Geostatistical and space geodetic methods on a 3D coseismic displacement field

Abstract

SummaryCoseismic displacements at the Earth's surface arise due to the release of energy by earthquakes. The energy, and hence the amount of displacement, depends mainly on the magnitude, depth and the movement mechanism of an earthquake. Coseismic displacements play a vital role in the characterization of geological faults and understanding of earthquake dynamics. This understanding is based in turn on the accurate measurement of coseismic displacements and creation of a continuous-field displacement map. The main objective of this research was to measure the coseismic displacements and to construct a 3D coseismic displacement field for an earthquake.Displacements are typically measured by comparing pre-and postearthquake radar images. In this study InSAR is used that measures phase (time) differences between pre-and post-earthquake SAR images resulting in displacements into line-of-sight (LOS) direction. The LOS is mainly sensitive in the vertical direction, moderately in the across flight direction and insensitive in the along flight direction. To measure the along-flight displacement component pixel tracking was used, providing azimuth offsets (AZO). Pixel tracking correlates the coregistered pre-and post-earthquake optical or SAR images. In this study pixel tracking was used for both SAR and optical data, whereas InSAR is used for SAR data only. Using the obtained displacement results, a 3D displacement field was generated from both descending and ascending image pairs bracketing the earthquake event.The study considered the 2003 Bam earthquake. The city is located on a geological fault that caused occurrence of the earthquake. From the measured displacements it appeared that the major part of the fault is located south of the city. It has a length of 13 km and the fault strike equals 170º, close to the along flight direction. The major vertical displacement occurred on east of the fault, showing a uplift in the south-eastern part of displacement map and subsidence in its north-eastern part. Horizontal displacements showed a relative movement along the fault in the along flight direction. The displacement was greater in horizontal direction than the LOS, revealing that the earthquake causative fault is a strike-slip fault.A displacement map commonly contains missing coseismic values due to lack of coherence between SAR images. In this study, missing displacement values were caused by non-coherence of pre-and post-earthquake SAR images for the area affected by the earthquake. These missing displacement values may be important for fault characterization, in particular if they occur along the earthquake-causative geological fault. Missing values were interpolated to evaluate the continuity of the earthquake causative fault underneath the city of Bam. i To assess the accuracy of the interpolated coseismic values, subsets of missing co-seismic values were selected and translated towards locations where displacement values exist. Here, missing values were generated artificially. These values were interpolate...