Review of works combining GNSS and insar in Europe

The Global Navigation Satellite System (GNSS) and Synthetic Aperture Radar Interferometry (InSAR) can be combined to achieve different goals, owing to their main principles. Both enable the collection of information about ground deformation due to the differences of two consequent acquisitions. Their variable applications, even if strictly related to ground deformation and water vapor determination, have encouraged the scientific community to combine GNSS and InSAR data and their derivable products. In this work, more than 190 scientific contributions were collected spanning the whole European continent. The spatial and temporal distribution of such studies, as well as the distinction in different fields of application, were analyzed. Research in Italy, as the most represented nation, with 47 scientific contributions, has been dedicated to the spatial and temporal distribution of its studied phenomena. The state-of-the-art of the various applications of these two combined techniques can improve the knowledge of the scientific community and help in the further development of new approaches or additional applications in different fields. The demonstrated usefulness and versability of the combination of GNSS and InSAR remote sensing techniques for different purposes, as well as the availability of free data, EUREF and GMS (Ground Motion Service), and the possibility of overcoming some limitations of these techniques through their combination suggest an increasingly widespread approach

VALIDATION OF FULL-RESOLUTION DINSAR-DERIVED VERTICAL DISPLACEMENT IN CULTURAL HERITAGE MONITORING: INTEGRATION WITH GEODETIC LEVELLING MEASUREMENTS

Towards revealing the potential of satellite Synthetic Aperture Radar (SAR) Interferometry (InSAR) for efficient detection and monitoring of Cultural Heritage (CH) encouraging resilient built CH, this study is devoted to the validation of InSAR-derived vertical displacements with a full-resolution perspective taking advantage of high-precision geodetic levelling measurements. Considering the Cathedral of Como, northern Italy, as the case study, two different Persistent Scatterer Interferometry (PSI) techniques have been applied to Cosmo-SkyMed high-resolution SAR images acquired in both ascending and descending orbit tacks within the time interval of 2010–2012. Besides using the simplified approach for obtaining the vertical displacement velocity from Line of Sight (LOS) velocity, a weighted, localized, multi-track Vertical Displacement Extraction (VDE) approach is proposed and evaluated, which uses the technical outcome of Differential InSAR (DInSAR) and spatial information. The results, using a proper PSI technique, showed that the accuracy level of extracted vertical displacement velocities in a full-resolution application is ca. 0.6 [mm/year] with a dense concentration of InSAR-Levelling absolute errors lower than 0.3 [mm/year] which are reliable and reasonable levels based on the employed validation framework in this study. Also, the weighted localized VDE can significantly decrease the InSAR-Levelling errors, adding to the reliability of the InSAR application for CH monitoring and condition assessment in practice

Data Processing and Modeling on Volcanic and Seismic Areas

This special volume aims to collecg new ideas and contributions at the frontier between the fields of data handling, processing and modeling for volcanic and seismic systems. Technological evolution, as well as the increasing availability of new sensors and platforms, and freely available data, pose a new challenge to the scientific community in the development new tools and methods that can integrate and process different information. The recent growth in multi-sensor monitoring networks and satellites, along with the exponential increase in the spatiotemporal data, has revealed an increasingly compelling need to develop data processing, analysis and modeling tools. Data processing, analysis and modeling techniques may allow significant information to be identified and integrated into volcanic/seismological monitoring systems. The newly developed technology is expected to improve operational hazard detection, alerting, and management abilities

Advances on the investigation of landslides by space-borne synthetic aperture radar interferometry

