Measurement of Along-track Displacements due to the M6.0 August 24, 2014 South Napa Earthquake Using X-band Multiple-Aperture SAR Interferometry

Multiple-aperture SAR interferometry (MAI) has been developed for measuring surface displacements in along-track direction as an alternative method of amplitude offset tracking method. Various studies on geological phenomena have been carried out using MAI technique with C-band and L-band SAR data, but application of MAI to X-band SAR is challenging due to its more severe temporal decorrelation compared to longer wavelength radar. The Italian Space Agency's (ASI) COSMO-SkyMed (CSK) mission consisting of four identical radar satellites has a powerful capability to minimize temporal baselines maintaining high coherence. This offers a good chance for MAI application for precise measurement of along-track displacements. In this study, we demonstrate the MAI performance of X-band SAR for the M6.0, South Napa Earthquake occurred on August 24, 2014. A coseismic CSK pair data (July 26 and August 27, 2014) acquired from descending orbit was used to show the along-track displacements in the fault zone. In order to evaluate the precision for measuring MAI deformation on the Napa Earthquake using CSK data, we produced a coherence map of the interferogram because the MAI precision is a function of interferometric coherence. However, we found that standard deviation of MAI phase does not coincide with the theoretical variation. The measurement uncertainty of along-track displacements was estimated by using the predefined empirical equation which was established through the performance test using multi-path CSK dataset at Kilauea Volcano region. The uncertainty map of the along-track displacements in the South Napa Earthquake region provides a reliable metric to estimate the variance/covariance of the data, useful for 3-D displacement field construction and geophysical modeling.