# Slip Modes Along a Structurally-Driven Earthquake Barrier in Chile

Diego Molina-Ormazabal, Mathilde Radiguet, Jannes Münchmeyer, Nicolas Hernandez-Soto, Adrien Vezinet, Lea Pousse-Beltran, Catalina Castro, Marie-pierre Doin, Juan Carlos Baez, Marcos Moreno, et al.

► To cite this version:

Diego Molina-Ormazabal, Mathilde Radiguet, Jannes Münchmeyer, Nicolas Hernandez-Soto, Adrien Vezinet, et al. Slip Modes Along a Structurally-Driven Earthquake Barrier in Chile. Journal of Geophysical Research: Solid Earth, 2025, 130 (6), 10.1029/2024JB030796. [https://doi.org/10.1029/2024JB030796](https://doi.org/10.1029/2024JB030796)

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# JGR Solid Earth

## RESEARCH ARTICLE

10.1029/2024JB030796

### Special Collection:

Slow to fast earthquakes and the geology, structure, and rheology of their host subduction zones

### Key Points:

* GNSS, InSAR and seismicity highlight how a subducting ridge, acting as seismic barrier, drives interactions between slow and fast slip

* A seismic sequence dynamically accelerated an ongoing slow slip event in a region where a subducted seamount is located

* The Copiapo ridge controls fault behavior by enhancing fluid circulation and providing clay-rich material, which reduce fault resistance

### Supporting Information:

Supporting Information may be found in the online version of this article.

### Correspondence to:

D. Molina-Ormazabal,
diego.molina@univ-grenoble-alpes.fr

### Citation:

Molina-Ormazabal, D., Radiguet, M., Münchmeyer, J., Hernandez-Soto, N., Vezinet, A., Pousse-Beltran, L., et al. (2025). Slip modes along a structurally-driven earthquake barrier in Chile. *Journal of Geophysical Research: Solid Earth*, 130, e2024JB030796. https://doi.org/10.1029/2024JB030796

Received 29 NOV 2024

Accepted 10 JUN 2025

### Author Contributions:

**Conceptualization:** Diego Molina-Ormazabal, Adrien Vezinet, Anne Socquet

**Investigation:** Mathilde Radiguet, Jannes Münchmeyer, Adrien Vezinet, Marie-Pierre Doin, Andres Tassara, Anne Socquet

**Methodology:** Diego Molina-Ormazabal, Mathilde Radiguet, Nicolas Hernandez-Soto, Lea Pousse-Beltran, Catalina Castro, Marie-Pierre Doin

© 2025. The Author(s).

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

<figure><img src="image_2.png" /></figure>

## Slip Modes Along a Structurally-Driven Earthquake Barrier in Chile

Diego Molina-Ormazabal¹ ID, Mathilde Radiguet¹ ID, Jannes Münchmeyer¹ ID, Nicolas Hernandez-Soto² ID, Adrien Vezinet¹ ID, Lea Pousse-Beltran¹, Catalina Castro³, Marie-Pierre Doin¹ ID, Juan Carlos Baez⁴, Marcos Moreno⁵ ID, Andres Tassara², Philippe Durand⁶ ID, and Anne Socquet¹ ID

¹CNRS, IRD, ISTerre, Université Grenoble Alpes, University of Savoie Mont Blanc, Université Gustave Eiffel, Grenoble, France, ²Departamento de Ciencias de la Tierra, Universidad de Concepción, Concepción, Chile, ³Departamento de Geofísica, Universidad de Concepción, Concepción, Chile, ⁴National Seismological Center, University of Chile, Santiago, Chile, ⁵Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile, ⁶Centre National d'Études Spatiales, Toulouse, France

**Abstract** Oceanic ridges often collocate with seismic barriers and episodic aseismic slip. However, how subducted seafloor topography drives interactions between slow and fast slip remains unclear. Here, using GNSS, InSAR and seismicity, we show interactions between a deep slow slip event (SSE) and a nearby shallow earthquake sequence that occurred in 2020 in northern Chile. These events overlap with the subducted Copiapo ridge, which has served as a barrier for historical earthquake ruptures. Gravity field data and seismic tomography reveal that the SSE nucleated in a region hosting a subducted seamount. Six months later, the seismic sequence dynamically triggered the acceleration and migration of the deep SSE, while afterslip and aftershocks propagated up to another subducted seamount at shallower depth. Our findings suggest that subducted seamounts influence fault hydromechanics, where high pore-pressure and rate strengthening material promote continuous slip release, reducing slip deficit. This process is modulated by SSEs and low magnitude seismic sequences.

**Plain Language Summary** Oceanic plates often have elevated seafloor structures, known as seamounts or ridges, subducting beneath continental plates. These structures influence fault behavior, potentially preventing earthquakes and hosting slow slip events (SSEs). However, the mechanisms by which these oceanic features control interactions between earthquakes and SSEs remain poorly understood. We focus in northern Chile to study a deep SSE and nearby earthquake sequence using geodetic and seismicity data. Both events occurred close to the Copiapo ridge, a feature with several seamounts and is historically known for stopping megathrust earthquakes. Our results reveals that a deep buried seamount was the starting point of the 2020 SSE. Six months later, a shallow earthquake sequence, close to the ridge, triggered aftershocks and afterslip motion that was halted by another shallower seamount. Simultaneously, the earthquake triggered the expansion of the ongoing deeper SSE. Our findings suggest that subducted topographic features affect fault behavior, by altering mechanical properties of the fault and increasing pore fluid pressure as evidenced by tomographic imaging. These conditions facilitate SSEs and prevent earthquake growing. Our work provides new insights on how elevated seafloor structures regulate earthquake activity, controlling interactions between fast and slow slip, thereby helping to better understand earthquake processes in subduction zones.

