University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
(2) University of Miami, Miami, Florida, United States
(3) University College London, London, WC1E 6BT, United Kingdom
(4) Univerity of Leeds, Leeds, LS2 9JT, United Kingdom


The slip rate of the left-lateral Altyn Tagh Fault (ATF) of Northern Tibet is poorly known and controversial. Geologic rates from Holocene and long-term offsets range from 10 to 30 mm/yr. Geodetic rates from GPS are towards the low end of this range (around 10 mm/yr). SAR data have been acquired over this region by the ERS satellites through the nineties and continue today with the ENVISAT platform. These datasets provide an opportunity to measure interseismic strain accumulation on faults. However, InSAR measurements vary from 5 mm/yr for the western ATF (Wright et al., 2004) to 17 mm/yr for the central ATF (Peltzer et al., AGU Fall Meeting 2006). An ongoing challenge for the accuracy of this geodetic technique is the variability of tropospheric water vapour, which induces unwanted phase delays. This is particularly pronounced in the case of the Altyn Tagh Fault system, which exhibits a 2-4 km step in relief between the Tarim Basin and Tibetan Plateau. We investigate the effects of water vapour on the inferred slip rate. An analysis of ERS data has highlighted a bias from the non-uniform distribution of SAR acquisitions through the year. Due to the lack of contemporaneous independent measurements of water vapour for ERS data, we attempt to mitigate tropospheric water vapour errors by solving for a linear fit with topography. A network of 59 interferograms is used to correct for orbital errors and water vapour, before inverting for a slip rate. We invert for a buried fault using a simple elastic dislocation model and assume pure strike-slip motion with no component of uplift. Using ERS data at 85E we find a slip rate of 10 +/- 5 mm/yr (1-sigma) with the water vapour correction, and 35 +/- 9 mm/yr without. Errors in the calculated slip rate are determined by Monte Carlo analysis using 100 noise simulations created and perturbed with characteristic orbital and atmospheric noise. Because of the large impact of the water vapour on the result, we tested for the effect of the temporal distribution of SAR acquisitions by systematically shifting the epochs through the year, using modelled weather data from the European Centre for Medium-Range Weather Forecasting to simulate the effects of tropospheric delay and using a known input slip rate to calculate synthetic interferograms. On repeating the Monte Carlo analysis, a clear seasonal bias is retrieved. In the worst cases, slip rates can be under- or over-estimated by as much as 15 mm/yr if the water vapour is not accounted for. When we solve for the water vapour contribution based upon height, the bias is removed.


Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry