<p>High-resolution geodetic measurements of crustal deformation from InSAR have the potential to provide crucial constraints on a region’s tectonics, geodynamics and seismic hazard. Here, we present a high-resolution crustal velocity field for the Alpine-Himalayan Seismic Belt (AHSB) derived from Sentinel-1 InSAR and GNSS. We create time series and average velocities from ~220,000 interferograms covering an area of 15 million km<sup>2</sup>, with an average of 170 acquisitions per measurement point. We tie the velocities to a Eurasian reference frame by jointly inverting the InSAR data with GNSS data to produce a low-resolution model of 3D surface velocities. We then use the referenced InSAR velocities to invert for high-resolution east-west and sub-vertical velocity fields for the entire region. The sub-vertical velocities, which also include a small component of north-south motion, are dominated by non-tectonic deformation, such as subsidence due to water extraction. The east-west velocity field, however, reveals the tectonics of the AHSB with an unprecedented level of detail.</p><p>The approach described above only provides good constraints on horizontal displacement in the east-west direction, with the north-south component provided by low-resolution GNSS measurements. Sentinel-1 does also have the potential to provide measurements that are sensitive to north-south motion, through exploitation of the burst overlap areas produced by the TOPS acquisition mode. These along-track measurements have lower precision than line-of-sight InSAR and are more effected by ionospheric noise, but have the advantage of being almost insensitive to tropospheric noise. We present a time series approach to tease out the subtle along-track signals associated with interseismic strain. Our approach includes improvements to the mitigation of ionospheric noise and we also investigate different filtering approaches to optimize the reduction of decorrelation noise. In contrast to the relative measurements of line-of-sight InSAR, these along-track measurements are automatically provided in a global reference frame. We present results from five years of data for the West-Lut Fault in eastern Iran and the Chaman Fault in Pakistan and Afghanistan. Our results agree well with independent GNSS measurements; however, the denser coverage of the technique allows us to also detect the variation in slip rate along the faults.</p><p>Finally, we demonstrate the improvement in the resolution of horizontal strain rates when including these along-track measurements, in addition to the conventional line-of-sight InSAR measurements.</p>

With the launch of the Copernicus Sentinel-1 Synthetic Aperture Radar (SAR) mission, the geoscience community acquired a unique tool for making precise measurements of large-scale surface deformation. The Center for Observation and Modeling of Earthquakes, Volcanoes and Tectonics (COMET) LiCSAR system (Lazecký et al., 2020) routinely generates Sentinel-1 differential interferograms over tectonic and volcanic areas, and carries out interferometric (InSAR) time series analyses to measure surface deformation in the satellite line-of-sight direction. When the satellites are traveling south to north (ascending), the look direction is downwards and slightly north of eastwards, and when moving north to south (descending) it is downwards and slightly north of westwards. The two geometries can therefore be used to accurately constrain the vertical and east-west components of any motion, but the line-of-sight sensitivity is very low for the north-south component, which is typically estimated using available GNSS data (Weiss et al., 2020).It is possible to estimate along-track displacements, which are sensitive to northward motions, by exploiting spectral diversity in the along-track, or azimuth, direction (Bechor & Zebker, 2006). This involves forming two sub-apertures for each SAR image, one with a look direction pointing slightly forwards and one pointing slightly backwards. Differencing interferograms formed from the forward-looking images and backward-looking images, results in phase change due to along-track displacement only, as displacement perpendicular to the flight direction cancels. The precision of these measurements is poor, however, as the difference in angle between the two sub-apertures is very small, particularly for the Interferometric Wideswath mode of Sentinel-1, which is the standard mode over land, due to the reduced Doppler spectrum available for each subswath.