## 1. Introduction

The lateral extent of large megathrust earthquakes is limited by areas that act as barriers to rupture propagation (Molina-Ormazabal et al., 2023; Philibosian & Meltzner, 2020), which typically display low interseismic coupling (Bürgmann et al., 2005; Metois et al., 2016; Wang & Bilek, 2014), high rate of microseismicity (e.g., González et al., 2023) and are prone to hosting slow slip events (SSEs) (Vaca et al., 2018). The interactions between seismic ruptures and adjacent SSEs, especially where structural barriers inhibit large earthquakes, has been sparsely explored (Itoh et al., 2023; Meng & Duan, 2023), obscuring the underlying mechanism behind rupture arrest.

These seismic barriers are often collocated with prominent oceanic relief underthrusting the forearc, suggesting that barrier behavior might be structurally controlled and persistent through multiple seismic cycles (Bassett &
**Resources:** Catalina Castro, Juan

Carlos Baez, Philippe Durand

**Software:** Jannes Münchmeyer,
Nicolas Hernandez-Soto, Marie-
Pierre Doin, Philippe Durand

**Supervision:** Mathilde Radiguet,
Lea Pousse-Beltran, Anne Socquet

**Validation:** Mathilde Radiguet,
Jannes Münchmeyer, Nicolas Hernandez-
Soto, Lea Pousse-Beltran, Catalina Castro,
Marie-Pierre Doin, Juan Carlos Baez,
Marcos Moreno, Andres Tassara

**Visualization:** Diego Molina-Ormazabal,
Nicolas Hernandez-Soto, Adrien Vezinet

**Writing – original draft:** Diego Molina-
Ormazabal

**Writing – review & editing:**

Diego Molina-Ormazabal,
Mathilde Radiguet, Jannes Münchmeyer,
Adrien Vezinet, Lea Pousse-Beltran,
Marie-Pierre Doin, Marcos Moreno,
Andres Tassara, Anne Socquet

Watts, 2015a; Contreras-Reyes & Carrizo, 2011; Molina et al., 2021; Philibosian & Meltzner, 2020; Sparkes et al., 2010).

Overall, it has been proposed that seafloor roughness severely alters the mechanical regime of the plate interface by producing changes in normal stress (Scholz & Campos, 2012; Tobin & Saffer, 2009). In addition, subducting seafloor topography has been shown to provide fluids into the shallow megathrust, increasing fault pore pressure which in turn affects effective normal stress (Gase et al., 2023; Saffer & Tobin, 2011). However, how seafloor structures and fluid release jointly modulate fault behavior remains enigmatic and until now not well understood.

Several works have documented shallow SSEs in the vicinity of subducted seamounts (Bangs et al., 2023; Saffer & Wallace, 2015; Shaddox & Schwartz, 2019; Todd et al., 2018; Wallace et al., 2017). The underlying mechanisms by which these features drive interactions between earthquake arrest and recurrent SSEs remains elusive. This is partially due to the lack of observations documenting earthquake arrest driven by enhanced seafloor topography undergoing aseismic slip, with few cases reported in the literature (Rolandone et al., 2018). Further, the mapping of subducting structures is limited mostly to the shallow part of the megathrust offshore and many deep structures are not yet imaged, making it difficult to determine their control in slip interactions.

To elucidate precisely the role of oceanic structures in barrier behavior, we focus here on the Atacama segment in northern Chile, a well-recognized seismic gap where no large earthquake has been registered since 1922 (Figure 1a). In this region, an incoming bathymetric anomaly, the Copiapo Ridge, has been identified to act as a seismic barrier for earthquake propagation (Molina et al., 2021; Vigny et al., 2024). The ridge, originally formed by a hotspot, is characterized by a seamount chain visibly protruding ~2.5 km above the Nazca plate (Figure 1a), causing a complex fracture network over the oceanic slab (Maksymowicz et al., 2024).

In this study, we compute residual gravity anomalies (RGA) and tomography images to characterize the subducted structure of the ridge. By combining these approaches, we image the existence of buried seamounts embedded in a fluid-rich medium. Then, we explore the impact of the Copiapo ridge collision on slip behavior, by analyzing a large data set that includes GNSS and InSAR time series, along with a seismicity catalog. In particular, we focus in the description of the slip behavior of a deep, long-term SSE that occurred in 2020 (Figure 1). This event was interspersed by an earthquake sequence in the southern adjacent segment of the subducting ridge (Klein et al., 2021, 2023) (Figure 1a). This dual slip behavior, spatially coinciding with the subduction of seafloor relief offers a unique opportunity to investigate the links between slow and fast slip, and how oceanic structures influence slip. Our findings suggest that the ridge promotes slip segmentation through both subducted seamounts and fluid circulation. We explore the fluid sources along the ridge, potentially governed by metamorphic processes over water-altered rock composition. Our results help to clarify how fluids and enhanced oceanic relief control fault slip behavior.

## 1.1. Imaging the Along-Dip Depth Structure of the Copiapo Ridge

The ridge is characterized by a seamount chain subducting orthogonally beneath the South American plate (Figure 1a) and tectonic plate motion models predict convergence toward the area where episodic deep SSEs occur (Álvarez et al., 2015; Gimenez et al., 2019; Klein et al., 2018). Despite this, no shallow subducted seamount has been identified along the subducted ridge. To locate buried seamounts, we determine Residual Gravity Anomalies (RGA), a technique that has been used to image subducted plate structure, including seamounts and hidden ridges (Bassett and Watts, 2015a, 2015b). We compute RGA following a strategy (Methods) (Figure S1 in Supporting Information S1) (Molina et al., 2021) that considers subtracting from the Free Air anomaly the regional variations of the gravitational field due to long-wavelength along-margin features related to the first order structure of the subduction system (Figure 2a). This allows the recovery of short wavelength gravity signals reflecting the density variability of the forearc crust and slab due to the heterogeneous geologic structure of both plates (Figure 2a). We focus on the area around the Copiapo ridge to explore in detail its buried structure.

