GD9.1 | Subduction zone processes along the western margin of South America
Subduction zone processes along the western margin of South America
Co-organized by SM4/TS2
Convener: Christian Sippl | Co-conveners: Andres Tassara, Anne Socquet, Sergio Ruiz, Marcos Moreno
Orals
| Fri, 19 Apr, 08:30–10:15 (CEST)
 
Room D1
Posters on site
| Attendance Thu, 18 Apr, 16:15–18:00 (CEST) | Display Thu, 18 Apr, 14:00–18:00
 
Hall X2
Orals |
Fri, 08:30
Thu, 16:15
The western South American subduction zone is among the largest subduction systems on the planet and stands out as the archetype of ocean-continent convergent margins. Compared to other subduction zones, the region is notable because it is associated with the largest accretionary orogen of the world (The Andes cordillera), it shows several regions of flat slab subduction, and it hosted some of the largest instrumentally recorded earthquakes. Over the last years and decades, significant progress has been achieved in characterizing and imaging the constituent parts of the South American subduction zone (downgoing oceanic plates, South American upper plate, plate interface between them, mantle wedge beneath the upper plate) as well in the understanding of geodynamic and seismotectonic processes shaping the convergent margin.

In this session, we aim to bring together scientists and contributions from a wide variety of disciplines that try to constrain and understand past and ongoing processes in this subduction zone. These can include, but are not limited to: seismo-geodetic studies of slow and fast deformation along the plate interface; geophysical studies of subduction zone structure, geometry and fluid processes; analog and numerical modeling studies of this subduction zone; studies on faulting or fluid processes in the upper plate; offshore studies on bathymetry and structure of the downgoing plate or the outer forearc; studies of Andean magmatism, volcanic processes and their link to tectonics.

Orals: Fri, 19 Apr | Room D1

Chairpersons: Christian Sippl, Andres Tassara, Anne Socquet
08:30–08:40
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EGU24-11363
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Highlight
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On-site presentation
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Audrey Margirier, Manfred R. Strecker, Stuart N. Thomson, Peter W. Reiners, Ismael Casado, Sarah George, and Alexandra Alvarado

The Cenozoic growth and uplift of the Andes has been strongly influenced by the subduction dynamics and the superposed effects of climate. Previous studies have shown that the arrival of oceanic ridges and slab flattening triggered regional uplift and exhumation in Peru and Chile. Recent studies suggest that the subduction of the Carnegie Ridge below the Ecuadorian Andes controlled the formation of a crustal sliver moving northward. However, the timing of the ridge’s arrival at the trench and its effect on topographic growth remain unclear.

New geo-thermochronological data allows us to investigate the possible role of ridge subduction in prompting the growth of the Ecuadorian Andes and to pinpoint the timing of the Carnegie Ridge subduction. Time-temperature inverse modeling of this new thermochronological dataset constrained two cooling phases in the Western Cordillera. The first phase occurred after the emplacement of intrusions, likely associated with magmatic cooling. The second phase began ~6 Ma, coinciding with the last cooling phase observed in the Eastern Cordillera and is likely to be associated with exhumation of the Western Cordillera. Based on our results and existing geological cross-sections we propose that recent crustal shortening and rock uplift led to exhumation of Ecuadorian Andes at ~6 Ma. We suggest that the onset of Carnegie Ridge subduction at ~6 Ma increased the coupling at the subduction interface, promoting shortening and rock uplift in the region.

How to cite: Margirier, A., Strecker, M. R., Thomson, S. N., Reiners, P. W., Casado, I., George, S., and Alvarado, A.: Tectonics and exhumation processes in the northern Andes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11363, https://doi.org/10.5194/egusphere-egu24-11363, 2024.

08:40–08:50
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EGU24-20861
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On-site presentation
Michele Paulatto, Yueyu Jiang, Audrey Galve, Mireille Laigle, Andreas Rietbrock, Monica Segovia, and Sandro Vaca

The subduction margin in Ecuador is dominated by the subduction of the Carnegie Ridge and associated oceanic plate relief. This region is also affected by complex slip behaviour including aseismic deformation, slow slip, and large earthquakes. We present new seismic and gravity data collected as part of the HIPER campaign in 2020 and 2022, covering the subduction margin at the northern edge of the Carnegie Ridge. Traveltime tomography of dense active source wide-angle seismic data from a trench perpendicular profile reveals the structure of this part of the margin. The slab crust thickens from 7.5 km at the western end of the profile to 15 km at the eastern end. The profile crosses two seamounts (Atacames seamounts), one currently impinging onto the margin (AS2) and the other already buried beneath the accretionary prism (AS1). The seamounts have low P-wave velocity roots and are associated with gravity anomaly highs. The forearc is uplifted in front of the subducted seamount AS1 and is affected by gravitational collapse in its wake. In the area affected by the seamounts, the interseismic plate coupling is reduced to almost zero likely because of the fracturing and disruption of the forearc and lubrication induced by enhanced fluid input. Further downdip the profile extends into the rupture area of the 2016 M7.8 Pedernales earthquake. This part of the plate interface is more laterally homogeneous and characterised by higher Vp. Our results confirm that rugged plate relief is associated with reduced interseismic coupling and that megathrust earthquake rupture areas tend to have high Vp and laterally homogeneous properties.

How to cite: Paulatto, M., Jiang, Y., Galve, A., Laigle, M., Rietbrock, A., Segovia, M., and Vaca, S.: The effect of subduction relief on megathrust slip properties in Ecuador, constraints from gravity anomalies and seismic tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20861, https://doi.org/10.5194/egusphere-egu24-20861, 2024.

08:50–09:00
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EGU24-5394
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ECS
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On-site presentation
Sara Ciattoni, Federico Cella, Stefano Mazzoli, Miller Zambrano, Robert Butler, Stefano Santini, Antonella Megna, and Claudio Di Celma

The Nazca Ridge's thickened subduction beneath the South American continental margin (10° to 15° S) is characterised by a flat-slab configuration. This peculiar geological setting strongly influences upper plate dynamics, significantly impacting stress distribution and seismicity in the South American plate. However, the effects of the Nazca Ridge subduction on the Peruvian forearc and Andean Cordillera development remain subjects of extensive debate. In this study we thoroughly investigate the general structure of the Nazca Ridge subduction zone producing an integrated two-dimensional structural-geological model of the south-Peruvian Andes. Combining surface geological data and geophysical information from existing literature, we delineated the crustal structure up to a depth of about 130 km along a ca. 1000 km-long transect, encompassing the Peruvian Forearc System and the Andean Cordillera. In order to improve the characterization of geological features and validate the model, we carried out forward modelling of the Bouguer anomaly in the region, integrating four distinct datasets. Subsequently, we formulated a two-dimensional density model to reproduce the observed gravity field, taking into consideration the petrological properties of the materials and the P-T condition in each area of the crustal section. The geometry of the structures was assessed by choosing the configuration that, honouring the geological and geophysical constraints upon which the initial model was based, also allowed maximising the fit between observed Bouguer anomaly values and the values computed during the forward modelling process. Our exhaustive approach allowed us to obtain a comprehensive model of the Nazca Ridge subduction zone, accurately defining both the deep lithosphere-asthenosphere system and shallow geological structures. This contribution will substantially enhance the ongoing debate on the tectonic evolution and geodynamics of Andean orogeny.

How to cite: Ciattoni, S., Cella, F., Mazzoli, S., Zambrano, M., Butler, R., Santini, S., Megna, A., and Di Celma, C.: Comprehensive two-dimensional structural-geological model of the Nazca Ridge subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5394, https://doi.org/10.5194/egusphere-egu24-5394, 2024.

