Nikos Svigkas | Istituto Nazionale di Geofisica e Vulcanologia (original) (raw)

Papers by Nikos Svigkas

In this work we describe the implementation of a processing chain for a fully automatic modeling ... more In this work we describe the implementation of a processing chain for a fully automatic modeling of the seismic source parameters and its slip distribution through the inversion of the InSAR displacements generated from the EPOSAR service. This processing chain consists of a suite of procedures and algorithms handling a sequence of steps: selection of the highest quality InSAR datasets, definition of the area of interest, image sampling, nonlinear and linear inversions to get, respectively, the source geometry and its slip distribution. A set of side procedures and interfaces also allows an interactive refinement and the publication of results, consisting of scientific data and graphical outputs. The whole procedure has been developed, tested and validated by considering 100 events with magnitudes between 5.5 and 8.2, worldwide distributed and covering an exhaustive range of mechanisms and tectonic contexts. Main aim of this work is describing the implementation of the automatic modeling procedures, used to produce solutions in real time, already during the emergency phase. These sources, validated by experts before their publication, can be a reference for operational purposes and initial scientific analyses. The creation of this repository sets also the framework to store, out of the emergency time, more sophisticated solutions, manually revised and/or with peer-review quality.

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Geophysical Journal International

SUMMARY On the 9th of January 2020, an Mw 6.4 strike-slip earthquake took place north of the Asia... more SUMMARY On the 9th of January 2020, an Mw 6.4 strike-slip earthquake took place north of the Asian margin of the Bering Sea. The earthquake occurred within the known reverse-right-lateral active fault zone, called Khatyrka–Vyvenka, which transverses the Koryak Highland from SE to NW and is thought to be a surface manifestation of the Asian portion of either the Bering plate boundary or the northern edge of the Alaskan stream. No other strong earthquake has ever been recorded in this remote uninhabited area and the few existing seismic stations provide poor quality earthquake locations. We adopt SAR interferometry (InSAR) technique to define an improved location of the Koryak 2020 earthquake and constrain the seismic source. The analysis of the 2020 event revealed a previously unknown active fault of left-lateral kinematics that is possibly hidden and strikes NW transversely to the Khatyrka–Vyvenka fault zone. Although several mechanisms could account for left-lateral kinematics of t...

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In March 2021 three strong earthquakes with magnitudes (Mw) of 6.3, 6.0, and 5.2 occurred in Thes... more In March 2021 three strong earthquakes with magnitudes (Mw) of 6.3, 6.0, and 5.2 occurred in Thessaly plain, Greece, on 3, 4, and 12 March, respectively. The modeling of all the three sources, by inversion of Interferometric Synthetic Aperture Radar and Global Positioning System data, indicates a northeast–southwest-trending extensional stress field with indications for northeast-dipping sources. The unmapped fault source of the first mainshock (Mw 6.3) is located approximately 6 km to the southwest of the known Larissa fault (LF). Moreover, the fault that was activated during the second mainshock (Mw 6.0) appears to be located more to the north, bordering the Titarisios river valley to the southwest, whereas the third mainshock (Mw 5.2) appears to be triggered at a fault segment located further to the northwest. The Coulomb stress analysis using the slip distributions of the three aforementioned mainshocks revealed a unilateral triggering of the second and third event toward the northwest, and explained the spatial development of the entire aftershock sequence. Furthermore, among the already known active faults in the broader area, only the LF was brought closer to failure as a result of the imparted stress changes.

