Combining the strapdown approach for InSAR data with one-dimensional tectonic strain analysis in the Irpinia region

The application of displacement time series derived from SAR Interferometry (InSAR) for tectonic strain estimation is constrained by the requirement that ground motion occurs predominantly within the east–vertical plane. Indeed, the near-polar orbit of most currently available SAR satellites makes the technique weakly sensitive to displacements in the northward direction. Since a single InSAR observation captures only the line-of-sight (LOS) projection of the displacement, data acquired from multiple viewing geometries, including at least one ascending and one descending, are required to approximately retrieve the east and vertical displacement components. To make this possible, the product between the northward displacement component and the coefficient that determines its contribution to the LOS projection is assumed to be zero. While this assumption may be acceptable when dealing with phenomena characterized by dominant east–up motion, in general, the accuracy of the estimates is suboptimal. 

Under these conditions, a-priori information on the displacement direction can improve the solution. In [1], a new method, the strapdown approach, was introduced. Starting from a-priori knowledge of the displacement direction, and particularly by imposing that the vector lies within a plane, this method allows the estimation of a locally two-dimensional but globally three-dimensional solution. The possibility to vary the plane’s orientation, together with the use of an appropriate stochastic model for both the InSAR observations and the angles defining the plane, enables the method to produce a spatially variable output in terms of both direction and accuracy. 

In this work, we apply the strapdown approach to highlight tectonic strain accumulation in southern Italy, specifically in the Irpinia area, characterized by high seismic hazard. We follow the one-dimensional procedure defined in [2], which focuses on detecting signs of tectonic strain accumulation by analyzing velocity variations along directions of interest, defined a-priori based on the tectonic knowledge of the area. In the absence of concomitant and spatially widespread phenomena (e.g., subsidence), these directions coincide with those along which the largest variations of velocity are expected. The one-dimensional procedure, originally applied using GNSS data, is therefore well-suited for use in combination with the strapdown approach when InSAR data are adopted as input. In this work, the proposed combined methodology is applied using time series provided by the European Ground Motion Service, considering the directions of interest initially defined in [2], which reflect the extensional tectonic regime perpendicular to the Apennine chain. Preliminary results highlight the potential of combining the strapdown approach with one-dimensional analyses of tectonic strains along known directions for seismic hazard monitoring. 

[1] Brouwer, Wietske S., and Ramon Hanssen. "Estimating three-dimensional displacements with InSAR: The strapdown approach." (2023).

[2] Crespi, M., Kossobokov, V., Panza, G. F., & Peresan, A. (2020). Space-time precursory features within ground velocities and seismicity in North-Central Italy. Pure and Applied Geophysics, 177(1), 369-386.