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Seismo-geodesy : integrating geodetic/seismological observations and analysis to probe the behavior of faults

Slip at faults generate ground motion over a wide range of time-scales, from milliseconds to decades and centuries. While geodesy and seismology have been used independently in the past to study the fault behavior, new capabilities in space geodesy as well as seismology analysis argue for a further integration of both disciplines. A non-limitative list of progresses include the development of continuous GNSS networks allowing to monitor motion over a frequency bandwidth overlapping with seismology, new radar satellite missions, optic imaging capabilities allowing to capture new signals, before, during and after earthquakes, new detection techniques of seismic signal and the ability to build high quality earthquakes, tremors and low-frequency earthquakes catalogues, opening new prospects for the study of the Earth’deformation and earthquakes.

This session will gather colleagues interested in integrating seismological and geodetic observations, data, and analysis to better understand the Earth’s deformation and earthquakes. Welcome are contributions about new technologies aiming at improving our ability to monitor ground motion at various time-scales inland and offshore at the sea-floor.
Comparison, validation and dissemination of high-quality accessible data and software are also encouraged. Finally, we encourage submission of joint analysis integrating seismology and geodesy to better understand earthquakes from regional to local approaches. Detection and characterization of transient slip through their joint geodetic and seismic signatures are most welcome.

This session is proposed by the joint IAG-IASPEI Seismo-geodesy sub-commission.

Co-organized by SM4, co-sponsored by IUGG
Convener: Jean-Mathieu Nocquet | Co-conveners: Takuya Nishimura, Haluk Ozener, Susanna Zerbini
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Wed, 28 Apr, 09:00–10:30

5-minute convener introduction

Kosuke Heki and Yuki Takasaka

Frequency spectra of seismic waves from a fault rupture reflects the size of the faults, i.e. relatively large amplitudes of long period waves are excited by larger earthquakes. Anomalies in rise times of the fault movements would also influence the spectra. For example, earthquakes characterized by slow faulting, known as tsunami earthquakes, excite large tsunamis for the amplitudes of short-period seismic waves. In this study, we compare amplitudes of long- and short-period atmospheric waves excited by vertical crustal movements associated with earthquake faulting. Such atmospheric waves often reach the ionospheric F region and cause coseismic ionospheric disturbances (CID) observed as oscillations in ionospheric total electron content (TEC), with ground Global Navigation Satellite System (GNSS) receivers. CID often includes long-period internal gravity wave (IGW) components in addition to short period acoustic wave (AW) components. The latter has a period of ~4 minutes and propagate by 0.8-1.0 km/s, while the former has a period of ~12 minutes and propagate as fast as 0.2-0.3 km/s. Here we compare amplitudes of these two different waves for five earthquakes, 2011 Tohoku-oki (Mw9.0), 2010 Maule (Mw8.8), 1994 Hokkaido-Toho-Oki (Mw8.3), 2003 Tokachi-oki (Mw8.0), and the 2010 Mentawai (Mw7.9) earthquakes, using data from regional dense GNSS networks. We found two important features, i.e. (1) larger earthquakes show larger IGW/AW amplitude ratios, and (2) Mentawai earthquake, a typical tsunami earthquake, exhibits abnormally large IGW amplitudes relative to AW amplitudes. These findings demonstrate that earthquakes with longer durations for faulting, or with longer times for vertical crustal movements, excite longer period atmospheric waves such as IGW more efficiently.

How to cite: Heki, K. and Takasaka, Y.: Slow earthquake signatures in the ratio between acoustic and internal gravity wave amplitudes in coseismic ionospheric disturbances, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-758, https://doi.org/10.5194/egusphere-egu21-758, 2021.

Yuta Mitsui et al.

Slow deformations associated with a subducting slab can affect quasi-static displacements and seismicity over a wide range of depths. Here, we analyse the seismotectonic activities at the Tonga-Trench subduction zone, which is the world’s most active area with regard to deep earthquakes, using data from GNSS and an earthquake catalogue. We find that trenchward transient displacements and quiescence of deep earthquakes, in terms of background seismicity, were bounded in time by large intraslab earthquakes in 2009 and 2013. We call this event as "slow deformation event”. It may have been triggered by a distant and shallow M8.1 earthquake, which implies a slow slip event at the plate interface or a temporal acceleration of the subduction of the Pacific Plate.

How to cite: Mitsui, Y., Muramatsu, H., and Tanaka, Y.: Slow deformation event between large intraslab earthquakes at the Tonga Trench inferred from geodetic and seismological data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8513, https://doi.org/10.5194/egusphere-egu21-8513, 2021.

Pierre Romanet et al.

Seismic tremor has previously been reported in the Marlborough (New Zealand) region, with detections made using the national GeoNet network. However, because of the sparsity of that network, only 40 tremors were detected using 6 stations. We conducted a similar analysis again, but this time using data from 4 stations from the GeoNet network as well as 16 stations from a local campaign network, bringing the total number of stations to 20. Our new tremor catalog contains 4699 tremors (around 100 times more events than the previous catalog) and spans the period 2013-2019 which include the major 2016 Mw7.9 Kaikoura earthquake. Based on our current knowledge, that makes the Marlborough region the most active region for tremors in New Zealand.


The observed tremor in the region are split into two clusters, separated by a gap of around 20 km. The South-West cluster has an elongated shape in the direction of the upper-plate dextral strike-slip (Hope and Clarence) faults. The occurrence of tremor before the Mw 7.9 Kaikoura earthquake is fairly constant over time. After the earthquake however we observe  a strong acceleration in the rate of tremor, that slowly recovers over time. At the end of the analysis (May 2019), more than 2 years after Kaikoura earthquake, the tremor burst rate has still not recovered to the previous rate before the earthquake. We also observe several episodes of tremor migration, with a migration velocity of around ~50km/day, most of the migration being from South-West to North-East.


This new tremor catalog provides a unique opportunity to better understand possible interaction of a major earthquake with the tremor activity and will help to better understand the local tectonic activity of the Marlborough region.