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The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran-Iraq border... more The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran-Iraq border on 12 November 2017. This M w 7.3 earthquake was followed by an intense seismic sequence. Implementing the double-difference earthquake location technique, we relocate 1069 events recorded by our local seismic network, deployed after the mainshock. The spatial distribution of the epicenters indicates linear alignments of the events nucleated along at least four notable clusters. The clusters are characterized by at least one significant earthquake, such as the Tazehabad earthquake of 25 August 2018 (M w 5.9) along a dense, east-west trending cluster and the Sarpol-e Zahab earthquake of 25 November 2018 (M w 6.3) along the cluster with a northeast-southwest trend. We use two-pass differential SAR interferometry (DInSAR) and Small BAseline Subset (SBAS) methods to study the coseismic permanent displacements of the Azgeleh, Tazehabad and Sarpol-e Zahab events as well as the one-year postseismic deformation field of the 2017-2018 seismic sequence, respectively. We use non-linear and linear optimization algorithms to derive the source geometry and the slip distribution along the fault planes. The inversion is conducted by introducing also seismological constraints, leading to the definition of a listric geometry for the Azgeleh mainshock rupture that accommodates the slip area at depth of 10-16 km along a sub-horizontal plane (dipping ~3 •) and a low-angle (~16 •) ramp. The thrust and dextral movements along this NNW-striking (~345 •) fault have triggered a tear fault responsible for the Tazehabad event ruptured an east-west trending (~267 •), north-dipping (~78 •) sinistral shear fault. We present the dextral slip distribution of the Sarpol-e Zahab event along a NE-striking (~34 •) fault, as a synthetic Riedel structure for the southern segment of the Khanaqin fault, dipping 63 • to the southeast. We find the postseismic deformation field associated with the seismic sequence is not confined only to the mainshock source (the Azgeleh fault), but also develops along the Tazehabad and Sarpol-e Zahab faults. We additionally propose afterslip along a duplex, flat-ramp-flat structure down-dip and up-dip of the Azgeleh coseismic slip area. The up-dip afterslip develops onto the shallow detachment (~3 •) at depth of ~8 km and the down-dip afterslip propagate onto the mid-crustal décollement level within the Pan-African basement. The Azgeleh, Tazehabad, Sarpol-e Zahab and Khanaqin faults mark the Lurestan Arc-Kirkuk Embayment sharp margin in the Northwest Zagros and play a key role in the lateral escape of the Lurestan Salient and vertical strain partitioning in the Zagros front.

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Low-to-moderate magnitude earthquakes often induce small ground displacement. For such events, gr... more Low-to-moderate magnitude earthquakes often induce small ground displacement. For such events, ground deformation fields detected by SAR interferometry can be masked or not clearly discernible from the fringes distribution because of the presence of error sources, such as atmospheric artifacts and topographic residuals. We show two examples of low-moderate magnitude earthquakes, for which we adopted a new automatic tool for calculating the fringe pattern stemming from seismological data. The tool estimates the extent and the geographic position of the deformation by running a forward model of the seismic source, thus identifying the best SAR frame to be collected and the expected surface effects to better figure-out the outcomes of InSAR processing. We present the Mw 5.7 occurred in Greece and the Mw 5.4 occurred in Zagreb on 21 and 22 March 2020, respectively. For the Greek earthquake, the tool predicted a deformation close to the InSAR product, and gave evidences of atmospheric disturbances, thus providing information for inverse source modelling. The tool in the Zagreb event was used to infer the extent and location of the ground motion, that were used to identify the best SAR pair to be processed, being the SAR frames edge very close to the epicenter.

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The active collision of the Apulian continental lithosphere with the Eurasian plate characterizes... more The active collision of the Apulian continental lithosphere with the Eurasian plate characterizes the tectonics of the Epirus region in northwestern Greece, invoking crustal shortening. Epirus has not experienced any strong earthquakes during the instrumental era and thus there is no detailed knowledge of the way the active deformation is being expressed. In March 2020, a moderate size (Mw 5.8) earthquake sequence occurred close to the Kanallaki village in Epirus. The mainshock and
major aftershock focal mechanisms are compatible with reverse faulting, on NNW-ESE trending
nodal planes. We measure the coseismic surface deformation using radar interferometry and investigate the possible fault geometries based on seismic waveforms and InSAR data. Slip distribution
models provide good fits to both nodal planes and cannot resolve the fault plane ambiguity. The results indicate two slip episodes for a 337◦ N plane dipping 37◦ to the east and a single slip patch for a 137◦ N plane dipping 43◦ to 55◦ to the west. Even though the area of the sequence is very close
to the triple junction of western Greece, the Kanallaki 2020 activity itself seems to be distinct from it, in terms of the acting stresses.