09:00–09:10
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EGU24-5819
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ECS
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On-site presentation
Arne Warwel, Dietrich Lange, Anke Dannowski, Sara Aniko Wirp, Eduardo Contreras-Reyes, Ingo Klaucke, Marcos Moreno, Juan Diaz-Naveas, and Heidrun Kopp

The subduction of the oceanic Nazca plate beneath the continental South American plate shapes the Chilean margin and is known to generate large megathrust earthquakes. Our study focuses on the region defined by the pre-collision and subduction of the Copiapó Ridge with the Chilean margin at ~27°S. This area has been a seismic gap since 1922, and little is known about the geometry and deep structures of the incoming plate, the overriding plate, and the processes related to the subduction of the Copiapó Ridge. 

We model the seismic structure in the region by using wide angle seismic data from a recent amphibious seismic refraction experiment. Thereby, we utilize seismic signals from both offshore airgun-shots and onshore mining blasts. Overall, we use 36 Ocean-Bottom-Seismometers and 10 temporal seismic land stations along an approximately 420 km long profile ranging from more than 300 km offshore up to more than 100 km landwards.

Our P-wave velocity model images the geometry and velocity structure of the incoming oceanic plate, including three seamounts belonging to the Copiapó Ridge, the marine and continental forearc, and the upper part of the downgoing slab. The model shows an oceanic crust with hardly any sediment cover (generally less than 10 m) and an average oceanic Moho depth of about 6.2 – 6.9 km below the seafloor, which increases to over 10 km below the seamounts of the Copiapó Ridge. The velocities beneath the seamounts are similar or slightly slower compared to the adjacent upper oceanic crust (Vp ranging from 3.5 to 6 km/s). This suggests that the Copiapó Ridge was predominantly formed by extrusive processes. In addition, the velocity model reveals a significant thinning (to less than 4 km) of the oceanic crust landwards of the trench axis.

Together with recently acquired bathymetry data, we will compare our findings to other studies north and south of the Copiapó region and discuss the structural and geometric along-strike variations of the northern Chilean subduction zone.   

How to cite: Warwel, A., Lange, D., Dannowski, A., Wirp, S. A., Contreras-Reyes, E., Klaucke, I., Moreno, M., Diaz-Naveas, J., and Kopp, H.: Structure and geometry of the Chilean subduction zone near Copiapó (~27°S) based on an amphibious seismic refraction experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5819, https://doi.org/10.5194/egusphere-egu24-5819, 2024.

09:10–09:20
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EGU24-5820
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On-site presentation
Bernd Schurr, Armin Dielforder, Lukas Lehmann, and Claudio Faccenna

Subduction zone forearcs deform transiently and permanently due to the frictional coupling with the converging lower plate. Transient stresses are mostly the elastic response to the seismic cycle. Permanent deformation is evidenced by forearc topography, upper plate faulting, and earthquakes; its relation to the megathrust seismic cycle is debated. Here we study upper plate seismicity, interplate earthquake slip vectors, and the GNSS strain field in the northern Chile subduction zone to deduce the stress field and to separate elastic and permanent strain. We find that seismicity is distributed unevenly and that high seismicity rates concur both with a break in the forearc topography and tectonics of the Coastal Cordillera and the onset of a change in subduction obliqueness. Earthquakes in the South American crust under the sea and the Coastal Cordillera show a remarkably homogenous north-south, i.e., trench-parallel, compressional stress field. The trench-parallel compression above the plate coupling zone, almost perpendicular to plate convergence direction, may be explained by strain resulting from a change in subduction obliqueness due to the concave shape of the plate margin, which we demonstrate by investigating inter-plate earthquake slip vectors. From these, we derive a strain rate estimate (-5×10e-8 /a) and compare it to one derived from upper plate earthquakes (-8×10e-9 /a). We argue that the dominance of trench-parallel compressive stresses over trench-perpendicular ones is due to canceling of the latter by tensional gravitational stresses due to the topographic gradient between the Andes and the Nazca trench. Based on the distribution of the type of faulting we investigate the trench-perpendicular stress field with a force-balance model. The observed deep strike-slip earthquakes, expression of trench-perpendicular tension, require the deepest extent of the megathrust to be very weak.

How to cite: Schurr, B., Dielforder, A., Lehmann, L., and Faccenna, C.: Spanning the Arc: Margin Geometry and Topography Control Upper Plate Deformation in the Central Andean Subduction Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5820, https://doi.org/10.5194/egusphere-egu24-5820, 2024.

09:20–09:30
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EGU24-8472
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ECS
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Highlight
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On-site presentation
Bertrand Lovery, Anne Socquet, Mohamed Chlieh, Marie-Pierre Doin, Mathilde Radiguet, Juan Carlos Villegas-Lanza, Juliette Cresseaux, and Philippe Durand

The Central Andes subduction has been the theater of numerous large earthquakes since the beginning of the 21th Century, notably the 2001 Mw8.4 Arequipa, 2007 Mw8.0 Pisco, and 2014 Mw8.1 Iquique earthquakes. A better knowledge of the interplate coupling distribution and seismic cycle in this area is thereby fundamental for improving our understanding of large earthquakes segmentation, and ultimately improving our knowledge of the seismic potential in the area. Interseismic models from inversions of 80 GNSS velocities in Central and South Peru (12–19°S) on a 3-D slab geometry indicate that the locking level is relatively high and concentrated between 20 and 40-km depth. Locking distributions indicate a high spatial variability of the coupling along the trench, with the presence of many locked patches that spatially correlate with the seismotectonic segmentation. Our study confirms the presence of a creeping segment where the Nazca Ridge is subducting, we also observe a lighter apparent decrease of coupling related to the Nazca Fracture Zone (NFZ). However, since the Nazca Ridge appears to behave as a strong barrier, the NFZ is less efficient to arrest seismic rupture propagation. Considering various uncertainty factors, we discuss the implication of our coupling estimates with size and timing of large megathrust earthquakes considering both deterministic and probabilistic approaches. We estimate that the South Peru segment, from the Nazca Ridge to the Arica bend, could have a Mw=8.4-9.0 earthquake potential depending principally on the considered seismic catalog and the seismic/aseismic slip ratio (Lovery et al., 2024).

We use large-scale InSAR Sentinel-1 time series, processed in the frame of the FLATSIM-Andes project (Thollard et al., 2021), encompassing the Central Andes (7–26°S) on the 2015-2021 period. These InSAR data provide a useful complementarity to the GNSS data, with a higher spatial resolution in exchange for a lower temporal resolution. Subsequently, it allows to better define the contours of the asperities, or the maximum locking depth. We modelled the effects of non-tectonic processes such as solid earth tides (SET), ocean tide loading (OTL), and ionospheric electronic content (TEC) on the ramps in range and azimuth, in order to measure ground deformation in a stable reference frame, with sufficient accuracy for large-scale tectonic applications, allowing vertical and horizontal decomposition.

In order to perform joint inversions of GNSS and InSAR interseismic velocities, we also develop finite element models of the subduction zone with more complex viscoelastic rheology (Maxwell and Burger laws). Viscoelastic models are expected to produce a broader displacement, with horizontal displacements extending further inland. Higher magnitudes of deformation in the late-stage of the interseismic period and a shallower optimal locking depth have also been reported for viscoelastic models. These features are key factors to make the link between short-term and long-term deformation, and to discriminate the slip on the slab interface from internal deformation. We investigate the viscoelastic effects associated with the great 2001 Mw8.4 Arequipa earthquake, in order to assess its impact on the interseismic loading estimate on the subduction megathrust.