How to cite: Romanet, P., Aden-Antoniow, F., Ando, R., Bannister, S., Chamberlain, C., Iio, Y., Matsumoto, S., Okada, T., Sibson, R. H., Toh, A., and Ide, S.: Detection of tremors in the Marlborough region and its relationship with the 2016 Mw 7.9 Kaikoura (New Zealand) earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-637, https://doi.org/10.5194/egusphere-egu21-637, 2021.

Leonard Seydoux et al.

Slow slip events are observed in geodetic data, and are occasionally associated with seismic signatures such as slow earthquakes (low-frequency earthquakes, tectonic tremors). In particular, it was shown that swarms of slow earthquake can correlate with slow slip events occurrence, and allowed to reveal the intermittent behavior of several slow slip events. This observation was possible thanks to detailed analysis of slow earthquakes catalogs and continuous geodetic data, but in every case, was limited to particular classes of seismic signatures. In the present study, we propose to infer the classes of seismic signals that best correlate with the observed geodetic data, including the slow slip event. We use a scattering network (a neural network with wavelet filters) in order to find meaningful signal features, and apply a hierarchical clustering algorithm in order to infer classes of seismic signal. We then apply a regression algorithm in order to predict the geodetic data, including slow slip events, from the occurrence of inferred seismic classes. This allow to (1) identify seismic signatures associated with the slow slip events as well as (2) infer the the contribution of each classes to the overall displacement observed in the geodetic data. We illustrate our strategy by revisiting the slow-slip event of 2006 that occurred beneath Guerrero, Mexico.

How to cite: Seydoux, L., Campillo, M., Steinmann, R., Balestriero, R., and de Hoop, M.: Observing seismic signatures of slow slip events with unsupervised learning, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5603, https://doi.org/10.5194/egusphere-egu21-5603, 2021.

Quentin Bletery and Jean-Mathieu Nocquet

Both laboratory experiments and dynamic simulations suggest that earthquakes can be preceded by a precursory phase of slow slip. Observing processes leading to an acceleration or spreading of slow slip along faults is therefore key to understand the dynamics potentially leading to seismic ruptures. Here, we use continuous GPS measurements of the ground displacement to image the daily slip along the fault beneath Vancouver Island during a slow slip event in 2013. We image the coalescence of three originally distinct slow slip fronts merging together. We show that during coalescence phases lasting for 2 to 5 days, the rate of energy (moment) release significantly increases. This observation supports the view proposed by theoretical and experimental studies that the coalescence of slow slip fronts is a possible mechanism for initiating earthquakes.

How to cite: Bletery, Q. and Nocquet, J.-M.: Slip bursts during coalescence of slow slip events in Cascadia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1194, https://doi.org/10.5194/egusphere-egu21-1194, 2021.

Axel Periollat et al.

Earthquakes are usually followed by a postseismic phase where the stresses induced by the earthquakes are relaxed. It is a combination of different processes among which aseismic slip on the fault zone (called afterslip), viscoelastic deformation of the surrounding material, poroelastic relaxation and aftershocks. However, little work has been done at the transition from the co- to the postseismic phase, and the physical processes involved.

We study the 2011 Mw 9.0 Tohoku-Oki earthquake, one of the largest and most instrumented recent earthquake, using GEONET GPS data. We focus on the few minutes to the first month following the mainshock, a period dominated by afterslip.

Based on the method developed by Twardzik et al. (2019), we process 30-s kinematic position time series and we use it to characterize the fast displacements rates that typically occur during the early stages of the postseismic phase. We quantify precisely the co-seismic offset of the mainshock, without including early afterslip, and we also characterize the co-seismic offset of the Mw 7.9 Ibaraki-Oki aftershock, which occurred 30 minutes after the mainshock. We analyze the spatial distribution of the co-seismic offsets for both earthquakes. We also use signal induced by the postseismic phase over different time windows to investigate the spatio-temporal evolution of the postseismic slip. We determine the redistribution of stresses to estimate the regional influence of the mainshock and aftershock on postseismic slip.

From a detailed characterization of the first month of postseismic kinematic time series, we find that the best-fitting law is given by an Omori-like decay. The displacement rate is of the type v0/(t+c)p with spatial variation for the initial velocity v0 and for the time constant c. We find a consistent estimate of the p-value close to 0.7 over most of the studied area, apart from a small region close to the aftershock location where higher p values (p~1) are observed. This p value of 0.7 shows that the evolution of the Tohoku-Oki early afterslip is not logarithmic. We discuss about the implications of these observations in terms of subduction interface dynamics and rheology. We also discuss about the different time-scales involved in the relaxation, and how this model, established for the early postseismic phase over one month, performs over longer time scales (by comparison with daily time series lasting several years).

Twardzik Cedric, Mathilde Vergnolle, Anthony Sladen and Antonio Avallone (2019), doi.org/10.1038/s41598-019-39038-z

Keywords: Early Postseismic, Afterslip, GPS, Kinematic, Omori Law

How to cite: Periollat, A., Radiguet, M., Weiss, J., Twardzik, C., Marill, L., Cotte, N., and Socquet, A.: The early postseismic phase of Tohoku-Oki earthquake (2011) from kinematics solutions: implication for subduction interface dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10720, https://doi.org/10.5194/egusphere-egu21-10720, 2021.

Roxane Tissandier et al.

The Mw 8.3 2015 Illapel earthquake ruptured a 190 km long segment of the Chilean subduction zone. In the past, this area ruptured several times through large and great earthquakes, the most recent event before 2015 being a Mw 7.9 earthquake in 1943. Here, we combine continuous and survey GPS ground displacements to perform a kinematic inversion of the two-months afterslip following the mainshock. We show that the postseismic slip developed South and North of the coseismic rupture, but also overlaps the deeper part of it. We estimate that two months after the large mainshock, the postseismic moment released represents 13% of the coseismic moment (the mainshock released 3.16x1021 N.m whereas the afterslip released 3.98x1020 N.m). At a first order, seismicity and areas experiencing afterslip match together and are concentrated at the edges of the coseismic rupture between 25 and 45 km depth. One interesting feature is the occurrence of two moderate size aftershocks on November, 11th at shallow depth North of the rupture. We investigate the relationship between the evolution of afterslip and these aftershocks. Finally, we interpret the result in the light of past earthquakes history and calculate the moment balance through the last centuries.