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We use Interferometric Synthetic Aperture Radar (InSAR) to study the Cephalonia-Lefkada Transform... more We use Interferometric Synthetic Aperture Radar (InSAR) to study the Cephalonia-Lefkada Transform Fault Zone (CTF) in the Ionian Sea. The CTF separates continental subduction to the north from oceanic subduction to the south, along the Hellenic Subduction Zone. We exploit a rich multi-modal radar dataset of the most recent major earthquake in the region, the 17 November 2015 Mw 6.4 event, and present new surface displacement results that offer additional constraints on the fault segmentation of the area. Based on this dataset, and by exploiting available information of earthquake relocation, we propose a new rupture process for the 2015 sequence, complementary to those published already. Our modelling includes an additional southern fault segment, oblique to the segment related with the mainshock, which indicates that the CTF structure is more complex than previously believed.

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In this study, an analysis of the November 12, 2017 northwestern Iran earthquake (Mw 7.3) is pres... more In this study, an analysis of the November 12, 2017 northwestern Iran earthquake (Mw 7.3) is presented. A multi-frequency Synthetic Aperture Radar Interferometry (InSAR) approach was adopted to retrieve the ground deformation due to the mainshock. Then, InSAR results were used as input for the source modeling of the seismic event. C-Band Sentinel-1 (IW TOPSAR acquisition mode) and L-Band ALOS-2 (Wide Swath) SAR data along both the ascending and descending orbits were processed. InSAR results show a displacement pattern in the epicentral area, with a maximum value up to ~1.0 meter. Preliminary analytical and numerical models suggest that the retrieved InSAR ground displacements are due to a slip dislocation (over 3.8 meters) on the causative fault and that dips to the east with a shallow angle of 16°.

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We studied the broader area of Thessaloniki in northern Greece and Chalkidiki and performed an In... more We studied the broader area of Thessaloniki in northern Greece and Chalkidiki and performed an InSAR campaign to study the surface deformation phenomena that have been known to exist for at least two decades. Sentinel-1 data (2015-2019) together with drill measurements were exploited to focus on specific sites of interest. Our results indicate an ongoing displacement field. At the region of Kalochori and Sindos-where intense subsidence in the 1990s was previously found to have had a natural surface rebound in the 2000s-a new period of subsidence, caused by the enlivenment of the groundwater overexploitation, was reported. The uplifting trend of Oreokastro is still active and subsidence in Anthemountas graben is ongoing; special focus was set on the Makedonia Airport, where significant displacement is occurring. The study also reveals a new area at Nea Moudania, that was not known previously to deform; another case corresponding to anthropogenic-induced surface displacement. Thessaloniki is surrounded by different persistent displacement phenomena, whose main driving mechanisms are anthropogenic. The sensitivity of the surface displacements to the water trends is highlighted in parts of the study area. Results highlight the plan of a water resources management as a high priority for the area.

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We study the surface deformation of a moderate size M5+ earthquake swarm-type activity which burs... more We study the surface deformation of a moderate size M5+ earthquake swarm-type activity which burst at the tip of the Biga peninsula (western Turkey) in early 2017. No previous M5+earthquakes have been recorded in the instrumental period on land, however, offshore normal faults, have ruptured to produce strong (M > 6) earthquakes. We use the Interferometric Synthetic Aperture Radar (InSAR) technique and exploit a data set of Sentinel-1 and ALOS-2 images, to successfully detect the surface deformation caused by three M5+ events of the sequence and constrain their source models, furtherly strengthened by seismic waveform analysis. The sequence occurred at shallow depths (<12 km) and is related with the activation of a normal fault. Our geodetic inversions constrained the rupture distribution of the main events and led us to conclude that they belong to a single fault plane, striking N110 • E and dipping ∼40 • to the SW, compatible with the regional tectonics. Furthermore, the InSAR analysis revealed that no aseismic transients occurred during the Biga swarm. InSAR applications in seismic swarms are limited mainly due to the low displacement signal they produce, which is not always easily detectable by radar satellites. Another obstacle is the time frequency of radar satellites acquisitions that often does not allow the temporal isolation of distinct seismic events. However, here we present a study that exploits InSAR data to isolate seismic sources of a swarm and proposes its use for the understanding of shallow seismic swarms in a systematic manner.