In this area, we identify two isolated neutral-positive gravity anomalies, labeled A1 and A2 in Figure 2a, that are aligned with the expected ridge path and are surrounded by negative RGA. Given the strong dependency of RGA on density heterogeneities, we interpret anomalies A1 and A2 as high-density seamounts likely embedded in a zone with decreased density. This lower density could be caused by major fracturing and fluid infiltration of the oceanic crust. Similar patterns of gravity anomalies have also been identified in previous works around the Copiapo Ridge (Álvarez et al., 2015; Bassett & Watts, 2015a). To gain further insight into structural variations
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Tectonic setting and geodetic data set used during the 2020 SSE. (a) Left: Longitudinal extent for historical and instrumental ruptures (Molina et al., 2021; Ruiz & Madariaga, 2018; Vigny et al., 2024). Right: Offshore bathymetry and onshore InSAR deformation associated with the 2020 SSE captured by Sentinel-1 descending track (D156). Blue and black arrows show the horizontal and vertical surface displacement respectively induced by the 2020 SSE obtained from permanent GNSS stations (Centro Sismológico Nacional stations). The filled circles are the GPS displacements projected in the Line of Sight to compare with the InSAR track. Gray squares depict GPS stations from Klein et al. (2023), that are only used for early SSE and afterslip characterization. The magenta thick lines contour the seismic sequence (solid) and 2006 seismic swarm (dashed) (Ojeda et al., 2023). Thin solid blue lines show contour for 2020 early afterslip (0.1–0.2–0.3 m) and 2006 postseismic phase respectively (0.3–0.4–0.5 m) (Ojeda et al., 2023). Gray star depicts the largest event of 2006 swarm. The red line contours the Slow Slip Event obtained from joint GNSS and InSAR inversion (this work). For reference, the black contour of the 2014 SSE is shown (Klein et al., 2018). Thin dashed lines are slab iso-depths every 20 km from Slab 2.0 (Hayes et al., 2018). (b) Normalized temporal evolution of the 2020 SSE recovered from GNSS and InSAR analysis over the main deformation area (Methods). The shaded color bands indicate the three-time windows analyzed: Before (green), during (blue), and after the 2020 seismic sequence (yellow).</figcaption></figure>

along the ridge, we derived a tomographic cross-section parallel to the ridge axis (EW at 27.5°S, coinciding with the ridge axis) of P-wave seismic velocity ($V_p$) and $V_p/V_s$ ratios (Methods) (Figures 2b and 2c). These parameters are useful to illuminate physical properties and conditions of the rock volume. Right above the megathrust patch located directly underneath gravity anomaly A2 (an area where our tomography has full resolution) we observe (Figure 2a) an anomalously low $V_p$ region ($dV_p/V_p < 10\%$) and significantly low $V_p/V_s$ ratio (ranging from 1.72 to 1.76) compared to regions immediately updip and downdip (Figure 2b).

Our seismic tomography is at the limit of resolution in the area near the plate interface underneath the gravity anomaly A1, but it suggests average values for $V_p$ and $V_p/V_s$. The region bounded by the two seamount anomalies
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Geophysical imaging of the Copiapo Ridge. (a) Residual Gravity Anomaly (RGA) with slip contours of Figure 1. Solid thick line represents the Slab-Moho intersection at depth (Van der Meijde et al., 2013). Thin dashed black lines correspond to the slab iso-depths every 20 km from Slab2.0. Green dashed line shows the inferred Copiapo Ridge path derived from RGA, dashed red circles the subducted seamounts A1 and A2 imaged by gravity. (b) Change in $V_p$ tomographic velocity anomaly with respect to initial 1D Model. (c) $V_p/V_s$ ratio. Circles depict the inferred Seamounts A1 and A2. Small black dots indicate the seismicity used to derive the tomography taken from González-Vidal et al. (2023). Dark triangles depict the coastline. The depth is relative to sea level. Solid lines correspond to the top of the slab computed from the seismicity distribution.</figcaption></figure>

is characterized by an elongated area of high $V_p/V_s$ ratio, peaking around 1.84. High $V_p/V_s$ ratios in subduction zones are inferred to represent elevated fluid content (Audet & Bürgmann, 2014; Halpaap et al., 2019; Kato et al., 2010). Based on this, the high $V_p/V_s$ values observed in our tomography suggest that the slab interface between 25 and 50 km depths is likely fluid-rich, while the areas near both subducted seamounts seem to be drier and therefore more intact. This region may be associated with hidden seamounts which we are unable to resolve from RGA, though a very low $V_p$ anomaly there might support their presence (Figure 2b).

At greater depth, between 60 and 100 km, we observe elevated $V_p/V_s$ ratio over a broad zone, implying that this area also might present fluids. These interpretations are consistent with the low RGA signature downdip the subducting A1 and A2 seamounts, indicating a medium with decreased density. Recently, the deeper structure A2 has been proposed based on $V_p$ perturbations (Pastén-Araya et al., 2022). However, the shallow seamount A1 had not been identified in previous studies, although it has been suggested that shallow seamounts might explain the different slip behavior observed there (Ojeda et al., 2023). Our findings support that A1 and A2 correspond to underthrusted seamounts passing beneath the forearc and are likely featuring different mechanical properties given the contrasting RGA signal and seismic wave velocities. In the following sections we link our detected seamounts with slow and fast slip modes along the Copiapo ridge, to later assess their structural control in megathrust behavior.