How to cite: Lovery, B., Socquet, A., Chlieh, M., Doin, M.-P., Radiguet, M., Villegas-Lanza, J. C., Cresseaux, J., and Durand, P.: Crustal Deformation Associated with the Seismic Cycle in the Central Andes from InSAR and GNSS Geodetic Time Series, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8472, https://doi.org/10.5194/egusphere-egu24-8472, 2024.

09:30–09:40
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EGU24-11322
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ECS
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Highlight
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On-site presentation
Caroline Chalumeau, Hans Agurto-Detzel, Louis De Barros, and Philippe Charvis and the Rapid Response Team of the 2016 Pedernales Earthquake

The Ecuador-Colombia subduction zone is a complex and spatially heterogeneous region that hosts both shallow aseismic slip and large megathrust earthquakes, and where both  inter-seismic and post-seismic seismicity have been linked to aseismic slip. Repeating earthquakes, which are the result of repeated loading and failure of single asperities on a fault, are a valuable tool in studying aseismic slip as well as in monitoring the evolution of fault properties over time. In this study, we search for repeating earthquakes within one year of aftershocks following the April 16th, 2016 Mw 7.8 Pedernales earthquake, and we analyze their relationship to afterslip and the evolution of their source properties. 

We calculate waveform cross-correlation coefficients (CC) on 4762 catalog events, and use a threshold CC of 0.95 to sort events into preliminary families, which are then completed using template-matching and relocated using HypoDD. In total, 376 earthquakes were classified into 62 families of 4 to 15 earthquakes. Additionally, the magnitudes, corner frequencies and stress drops of 136 repeaters were determined using spectral ratios.

We find an increase in the recurrence time of repeating events with time after the mainshock, highlighting a possible timeframe for the afterslip’s deceleration. However, repeating earthquakes appear to concentrate around the areas of largest afterslip release, where afterslip gradient is the highest. This suggests 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. We also find that repeaters in the region near the trench are unusual, in that their stress drops are anomalously low and systematically decrease over the postseismic period, hinting at a potential increase in pore fluid pressure in this region over time. 

How to cite: Chalumeau, C., Agurto-Detzel, H., De Barros, L., and Charvis, P. and the Rapid Response Team of the 2016 Pedernales Earthquake: What repeating earthquakes can tell us about postseismic slip and fluid circulation in the Ecuadorian subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11322, https://doi.org/10.5194/egusphere-egu24-11322, 2024.

09:40–09:50
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EGU24-20233
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ECS
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Highlight
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On-site presentation
Diego Molina, Jannes Münchmeyer, Mathilde Radiguet, Anne Socquet, and Marie-Pierre Doin

While subduction earthquakes are widely recognized for releasing seismic slip, aseismic slip can also be hosted on the megathrust by the occurrence of postseismic phase or Slow Slip Events (SSEs). SSEs have been reported along several subduction zones, preferably on the deeper zone and usually lasting months or even years (Draguert et al., 2001). Notably, in the Chilean subduction zone, deep SSEs have been observed in only a reduced area in Central Andes, specifically close to the Copiapo city. Recent studies report that this area is prone to host regular SSEs with a recurrence time of ~5 years and variable duration (Klein et al., 2021), which was confirmed by a new detected SSE in 2020 and 2023.

Notably, during the 2020 SSE, a seismic crisis with a main shock of Mw 6.9 took place on the zone (September 2020), likely provoking an interaction between the different slip modes.  In this work, we attempt to enhance the characterization of the temporal and spatial pattern-evolution of the SSE to elucidate whether there was a trigger mechanism for the seismic crisis or if the earthquake affected the SSE evolution.

To describe the seismic behavior of the area, we recur to the analysis of distinct data sets. On one hand, GNSS stations deployed by different institutions are used to characterize the temporal evolution and amplitude of the 2020-SSE and respective seismic crisis. On the other hand, the spatial pattern is recovered by InSAR data recorded by Sentinel-1 mission. Additionally, a seismicity catalogs coming from machine learning approach is used to investigate aseismic-seismic interactions.

Our analysis shows that the 2020 SSE triggered the seismic sequence in September of that year. We also observed that the aseismic deformation migrates, resulting in a total cumulative slip pattern similar to another SSE detected in 2014. Remarkably, our study evidences a clear segmentation along dip and strike affecting both, aseismic and seismic slip, which correlates with gravity anomalies.

This study suggests a tectonic control on the slip behavior characterizing the area and highlights the cinematic between slow and fast earthquakes hosted along the plate interface.

 

 

How to cite: Molina, D., Münchmeyer, J., Radiguet, M., Socquet, A., and Doin, M.-P.: Structural control on aseismic and seismic slip interactions during the 2020 SSE in the Atacama region, Chile., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20233, https://doi.org/10.5194/egusphere-egu24-20233, 2024.

09:50–10:00
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EGU24-6860
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ECS
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On-site presentation
Martin Riedel, Andrés Tassara, Nicole Catalán, and Rodolfo Araya

Subduction zones are complex geotectonic environments where multiple processes interact resulting in different kinds of seismicity. Among them is intermediate depth intraplate seismicity, which occurs within the subducting plate in conditions that should favor ductile shear rather than fragile faulting. A high variability in focal mechanisms and spatial distribution has been observed globally for this kind of events. In some subduction zones a double seismicity zone develops while in others not. Moreover, these earthquakes may occur in dry and hydrated conditions. Therefore, there is no consensus on the process that originate them.

In the context of hydrated subductions, such as is the Chilean case, the influence of fluids liberated through the metamorphism of the slab is generally considered as the main triggering factor. It is therefore important to know at what pressure and temperature and between which mineral associations these reactions occur.

Hacker et al. (2003) compiled information on average mineralogy and whole rock composition to create phase diagrams which have allowed the study of dehydration reactions and intraplate seismicity around the world. However, their work is based on data from the FAMOUS area in the Atlantic Ridge for the MORB and the Semail ophiolite for ultramafic rocks, which do not correlate to compositions in the Nazca Plate.

To better constrain the conditions on which dehydration reactions take place within the Nazca slab, we used PERPLE_X to calculate pseudosections with geochemical data more representative of it. We created a simple model of the plate consisting of a top layer with MORB compositions from drilled and dredged samples for the crust and a bottom layer with ultramafic rock compositions obtained from ophiolites from a geotectonic context consistent with that of the Nazca Plate. We then coupled the pseudosections with a kinematic thermal model of the Chilean subduction zone to create profiles of stable mineral associations and hydration gradient along the subducted slab.

We observe that, for constant PT, hydrated mineral stability is mainly controlled by the initial (pre-subduction) slab hydration percentage and in a much lesser extend by slab composition. For areas where slab hydration is constrained by geophysical data, we tested different slab compositions and found that modelling with data from Nazca Plate layer 2 basalts and a mid ocean ridge type ophiolites provides the best fit to seismic data. It appears that intraplate seismicity nucleates along areas with strong hydration gradients, i.e. where dehydration reactions occur. We then extrapolated these compositions to the rest of the plate and with the assumption that the correlation observed between hydration gradient and intraplate seismicity hypocenters is maintained along the margin, we estimated hydration percentages along 5 latitudinal profiles.  Although further work remains to improve our seismic catalogues and spatial resolution of the thermal model, preliminarily it seems that the Nazca Plate is more hydrated in northern Chile (~2.5%) and less to the south (~1%) and that the Chilean double seismicity zone only occurs where hydration is above ~2%.

How to cite: Riedel, M., Tassara, A., Catalán, N., and Araya, R.: Phase stability and metamorphic reactions related to intraplate seismicity in the Nazca Plate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6860, https://doi.org/10.5194/egusphere-egu24-6860, 2024.

