Researchers at the Institute of Geomatics — Research Centre of the Polytechnic University of Catalonia (UPC) and the Government of Catalonia — have estimated the ground deformation suffered in the area of Sendai, Japan, as a consequence of the earthquake of 11 March and its aftershocks. The estimate was derived from radar observations acquired by the Envisat satellite of the European Space Agency. According to this estimate, obtained over an area of approximately 300 x 100 km around the city of Sendai, the terrain has suffered a co-seismic deformation — permanent deformation of Earth’s surface — associated with the earthquake of up to 1.69 m.
This value refers to the line connecting the satellite and the observed area, which is tilted about 41º from the vertical. The estimated value of 1.69 m is a deformation value relative to the observed area: the absolute deformation (i.e., calculated over an area not affected by the earthquake) is surely larger and cannot be estimated with this technique.
The minimum distance from the epicentre to the study area is 100 km approximately. At these distances the vast majority of earthquakes do not cause co-seismic deformation. However, in this case, there has been considerable deformation due to the exceptional magnitude of this earthquake.
The deformation can be seen in the map shown above, where the displacements are represented in a colour scale from black (no relative motion) to red (area of maximum deformation, which is the closest to the epicentre). There is relative deformation of 1.69 m from the area that appears black to the maximum area in red.
This study conducted by the Institute of Geomatics, and led by Dr. Michele Crosetto, head of the Remote Sensing Unit, is based on a well-known satellite-based microwave remote sensing technique called differential interferometry SAR (Synthetic Aperture Radar).
What does the observed deformation entail?
As mentioned above, the estimated 1.69 m deformation refers to the line connecting the satellite and the observed area. We know from other sources, such as measures of GPS (Global Positioning System), that the horizontal displacement has been of approximately 3 m eastwards, 0.5 m northwards and the vertical displacement varies between 0.5 and 1 m. The most important of these three components is probably the subsidence as it affects land elevation above sea level. Coastal areas are likely to be the most affected: in the worst case scenario, subsidence might lead to land loss. This is a phenomenon similar to what might result from climate change, although in this case, land would be lost as a result of sea-level rise.
How were the results derived?
SAR interferometry is a remote sensing technique used to monitor surface deformations. This technique, based on the analysis of SAR satellite data, has been used in various applications such as studying the dynamics of glaciers, earthquakes, volcanoes, mining, civil works, landslides and deformations due to exploitation of aquifers.
The results of this study were obtained using SAR data from the sensor ASAR mounted on the satellite Envisat of the European Space Agency. Specifically, in this case, two SAR images acquired before (19/02/2011) and after the seismic event (03/21/2011) have been used.