## 1.2. Characterizing Slip Modes During the 2020 Seismic/Aseismic Sequence

To explore the role of oceanic-plate features on fault behavior, we analyze the spatio-temporal relationship between the 2020 SSE and the associated megathrust earthquake sequence using geodetic and seismic observations (Figure 1a) (Methods). The area is affected by several tectonic and non-tectonic processes such as surface loading and seasonal modulation, which are monitored by permanent GNSS stations (Socquet et al., 2023) (Figure 1a). To isolate the different slip modes occurring during the SSE from these unrelated phenomena, we employ the variational Bayesian Independent Component Analysis (vbICA), a technique widely used to separate distinct geophysical signals on GNSS time series (Gualandi et al., 2016; Michel et al., 2019). Our analysis allows us to describe the slip evolution during three different time periods: before, during and after the seismic sequence (Methods) (Figure 1c).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Slip characterization along the plate interface. Upper panel: (Left) Cumulative aseismic slip associated with the 2020 SSE (01/03/2020–01/09/2021). Thin cyan line represents the 2014 SSE slip solution, with a contour of 0.3 m (Klein et al., 2018). Inner and outer circles depict the observed and predicted vertical displacement respectively. (Middle) Observed InSAR deformation field jointly inverted with GPS data (Methods). (Right) Predicted InSAR from slip model. The GPS observation and predictions are shown in LOS projection for comparison in middle and right panels. Lower panel: (Left) Cumulative coseismic slip distribution for seismic sequence (01/09/2020) inverted from InSAR and GPS displacement. (Middle) InSAR observation jointly inverted with GPS data. (Right) The GPS observation and predictions are shown in LOS projection for comparison in middle and right panels.</figcaption></figure>

Additionally, we compute an InSAR time series using Sentinel-1 data with the NSBAS chain (Doin et al., 2011), and postprocess it by applying a parametric decomposition that allows us to separate the displacement signal related to the 2020 SSE and seismic sequence (Methods) (Daout et al., 2023; Maubant et al., 2022). We then invert the displacement field from InSAR and static GNSS data to characterize the slip amplitude along the plate interface, assuming an elastic half-space and 3D fault geometry (Methods). In parallel, we create a dense earthquake catalog with high location accuracy using deep learning detection methods (Michelini et al., 2021; Münchmeyer et al., 2022; Woollam et al., 2022) (Methods), which enables to describe seismicity evolution during the 2020 SSE and seismic sequence. Combining GNSS, InSAR and seismicity data, we characterize the fault behavior during the 2020 SSE and nearby seismic sequence.

Our analysis of the 2020 SSE and seismic sequence shows that the surface deformation is dominated by a long-lived transient that started in early March 2020 and lasted roughly 1.5 years (Figures 1 and 3), resulting in cumulative surface displacement close to 4 cm at the nearest stations and 5 cm in the line-of-sight (LOS) (Figures 1 and 3). The associated cumulative aseismic slip inverted jointly from InSAR and static GNSS is characterized by two main slip patches, both with maximum slip around 30 cm (Figure 3). The slip pattern is very similar to the 2014 SSE (Klein et al., 2018) slip model. We decomposed the 2020 SSE into 3 stages (Figure 3): before, during and after the seismic sequence.

Our inversion indicates that the 2020 SSE initiated at depths around 40–50 km, in the area where the deeper seamount A2 is located (Figures 4a and 4c). Six months later, on September 1st, a seismic sequence lasting one
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Slip rate and seismicity evolution during the 2020 SSE. (a) Detrended east component GNSS time series during the period covered by the slow slip event. Shaded color bands highlight the first (in green) and second part (in yellow) of the SSE as well as the early postseismic phase following the 2020 seismic sequence (in blue). (b) Latitude-time evolution of the seismicity. (c–e) Slip rates of the different slow slip patterns recovered for each time window (green SSE initiation-blue early postseismic phase-yellow SSE continuation). Red and yellow arrows correspond to observed and predicted GPS displacements. Dots depict the seismicity during each time period color-coded by the vertical distance to the interface computed from seismicity distribution. Black dashed lines depict slab isodepths with intervals of 20 km (Slab2.0). The magenta bold lines show coseismic slip contours every 0.1 m. The thin magenta lines on panel (e) correspond to afterslip contour every 10 mm, starting from 30 mm.</figcaption></figure>

day, broke a shallower portion of the megathrust (Figure 3). This sequence consisted in three major earthquakes up to Mw 7 with cumulative coseismic slip of 0.3 m and geodetic moment magnitude of Mw 7.1. It occurred approximately 100 km southwest of the SSE, roughly beneath the coastline at 35 km depth (Figure 3). The seismic rupture is limited in the north by the Copiapo ridge (Figure 3c and Figure S12 in Supporting Information S1) and it broke near the northern edge of the 1922 rupture (Figure 1a).

During the 3 weeks immediately following the seismic sequence, the GNSS stations recorded increased seaward motion (Figure 4a). By inverting this displacement field, we found that it corresponds to early afterslip occurring north and updip of the seismic rupture (Figure 4d). Intriguingly, the deeper region of the ongoing 2020 SSE around the seamount A2 also exhibits a pulse of aseismic slip following the earthquake, with a higher slip rate compared to the preceding 6 months (Figure 4d). After the postseismic phase, the GNSS time series continues to show trenchward motion until at least September 2021, although at a slower slip rate (Figure 4a). Our inversion results indicate that this signal corresponds to the northward propagation of the 2020 SSE (Figure 4e).

Tracking seismicity during these three phases highlights the spatial and temporal evolution of earthquakes both along the interface and within the slab (Figures 4b and 5). During the first 6 months of the SSE, increased seismicity stands out just downdip the SSE area, at 60 km depth, predominantly intraslab earthquakes (Figures 4c and 5b). The epicentral area of the September 1st seismic sequence seems to be characterized by persistent small
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Cumulative aseismic slip and seismicity during the 2020 SSE. (a) Cumulative slow slip of the 2020 SSE considering both InSAR and GPS data. Note that the coseismic slip does not contribute to the total aseismic slip (Cyan contour of the 2014 SSE solution is shown for reference (Klein et al., 2018). Magenta thick line contours display the 2020 seismic sequence. Thin solid and dashed magenta lines show contour for 2020 afterslip (30–40–50 mm) and 2006 postseismic phase respectively (10–20–30 mm) (Ojeda et al., 2023). Green dashed lines represent the expected ridge path in depth. (b) Depth cross section along the expected ridge path, showing earthquakes color-coded by time spanning 2020–2022. Gray dots depict seismicity spanning 2014–2020. Magenta and blue ellipses depict the after-slip and SSE area. The gray dashed line represents the Moho discontinuity (Reguzzoni et al., 2013). The gray solid line corresponds to the top of the slab computed from the seismicity distribution. (c) Number of earthquakes along longitude (From b) compared with averaged aseismic slip during 2020–2022 (shallow after slip and deeper SSE). The area from where seismicity has been extracted correspond to profile along the Copiapó ridge (27.45°–27.55°S). A1 and A2 indicate the imaged seamounts.</figcaption></figure>