How to cite: Tissandier, R., Nocquet, J.-M., Klein, É., and Vigny, C.: The Mw 8.3 2015 Illapel afterslip imaged through a time-dependent inversion of continuous and survey GPS data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4541, https://doi.org/10.5194/egusphere-egu21-4541, 2021.

Caroline Chalumeau and the Pedernales research team

Repeating earthquakes are earthquakes that repeatedly break a single, time-invariant fault patch. They are generally associated with aseismic slip, which is thought to load asperities, leading to repeated rupture. Repeating earthquakes are therefore useful tools to study aseismic slip and fault mechanics, with possible applications to earthquake triggering, loading rates and earthquake forecasting.

In this study, we analyze one year of aftershocks following the 16th April 2016 Mw 7.8 Pedernales earthquake in Ecuador to find repeating families, using data recorded by permanent and temporary seismological stations. In our area, seismicity during both the inter-seismic and post-seismic periods has been previously linked to aseismic slip. We calculate waveform cross-correlation coefficients (CC) on all available catalogue events, which we use to sort events into preliminary families, using a minimum CC of 0.95. These events were then stacked and used to perform template-matching on the continuous data. In total, 376 earthquakes were classified into 62 families of 4 to 15 earthquakes, including 8 from the one-year period before the mainshock. We later relocated these earthquakes using a double-difference method, which confirmed that most of them did have overlapping sources.

Repeating earthquakes seem to concentrate largely around the areas of largest afterslip release, where afterslip gradient is the highest. We also find an increase in the recurrence time of repeating events with time after the mainshock, over the first year of the postseismic period, which highlights a possible timeframe for the afterslip’s deceleration. Our results suggest that while most repeating aftershocks are linked to afterslip release, the afterslip gradient may play a bigger role in determining their location than previously thought.

How to cite: Chalumeau, C. and the Pedernales research team: Repeating earthquakes follow afterslip gradient in the aftermath of the 16th April 2016 M7.8 Pedernales earthquake in Ecuador, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2918, https://doi.org/10.5194/egusphere-egu21-2918, 2021.

Jean-Mathieu Nocquet et al.

We use 40 continuous GPS stations in Ecuador to quantify 3 years of the  post-seismic deformation that followed the Mw 7.8 April 16 Pedernales earthquake. We perform a kinematic inversion solving for the daily slip along the subduction to retrieve the afterslip evolution through time and space.

Rolandone et al. (2018) had found that the afterslip during the first 30 days following the earthquake was abnormally large and rapid, mainly developing at discrete patches north and south updip of the co-seismic rupture. We find that large slip and slip rate continue at both location, decreasing through time. However, models suggest that modulations of slip rate occur within those areas, with episods of slip acceleration sometimes associated with the occurrence of moderate size aftershocks.  Aside these patches, afterslip developed updip the co-seismic rupture between the patches and downdip of the coseismic rupture, with little slip occurring within the co-seismic rupture.

The overall model confirms a model of a seismic asperity encompassed in a subduction interface releasing stress through aseismic processes. However, some areas experiencing afterslip appear to be locked before the earthquake. Furthermore, those areas experienced SSE before the earthquake and during the afterslip period, raising the question of the friction parameter controlling their behavior.

In terms of moment, the amount of afterslip after 3 years is equivalent to 90% of the moment released by the Pedernales earthquake. This observation highlights that aseismic slip has an important contribution to the balance of slip during the earthquake cycle along the central Ecuador segment. This observation strengthens the proposed hypothesis of earthquake an super-cycle in central Ecuador (Nocquet et al., 2017), by confirming that the occurrence of three successive major earthquakes within 110 years exceeds the moment accumulation as derived from a decade of interseismic coupling models spanning a decade before the 2016 earthquake.

How to cite: Nocquet, J.-M., Rolandone, F., Mothes, P., and Jarrin, P.: Spatio-temporal evolution of afterslip following the Mw 7.8 Pedernales earthquake, Ecuador, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10787, https://doi.org/10.5194/egusphere-egu21-10787, 2021.

Fabio Corbi et al.

Finding a deformation pattern that is representative of a given stage of the seismic cycle of subduction megathrusts is crucial as this might provide clues about the upcoming earthquake. Here we focus on the short term interaction between seismic asperities and in particular on how geodetic velocities change in response to ruptures of an along-strike neighbor portion of the megathrust. Enhanced megathrust coupling, slab acceleration, in plane bending of the overriding plate, continental-scale viscoelastic mantle relaxation have been proposed as potentially responsible driving mechanisms. However, the paucity of observations from natural cases and the multiple- interrelated contributions that act at different spatial and temporal scales complicate the understanding of this process.

We use an analog model that simulates a series of laterally partial ruptures and analyze systematically the effect of slip episodes on deformation history of the neighbor “receiver” region. The analog model has the advantage of reproducing tens of seismic cycles with well controlled boundary conditions. The model shows that the deformation pattern associated to slip episodes has a characteristic twisting about a vertical axis. Such twisting interfere positively (causing velocity increase) or negatively (causing velocity decrease) with local interseismic velocity field depending on time since the last earthquake. Identifying accelerating or decelerating velocities in geodetic timeseries could be therefore informative of the seismic evolution of a subduction zone.

How to cite: Corbi, F., Poli, P., Bedford, J., and Funiciello, F.: Coseismic displacement interferences and patterns along subduction megathrusts: insights from analog modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3010, https://doi.org/10.5194/egusphere-egu21-3010, 2021.

Mario D'Acquisto et al.

Seismological and geodetic observations indicate that similar physical processes are active at different subduction margins and provide information about the deformation at the different stages of the earthquake cycle. We analyze geodetic observations along sections of the South American subduction zone during the inter-seismic stage. Results show that overriding plates shorten from the trench to a “backstop”, where horizontal inter-seismic velocities become close to zero. In most, but not all regions, the backstop location from trench-perpendicular GPS velocities agrees with that from trench-parallel velocities. The distance of the backstop from the trench varies along the western South America margin. Backstop locations shows some correlation with gradients in the effective elastic thickness of the overriding plate. An apparently conflicting observation is that co-seismic and early post-seismic GPS-displacements during the 2010 Maule earthquake extended well beyond the backstop into eastern South America. Similarly conflicting observations were made in the overriding plate of the 2004 Sumatra earthquake and the 2011 Tohoku earthquake.