Landslides are destructive geohazards to people and infrastructure, resulting in hundreds of deaths and billions of dollars of damage every year. Therefore, mapping the rate of deformation of such geohazards and understanding their mechanics is of paramount importance to mitigate the resulting impacts and properly manage the associated risks. In this paper, the main outcomes relevant to the joint European Space Agency (ESA) and the Chinese Ministry of Science and Technology (MOST) Dragon-5 initiative cooperation project ID 59,339 “Earth observation for seismic hazard assessment and landslide early warning system” are reported. The primary goals of the project are to further develop advanced SAR/InSAR and optical techniques to investigate seismic hazards and risks, detect potential landslides in wide regions, and demonstrate EO-based landslide early warning system over selected landslides. This work only focuses on the landslide hazard content of the project, and thus, in order to achieve these objectives, the following tasks were developed up to now: a) a procedure for phase unwrapping errors and tropospheric delay correction; b) an improvement of a cross-platform SAR offset tracking method for the retrieval of long-term ground displacements; c) the application of polarimetric SAR interferometry (PolInSAR) to increase the number and quality of monitoring points in landslide-prone areas; d) the semiautomatic mapping and preliminary classification of active displacement areas on wide regions; e) the modeling and identification of landslides in order to identify triggering factors or predict future displacements; and f) the application of an InSAR-based landslide early warning system on a selected site. The achieved results, which mainly focus on specific sensitive regions, provide essential assets for planning present and future scientific activities devoted to identifying, mapping, characterizing, monitoring and predicting landslides, as well as for the implementation of early warning systems.This work was supported by the ESA-MOST China DRAGON-5 project with ref. 59339, by the Spanish Ministry of Science and Innovation, the State Agency of Research (AEI), and the European Funds for Regional Development under grant [grant number PID2020-117303GB-C22], by the Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital in the framework of the project CIAICO/2021/335, by the Natural Science Foundation of China [grant numbers 41874005 and 41929001], the Fundamental Research Funds for the Central University [grant numbers 300102269712 and 300102269303], and China Geological Survey Project [grant numbers DD20190637 and DD20190647]. Xiaojie Liu and Liuru Hu have been funded by Chinese Scholarship Council Grants Ref. [grant number 202006560031] and [grant number 202004180062], respectively

The determination of subtle deformation signals using a permanent CGPS network in the Aegean

Geophysical motions can occur over a broad temporal spectrum, from high frequency seismic movements to very long period tectonic deformation. The Aegean region is tectonically one of the most active areas on Earth. There have, over the past 15 years, been a range of campaign style GPS studies which have looked to increase our knowledge of the area and better define the geodynamic processes involved. In 2002 the Center for the Observation and Modelling of Earthquakes and Tectonics (COMET) established a network of continuously operating GPS receivers (CGPS) throughout the region in order to add to the knowledge gained from previous studies. This thesis focuses on which tectonic motions can be observed using the COMET continuous GPS network. Approaches for the precise analytical estimation of subtle tectonic motion are presented. Daily coordinate estimates of COMET sites and a number of ITRF (International Terrestrial Reference Frame) sites around Europe were calculated using a precise point positioning strategy and ambiguity resolution using NASA’s GIPSY – OASIS II processing software and IGS (International GPS Service) precise products. Time series produced showed post fit standard deviations of 2-3 mm in the horizontal and 6-8 mm in the vertical. Significant annual periodic variation is observed in the time series. The coordinate time series studies were further refined using a selection of filters. Firstly, gross and sigma filters were applied to remove outliers, the data then had a range of regional filters applied looking to best define and remove the common mode error in the area. These filters produced mixed results with time series improvement occurring on a site by site basis. In some cases noise was reduced by a factor of 2 whilst in other cases there was little or no improvement. This combined with a lack of knowledge of the individual site movements led to the use of a filtered baseline method, whereby common mode error was removed purely on a site by site basis. This method revealed expansion across the Hellenic arc of the order of a few millimetres per year and sub millimetre north-south compaction behind the arc. It also revealed first evidence of transient motion at a number of sites parallel to the Hellenic arc. The transient signals occurred every 12 months ±1.5 and lasting for 40 – 100 days. These signals were not so much a reversal of tectonic motion akin to the silent earthquakes observed in Cascadia, Japan and Mexico, instead they appeared more as a pause in the otherwise consistent movement of the Aegean microplate overriding the subducting African lithosphere. In addition to the observed tectonic signals, the effects and implications of the two post processing strategies are analysed and discussed. Higher temporal frequency positioning is carried out on seismic events (Mw 6.7 earthquake Kithera, Mw 8.1 and Mw 6.7 earthquakes, Macquarie island) using instantaneous positioning followed by “sidereal filtering” whereby integer-cycle phase ambiguities are resolved using only single epochs of dual frequency phase and pseudorange data. These positions are then siderealy stacked to reduce the effects of geometry related error. The technique reduces geometry related noise by a factor ≈2 using epoch by epoch 30 second data. The feasibility of the technique for observing pre, co and post seismic signals is demonstrated. A visualisation tool was developed to allow the simultaneous observation of the tectonic motion of a CGPS network data over any spatial and temporal regimes