interplate earthquakes activity prior to the rupture (Figures 4b and 4c). Right after the September 1st seismic sequence, most aftershocks are observed up dip of the epicentral area along the interface (Figures 4b and 4d), in the southern edge of the afterslip patch (Figure 4d).

Zooming in on the aftershock sequence during the first 3 weeks (Figure 6), we observe a group of aftershocks propagating northward with a velocity of 9 km/day until they reach S27.2 latitude, coinciding with the Copiapó Ridge (Figures 4b, 4d, and 6). These aftershocks are mostly located at the updip edge of the afterslip patch (Figure 4d). We interpret this as a coeval migration of aftershock and afterslip, that stops on the Copiapó Ridge, where the shallow seamount A1 has been identified at a depth of ~15 km. A few interplate aftershocks keep

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Aftershock sequence. (a) Normal stress changed induced by the earthquake sequence. Filled circles show the aftershock distribution colored by time. Magenta thick line contours display the 2020 seismic sequence every 0.1 m. Green dashed lines represent the expected ridge path in depth. The rectangle highlights the selected area shown in panel (b). Most of the aftershocks concentrate in a region where normal stress change is negative. (b) Circles display the aftershock distribution as a function of latitude and time. Circles are colored by depth. The bright green shade region represents the Copiapó ridge location. Two pulses of aftershock migration are revealed with different slip rate propagation.</figcaption></figure>
propagating slightly further north, outside the northern seamount A1 flank, but at much lower rate (1.2 km/day) (Figure 6) and where previous swarms have occurred in the last decades (Figure 4e) (Comte et al., 2002; Holtkamp et al., 2011). Finally, after the postseismic phase, the SSE experiences northward propagation, in a region where almost no earthquakes are reported (Figure 4e).

## 1.3. Structural Control on Slip Mode Segmentation

The 2020 seismic sequence and other past megathrust events appear to have been arrested or size-limited by the Copiapo ridge, which was identified as a frequent barrier (Figure 1a) (Vigny et al., 2024). The imaged seamount A1 is found within the shallower section of the ridge (Figure 5a) and is probably responsible for earthquake segmentation. It is well established that underthrusted seamounts reduce the mechanical coupling along the plate interface by lowering normal stress (Duarte et al., 2015; Scholz & Campos, 2012; Wang & Bilek, 2011, 2014) which in turn controls fault strength. Accordingly, a notable feature of the Copiapo Ridge is its low interseismic coupling, particularly around the seamount A1 location (González-Vidal et al., 2023; Yáñez-Cuadra et al., 2022). It is therefore reasonable to conclude that the Copiapo ridge subduction induces lateral changes in normal stress, and therefore fault strength, promoting arrest of seismic ruptures (Molina-Ormazabal et al., 2023) and leading to the observed earthquake segmentation.

Our observations also reveal the complex variability in the slip mode that the ridge can host along the dip direction. The coeval northward migration of aftershock and afterslip led by the sequence stops on the Copiapo Ridge, where the shallow seamount A1 has been identified at depth of ~15 km (Figures 4d and 5). In contrast, the deeper seamount A2, imaged at ~50 km depths, overlaps with the region hosting the 2020 SSE (Figures 4c–4e and 5). The two regions hosting A1 and A2 also exhibit a reduced number of earthquakes (Figure 5), suggesting that these portions of the megathrust might be dominated by a more stable slip regime (aseismic slip under slow tectonic loading).

Additionally, the area bounded by the two seamounts hosted afterslip induced by the 2006 seismic swarm at ~25 km depth (Figures 5a and 5b) (Ojeda et al., 2023) but also exhibits persistent seismicity, as demonstrated by the numerous interplate earthquakes between 25 and 45 km depth (Figure 5). This intermittency between aseismic slip and enhanced seismicity downdip of A1 might imply a fault portion with a conditionally stable regime, that is, slip behavior may vary depending on local stress conditions and loading rates (Scholz, 1998). Such variable slip distributions linked to oceanic structures suggest that fault behavior is likely controlled by structural heterogeneities.

Numerical modeling of subducting topography indicates that the wake of seamounts corresponds to a stress shadow characterized by enhanced damage and a relative extension, which significantly decreases normal stress ($\sigma_n$), but the opposite occurs in their leading edges, where increased $\sigma_n$ is observed (Sun et al., 2020). Hence, the spatial variability in stress regime induced by the seamounts could be modulating the different slip regimes observed in the area. To investigate the physical properties influencing fault behavior in the region, we study the time-space evolution of the different slip patches and the concomitant seismicity along and around the Copiapo ridge.