We use cyclic 3D numerical models with dynamically driven co-seismic and afterslip to test the hypothesis that lateral contrasts in the thickness and/or elasticity of the overriding plate explain the observations. The model setup allows us to explore the sensitivity of geodetically observable surface motion to the mechanical structure of the subduction system during all parts of the earthquake cycle. We conclude that the observations can be explained by a lateral contrast. Such contrast restricts inter-seismic horizontal velocities in the region between the trench and the backstop, controlling their gradient, while allowing deformation due to coseismic slip and afterslip to reach well into the far field. One particularly interesting finding from our models is that stress accumulation in the overriding plate is controlled by the distance to the backstop.

How to cite: D'Acquisto, M., Broerse, T., and Govers, R.: Presence and significance of backstops in the overriding plate during the megathrust earthquake cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7344, https://doi.org/10.5194/egusphere-egu21-7344, 2021.

Alessandro Caporali et al.

Seismicity in the Central Apennines is characterized by normal faulting with dip NE-SW near 45°. We show that if the stress at the hypocenter of the 2016 Norcia (Mw=6.5) and 2009 L’Aquila (Mw=6.3 on the Paganica fault) earthquakes originated only from stress transfer from previous historical events occurred in 1315 and 1461 (L’Aquila), 1703 (Montereale plain) and 1703 (Norcia/Valnerina), then the orientation of the principal stress axes would be inconsistent with the observed tensional regime. The additional contribution of a regional stress is thus required to properly align the principal stress axes to those of the moment tensor, but GNSS geodesy provides only stress rates. We empirically estimate a time multiplier for the regional stress rate, computed with a dense GNSS network, such that the principal stress axes resulting from the sum of the stress transferred by previous events and the regional stress rate multiplied by the empirical temporal scale are consistent with normal faulting, both at the L’Aquila and Norcia hypocenters. Based on a Catalogue of 36 events of magnitude larger than 5.6 we estimate the total Coulomb stress at depths and along planes parallel to those of L’Aquila and Norcia. We provide evidence of an asymmetry of the Coulomb stress leading to a stress concentration near the hypocenter of the two events just prior of the 2009 and 2016  earthquakes. This stress anomaly disappeared after the two events. Similar stress patterns are observed for earlier events which took place in 1461 at L’Aquila, 1703 on the Montereale plain and in 1703 at Norcia/Valnerina. The 1997 sequence of Colfiorito exhibits a similar, anisotropic Coulomb stress pattern. Based on the Database of Individual Seismogenic Sources DISS 3.2.1 of INGV we identify as areas of maximum Coulomb stress at present (>2016) the Gran Sasso , the Camerino and Sarnano areas and the area between the San Pio delle Camere, Tocco da Casauria and Sulmona faults.

How to cite: Caporali, A., Zurutuza, J., and Bertocco, M.: A time dependent model of elastic stress in the Central Apennines, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-653, https://doi.org/10.5194/egusphere-egu21-653, 2021.

Jonathan Bedford et al.

GNSS derived displacement time series are used to track plate tectonics and the associated motions across major plate boundaries. With a growing number of continuous GNSS observations, it is becoming increasingly apparent that plate trajectories rarely conform to standard trajectory models. The deviations from these expected trajectories can be considered as transient motions, some being tectonically related, and others being related to geophysical fluid loading, local site effects, and artifacts of the GNSS processing. As we increasingly inspect the transient motions of GNSS displacement time series, we have to take care that the GNSS processing choices, such as the reference frame, are not introducing non-tectonically related artifacts into the eventual isolated tectonic signals.

Here we explore the effects that different solutions and processing strategies have on our eventual daily GNSS displacement time series - the aim being to explain how artifacts arise and to determine which strategies best mitigate these artifacts. We compare displacement time series made from both Precise Point Positioning and network (double-differenced) solutions that are provided in the latest official reference frame solution ITRF2014, and in JTRF2014 based on Kalman filtering.

In our analyses, we use approximately one hundred GNSS stations from South America, with a focus being to identify transient tectonic activity related to the subduction of the Nazca plate under Chile over the past decade.


How to cite: Bedford, J., Glaser, S., and Männel, B.: Impact of solutions on the identification of tectonic transients: A case study using the last decade of GNSS data in South America, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13257, https://doi.org/10.5194/egusphere-egu21-13257, 2021.

Laëtitia Lemrabet et al.

The global and systematic coverage of Sentinel-1 radar images allows characterizing, by radar interferometry (InSAR), surface deformation on a continental scale.

Our study focuses on the eastern part of the Tibetan plateau, where a combination of major strike-slip and thrust fault systems accommodates part of the deformation related to the collision between the Indian and Eurasian plates.

We use an automated Sentinel-1 InSAR processing chain based on the NSBAS approach (Doin et al., 2011, Grandin, 2015) to measure the interseismic deformation across these fault systems. Processing is made on the CNES high-performance computer center in Toulouse in the FLATSIM project framework (ForM@Ter LArge-scale multi-Temporal Sentinel-1 Interferometric Measurement, Durand et al., 2019). We perform a time series analysis of the 2014-2020 Sentinel-1 InSAR data set, for 1200 km-long tracks (acquired along 7 ascending and 7 descending orbits), covering a 1 700 000 km2 area, with a 160 m spatial resolution. From about 130 acquisitions per track, we perform about 600 interferograms, with short, three months, and one-year temporal baselines. After inversion, we obtain time series of line-of-sight (LOS) delay maps, including residual atmospheric delay and network misclosure measurements. The time series are fitted by a seasonal signal plus a velocity trend. The velocity field on overlap areas agrees within less than 1~mm/yr.

Finally, we decompose the LOS velocity maps into a vertical and a horizontal contribution.

InSAR velocity maps highlight surface deformation patterns mostly localized on known major faults, short-wavelength patterns attributed to slope instabilities phenomena, and hydrological signals.