Mapping and modelling the spatial variation in strain accumulation along the North Anatolian Fault

Since 1900, earthquakes worldwide have been responsible for over 2 million fatalities and caused nearly $2 trillion of economic damage. Accurate assessment of earthquake hazard is therefore critical for nations in seismically active regions. For a complete understanding of seismic hazard, the temporal pattern of strain accumulation, which will eventually be released in earthquakes, needs to be understood. But earthquakes typically occur every few hundred to few thousand years on any individual fault, and our observations of deformation usually only cover time periods of a decade or less. For this reason, our knowledge of the temporal variation in strain accumulation rate is limited to insights gleaned from kinematic models of the earthquake cycle that use measurements of present-day strain to infer the behaviour on long time scales. Previous studies have attempted to address this issue by combining data from multiple faults with geological estimates of long-term strain rates. In this thesis I propose a different approach, which is to observe deformation at multiple stages of the earthquake cycle for a single fault with segments that that have failed at different times. In the last century the North Anatolian Fault (NAF) in Turkey has accommodated 12 large earthquakes (Mw >6.5) with a dominant westward progression in seismicity. If we assume that each of these fault segments are at a different stage of the earthquake cycle then this provides a unique opportunity to study the variation in along-strike surface deformation, which can be equated to variation of deformation in time. In this thesis I use Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) observations to examine the spatial distribution of strain along the NAF. InSAR is an attractive technique to study surface displacements at a much higher spatial resolution (providing a measurement every 30 m) compared to established GNSS measurements, with station separations between 10 km to 100 km in Turkey. I specifically address a key technical challenge that limits the wide uptake of InSAR: phase unwrapping, the process of recovering continuous phase values from phase data that are measured modulo 2π radians. I develop a new unwrapping procedure for small baseline InSAR measurements that iteratively unwraps InSAR phase. For each iteration, this method identifies pixels unwrapped correctly in the previous iteration and applies a high cost to changing the phase difference between these pixels in the next iteration. In this way, the iterative unwrapping method uses the error-free pixels as a guide to unwrap the regions that contained unwrapping errors in previous iterations. I combine measurements of InSAR line-of-sight displacements with published GNSS velocities to show that an ∼80 km section of the NAF that ruptured in the 1999 Izmit earthquake (Mw 7.4) is creeping at a steady rate of ∼5 mm/yr with a maximum rate of 11 ± 2 mm/yr near the city of Izmit within the observation period 2002-2010. I show that in terms of the moment budget and seismic hazard the effect of the shallow, aseismic slip in the past decade is small compared to that from plate loading. Projecting the shallow creep displacement rates late into the earthquake cycle does not produce enough slip to account for the 2-3 m shallow coseismic slip deficit observed in the Izmit earthquake. Therefore, distributed inelastic deformation in the uppermost few kilometers of the crust or slip transients during the interseismic period are likely to be important mechanisms for generating the shallow slip deficit. I used similar techniques to confirm that a ∼130 km section of the central NAF near the town of Ismetpasa, is also undergoing aseismic creep at a steady rate of 8±2 mm/yr. Using simple elastic dislocation models to fit fault perpendicular velocities I show that there is an eastward decreasing fault slip rate in this region from ∼32 mm/yr to ∼21 mm/yr over a distance of about 200 km. The cause of this decrease remains unclear, but it could be due to postseismic effects from the 1999 Izmit and Duzce earthquakes and/or long-term influence from the 1943 (Mw 7.4) and 1944 (Mw 7.5) earthquakes. Finally, I combine line-of-sight displacements from 23 InSAR tracks to produce the first high resolution horizontal velocity field for the entire continental expression of the NAF (∼1000 km). I show that the strain rate does not vary significantly along the fault, and since each segment of the NAF is at a different stage of the earthquake cycle, the strain rate is invariant with respect to the time since the last earthquake. This observation is inconsistent with viscoelastic coupling models of the earthquake cycle, which predict a decreasing strain rate with time after an earthquake. My observations imply that strain accumulation reaches a steady-state fairly rapidly after an earthquake (<7-10 years) after which strain is localised on a narrow shear zone centred on the fault and does not vary with time. A time-invariant strain rate is consistent with a strong lower crust in the region away from the fault with a viscosity ≥1020 Pas. My results imply that short term snapshots of the present-day strain accumulation (as long as it is after the postseismic period) are representative of the entire earthquake cycle, and therefore geodetic estimates of the strain rate can be used to estimate the total strain accumulation since the last earthquake on a fault, and be used as a proxy for future seismic hazard assessment. The techniques I developed to explore the spatial and temporal pattern of aseismic fault creep and long-term strain accumulation along the NAF are general and can be ap- plied to all strike-slip faults globally. The archived ERS-1/2 and Envisat satellite data are an extremely valuable resource that can and should be used to extend InSAR time series measurements back to the early 1990s. Together with the new Sentinel-1 data sets, this provides an unprecedented opportunity to explore tectonic deformation over several decades and on continental scales. Despite the availability of numerous correction techniques (in this thesis I use global weather models to calculate the atmospheric contribution), atmospheric delays remain the major challenge to exploiting Sentinel-1 data for global strain mapping, the mitigation of these delays are an important goal for the InSAR community