## 1.4. Seismic and Aseismic Interactions Around a Barrier

Our observations show that the 2020 earthquake sequence occurs in a shallow portion of the interface, about ~150 km away from the deep Slow Slip zone (Figure 5a). Previous studies show evidence that some SSEs can lead to subsequent large earthquakes (Dascher-Cousineau & Bürgmann, 2024; Obara & Kato, 2016; Radiguet et al., 2016; Socquet et al., 2017; Voss et al., 2018). One proposed explanation is that proximal SSEs increase Coulomb Stress Change (CSC) in the earthquake epicenter area (Radiguet et al., 2016). In our case, we estimate that CSC induced by the SSE in the hypocentral area is ~2 kPa (Figure S2a in Supporting Information S1), much lower than the 40 kPa documented in cases where the SSE and the earthquake are closer to each other (Radiguet et al., 2016). Despite the low CSC in our case, small stress variations have been shown to influence seismic activity (Bucholc & Steacy, 2016; Stroup et al., 2007). In our case, a CSC as low as 2 kPa could have perturbed the fault enough to trigger the 2020 earthquake, especially in a weak region that is critically stressed, as suggested by the persistent seismic activity there (Figures 4b and 4c). This has important implications for seismic potential in the area: as the region is prone to being perturbed by nearby aseismic slip patches, the likelihood of a large megathrust earthquake (similar to the 1922 event) in the shallow, highly coupled patches adjacent to the Copiapo
ridge (Yáñez-Cuadra et al., 2022) could be increased. Indeed, recurrent SSEs could increment the stress build up in shallower asperities, bringing the segments closer to failure and potentially triggering a runaway rupture (Meng & Duan, 2023; Voss et al., 2018).

Following the September 1st earthquake, the afterslip propagation from the south also perturbed the shallower section of the Copiapo ridge (Figures 4b, 4d, and 6). Initially, the aftershock distribution during the onset of the 2020 postseismic phase overlap with a region of negative normal stress change $\Delta\sigma_n$ caused by the coseismic rupture, as shown by our CSC analysis (Figure 6a). Negative $\Delta\sigma_n$ can induce fault unclamping, bringing the segment closer to failure (Perfettini et al., 1999). This mechanism helps to explain the concentration of after-shocks in an area of negative $\sigma_n$ (Figure 6a).

As the postseismic phase progressed, the afterslip pulse and many aftershocks propagated northward, until they were arrested by the seamount A1 (Figures 4d and 6b). The linear aftershock migration, with a velocity of 9 km/day (Figure 6b), suggests that this migration is not driven by a diffusive process like fluids but rather a linear process like afterslip. The slower migration in the northern flank of the seamount A1 (Figure 6b) implies that another mechanism, such as fluid propagation, might be involved (Ross et al., 2017). This area has experienced several swarms, which are typically associated to the presence of fluids (Nishikawa & Ide, 2017). Two swarms have been well documented: one in 2006, presumably accompanied with a shallow SSE (Ojeda et al., 2023), and another occurred in 2023. This swarm clearly coincides with an SSE expanding bilaterally from the A1 location and has been linked to fluid presence (Münchmeyer et al., 2025). These observations suggest that both fluid diffusion and slow slip might be also involved in the 2020 post-seismic episode, but it is difficult to uniquely identify the mechanisms underlying the apparent propagation of seismicity and slip.

At greater depths, the potential influence of the earthquake on the slow slip is not straightforward. Previous studies of the 2020 SSE did not investigate its potential continuation after the seismic sequence (Klein et al., 2023). Our slip models indicate that not only the slip continued around the deeper seamount (A2) after the earthquake, but also the slip rate increases in the deep SSE zone from ~0.3 to ~0.5 m/year (Figure 4d). This is followed by a northward migration of the SSE immediately after the 2020 earthquake (Figures 4a and 4d). Our findings show that static CSC induced by the earthquake in the deep SSE area is around 0.1 kPa (Figure S2 in Supporting Information S1), suggesting that the static stress perturbation alone is unlikely to have triggered the continuation of the SSE.

In contrast, in several other regions, it was shown that distant earthquakes have triggered or hastened episodic SSEs (Cruz-Atienza et al., 2021; Hirose et al., 2012; Wallace et al., 2017) or changes in coupling (Materna et al., 2019) through dynamic stress change caused by the passing of seismic waves (Itaba & Ando, 2011; Wei et al., 2018; Zigone et al., 2012). For instance, in Mexico, a shallow megathrust earthquake of 7.2 Mw dynamically triggered a deeper SSE in an area ~150 km away at ~50 km depth (Cruz-Atienza et al., 2021), a tectonic configuration similar to our case. Therefore, the passage of seismic waves radiated from the 2020 seismic sequence might have dynamically triggered the slip acceleration, particularly in the region where the deeper seamount is found. Importantly, regions that are critically stressed, like those with elevated pore pressure, are more sensitive to dynamic wave (Peng & Gomberg, 2010). This is consistent with the observations that deep seamounts enhance fluid mobilization (Singh et al., 2011). Therefore, the presence of seamount A2 potentially provide physical conditions to facilitate the dynamic triggering of slip.

In summary, our analysis suggests that the deep SSE might have played a role in triggering the shallow earthquake sequence which was arrested in the north by the ridge. The seismic sequence nucleation perturbs the fault portions hosting the shallow and deep seamounts, promoting aseismic slip in the form of afterslip propagation and SSE acceleration in deeper zones. Such observations highlight the complex interactions between slow and fast slip occurred during 2020–2022 around a subducted ridge, suggesting that both subducted seamounts and the presence of fluids persistently govern the fault behavior (Münchmeyer et al., 2025).