The seasonal signal combines residual atmospheric phase delays and widespread hydrological phenomena in sedimentary basins, which we interpret in parallel with the regional geological map.  Masking areas affected by dominant gravitational slope or hydrological deformation allows to better focus on tectonic deformation.

We finally discuss slip partitioning on the various fault systems from the velocity maps and 2D profiles’ analysis.

How to cite: Lemrabet, L., Doin, M.-P., Lasserre, C., Replumaz, A., Métois, M., Leloup, P.-H., Chevalier, M.-L., and Sun, J.: Analysis of the hydrological and tectonic deformation in the eastern part of the Tibetan plateau, from FLATSIM automated time series analysis of Sentinel-1 InSAR, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12866, https://doi.org/10.5194/egusphere-egu21-12866, 2021.

Letizia Elia et al.

We investigated a large network of permanent GPS stations to identify and analyse common patterns in the series of the GPS height, environmental parameters, and climate indexes.

The study is confined to Europe, the Mediterranean, and the North-eastern Atlantic area, where 114 GPS stations were selected from the Nevada Geodetic Laboratory (NGL) archive. The GPS time series were selected on the basis of the completeness and the length of the series.

In addition to the GPS height, the parameters analysed in this study are the atmospheric surface pressure (SP), the terrestrial water storage (TWS), and a few climate indexes, such as MEI (Multivariate ENSO Index). The Principal Component Analysis (PCA) is the methodology adopted to extract the main patterns of space/time variability of the parameters.

Moreover, the coupled modes of space/time interannual variability between pairs of variables was investigated. The methodology adopted is the Singular Value Decomposition (SVD).

Over the study area, main modes of variability in the time series of the GPS height, SP and TWS were identified. For each parameter, the main modes of variability are the first four. In particular, the first mode explains about 30% of the variance for GPS height and TWS and about 46% for SP. The relevant spatial patterns are coherent over the entire study area in all three cases.

The SVD analysis of coupled parameters, namely H-AP and H-TWS, shows that most of the common variability is explained by the first 3 modes, which account for almost 80% and 45% of the covariance, respectively.

Finally, we investigated the relation between the GPS height and a few climate indexes. Significant correlations, up to 50%, were found between the MEI (Multivariate Enso Index) and about half of the stations in the network.

How to cite: Elia, L., Zerbini, S., and Raicich, F.: Vertical crustal deformations and climate variability through PCA and SVD, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13013, https://doi.org/10.5194/egusphere-egu21-13013, 2021.

Sanja Tucikešić et al.

Meet the authors in their breakout text chats

Wed, 28 Apr, 11:00–11:45

Sigurjón Jónsson et al.

On 20 October 2020, Reykjavík was rocked by the largest earthquake in southwest Iceland in over a decade when a magnitude 5.6 event occurred only 25 km from the city. The earthquake caused movement on multiple surface fractures, distributed over an 8-km-long north-south oriented area, indicating the location of the underlaying right-lateral strike-slip fault rupture. We mapped the coseismic surface fractures and deformation using Sentinel-1 and TerraSAR-X InSAR data, selecting with a new method the best pre- and post-earthquake SAR scenes from analyzing the tropospheric signals on each SAR image. This method does not require masking out deformed areas when determining the InSAR covariance structure and thus yields better earthquake source estimations. As the InSAR data are primarily sensitive to east-west and vertical displacements, we additionally used split-beam interferometry to obtain more information about north-south displacements. For this, we used burst-overlap interferometry (BOI), in the case of Sentinel-1 data, and multiple-aperture interferometry (MAI) on the TerraSAR-X data. Together with the standard InSAR data, we estimated the full 3D coseismic surface displacement field of the earthquake. The results show that most of the fractures had limited surface offsets, apart from a 2-3 km long north-south trending segment just north of the epicenter that was right-laterally offset by about 15 cm. Source modeling of the earthquake shows that the deformation is consistent with a near vertical north-south striking fault with up to ~30 cm of slip located at roughly 3 km depth below the surface. The estimated geodetic moment of the model amounts to a magnitude 5.6 earthquake, consistent with seismological estimates. Most of the modeled fault slip and mapped surface fractures are located north of the earthquake epicenter, indicating that the earthquake ruptured unilaterally from south to north, which agrees with the more severe surface effects and shaking reported from near the northern end of the earthquake rupture.

How to cite: Jónsson, S., Cao, Y., Vasyura-Bathke, H., and Li, X.: Surface fractures and deformation of the magnitude 5.6 earthquake near Reykjavík on 20 October 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12845, https://doi.org/10.5194/egusphere-egu21-12845, 2021.

Adriano Nobile et al.

We used teleseismic waveforms and ground deformation data from GNSS and InSAR to estimate source fault parameters of the Mw6.4 earthquake that occurred just offshore southwestern Puerto Rico on 7 January 2020. The mainshock was a part of an energetic seismic sequence that started on 28 December 2019 and led to a Mw5.8 earthquake on 6 January 2020, a day before the Mw6.4 mainshock. The ground-shaking due to the largest earthquakes of the sequence caused significant damage to buildings and infrastructures in Puerto Rico and one casualty was reported by the local media. The mainshock was followed by a strong aftershock sequence that included four Mw ≥ 5 events within the first 3 hours. In the first 40 days of the seismic sequence, data from the Puerto Rico Seismic Network were used to locate ~3800 earthquakes of magnitude > 2, illuminating an east-west elongated 30x50 km2 area, just offshore the southwestern coast of Puerto Rico. The region affected by this activity was before characterized by relatively low seismicity rates, even if a system of active faults, both onshore and offshore, had been mapped. The sequence is peculiar due to its complex development and many large aftershocks (magnitude > 4.5), with the mainshock releasing only  ~60% of the total seismic moment.

We estimated the key source parameters of the mainshock using teleseismic data, GNSS data from the Puerto Rico Geodetic Network, and InSAR data from the Sentinel-1 and ALOS-2 satellites. The modeled source is consistent with a ~15 km long and ~11 km wide blind fault, oriented roughly east-west and dipping 46o towards north, and with up to 1.1 m of oblique normal and left-lateral strike-slip.