GPS Imaging of Vertical Land Motion and Earthquake Coseismic Displacements in the GPS Mega-Network

The Nevada Geodetic Laboratory’s (NGL) Global Positioning Systems (GPS) worldwide data holdings number nearly 21,000 GPS stations that comprise the GPS Mega-Network today. Advances in data processing software, final orbit and clock products, atmospheric modeling, and reference frames have improved the precision and accuracy of GPS positioning solutions to the sub-millimeter level. The rates of change in these GPS position time series can be calculated by the MIDAS robust trend estimator to identify the patterns and styles of crustal deformation. Additionally, the large number of global stations improves the spatial resolution of observable geophysical signals. Together, these improvements helped motivate the GPS Imaging technique, an analysis method that interpolates spatiotemporal GPS trends between stations to construct a crustal velocity field representative of coherent movement of the solid Earth. The research presented in this dissertation uses the GPS Imaging technique to identify and analyze a number of geophysical signals related to vertical land motion and earthquake deformation. Two studies examine vertical land motion trends in regions of the United States and try to pinpoint the underlying geological sources for their signals. In the first study, GPS Imaging is used to identify the scope and extent of a subsidence signal observed in the Pacific Northwest. This signal is subsiding at approximately –2 mm/year, a rate higher than surrounding subsidence, and is located at latitudes corresponding to the Cascadia subduction zone and approximate longitude of the Cascadia arc. Several methods tested the resolution of GPS Imaging and changes to the regional signal over time. GPS data was then compared to predictions of various hypothesized loading sources that might contribute to the subsidence feature. GPS Imaging and realistic regional geological properties constrained volcanic loading and end loading models. This revealed that both styles of loading matched the width of the subsidence feature. A postseismic relaxation model from the 1700 M9.1 Cascadia Earthquake was compared to the GPS Imaging result, and accounted for approximately half of the subsidence signal concentrated around the Cascadia arc. Glacial isostatic adjustment modeling of the region determined that lithospheric flexure contributes about –1 mm/year of subsidence to the region. By combining the postseismic relaxation and glacial isostatic adjustment models, the subsidence feature was removed, suggesting that these two processes are likely the dominant sources of the subsidence signal. However, climatic and hydrological data compared to vertical land motion trends indicate possible contributions from hydrological loading. This work demonstrates a way to analyze subsidence signals in geologically complex regions, and laid important groundwork for other vertical land motion research. The second vertical land motion study was located in the Great Plains, United States. Vertical velocity data indicated there was an enigmatic source of regional uplift of approximately ~2 mm/year centered around the Texas Panhandle, with uplift extending through to the surrounding ~670 km x 280 km area. This region is home to the High Plains aquifer, the largest aquifer in the country and a major source of groundwater for agriculture. Water levels for the southern part of the aquifer have declined over 45 m, with greatest declines centered near the Texas Panhandle. Hydrological unloading was investigated as the principal source of the uplift signal. Climatic and hydrological data indicate a correlation between periods of drought and an increased rate of uplift observed by GPS data in the region. A hydrological unloading model was constrained