## 1.5. Hydromechanics of the Copiapo Ridge

The subduction of aseismic ridges and seamounts has been linked with low interseismic coupling, SSEs and creep (Lovery et al., 2024; Metois et al., 2016; Wang & Bilek, 2014), yet the underlying mechanisms of how subducting topography alter the kinematic and mechanical state of the plate interface remains debated. The seismic tomography and gravity anomalies provide valuable constraints regarding the fault properties along the subducting
<figure><img src="image_9.png" /></figure>

**Figure 7.** Schematic representation of structural and petrological control on fault behavior (Not to scale). Underthrusted seamounts, enriched in volcaniclastic material, suffer significant rock alteration respect to nearby seafloor areas, forming hydrous minerals (Clays) (Hacker et al., 2003; Staudigel et al., 2010). Subducted clays, supplied by seamounts, are shown to be velocity strengthening (Volpe et al., 2024). Further, mechanical loading of the ridge facilitates slab hydration. After subduction, the ridge acts as a major source of fluids caused by mineral dehydration, changing the hydromechanical properties of the fault. The seamounts and ridge structure, with areas highly hydrated (no identified seamounts) and others drier (seamounts), promote fluid mobilization and provide clay-rich material in shallower regions. All these physical conditions highlight how subducting topography drive slow and fast slip interactions while also promoting earthquake arrest. The P-T diagram modified from Behr and Bürgmann (2021), provides a rough view of metamorphic transitions, potentially associated to slab dehydration at different depths. The P-T shade dashed cyan field is based on thermally predicted conditions for MORB in Central-North Chile (Figure 8 in Ji et al. (2019)). A thermal-mechanical model would be needed to determine more accurately at what depth altered-rock-composition of seamounts contribute to fluid release.

Copiapó ridge. Notably, the high $V_p/V_s$ anomaly and negative RGA along the slab, between 25 and 45 km depth, suggests a fault interface rich in fluids, while the low $V_p$ and negative RGA on the seamounts inferred location indicates locally drier and possibly highly fractured zones (Bangs et al., 2023; Ellis et al., 2015) (Figure 2).

Typically, SSEs near seamounts have been explained by fluids liberated from water-enriched subducted sediments facilitated by these oceanic structures (Bassett & Watts, 2015a; Saffer & Wallace, 2015; Takemura et al., 2023). However, the low amount of sediments at the trench in north Chile due to its arid climate and the thin pelagic sediment layer above the unsubducted ridge (Báez et al., 2023; Contreras-Reyes & Carrizo, 2011; Maksymowicz et al., 2024) suggest that fluids supplied by wet sediments at the plate interface is an unlikely major source of fluids in this region. Interestingly, the load of the Copiapó ridge above the oceanic crust induces intense faulting and fracturing which results in enhanced water percolation into the slab (Maksymowicz et al., 2024), facilitating its hydration (Boneh et al., 2019) (Figure 7). Further, drilling and seismic experiments have found that seamounts have a high content of hydrous minerals such as clays which are produced through water-alteration of volcanoclastic material (Gase et al., 2024; Staudigel et al., 2010) (Figure 7). These two factors imply that the Copiapó ridge might be a significant water reservoir compared to adjacent seafloor areas (Gase et al., 2023). We proposed that this enhanced water content in the ridge is a primary source of fluids. Once the ridge begins to subduct, the water is progressively released from the breakdown of the hydrous minerals due to metamorphic reactions (Hacker et al., 2003). The high $V_p/V_s$ anomaly observed between 25 and 45 km (Figure 2c) might reflect dehydration processes along this domain (Figure 2c), supported by the negative RGA patches, which might indicate fluid-induced low-density zones (Figure 2a).

The presence of fluids at the plate interface influences fault behavior by increasing pore-pressure and reducing normal stress. At the same time, the downdip front of subducted seamounts might promote normal stress
concentration (Sun et al., 2020). The interplay between these two mechanisms could lead to spatio-temporal fluctuations in normal stress which could modulate frictional stability, resulting in conditionally stable fault properties that depends strongly in normal stress (Scholz, 1998). This might help to explain the aseismic-seismic behavior documented in regions downdip the seamount A1, where afterslip and enhanced interplate seismicity are observed (Figure 5).

At shallower regions, where the seamount A1 has been detected, the intermediate low $V_p$ and positive RGA might reflect the presence of a relative dry zone. This may suggest that fluids might still be confined within the structure of hydrous minerals and therefore another factor might be modulating fault behavior at these depths. In particular, the potential high content of clays might play a key role (Figure 7). At such depths, clays have been shown to exhibit velocity strengthening behavior (Saffer & Marone, 2003), that is aseismic slip under slow tectonic loading. Recent studies also have suggested that such material can sustain instabilities in the form of slow slip events triggered by pore-fluid pressure perturbations (Perez-Silva et al., 2023; Volpe et al., 2024). Further, velocity strengthening material promotes earthquake arrest and low interseismic coupling (Kaneko et al., 2010). Considering this, we hypothesize that the likely high content of clay strengthening material, combined with enhanced damage and fluid circulation favored by the subducted Copiapo ridge contribute to aseismic fault slip (Chesley et al., 2021; Nishikawa & Ide, 2017; Wang & Bilek, 2011). This explains well the diverse slip patterns observed around seamount A1, including afterslip, SSEs and seismic swarms, as well as the low interseismic coupling and long term barrier behavior featured by the Copiapo ridge (Figure 7).

At greater depths (>45 km), a high $V_p/V_s$ ratio, including in intraslab areas, suggest that the medium is enriched in free fluids (Figure 2c). At ~50 km depths, gravity data indicates that the Moho-slab intersection defined also as the petrological boundary separating a dry lower crust from a deeper partially serpentinized mantle wedge, takes place (Bostock et al., 2002). Notably, deeper oceanic slab dehydration has been proposed to promote fluid release which contribute, at least partially, to the serpentinization of the mantle wedge (French et al., 2019; Schwartz et al., 2013; Wang et al., 2020). In the case of the Copiapo ridge, we attribute the high $V_p/V_s$ anomaly between 75 and 100 km depth, to fluids originated by mineral dehydration (Figure 2c). This is supported by enhanced intraslab seismicity at similar depths (Figure 5b), which is often linked to metamorphic reactions releasing fluids (Hacker et al., 2003; Halpaap et al., 2019; Pastén-Araya et al., 2022; Preston et al., 2003). The large number of intra-slab earthquakes is more pronounced where faulting structures and slab bending facilitates oceanic crust hydration prior to subduction (Boneh et al., 2019), as in the case of the Copiapo ridge. Part of these fluids are expected to be channeled along the plate interface toward the mantle wedge corner, increasing pore fluid pressure there (Behr & Bürgmann, 2021; Kawano et al., 2011; Mizukami et al., 2014; Ramachandran & Hyndman, 2012). The resultant low normal stress and antigorite-rich serpentinite has been shown to cause brittle-ductile viscous flows, which may explain the occurrence of many deep SSEs in such deep regions (Audet & Bürgmann, 2014; Goswami & Barbot, 2018).