10:00–10:10
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EGU24-13804
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On-site presentation
Nipaporn Nakrong, Marnie Forster, Wim Spakman, Hielke Jelsma, and Gordon Lister

Here we present a 3D-time reconstruction of the tectonic evolution of the Nazca and South American plates. The geometry of subducted slabs was modelled down to a depth of ~1950km using UU-P07 global tomographic model. Our approach integrated geochronological records, geological history, and seismotectonic data. Furthermore, our proposed slab models incorporated both velocity and temperature gradients to determine the mid-slab surface accurately. To reconstruct these slabs with minimal distortion back to the Earth's surface, we employed a reverse engineering method. The positions of potential tears in the subducted slabs can then be recognized by the induced distortions. We identified at least three down-dip tears, which significantly influence subduction behaviour. We then integrated the floated or pre-subducted slabs into a 2D-time tectonic reconstruction and tracked the subduction interface over time. Our reconstruction reveals that the pre-subducted slabs accurately mimic the shape of the Andes during the Oligocene-Miocene boundary. However, the remnants of slabs subducted before that time are no longer connected to the entire slab. To the north of the Nazca tear, which coincides spatially with the Nazca fracture zone, the continuous slab has subducted to a depth of ~1950km. To the south, the downgoing slab has been segmented into three distinct zones, with tears localized along the two arms of the extrapolated Juan de Fernández ridge and the inferred Challenger fracture zone. Moving from north to south, the slab in these zones detached at some point after 22Ma, 15Ma, and 12Ma, respectively. Each slab segment exhibits variations in geometry, with flat slab and steep slab portions, as well as differences in penetration depth. Notably, the location of the Nazca down-dip tear coincides with the initial location of the eastward spanning of the Cu belts.

How to cite: Nakrong, N., Forster, M., Spakman, W., Jelsma, H., and Lister, G.: 3D-time slab deconstruction and ore deposit localization in South America, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13804, https://doi.org/10.5194/egusphere-egu24-13804, 2024.

10:10–10:15

Posters on site: Thu, 18 Apr, 16:15–18:00 | Hall X2

Display time: Thu, 18 Apr, 14:00–Thu, 18 Apr, 18:00
Chairpersons: Christian Sippl, Sergio Ruiz, Marcos Moreno
X2.82
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EGU24-990
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ECS
Valentina Cortassa, Robert Ondrak, Stefan Back, Cecilia del Papa, and Eduardo Rossello

The Andean Foreland Basin in the Chaco-Pampean Plain of North-West Argentina is thought to have been tectonically inactive during the Cenozoic. However, re-interpreted industry seismic-reflection data and borehole information document a complex tectonic history in the subsurface at least until Palaeogene times. Data synopsis and re-analysis reveal two regionally extensive and approximately NW-SE-oriented basement highs beneath the flat present-day surface, the Quirquincho (or Rincón Caburé) High and the Pampeano-Chaqueño High. These large geological structures were described previously, but the mechanism that elevated these features relative to the surrounding stratigraphy and the timing of uplift has remained elusive.

This study documents and describes of the Quirquincho and Pampeano-Chaqueño Highs and their relationship to the depocenters around and the sedimentary successions of the Chacoparanaense, Salta Rift and Andean Foreland Basins. We studied palaeo-basin morphology, stratigraphy, stratal terminations and distance to the Andes to unravel viable mechanisms that influenced the genesis of the two structural highs. The work presented is based on the re-interpretation of a large set of subsurface information (2D seismic-reflection profiles and well reports) using a regional approach that considers the Chacoparanaense, Salta Rift and Andean Foreland Basins as a complex lateral arrangement of basins varying in activity through time, depending on their relative location concerning orogens and rifted ocean margins.

Our research reveals that the Quirquincho and Pampeano-Chaqueño Highs were elevated features from the Late Palaeozoic to the Palaeogene, strongly influencing the deposition of Mesozoic and Palaeogene sediments. The tectonic mechanism controlling the rise of the Quirquincho and Pampeano-Chaqueño Highs was initially flexural deformation in the foreland of the Gondwanide orogen in the Permian, subsequently influenced in the Mesozoic by the opening of the Southern Atlantic Ocean.

How to cite: Cortassa, V., Ondrak, R., Back, S., del Papa, C., and Rossello, E.: Quirquincho and Pampeano-Chaqueño Highs: forebulge or Palaeozoic structures?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-990, https://doi.org/10.5194/egusphere-egu24-990, 2024.

X2.83
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EGU24-12834
Andreas Kammer, Camilo Andrés Betancur, and Camilo Conde

The tectonic setting of the Northern Andes is delineated fundamentally by a western oceanic terrane that was juxtaposed to the continental margin along the now fossilized interandean Romeral suture since the Early Cretaceous. This constellation and the connection to the Caribbean Large Igneous province have been attributed to a far-travelled and now partially subducted, formerly coherent terrane with a trailing edge represented by the Panama-Choco block. A former disconnection between oceanic terrane and South American plate may, however, be contended by considering continental provenance data of siliciclastic and volcanic rock units and a widely distributed geochemical arc signature of the effusive rock series. Moreover, the emplacement of the basic igneous sequences was strongly controlled by extensional tectonics and subduction correlates in its lifetime with the production of oceanic crust, suggesting a coupling between intrusive activity and convergence. In this contribution, we examine apparently conflicting structural deformations that may be reconciled, however, with the opening of a forearc basin and a deformational imprint that affected extensively the continental margin, supposing the existence of a subduction system composed of two continentward dipping slabs.

How to cite: Kammer, A., Betancur, C. A., and Conde, C.: Lower plate retreat and opening of a Cretaceous forearc basin, Northern Andes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12834, https://doi.org/10.5194/egusphere-egu24-12834, 2024.

X2.84
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EGU24-12909
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ECS
Mara A. Figueroa, Franco S. Sobrero, Demián D. Gómez, Robert Smalley Jr., Michael G. Bevis, Dana J. Caccamise II, and Eric Kendrick

The Central and South-Central Andes form a “two-sided” mountain belt bounded by distinct zones of convergence in the western forearc and eastern foreland flanks. Previous geodetic studies of interseismic deformation in the Bolivian Subandes and the Argentine Precordillera found that the forearc to foreland velocity field decayed too slowly to be explained purely by elastic shortening driven by locking of the Nazca megathrust. The velocity field is more precisely explained if elastic deformation is augmented by eastward displacement of the entire Andes. Here, we extend the earlier interpretation of interseismic motion and argue that foreland décollements can participate in the co- and postseismic phases of the earthquake deformation cycle associated with the Nazca megathrust. These findings have direct implications in estimating recurrence interval, slip rate, and probabilistic seismic hazard analysis on both sides of the orogen.

How to cite: Figueroa, M. A., Sobrero, F. S., Gómez, D. D., Smalley Jr., R., Bevis, M. G., Caccamise II, D. J., and Kendrick, E.: Microplate behaviour of the Andes during co- and early postseismic phases of the seismic cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12909, https://doi.org/10.5194/egusphere-egu24-12909, 2024.

X2.85
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EGU24-8006
|
ECS
Dominika Godová and Christian Sippl

The South American active margin is one of the most important and well-studied subduction zones on the Earth. In the last decade, our knowledge about the geometry of its respective constituent parts in Northern Chile has been significantly expanded thanks to seismicity data from a large network of permanent seismic stations. That calls for an effort to summarize these diverse constraints in a single 3D model, which can be validated and optimized using satellite gravity data.