by GPS Imaging by locating the greatest water mass loss where the uplift signal was ≥1 mm/year. Results indicated that a water volume loss of –5.1 km3/ year was sufficient to create the uplift signal observed by GPS Imaging, and this unloading rate is substantiated by other estimated rates of High Plains aquifer depletion. Our results indicated that hydrological unloading from aquifer deletion from climatic and anthropogenic influences is causing vertical land motion in the southern High Plains aquifer. This challenges the common conception that aquifer depletion equates to a subsidence signal, and also proves that GPS Imaging can be used as a tool to monitor groundwater changes remotely. The last study shifts away from regional vertical land motion investigations to apply GPS Imaging to global earthquake research. Some of the ~21,000 GPS stations in GPS Mega-Network are situated in earthquake prone regions experiencing tectonic deformation from plate interactions and/or induced seismicity. Earthquakes captured by the GPS Mega-Network are recorded in GPS time series as immediate discontinuities that represent coseismic displacement. Several different strategies are first tested to estimate coseismic displacements for the NGL. Analysis of coseismic displacements, aided by GPS Imaging, suggests that estimations are improved by a hierarchical strategy and radius of influence used to approximate which stations may be potentially affected by an earthquake. Next, the coverage, completeness, and resolution of coseismic displacements in the GPS Mega-Network is examined using the GPS Global Earthquake Catalog built from the coseismic displacement data. Comparisons of the GPS Global Earthquake Catalog to the USGS National Earthquake Information Center Earthquake Catalog for events occurring between 1 Jan. 1994–20 Apr. 2022 reveal that the GPS Mega-Network’s ability to capture global earthquake activity has increased over time and that the availability of estimated GPS coseismic displacements is greatest for earthquakes M≥7. Of the 427 earthquakes M≥7 recorded by the USGS, 93% of earthquakes 7≤M<7.5 have estimated GPS displacements, and 100% of earthquakes 7.5≤M≤9.1 have coseismic displacement data available

D10.1 Report on the dissemination activities and Conference organisation

This deliverable provides an extensive analysis of the dissemination activities and workshops organisation of the EXCELSIOR H2020 Teaming Project. The analysis starts with the report on our participation in conferences (11) and how the project was promoted through it. Then, we explain about the participation of our team members in talks (17), workshops (7) and seminars (12) as invited speakers. The deliverable continues with a thorough presentation of the lectures by invited speakers (8), the webinar (1) and the workshops (2) organized by our team. Additionally, we document about our participation in other events (i.e., European Researcher’s Night 2021 and SpaceUPCyprus 2021 Live). The last chapter provides the publications, journal papers, conference papers, and book sections for the reporting time period. The deliverable concludes by providing information on the outcome of the reported activities and how they have contributed to the progress of the EXCELSIOR H2020 Teaming Project. It is concluded that there is a strong need to establish links in the EMMENA region and connect with them. This has not been achieved yet, but a strategy was prepared to raise awareness about the EXCELSIOR Project in the EMMENA region and establish partnerships, starting with targeted stakeholders’ workshop in autumn 2021, where selected stakeholders from the region will be invited to be informed them about the project and provide them the space to discuss their needs and identify common scientific interests and ways of collaboration