Fast subsidence in downtown of Seattle observed with satellite radar

Abrupt change in the long term displacement trend was observed in the downtown of Seattle (Washington, USA) with the satellite Differential Interferometric Synthetic Aperture Radar (DInSAR). The forty nine ascending and forty six descending RADARSAT-2 Spotlight SAR images with range-azimuth resolution of 1.6×0.8 m and ground coverage of 18×8 km were collected during June 6, 2012-August 16, 2015. The vertical deformation time series were computed from the ascending and descending data sets with the advanced Multidimensional Small Baseline Subset (MSBAS) software. During June 6, 2012-August 31, 2014 subsidence occurred along the two transportation routes, running north-south parallel to the coast line, reaching 2.5 cm. The long term deformation rate did not exceed 1.2 cm/year and was nearly constant during that time. After August 31, 2014, rapid subsidence appeared in the downtown of Seattle, in a region where it previously was not observed. Over the next four months during August 31, 2014-December 29, 2014 cumulative subsidence reached approximately 2.5 cm, which corresponds to an annual rate of 10 cm/year, about eight times faster than during the previous two years. The rate of subsidence remained high for an additional few months before converging to its secular value. The cause of subsidence was linked to the recent anthropogenic activities related to tunnel boring for Alaskan Way Viaduct replacement project. The elliptical model for subsidence due to groundwater pumping proposed by Tiampo et al. (2012) was used to invert for the physical parameters of the associated groundwater pumping. The results are in good accordance with the location of pumping operations and the groundwater bearing strata in the region. Over a long period of time such fast subsidence will cause additional strain on urban infrastructure, particularly older, historic structures and high-rise buildings. This work provides an excellent example of the importance and applicability of rapid, accurate monitoring of ongoing, large infrastructure projects using DInSAR techniques in order to make optimal use of limited resources and perform early intervention and mitigation strategies.