Integrated geophysical modelling is primarily based on gravity data, which bring information about different crustal density inhomogeneities and their sources. As an inverse geophysical problem, gravity modelling is ambiguous, and therefore it is necessary to include geometry constraints from other geophysical data as well as geological information. Different types of seismic data offer the most commonly used constraints due to their depth range and the relatively well-described relation between seismic velocities and densities.

We aim to compile a 3D integrated geophysical model for Northern Chile in the IGMAS+ software, based on gravity data of the global gravitational (or geopotential) model EIGEN-6C4, which include terrestrial, satellite and altimetry data to a high degree and order of spherical harmonic expansion. As the main geometry constraints of the model, we use the newest available seismicity catalogs in the study area together with crustal thickness values from receiver functions. Starting with the geometries of previously published density models in the area of the Central Andes, especially those located at least partly in Northern Chile, we will modify these models guided by the geometry constraints from seismic data and will also use regional and global models of crustal and lithospheric interfaces, such as the top of basement in sedimentary areas, plate interface geometry, depth to continental Moho, and the lithosphere-asthenosphere boundary (LAB). Densities of the modeled bodies will be selected based on previously published models or estimated from seismic velocities. We also plan to study the gravity effect of the different geometries deduced from different generations of seismic data.

Our contribution provides an overview of evidence compiled in previous studies and adds new information on the deep lithospheric structure of the North Chilean margin by integrating them into a single model.

How to cite: Godová, D. and Sippl, C.: Towards a 3D integrated geophysical model of Northern Chile, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8006, https://doi.org/10.5194/egusphere-egu24-8006, 2024.

X2.86
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EGU24-17042
|
ECS
Audrey Chouli, Lucile Costes, David Marsan, Jannes Münchmeyer, Sophie Giffard-Roisin, and Anne Socquet

Repeating earthquakes, corresponding to the rupture of the same asperity over time at more or less regular intervals, can be used to estimate the slip rate on a subduction plate interface. The purpose of this work is to build a catalog of repeaters for the central part of the Chilean subduction zone, extending from 24°S to 33°S latitude and centered on the Copiapo seismic gap. As a basis for our study, we used the seismicity catalog from the Centro Sismológico Nacional (CSN).

The similarity between waveforms gives a good criterion to assign earthquakes to a similar asperity. To measure it, we calculated for each pair of events the coherency, correlation and associated time lag between the vertical components of their P waves, on a 5 s window starting 1 s before the P arrival. We tested different frequency bands (1-4 Hz, 3-12 Hz, 5-20 Hz, ...) and kept each time the one with the best coherency value. We selected all pairs of earthquakes with coherency higher than 0.95, at three or more stations. To ensure a stable measurement, we imposed that the time lags from cross-correlation and coherency differ by less than 0.01 s. To verify that the earthquakes of a repeaters family take place at the same location, we relocated the events using a double-difference method and created clusters based on both coherency and location similarity. As coherency values are calculated on a 5 s window, we relocate the centroids of the events, i.e. the center of mass of the rupture. To estimate the surface rupture, we calculate the rupture radius based on the seismic moment and stress-drop values (Eshelby 1957), estimating the stress-drop and seismic moment with SourceSpec (Satriano 2023).          

As preliminary results, we found 347 families, mostly located between 30-60 km deep, and between 29-33.5°S. Almost no repeaters were found before 2015 due to the lack of available stations. Obtained families contain a few events, with 11 earthquakes in the biggest one. We compared the obtained repeaters with the coupling along the plate interface.  Most repeaters are located at the transition between strong and low coupling zones in the Illapel area, making a circle shape around the deep part of the Illapel coseismic slip. Furthermore, we investigated the evolution of the number of repeaters with time in different areas and found potential aseismic slip marked by repeaters' activity consistent with previous observations, such as before the 2017 Valparaiso sequence (Ruiz et al., 2017) or after the Atacama sequence (Klein et al., 2021).

In order to obtain more complete repeaters families, we created a new machine learning based earthquake catalog for the study area with SeisBench (Woollam et al., 2022), using data from permanent and temporary networks in Northern and Central Chile. We are currently applying our analysis to this new catalog.

How to cite: Chouli, A., Costes, L., Marsan, D., Münchmeyer, J., Giffard-Roisin, S., and Socquet, A.: Search for repeaters in the central part of the Chilean subduction zone , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17042, https://doi.org/10.5194/egusphere-egu24-17042, 2024.

X2.87
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EGU24-7484
Christian Sippl

The use of machine-learning based tools for phase picking and association is in the process of revolutionizing the field of seismicity analysis, leading to the simplified creation of “deep” seismicity catalogs often containing 10s or 100s of thousands of events. Having such catalogs as an available resource opens the field for novel ways of combining seismicity information with other types of datasets. At the same time, the sheer amount of data poses challenges for the visualization as well as joint analysis with other constraints.

In this contribution, I want to explore different ways of using and visualizing large seismicity catalogs, using a range of different recently compiled earthquake catalogs from the Chilean margin as showcase examples. Moreover, I attempt to find efficient ways of cross-plotting seismicity data from “deep” catalogs with other datasets such as interplate coupling maps, seismic velocity distributions, temperature models or inferred mineralogy maps.

How to cite: Sippl, C.: Using large microseismicity catalogs to constrain subduction zone processes – examples from the Chilean margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7484, https://doi.org/10.5194/egusphere-egu24-7484, 2024.

X2.88
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EGU24-9019
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ECS
Nooshin Najafipour, Jorge Antonio Puente Huerta, Christian Sippl, Javad Kasravi, Jonas Folesky, and Bernd Schurr

Northern Chile is one of the most seismogenic regions on the planet, and has been monitored by a permanent network of seismic stations since 2007. We here present a first step towards a new, more complete seismicity catalog for this region, leveraging modern deep-learning based algorithms for phase picking and association.

We first assessed the performance of EQTransformer, a deep learning based phase picker, in detecting and phase picking seismic data from the Northern Chile Subduction Zone by comparison with a large, meticulously handpicked dataset. We found that the "INSTANCE" model within SeisBench yielded the best performance for our study area. Through systematic threshold variations, we determined the optimal values using Precision-Recall curves (0.4 for event detection, 0.1 for P and S picks). Subsequently, we applied GaMMA, identified as the best performing phase associator in synthetic tests, coupled with NonLinLoc for initial event location. One of GaMMA's key operational criteria is the association threshold, where we required a minimum of five seismic phases to define an event, which yielded a high reliability in the phase association process. Moreover, we refined the catalog by automatically identifying and removing duplicate events. All associated events were consecutively relocated in a 1D and a 2D velocity model, using the VELEST and simul2000 algorithms. Events with disproportionally high RMS residuals as well as single picks with high residuals were removed in the process. In a final step, events were relocated with a double-difference approach.

A first application of this combined approach for the year 2020 yielded 2,838,080 P and S picks in the picking stage, with a total of 83,194 events after association and relocation. This is a nearly tenfold increase in event numbers compared to the IPOC catalog of Sippl et al. (2023), which contains 8,716 events for the same time interval.

In this contribution, we present results from a larger-scale application of our procedure to several years of IPOC data, and compare retrieved geometries as well as event numbers to the previously published IPOC catalog. Our findings demonstrate the potential of modern deep-learning algorithms in the creation of larger and more complete earthquake catalogs. Moving forward, our goal is to extend this preliminary catalog to span the entire 15 years of IPOC operation, facilitating in-depth analysis of regional processes.

How to cite: Najafipour, N., Puente Huerta, J. A., Sippl, C., Kasravi, J., Folesky, J., and Schurr, B.: The IPOC catalog goes deep: preliminary results, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9019, https://doi.org/10.5194/egusphere-egu24-9019, 2024.