The optimal fault plane source indicates that it is an offshore continuation of the mapped North Boquerón Bay - Punta Montalva fault zone, supported by the large number of the aftershocks that trend along the same direction. However, most of the aftershocks, even those of magnitude > 5, occurred on other nearby faults, highlighting the complexity of this fault zone area.

How to cite: Nobile, A., Viltres, R., Vasyura-Bathke, H., Trippanera, D., Wenbin Xu, W., Passarelli, L., and Jónsson, S.: Fault parameters of the Mw 6.4 January 7, 2020, Puerto Rico earthquake estimated from teleseismic, GNSS and InSAR data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11166, https://doi.org/10.5194/egusphere-egu21-11166, 2021.

Bilal Mutlu et al.

The earthquake with a magnitude of Mw 6.9 (according to Kandilli Observatory and Earthquake Research Institute-KOERI) occurred 8 km north of Samos Island at a depth of 16 km, on 30.10.2020, at 11:51:24 UTC. It took place on the north-dipping normal fault zone of approximately 40 km length in the sea between Samos Island of Greece and Kuşadası Bay of Turkey. After the mainshock, a tsunami with the height exceeding 1 meter occurred in Seferihisar region, south of Izmir, and north side of Samos Island. In this study, a geodetic investigation of the Samos-Izmir earthquake using GNSS and SAR techniques was carried out. Within the scope of this study, 1Hz observations of Turkey National Continuous GNSS Network-Active (TUSAGA-Aktif) stations in the earthquake zone, were used, and it was aimed to reveal the co-seismic deformation caused by the earthquake. In addition to GNSS data, the InSAR process has been performed by using ESA Sentinel-1 SAR data, and the vertical deformations were clarified with the unwrapped interferogram. The GNSS data were processed using web-based online processing services according to the relative and absolute positioning techniques as static and kinematic modes. In conclusion, considering the absolute and relative static processing of pre- and post-earthquake GNSS data, the maximum horizontal deformations were observed at CESM and IZMI GNSS stations located in the north of the fault. Due to the earthquake, these points moved to the north direction and the maximum horizontal deformations were found as 5.5 cm and 3.5 cm, respectively. According to the kinematic processing of the GNSS data, instantaneous horizontal movements of 12 cm and 4 cm towards the north were observed at the same stations, respectively, at the time of the earthquake. On the contrary, DIDI and AYD1 GNSS stations, which are located in the south of the fault, moved to the south-east direction and the magnitude of horizontal deformations were smaller. Considering the InSAR results, it was seen a 10 cm uplift in the west of the island of Samos and a 10 cm subsidence at the northernmost part. Besides this, a 5 cm subsidence was observed in Izmir territory, the north side of the fault, by means of the interferogram.

How to cite: Mutlu, B., Erol, S., Çevikalp, M. R., and Erol, B.: Geodetic Investigation of the 30 October 2020 Mw 6.9 Samos-Izmir Earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12219, https://doi.org/10.5194/egusphere-egu21-12219, 2021.

Hamid Sana et al.

We are using InSAR time-series analysis to measure the interseismic deformation across various faults of the Kashmir Himalaya. Active faults reaching the surface include the Main Boundary Faults, Bagh-Balakot Fault, which ruptured in the 2005 Kashmir earthquake (Mw 7.6), Jhelum Fault, Reasi Thrust and intra-Kashmir basin faults. We concentrate on these shallow faults that are closest to the people living in Kashmir. The Main Boundary Faults and other faults likely connect to the Main Himalayan Thrust (MHT) that is the plate-boundary megathrust beneath Kashmir and the rest of the Himalayas. The MHT has been suggested as a possible source for Mw 8 to Mw 8.5 earthquakes in this area. We have processed interferometric pairs from the Japan Aerospace Exploration Agency ALOS-2 L-band (24 cm wavelength) Synthetic Aperture Radar (SAR) wide-swath (ScanSAR) data acquired between 2015 and 2020. Initial interferometric SAR (InSAR) processing was carried out using the alos2App application of the InSAR Scientific Computing Environment (ISCE2) package, with ionospheric corrections enabled. We found that many scenes acquired in the winter form pairs that have low coherence due to snow cover in the High Himalayas and Pir Panjal Range. We also found that phase unwrapping in the mountains was improved by taking 10 range and 56 azimuth looks from the full-aperture ScanSAR for an effective resolution of about 200 meters. We are running a co-registered stack processing of the ALOS-2 SAR data, with self-consistent ionospheric corrections estimated using the split-spectrum method, using the new alosStack application of ISCE2 package to carry out time-series InSAR analysis, using an open-source Python toolbox, MIntPy.

How to cite: Sana, H., Fielding, E., Liang, C., and Yunjun, Z.: Strain accumulation along various faults in the Kashmir Himalaya from InSAR, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13780, https://doi.org/10.5194/egusphere-egu21-13780, 2021.

Marco Meschis et al.

Geologic studies of preserved stairs-like uplifted marine terraces and continuous GPS data collected in subduction zones provide a unique opportunity to investigate, on different time scales, crustal deformation resulting from upper‐plate extension. The West Crati Fault in Calabria, southern Italy, is a normal fault located within the seismically extending upper plate above the Ionian subduction zone. It is of interest because a thorough comparison of the extension rates inferred from geologic and GPS data has not yet been performed. This E-dipping fault lies in an area where a few historical damaging earthquakes occurred, examples are those in 1184 (M 6.7) and 1638 (M 6.7). Fault slip-rates and earthquake recurrence intervals for the West Crati fault are still subject of debate. We investigated raised marine terraces along the strike of the fault, on its footwall over its tips, located above the Ionian subduction zone, to derive refined uplift rates and study the role that known extensional faults contribute to observed coastal uplift. We also estimated short-term vertical and horizontal movements on the hangingwall of this fault by analyzing the data of 7 permanent GPS stations located along the N-S oriented strike of this fault.