Indeed, the 2020 SSE beneath the mantle wedge corner, suggests that such rheological conditions might explain its nucleation. Unfortunately, we cannot assess whether wedge serpentinization dominates along deep sections of the megathrust, preventing us to conclude that this is the most relevant factor behind the recurrent SSEs reported in the area over the last years. However, what is clear from our results is that the position of a subducted seamount coincides with the aseismic zone (Figures 2 and 7), being one of the few examples where a deep seamount (>40 km) is linked to episodic SSEs. The low $V_p$ anomaly and $V_p/V_s$ ratio here suggest that seamount A2 is characterized by enhanced damage and dry material (Ellis et al., 2015), creating an anomalous high-density zone evidenced by the positive RGA (Figure 2). These features likely promote fluctuations in permeability (Fisher & Wheat, 2010; Singh et al., 2011), facilitating fluid mobilization from slab dehydration toward the mantle wedge base and thereby promoting heterogenous fluid distribution (Figure 7). Further, seamount A2 could act as a weak asperity with enhanced loading rate, a process that also contributes to spontaneous SSE nucleation (Schmidt & Gao, 2010). As a result, the combined ubiquitous metamorphic fluid environment together with the mechanical effect led by seamount A2 passage, might create a suitable environment to host the 2020 long-term SSE (Figure 7), providing an alternative perspective in deep SSE mechanics in this region.

Taken together, our results suggest that the slow slip behavior and low interseismic coupling observed along subduction zones might not be only restricted to subducting oceanic relief facilitating wet sediment supply such as in New Zealand or Nankai (Saffer & Wallace, 2015; Takemura et al., 2023). Remarkably, subducting topography itself could also supply friction-strengthening clay minerals, to reduce fault resistance which also
facilitates SSEs and microseismicity (Figure 7). Further, the ridge structure acts as a major source of fluid after subduction. These combined mechanical effects of fluid circulation and subducted seamounts play a primary role in promoting different slip modes by altering hydro-mechanical properties of the fault. Such conditions likely make the fault sensitive to small stress perturbations, driving aseismic-seismic interactions as revealed by our slip analysis. To evaluate more precisely the connection between rock composition and fluid release with slip mode, future studies incorporating a thermal model constraining more accurately the depths at which dewatering and mineralogical reactions occur would be needed, especially for altered-rock composition in seamounts.

## 2. Conclusions

We have demonstrated the significant role of subducted oceanic-plate structures in controlling slow slip patterns and earthquake arrest. Our findings reveal that the 2020 SSE nucleated in a region near to a subducted seamount and was later perturbed by a shallower seismic sequence, triggering its acceleration and northward migration. At a shallower depth, the postseismic afterslip following the seismic sequence propagated toward another subducted seamount, where ultimately is stopped. These observations indicate that both the deep and shallow seamounts play a key role in localizing aseismic slip.

The combined analysis of RGA and seismic tomography suggest that the plate interface along the subducted ridge is characterized by a heterogenous fluid distribution, promoting along dip variations in fault frictional properties. These finding reinforce the idea that bathymetric anomalies induce hydromechanical changes that modulates slip segmentation, acting as a key factor in megathrust behavior.

We provide evidence that such similar mechanisms might operate in other subduction zones where SSE and seismicity patterns spatially correlate with subducted plate tectonic features. Finally, we highlight the interplay between rock composition and fluid presence governing fault mechanics, underscoring the complex interactions that control slip behavior in hydrated subduction environments.

## Data Availability Statement

Daily solution of GNSS time series for South America (Socquet et al., 2023) were used to capture the different slip processes analyzed in this manuscript. The seismic catalog shown in the manuscript is attributed to Münchmeyer et al., 2025. Gravity anomaly grids, the co-seismic interferogram computed with GDM-SAR, and the Sentinel D156 InSAR time series from 2016 to 2022 on the area of interest are provided in the Zenodo repository https://doi.org/10.5281/zenodo.15130537.

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## Acknowledgments

This work was supported by funding from European Research Council (ERC). A. Socquet received funding from the ERC CoG 865963 DEEP-trigger. D. Molina-Ormazabal salary was covered by a grant from CNES. J. Münchmeyer salary was covered by the European Union Grant n° 101104996 ("DECODE"). Adrien Vezinet is supported by a grant from the European Research Council (ERC) under the European Union's Horizon H2020 research and innovation program (Synergy Grant MEET, Grant agreement no. 856555). Juan-Carlos Baez acknowledges support from ANID FONDECYT N°1240501 project. M. Moreno acknowledges support from FONDECYT 1221507 ANID project. We are thankful to Emily Klein and Centro Sismológico Nacional for providing GNSS data used in this work. We thank the engineers who worked on the operational implementation of the GipsyX GNSS processing at ISTerre for providing the GNSS time series. We thank the FLATSIM Team (Marie-Pierre Doin and Cécile Lasserre, Project Leaders, Catherine Proy, Project Manager, Michel Diament and P. Durand, Supervisors, Franck Thollard, Christophe Laurent, Erwan Pathier, Raphaël Grandin, Elisabeth Pointal, Emilie Deschamps-Ostanciaux, Dominique Clesse, project members) for building the FLATSIM service. InSAR data were pre-processed using the online service GDM-SAR supported by Formater (poletterresolide.fr), ISDeform (https://cnes.fr/fr). Finally, we thank to the reviewers Roland Bürgmann and Matt Wei for their valuable comments and feedback which contribute to the improving of this manuscript.
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