X2.89
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EGU24-14050
Andres Tassara, Christian Sippl, Martin Riedel, Catalina Castro, and Favio Carcamo

Asperities inside the seismogenic zone of subduction megathrust are regions where specific frictional properties allow a stick-slip behavior characterized by the accumulation of slip deficit over decades to centuries and its sudden release during earthquakes. Despite its major role on the occurrence of the most devastating earthquakes and tsunamis on the planet, the physical nature of asperities and their limiting barriers is still unclear. This is partially due to an, often, ambiguous interpretation of individual geophysical proxies that are theoretically connected with the frictional structure of the megathrust at quite different time scales, ranging from 100-102 yrs (seismicity patterns, geodetic locking, Vp/Vs and MT anomalies) to 105-107 yrs (coastal geomorphology, forearc wedge geometry and associated basal friction, magnetic and gravity anomalies). If transient phenomena, like slow slip events (SSEs) or stress shadows created by previous earthquakes, do not dominate the seismogenic behavior of the megathrust, then short- and long-term frictional proxies should coincidently illuminate the location of asperities and barriers. Moreover, this would imply that the nature of this features must be connected to the geology structure of both converging plates, with strong implications to seismic hazard assessment. A number of previous studies have explored a combination of several geophysical proxies for megathrust frictional structure, most of them along the Chilean margin. Here we expand over the work of Molina et al. (2021) and Sippl et al. (2021) by performing an integrated analysis of gravity anomalies, friction from critical wedge theory, geodetic locking and seismicity patterns for the entire 4000-km long Chilean megathrust. Particularly, we use available (micro)seismicity catalogues to compute maps of the b-value of the frequency-magnitude relationship. This parameter contributes with an independent short-term proxy for the stress state of the megathrust and we treat it as an additional continuous field into a principal component analysis (PCA) similar to Molina et al. (2021) that aims to quantify the main spatial correlation between the proxies. We will also test other techniques to measure the degree of spatial correlation, like AI-based pattern recognition methods. This integrated analysis will also consider rupture length of historical earthquakes over the last 500 yrs and slip distribution of instrumental earthquakes and SSEs. This will allow us to test contrasting hypothesis about the nature of seismic asperities and barriers along the Chilean megathrust and elsewhere.

How to cite: Tassara, A., Sippl, C., Riedel, M., Castro, C., and Carcamo, F.: Nature of asperities and barriers along the Chilean megathrust unveiled by an integrated analysis of seismicity, gravity, geodetic locking and wedge geometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14050, https://doi.org/10.5194/egusphere-egu24-14050, 2024.

X2.90
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EGU24-12800
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ECS
Javiera Álvarez, Ignacia Calisto, Jorge Crempien, Joaquín Cortés, Claudio Faccenna, and Rodolfo Araya

The characterization of the spatial distribution of historical earthquake ruptures in a seismic segment plays a fundamental role in our understanding of the seismic cycle of significant earthquakes and in assessing the potential hazards associated with future events of this nature.

Due to its tectonic behavior, Chile has been impacted by megathrust earthquakes of considerable magnitude, such as the Valdivia 1960, Maule 2010, and more recently, the Illapel 2015 events. However, there are certain areas where no large earthquakes have occurred and are thus considered to be in a seismic gap. Despite experiencing some significant events, they do not manifest the required energy release properties and depth to compensate the accumulated friction. All these earthquakes, which represent varying stages of the seismic cycle, interact with different geological characteristics of the segment. This is evident in the central zone of Chile, specifically in the Valparaíso region, which has been in a seismic gap since the last major surface-rupturing earthquake of 1730.

During the Maule 2010 and Illapel 2015 earthquakes, rupture occurred only in the southern and northern segments in the mentioned area. Despite seismic activity in 1822, 1906, 1985, and 2017, and the presence of the Marga-Marga crustal fault in Viña del Mar, the energy release has not been sufficient to trigger the expected seismic sequence. It is worth noting that the fault is dangerously located in the most densely populated and frequented area of the city of Viña del Mar, presenting a threat to the surrounding population greater than what could be expected from a subduction earthquake itself.

This research aims to identify and quantify the interaction between the subducting and the Marga-Marga faults in order to assess the potential seismic activity in the area, considering that the crustal earthquakes caused by faults such as Marga-Marga are potentially more destructive than subduction earthquakes of equal magnitude. A relevant precedent is the interaction between the rupture of the Maule 2010 earthquake and the active fault segment of Pichilemu, which triggered a seismic swarm in 2011.

To achieve this, a study was conducted to characterize the slip associated with tsunamigenic events that occurred in the Central Chile segment in 1730, 1906, and 1985. The study revealed deformation patterns, indicating that the last shallow movement occurred in 1730, followed by deep patterns along the coast for subsequent events. Historical data was collected, and a stochastic modeling methodology was applied to comprehensively reconstruct the events. The Coulomb stress transmission between the Marga-Marga fault and subduction events, such as the one in 1906, was then characterized using the newly acquired information from historical deformation to identify potential activation zones of the crustal fault. Currently, efforts are underway to implement a methodology that uses computational simulation tools to visualize the impact of a coseismic event, such as the one in 1730, on the crustal fault and the surrounding region. The aim is to understand the past behavior of the region to be prepared for potential future activations.

How to cite: Álvarez, J., Calisto, I., Crempien, J., Cortés, J., Faccenna, C., and Araya, R.: Interaction between historical earthquakes in the seismic gap of central Chile and the Marga-Marga crustal Fault: The seismic potential of the Valparaiso region., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12800, https://doi.org/10.5194/egusphere-egu24-12800, 2024.

X2.91
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EGU24-8443
Sui Tung and Timothy Masterlark

We derive a co-seismic slip model of the 2015 Mw 8.3 Illapel, Chile earthquake constrained by line-of-sight displacements from Sentinel-1 interferograms. Greens functions are calculated with 3D finite element models (FEMs). The FEMs simulate a non-uniform distribution of elastic material properties and a precise geometric configuration of the irregular topographical surface. The rupturing fault follows the curvilinear Peru-Chile Trench and Slab1.0. The optimal model that inherits heterogeneous material properties, provides a significantly better solution than that in a homogenous domain at the 95% confidence interval. The best-fit solution for the domain having a non-uniform distribution of material properties reveals a triangular slip zone. Slip is concentrated near the trench with a dip-slip up to 7.75 m, giving rise to a moment magnitude of Mw8.22 in general agreement with the seismological estimate. This methodology allows us to integrate multiple datasets of geodetic observations with seismic tomography, to achieve a better understanding of seismic ruptures within crustal heterogeneity and fault curvature.

How to cite: Tung, S. and Masterlark, T.: Revisit the coseismic slip model of the 2015 Mw 8.3 Illapel, Chile earthquake with curvilinear fault rupture, finite element model, and InSAR observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8443, https://doi.org/10.5194/egusphere-egu24-8443, 2024.