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The focus of this study is Anthemountas basin in northern Greece, where land subsidence phenomena... more The focus of this study is Anthemountas basin in northern Greece, where land subsidence phenomena had been previously
identified. The basin has a NNW-SSE orientation, which is optimal to the currently acting ~ N-S extension field in the back-arc
Aegean area and is bordered to its north and south edge by two, well-pronounced faults. The region of study is located close to the
greater metropolitan city of Thessaloniki and comprises a number of critical facilities including the city’s international airport.
The research object is the monitoring of the evolution of the displacement at Anthemountas graben, using InSAR time-series and
the investigation of the dominant driving mechanism. We show new surface displacement results from the ENVISAR satellite,
and overall, an enhanced dataset of SAR time-series is presented, spanning the period 1992 to 2010, by using the satellites ERS1,
ERS2 and ENVISAT. Results indicate a continuously deforming environment in both decades, with an increasing magnitude.
The detailed study of the deformation pattern together with the analysis of in situ data defines aquifer overpumping as the main
cause of the detected displacement of both decades. Critical regions are examined in detail, as for example: an area close to
Thermi, the town of Perea and the Thessaloniki’s international airport, a major hub of the Balkans. The latter was subjected to an
increasing deforming velocity during the monitoring period. Overall, we conclude that a water management plan should be of
high priority for the area.

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The industrial area of Oreokastro, NW of the city of Thessaloniki, is monitored using radar inter... more The industrial area of Oreokastro, NW of the
city of Thessaloniki, is monitored using radar interferometry
to determine the spatial evolution of the underlying
ground deformation trends. Previous studies, using SAR
data acquired between 1992 and 1999, have revealed
subsidence; however, the driving mechanism has not been,
so far, solidly explained. Here, SAR satellite data from
ERS 1, 2 and ENVISAT missions, acquired between 1992
and 2010, are analysed to enhance our understanding of the
ground displacement trends and provide a thorough interpretation
of the phenomena. The analysis confirms a subsiding
displacement pattern from 1992 to 1999, whereas
the recent data indicate that after 2003 the motion direction
has changed to uplift. This whole monitoring of subsidence
and the subsequent uplift is a rarely documented phenomenon,
and in the case of Oreokastro is not reflecting a
natural process; on the contrary, the driver is anthropogenic,
related to the regional aquifer activity. Our study
also highlights the fact that the local faults act as
groundwater barriers and captures the existence of a possible
previously unknown tectonic structure.

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Land subsidence in the broader Kalochori village region, at the west side of Thessaloniki, has be... more Land subsidence in the broader Kalochori village region, at the west side of Thessaloniki, has been recorded since the early 1960s reaching gradually, next to the coastline, maximum values of 3–4 m. Temporal monitoring of terrain movements are exploited and combined with in-situ data to enhance understanding of the deformation signals. Persistent Scatterer Interferometry (PSI) and Small Baseline Subset (SBAS) multi-temporal Interferometric approach are applied for the analysis of a 20 year ERS 1, 2 and ENVISAT dataset. The velocities estimated for the ERS dataset are in excellent accordance with previous studies, depicting subsidence with magnitude up to 35 mm/year. The intriguing output of the ENVISAT data archive (2003–2010) shows that, during the second decade , there was a change in motion trend, from subsidence to uplift. The fact that this uplifting trend of the second decade is well correlated with hydrogeological data of the area that show a synchronous rise of the aquifer level, verifies the dominating driver of the human factor concerning the land subsidence phenomena taking place the last 55 years. This conclusion is further supported by the fact that since 2007 the uplifting signal becomes smoother, following the smoother recovery of the aquifers.

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Proceedings of Fringe 2015: Advances in the Science and Applications of SAR Interferometry and Sentinel-1 InSAR Workshop, 2015

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Geophysical Journal International

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Proceedings of Fringe 2015: Advances in the Science and Applications of SAR Interferometry and Sentinel-1 InSAR Workshop, 2015

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