Our preliminary results demonstrate that (i) GIS-based elevations of Middle to Late Pleistocene marine terraces, as well as temporally constant uplift rates, vary along the strike of this fault, mapped on its footwall; (ii) rates of short-term vertical movements vary along the strike of this fault on its hangingwall. This confirms active deformation, on different time scales, along the E-dipping West Crati Fault, suggesting that the fault slip-rate governing seismic hazard has also been constant through time. Our preliminary results show the importance of mapping crustal deformation within the upper plate above subduction zones to avoid unreliable interpretations concerning the mechanism responsible for regional uplift.

How to cite: Meschis, M., Zerbini, S., Lattanzi, G., Di Donato, M., and Castellaro, S.: The West Crati Fault, Calabria (southern Italy): refined crustal extension rates constrained by geologic and GPS data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2279, https://doi.org/10.5194/egusphere-egu21-2279, 2021.

Estelle Hannouz et al.

       The Belledonne region, located on the western edge of the French Alps, behaves as a deformation transfer zone between the inner part of the western Alps, where geodesy and seismicity show extensional deformation, and its compressional surrounding basin (the Rhône Valley). Seismological and geodetic networks are less dense and younger in the Rhône Valley, which makes it more difficult to characterize its deformation. Nevertheless, these two regions have a moderate historical and instrumental seismicity. A large part of these earthquakes is concentrated on the Belledonne range and accommodated by the active NE–SW Belledonne fault, located at the western foot of this chain. The fault characteristics, such as its connection at depth with surrounding fault systems (e.g. Cléry fault), still need better constraints. The dense seismological network present in the Alpine region has made it possible to highlight its dextral strike-slip kinematics. To complete these observations, we present here an update of the geodetic velocity field around this fault from GNSS data recorded over the last two decades.

To do so, we first computed daily positions for a total of about 200 stations provided by different European networks (IGS, RENAG, RGP, GAIN, DGFI networks) over a period of 23 years (from 1997 to 2020), by using a double-difference processing with the GAMIT software (Herring et al. 2015). Then, we constrained a velocity field with the Kalman filter GLOBK with respect to the fixed European plate. We finally analyzed the residual motions in our area of interest with respect to stable Europe, as provided by our updated velocity field.

Across the Belledonne range, our results show a deformation pattern consistent with the dextral strike-slip mechanism observed by the current seismicity. Methodological studies concern the expected decrease of uncertainty on the velocity field thanks to the increase of recordings through time. These tests aim at quantifying the Belledonne fault present-day slip rate, including a well-constrained velocity uncertainty. We also exploit the new 3D velocity field to confirm and precise the local amplitude, in the Belledonne area, of the general uplift of the Alpine belt, as observed by previous geodetic studies.

How to cite: Hannouz, E., Walpersdorf, A., Sue, C., Mathey, M., Baize, S., and Lemoine, A.: Up to date geodetic velocity field of the Belledonne region (Western Alps, France), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12715, https://doi.org/10.5194/egusphere-egu21-12715, 2021.

Mustapha Meghraoui et al.

The kinematic of tectonic motions between the African (Sahara) platform and the Maghrebian thrust belt remained unexplored since the onset of space geodesy. Here, we use data of 6 permanent GNSS stations located north and south of the Atlas thrust belt in Algeria to constrain shortening and transpression at the tectonic boundary. The permanent GPS data and results are obtained from the network in Algeria operative from 2013 to 2019, presented with the results of the REGAT network in Algeria since 2007. The south Atlas suture zone constitutes the limit between African (Sahara) shield domain considered as a stable continental interior and the Sahara Atlas that belong to the Alpine orogeny. The tectonic boundary is marked by a E-W to ENE-WSW, en echelon fold belt system with deformed Plio-Quaternary formations to the North and flat laying Mesozoic and Tertiary sedimentary units south of the suture zone. The GNSS data are processed using Gamit-GlobK and results show tectonic motions with a predominant 5 to 6 mm/yr velocities trending NNW-SSE to NW-SE (westward) in the Sahara Platform. The GPS velocities show uniform trend in the African platform from which we infer 0.5 to 1.0 mm/yr convergence across the south Atlas suture zone. The intraplate convergence is attested by the moderate but permanent seismic activity at the tectonic boundary.

How to cite: Meghraoui, M., Abdellaoui, H., and Masson, F.: Constraint of Active Deformation between the African Platform and the Maghrebian Thrust Belt: Current Plate Motion from Permanent GNSS data in Algeria, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12969, https://doi.org/10.5194/egusphere-egu21-12969, 2021.

Juan José Portela Fernández et al.

The Guaycume fault is a right-lateral strike-slip structure located in Western El Salvador, within the El Salvador Fault Zone (ESFZ). The ESFZ consists of a strike-slip fault system extending through the Central American Volcanic Arc, on the western margin of the Chortís block, where the Cocos plate subducts under the Caribbean plate.

The Guaycume fault has been proposed as a possible source for the Mw 6.4 1917 El Salvador destructive earthquake, presenting high seismic potential in close proximity to San Salvador (Alonso-Henar et al., 2018). Its geomorphological expression has been clearly identified (Martinez-Diaz et al., 2016); however, few specific studies are currently published, and its behaviour and kinematics remain widely unknown. Notably, there is a lack of precise information about the amount of deformation that this fault currently absorbs of the westward movement (relative to the Chortís block) of the forearc sliver.

We process GNSS data in the area from 2007 to 2020 in order to retrieve the GNSS velocity field surrounding the Guaycume fault. We use these data to perform a thorough kinematic study, updating the previously existing slip rates (Staller et al., 2016). Combined with seismological data, this information allows us to understand the seismic cycle of the fault to a better extent, thus leading to a better comprehension of its seismic potential.

How to cite: Portela Fernández, J. J., Staller Vázquez, A., Béjar-Pizarro, M., Martínez-Díaz, J. J., Álvarez-Gómez, J. A., and Hernández, D.: Preliminary results on the characterisation of the Guaycume fault (El Salvador) from geodetic and seismological data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6514, https://doi.org/10.5194/egusphere-egu21-6514, 2021.

Paul Jarrin et al.