X2.92
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EGU24-12727
Ignacia Calisto, Rodrigo Cifuentes, Javiera San Martín, Javiera Alvarez, Lisa Ely, Breanyn MacInnes, Jorge Quezada, and Daniel Stewart

Characterizing the spatial distribution of ruptures from historical and recent earthquakes is key to understanding the seismic cycle of large earthquakes in subduction zones, and thus to assessing the potentialrisks associated with future earthquakes. Central Chile (35°S - 38°S) has been continuously affected by large earthquakes, such as the 2010 Maule (Mw 8.8) and the 1835 earthquakes witnessed by Robert Fitzroy (HMS Beagle captain). Here, we identify the rupture pattern and tsunami propagation of the 1751, 1835, and 2010 mega-earthquakes, events that overlapped in central Chile, by  compiling historical records and applying robust statistical tools. We used an adaptation of a logic tree methodology to generate random sources of slip distribution for each event, constrained by tsunami and deformation data. We find that the three events studied have different slip peaks. The 1751 earthquake has the largest slip with a maximum patch of ∼ 26 m, while the 2010 and 1835 earthquakes reach slips of ∼ 16 m and ∼ 10 m, respectively. Our results show that a part of the segment between 36◦S and 37◦S was consistently affected by large earthquakes, but with different slip and depth. The northern part of the segment accumulated energy for at least 300 years and was released by the 2010 earthquake. This work provides important information for identifying rupture patterns between historical and recent earthquakes, and highlights the importance of extending the time scale of earthquake slip distribution analyses to multiple cycles to describe both earthquake characteristics and their spatial relationship, and thus gain a better understanding of seismic hazard.

How to cite: Calisto, I., Cifuentes, R., San Martín, J., Alvarez, J., Ely, L., MacInnes, B., Quezada, J., and Stewart, D.: Characterizing past earthquakes through historical observations and logic tree approximation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12727, https://doi.org/10.5194/egusphere-egu24-12727, 2024.

X2.93
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EGU24-13893
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ECS
Rogelio Torres and Sergio Ruiz

In recent years, several historical earthquakes have been studied in Chile to understand the seismotectonic context and anticipate the ground motion of these natural phenomena. One of the first great earthquakes documented by the Global Digital Seismographic Network (GDSN) occurred on March 3, 1985, off the coast of Valparaiso, with a moment magnitude (Mw) 8.0.

Several researchers have modeled the slip distribution at the seismic source, obtaining satisfactory results and fits, mainly at low frequencies and in far field. However, a discrepancy has been observed between the areas of maximum slip and the accelerations recorded in the near field.

In this study, the code proposed by Ruiz and Otarola (2016) was employed to stochastically generate synthetic accelerograms capable of accurately replicating the accelerations observed during near-field ground motion. This approach provides a realistic simulation of earthquake characteristics, source, path, and site.

The importance of generating synthetic accelerograms extends to critical sectors such as civil engineering, geophysics, construction, and urban planning. These simulations play a critical role in understanding ground behavior, predicting large seismic movements, and improving the development of earthquake-resistant structures. Furthermore, in the fields of construction and urban planning, synthetic accelerograms are essential for assessing the vulnerability of specific areas, diversifying applications in industry, and facilitating a more resilient design approach for future seismic events.

The results obtained by generating synthetic accelerograms can replicate the spectral and temporal shape, in agreement with the records provided by the National Seismological Center (CSN) network. Stochastic simulations have been run both in rocky environments and in areas with site effects.

How to cite: Torres, R. and Ruiz, S.: Stochastic Strong-Motion Simulation of Valparaiso 1985 Mw 8.0 Chile Earthquake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13893, https://doi.org/10.5194/egusphere-egu24-13893, 2024.

X2.94
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EGU24-9376
Ariane Maharaj, Steve Roecker, Diana Comte, Mauro Saez, Sol Trad, Gustavo Ortiz, and Martin Fernadez

The Andean Margin hosts alternating regions of “flat” and “normal” subduction, which includes the Pampean flat slab that extends from central Chile to Argentina. The discovery of an unusual travel time anomaly beneath the high Andes above the flat slab motivated a study to investigate the lithosphere in this region. Leveraging extensive archived seismic data from both Chile and Argentina, we performed a large-scale joint inversion of P and S body wave arrival times from earthquakes, and surface wave dispersion measurements from earthquakes and ambient noise. We created 3D Vp, Vs and Vp/Vs models using at least an order of magnitude more data than previous studies with about an 80% reduction in grid spacing. Our models corroborate results from previous studies: (1) a high velocity, high Vp/Vs region associated with a cool, slightly hydrated and depleted mantle above the flat slab, and (2) a low velocity structure beneath the high Andes interpreted as an overthickened crustal root, with our results showing that the root extends to just above the flat slab. Curiously, our models also reveal two low velocity zones within and below the flat slab seismic zone that have not been previously reported. Notably, the decrease in velocity is more pronounced in Vp than Vs. We postulate that the eastern low velocity anomaly is likely due to hot asthenosphere heating the slab, although no melting is occurring as the Vs is not significantly reduced. The western low velocity anomaly, which spatially correlates with the Juan Fernandez Ridge (JFR), we postulate is either due to the presence of supercritical fluids trapped within the JFR or an increase in silica content possibly linked to petit spot volcanism.

How to cite: Maharaj, A., Roecker, S., Comte, D., Saez, M., Trad, S., Ortiz, G., and Fernadez, M.: Joint Tomographic Inversion of the Pampean Flat Slab: Insights from Extensive Archived Seismic Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9376, https://doi.org/10.5194/egusphere-egu24-9376, 2024.

X2.95
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EGU24-7958
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ECS
Ignacio Castro-Melgar and Christian Sippl

In subduction zones, intermediate-depth earthquakes typically occur in two discrete layers, delineating an upper and a lower seismicity plane with a separating aseismic or minimally seismic region, a phenomenon named Double Seismic Zone (DSZ). However, the seismicity pattern in Northern Chile features two parallel planes of seismic activity only in the shallower section of the slab. Around depths of 80–90 km, the seismicity undergoes a transition to a significantly seismogenic zone approximately 25–30 km thick, effectively connecting the initial seismicity planes. This variation presents a distinct form of intraslab seismicity that deviates from the traditional DSZ structure and prompts further investigation into its underlying mechanisms and implications for regional seismic hazard assessment. Insights derived from this region's seismicity could provide pivotal constraints and enhance our understanding of the complex interplay between geological processes, mineral transformations, and fluid migrations in shaping subduction zone seismicity.

The attenuation of seismic waves in a rock volume is a property that is highly sensitive to the presence and concentration of fluids as well as spatial variations of temperature. As intermediate-depth seismicity is thought to originate from dehydration processes in the downgoing slab, along-strike or along-dip changes in slab seismicity should have a signature in seismic attenuation of the slab as well as the overlying mantle wedge. We hence aim at better understanding the aforementioned seismicity configuration in Northern Chile by acquiring a 3D image of its attenuation signature. 

The primary dataset for our analysis comes from the seismic stations of the Integrated Plate boundary Observatory Chile (IPOC) network in Northern Chile's forearc, augmented by additional data from different temporary deployments. Using the extensive seismicity catalog of Sippl et al. (2023), we have about 180,000 events at over 50 seismic stations at our disposal from the period 2007 to 2021; we select only the high quality traces for the analysis. The rays are traced in a 3D velocity model. We invert the spectral ratios obtained with the coda normalization method to obtain total-Q values. We present images of the 3D attenuation structure of the Northern Chile Forearc between 21ºS and 23ºS, which are obtained with measurements of the coda normalization method using the Multi-Resolution Attenuation Tomography algorithm (Sketsiou et al., 2021).

How to cite: Castro-Melgar, I. and Sippl, C.: Towards 3D attenuation tomography of the Northern Chile forearc, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7958, https://doi.org/10.5194/egusphere-egu24-7958, 2024.

