Elastic versus permanent along-strike deformation at the North Chilean forearc, observed by radar interferometry

Abstract

Along-strike seismogenic behavior of subduction megathrusts may feed back into the forearc deformation pattern as elastic and permanent deformation. The Chilean subduction zone, including the megathrust and the forearc, shows along-strike variations in both short-term behavior observed from seismicity and geodesy as well as from long-term records (upper-plate faults and topography) archived in the forearc. Unlike elastic deformation, which accumulates temporarily during the earthquake cycle, permanent deformation is reflected in the topography. Over which stage of the earthquake cycle permanent deformation occurs, however, remains unclear. Also, the connection between short-term (mainly elastic) and long-term (permanent) deformation in the forearc remains unclear.To evaluate the forearc deformation, we analyze interseismic surface deformation data of the coast and coast range (i.e., Coastal Cordillera), covering the North Chilean forearc from the south of Iquique to south of Taltal (Latitude: -20.5 to -26). We tie displacement rates obtained from five years of Sentinel-1 radar interferometric (InSAR) time series, and two view angles to a uniform reference frame spanned by accurate positioning rates and decompose the InSAR to east and vertical components. We evaluate the correlation between interseismic deformation, topography, and the activity of forearc faults. This involves an attempt to subtract the elastic vertical component, assuming a small percentage of interseismic permanent deformation. We assess the conversion of interseismic vertical deformation into permanent deformation along the coast (i.e., examining available uplifted marine terraces data) and Coastal Cordillera topography. Our preliminary findings propose a systematic change in vertical deformation from the coast to the Coastal Cordillera during the interseismic period: the coast (a narrow zone) is mainly experiencing subsidence, whereas the Coastal Cordillera is undergoing uplift. Uplift rates at the Coastal Cordillera vary along strike and are highest at the northern and southern regions of the Mejillones peninsula. Here, our elastic interface locking model fails to predict the uplift rates, implying additional processes governing uplift. Subsurface data (e.g., seismicity and seismic tomography) are required to examine the processes involved in the uplift pattern. Known as a barrier for megathrust events, the Mejillones peninsula exhibits maximum ongoing subsidence. Although interface locking is probably the primary process controlling the subsidence, our preliminary results imply the potential contribution of upper-plate faults in amplifying the subsidence rate.