The North Andean Sliver (hereinafter NAS) lies at the northwestern end of the South American plate (hereinafter SOAM). This extensive area exhibits a complex deformation process controlled by the interactions of Nazca, Caribbean, South America plates, and Panama block, producing crustal seismicity, arc-continental collision, and subduction processes. Previous models based on partial GPS data sets have estimated the NAS kinematics as a single rigid block moving towards northeast  at 8-10 mm/yr (Nocquet et al. 2014, Mora-Paez et al 2019). By contrary, geologic interpretations as well as seismotectonic data propose more complex kinematic models based on the interaction of several blocks (Audemard et al 2014, Alvarado et al 2016).  Here, we present an updated and most extensive interseismic horizontal velocity field derived from continuous and episodic GPS data between 1994 and 2019 that encompasses the whole North Andean Sliver.  We then interpret it, developing a kinematic elastic block model in order to simultaneously estimate rigid block rotations, consistent slip rates at faults and the spatial distribution of interseismic coupling at the Nazca/NAS megathrust interface. Our model is not constrained either by a priori information derived from geologic slip rates or by a priori information of creeping faults. In contrast with previous simplest models, our model will allow us to estimate the degree of slip partitioning more precisely along the NAZCA/SOAM convergence as well as an improved model of interseismic coupling. We will discuss our coupling distribution with respect to previous models, and our block geometry quantifying the goodness of fit, resolution,  and considering its consistency with geological interpretations.

How to cite: Jarrin, P., Nocquet, J.-M., Rolandone, F., Mora-Paez, H., and Mothes, P.: Elastic Block Model in the North Andean Sliver, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7995, https://doi.org/10.5194/egusphere-egu21-7995, 2021.

Takuya Nishimura

In Japan, the Headquarters for Earthquake Research Promotion has developed a nationwide probabilistic earthquake model called “National Seismic Hazard Maps for Japan” since the destructive 1995 Kobe earthquake. This model covers both subduction and crustal earthquakes based on a history of past large earthquakes from seismological, archaeological, and geological data. The model for crustal earthquakes relies on geological and geomorphological data of active faults and never use geodetic data, whereas contemporary deformation of the Japanese Islands has been observed by a dense GNSS network. Here, we attempt to develop a preliminary forecast model of shallow crustal earthquakes using GNSS velocity data.

We follow the procedure of Shen et al.(2007) to calculate the forecast model. The GNSS velocities at continuous GNSS stations from April 2005 to December 2009 are used for the model in southwest Japan. Elastic deformation due to interplate coupling along the Nankai Trough is removed using the block model of Nishimura et al. (2018). Strain rate field is calculated at a grid point of 0.2º x 0.2º by a method of Shen et al (1994). The strain rates are converted to geodetic moment rates by a formula proposed in Savage and Simpson (1997). The thickness of a seismogenic layer, rigidity, b value of the Gutenberg-Richter law, and magnitude of the maximum earthquake are assumed to be 12 km, 30 GPa, 0.9, and 7.5, respectively. They are uniform in the modeled region. Previous studies (e.g., Shen-Tu et al., 1994) revealed that geodetic strain rates were much larger than seismological ones in southwest Japan because geodetic strain includes both elastic and inelastic strain. Elastic strain rates presumably equal to seismological ones on a long-term average. We compared seismic moment rates released by shallow historical earthquakes since AD1586 with the geodetic moment rates. Their ratio is 0.24 and 0.16 in the Chubu, Kinki, and Chugoku region and the whole southwest Japan. This difference is probably attributed to the distribution of historical documents and may also reflect the regionality of the ratio between elastic and inelastic strain. Applying 0.16 for calculating elastic rates and the stationary Poisson process of the earthquake occurrence, a probability of M≥6 earthquakes for 30 years ranges from 5.1 % to 0.2 % in each 0.2º x 0.2º grid of southwest Japan. We verify this probability model by using shallow (Depth≤ 20 km) M≥5 earthquakes occurred in 2010-2019, which is a period after the used GNSS data. The number of earthquakes was 36, which is roughly concordant to the predicted number of the model (3.04 per year). About 58 % of the earthquakes occurred with 25 % of the area with the highest strain rates, which suggests many crustal earthquakes occur in high strain-rate regions. The verification suggests the preliminary forecast model has the predictive power reasonably.

How to cite: Nishimura, T.: Preliminary forecast model of crustal earthquakes in southwest Japan based on GNSS data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10459, https://doi.org/10.5194/egusphere-egu21-10459, 2021.

Jesus Piña-Valdés et al.

The impact of the crust deformation on the processes that control the seismic activity is still controversial. The seismic activity is usually thought to be associated with the active tectonic deformation as estimated from the horizontal displacements field: seismic active regions are usually dominated by important horizontal deformation controlled by tectonic activity. But this is not so clear on regions of low to moderate seismicity, where small horizontal deformation rates are commonly observed, similar to the rates detected for regions of no seismicity. In those regions, the non-tectonic processes such as the Glacial Isostatic Adjustment (GIA), may have a significant impact on the seismicity.

Since the deformation of low tectonic activity in Europe is usually piecewise, we combined 10 different GNSS velocity field solution to generate a dense GNSS solution to derive the 3D strain rate at continental scale: using the velocity solutions of common stations, the different datasets were converted to a common reference frame. Their uncertainties were homogenized, and a combined velocity field was computed considering the homogenized uncertainty of each independent solution. Finally, an automatized criterion of identification and outliers removal was applied, as well an adaptive smoothing scheme that depends on the station density, the noise, and the local tectonic deformation rate

The resulting 3D combined GNSS velocity field was interpolated and the horizontal strain rate was derived. Then, assuming the Hooke law for the earth crust, we decompose the vertical velocity field into a component due to tectonic deformation and a component due to isostatic rebound. To better understand the effects of horizontal tectonic deformation versus the flexure generated by GIA on the seismicity, the spatial distribution of the seismicity is compared to the strain rate map and the vertical velocity fields.

How to cite: Piña-Valdés, J., Socquet, A., Beauval, C., Bard, P.-Y., Doin, M.-P., D’Agostino, N., and Shen, Z.: Role of the Crustal deformation processes on the seismicity: An approach, using combined dense GNSS velocity field in Europe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15288, https://doi.org/10.5194/egusphere-egu21-15288, 2021.

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