X2.96
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EGU24-5730
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ECS
Nazia Hassan, Sally Henry, and Christian Sippl

Intermediate-depth seismicity in Northern Chile shows a pattern which is distinct from a conventional double seismic zone (DSZ) setting. While two distinct seismicity planes are present in the updip part of the slab, there is a sharp transition to a highly seismogenic cluster of 25–30 km thickness at 80-90 km depth, extinguishing the gap between the two seismicity planes. As seismic velocities can be used to constrain mineralogy and fluid content, characterizing seismic wavespeeds of this subduction zone segment using local earthquake tomography can provide important constraints on the mineralogical processes that produce the seismicity pattern seen here.

We used the catalog of Sippl et al. (2018), which contains arrival time data from permanent stations of IPOC (Integrated Plate Boundary Observatory Chile), complemented by several temporary deployments spanning shorter time sequences. The catalog contains more than 100,000 earthquakes and 1,200,404 P- and 688,904 S-phase picks for the years 2007 to 2014. In order to use the best available picks for tomography, we limit our analysis to events that have more than 14 P-arrivals as well as more than 7 S-arrivals, leading to a total of 8883 events with 213,908 P- and 99466 S-arrivals.

Parallelly, we also attempt to obtain an estimate of the possible mineral compositions at the depths and P-T conditions relevant to our study in the DSZ setting. For this, we assume a simple model where the upper plane of the DSZ is considered to be evolving from MORB-like composition and the lower plane of the DSZ from depleted-mantle composition. These global average compositions are then fed into Perple X (Connolly & Kerrick, 1987) as starting compositions and pseudosections of possible mineral assemblages are constructed for P-T conditions significant to this study. We calculate the theoretical Vp and Vp/Vs values for those P-T conditions using the same software.

We present 3D models of P- and S-wavespeeds from the Northern Chile forearc between about 20.4° S and 22.5° S, as well as images of ray coverage, relocated seismicity, and synthetic resolution tests. Tomography models for different choices of grid spacing and damping-smoothing parameters are compiled and compared to derive the optimal settings for the inversion. The seismic velocity distribution obtained through tomography is compared with the aforementioned theoretical wavespeeds to narrow down the range of possible reactions occurring at depth.

How to cite: Hassan, N., Henry, S., and Sippl, C.: Combining local earthquake tomography and petrological models to constrain wavespeeds in the subducting Nazca Plate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5730, https://doi.org/10.5194/egusphere-egu24-5730, 2024.

X2.97
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EGU24-8641
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ECS
Zixin Chen, Haijiang Zhang, and Lei Gao

We collected earthquake waveform data recorded by permanent seismic stations in northern Chile from 2014 to 2019 to construct a new earthquake catalog, and integrated them with the previous catalog data. In total, the new catalog consisted of 536342 P and 453920 S arrival times from 52165 earthquakes and 245 stations. We resolved Vp, Vs, and Vp/Vs models and seismic locations for northern Chile by using a new version of double-difference seismic tomography method based on Vp/Vs model consistency constraint (Guo et al., 2018). The new velocity models provide a refined structure of the subducting slab down to 350 km.

The earthquake relocations reveal a distinct double seismic zone in northern Chile, but the gap between the two seismic planes disappears at a depth of approximately 100 km and replaced by a concentration of seismic cluster. Under this intermediate-depth seismic cluster, several isolated small seismic clusters remain. The tomography results indicate a strong correlation between seismicity distribution and high-velocity anomalies. The subducting Nazca Plate presents stripe-like high-velocity anomalies with clear segmentations, potentially related to the weakening at the outer-rise of the trench. Furthermore, our Vp/Vs model indicates that the upper seismic plane exhibits high Vp/Vs anomalies, which may indicate the presence of fluids released from dehydration reactions of various hydrous minerals. In contrast, lower seismic plane and deep seismic clusters are associated with low Vp/Vs anomalies, which could be related to supercritical fluids. Additionally, the enhanced seismicity and velocity anomalies in the region of 21-22ºS along the strike suggest a potential influence of the subduction of the Iquique Ridge of the Nazca Plate.

How to cite: Chen, Z., Zhang, H., and Gao, L.: Joint inversion for Vp, Vs, and Vp/Vs of subduction zone in northern Chile, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8641, https://doi.org/10.5194/egusphere-egu24-8641, 2024.

X2.98
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EGU24-20700
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ECS
Nicolás Hernádez-Soto, Matthew Miller, Marcos Moreno, Dietrich Lange, Anne Socquet, Christian Sippl, and Diego González-Vidal

Between 2020 and 2022 the ANILLO+DEEPtrigger (Y6+XZ) Seismic Network, comprising 108 seismic stations, operated for eighteen months in Northern-Central Chile (24.5°S - 29°S). Employing Deep Learning (EQTransformer, Mousavi et al., (2022)) and Phase Association (GaMMA, Zhu et al. (2021)) algorithms, we identified over 30,000 seismic events in an area with a notable absence of moderate-to-large events in the past century, since the 1922 M8.5 Atacama earthquake.  
From the initial catalog, we selected a well-distributed subcatalog of 1000 earthquakes, consisting of 26,570 P- and 22,109 S-wave arrival times, by selecting for events with an optimal spatial distribution, small residuals, and abundant P- and S-arrivals. These selected events served as input for VELEST (Kissling et al., 1994) to compute a new 1-D velocity model representative of this region by minimizing the subset residuals. To reduce both residuals and location errors associated with the seismicity, we relocated the entire catalog using staggered tomographic inversions based on SIMUL2000 (Thurber & Eberhart-Phillips, 1999), simultaneously inverting for seismic velocity models and hypocentral parameters within the iterative damped least squares method. Following the proposed method, we gradually increased model complexity, transitioning from 2-D Vp and Vp/Vs to ultimately a 3-D fine Vp and Vp/Vs solution with low node separation.
Next, synthetic resolution tests were conducted to assess the reliability of the spatial limits and boundaries within the solutions. In this context, distinctive patterns were identified for each profile of the three-dimensional model, revealing enhanced horizontal and vertical resolution in the central region beneath the network. Conversely, a decline in resolution was noted at the peripheries, primarily attributable to reduced station coverage causing poorer seismic event relocations.
Our results reveal both regional and local patterns. We observed a mantle wedge with vertical thicknesses ranging from ~35km in the southernmost profiles less than 25 km in the northern region, consistent with previous seismic tomography observations in northern Chile (Pastén-Araya et al., 2021). The Vp/Vs ratio and Vp values allow us to discern the distribution of the hydrated slab, which, spatial correlated with seismicity, provides evidence of irregular dehydratation processes along both dip and strike directions.
Relocated seismicity exhibits some noteworthy features. Shallower crustal sesmicity is predominantly related to high rates of mining activity. In the subduction areas, the most prominent cluster is located at depths of 20-50 km, delineating the seismogenic zone. At greater depths, double and even triple seismic bands add structural complexities to the observations.
From 26.5°S to 29.5°S, between 20 km and ~75 km depth, seismicity predominantly aligns with the interplate contact defined by SLAB2 (Hayes et al., 2018). In contrast, northward from 26.5°S, our deepest seismicity, situated between 75 and 125 km depth, diverges from SLAB2, depicting a steeper dip angle.
Lastly, we recommend integrating OBS and back-arc stations, whose data would improve off-shore and back-arc resolution, contributing to a more comprehensive understanding of seismotectonic environments. Non-supervised Deep Learning results can provide exceptional databases for tomographic studies, yielding residuals similar to human-picked databases but within shorter timeframes.

How to cite: Hernádez-Soto, N., Miller, M., Moreno, M., Lange, D., Socquet, A., Sippl, C., and González-Vidal, D.: From regional to local structures imaged by seismic tomography at the Atacama seismic gap, Central-Northern Chile (24.5-29°S), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20700, https://doi.org/10.5194/egusphere-egu24-20700, 2024.