# Seismic Swarms Unveil the Mechanisms Driving Shallow Slow Slip Dynamics in the Copiapó Ridge, Northern Chile

Jannes Münchmeyer, Diego Molina-ormazabal, Mathilde Radiguet, David Marsan, Juan-carlos Baez, Francisco Ortega-culaciati, Andres Tassara, Marcos Moreno, Anne Socquet

► To cite this version:

Jannes Münchmeyer, Diego Molina-ormazabal, Mathilde Radiguet, David Marsan, Juan-carlos Baez, et al.. Seismic Swarms Unveil the Mechanisms Driving Shallow Slow Slip Dynamics in the Copiapó Ridge, Northern Chile. Geophysical Research Letters, 2025, 52 (8), pp.e2024GL113953. 10.1029/2024GL113953. hal-05350506

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Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2024GL113953

**Special Collection:**

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

Jannes Münchmeyer and Diego Molina-Ormazabal contributed equally to this work.

**Key Points:**

* We observe a month-long slow slip event $M_w = 6.6$ in Northern Chile, accompanied by dense, migrating seismic swarm activity

* The slow slip initiation is likely controlled by migrating fluid overpressure on the segmented interface around a subducted seamount

* Historic earthquake swarm activity provides evidence for recurrent slow slip with variable magnitude controlled by the interface structure

**Supporting Information:**

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

**Correspondence to:**

J. Münchmeyer,
munchmej@univ-grenoble-alpes.fr

**Citation:**

Münchmeyer, J., Molina-Ormazabal, D., Radiguet, M., Marsan, D., Baez, J.-C., Ortega-Culaciati, F., et al. (2025). Seismic swarms unveil the mechanisms driving shallow slow slip dynamics in the Copiapó ridge, Northern Chile. *Geophysical Research Letters*, *52*, e2024GL113953.
https://doi.org/10.1029/2024GL113953

Received 27 NOV 2024

Accepted 18 MAR 2025

© 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.

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## Seismic Swarms Unveil the Mechanisms Driving Shallow Slow Slip Dynamics in the Copiapó Ridge, Northern Chile

Jannes Münchmeyer¹ ID, Diego Molina-Ormazabal¹ ID, Mathilde Radiguet¹ ID, David Marsan¹ ID, Juan-Carlos Baez², Francisco Ortega-Culaciati³ ID, Andres Tassara⁴ ID, Marcos Moreno⁵ ID, and Anne Socquet¹ ID

¹Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, Université Gustave Eiffel, ISTERre, Grenoble, France, ²National Seismological Center, University of Chile, Santiago, Chile, ³Department of Geophysics, Faculty of Physical and Mathematical Sciences, University of Chile, Santiago, Chile, ⁴Departamento Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile, ⁵Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile

**Abstract** Like earthquakes, slow slip events release elastic energy stored on faults. Yet, the mechanisms behind slow slip instability and its relationship with seismicity are debated. Here, we use a seismo-geodetic deployment to document a shallow slow slip event (SSE) in 2023 on the Chile subduction. We observe dense, migrating seismic swarms accompanying the SSE, comprised of interface activity and upper plate splay faulting. Our observations suggest that the slow slip initiation is driven by structurally confined fluid overpressure in the fluid-rich surroundings of a subducted seamount. The fluid pressure migration interacts with the slow shear deformation increasing the interface permeability and facilitating a large-scale SSE migration. Historical earthquake swarms highlight the persistent structural control and recurrent nature of such slow slip events. Our observations provide insight into the interactions between slow slip and seismicity, suggesting they are controlled by creep on a fluid-infiltrated fault with fractally distributed asperities.

**Plain Language Summary** Subduction zones continuously accumulate stress from tectonic plate movement. Part of this stress is released in earthquakes, but another part is released through transient deformations, so-called slow slip events (SSEs). SSEs lead to the same motion on the plate interface as regular earthquakes, but happen over a much longer timeframe of days to months. SSEs do not cause ground shaking, but they are important to understand the mechanisms leading to major earthquakes. Here, we report on the first shallow SSE observed in Northern Chile, in the area of the subducted Copiapó ridge. We combine geodetic observations with a high-resolution earthquake catalog to study the mechanisms of slow slip. We find dense earthquake swarms, migrating together with the slow slip. Our findings suggest that the SSE and swarms are both caused by migrating fluid pulses traveling through a fluid-rich plate interface. The fluid-rich environment results from a subducted seamount on the Copiapó ridge. The partition of stress release into slow slip and micro-earthquakes can be explained with a segmentation of the plate interface into an aseismic medium with embedded seismic patches. Our results provide new insight into the mechanisms of SSE initiation and the interplay between slow slip and regular seismicity.

## 1. Introduction

Subduction plate interfaces continuously accumulate stress due to tectonic forcing. This stress is released through earthquakes or slow slip events (SSEs), which last from days to months and typically do not emit strong seismic waves (Delahaye et al., 2009; Ide et al., 2007; Takemura et al., 2023). The physical parameters that determine whether a rupture is slow or fast, and the mechanisms driving slow slip instability are still debated (Ide et al., 2007; Kato & Ben-Zion, 2021; Molina-Ormazabal et al., 2023; Nielsen, 2017; Obara & Kato, 2016). Furthermore, although slow slip events have been involved in the triggering (Radiguet et al., 2016; Uchida et al., 2016; Voss et al., 2018), nucleation (Itoh et al., 2023; Nishikawa & Ide, 2018; Obara & Kato, 2016) or arrest (Itoh et al., 2023; Rolandone et al., 2018; Uchida & Matsuzawa, 2013) of large earthquakes, how slow slip is related to seismic rupture is still poorly understood.

Several studies suggest that the megathrust interface is segmented into slow and fast slipping regions (Kaneko et al., 2010; Molina et al., 2021; Molina-Ormazabal et al., 2023) down to small-scale (Behr et al., 2021; Lavier et al., 2021; Seno, 2003; Yabe & Ide, 2014). These segments differ in terms of lithology, geometry, stress
heterogeneity, and fluid availability (Gosselin et al., 2020; Manga et al., 2012; Nakajima & Uchida, 2018; Warren-Smith et al., 2019). While the origin of this segmentation is not fully understood, a commonly proposed mechanism is a structural control by subducted oceanic structures, such as seamounts (Barnes et al., 2020; Contreras-Reyes & Carrizo, 2011; González-Vidal et al., 2023; Singh et al., 2011; Yáñez-Cuadra et al., 2022). SSEs are primarily controlled by variations in the friction coefficient and the effective normal stress (Ozawa et al., 2024). Two counteracting mechanisms are required to initiate and maintain an SSE: one accelerating the slip above the background rate and one preventing a runaway rupture. Purely frictional models suggest slip velocity dependent transition from velocity weakening to velocity strengthening (Liu & Rice, 2007), potentially combined with geometrical complexities (Romanet et al., 2018) or thermally activated friction (Wang & Barbot, 2023). Effective normal stress variations can be explained, for example, through thermal pressurization (Garagash, 2012), or through fluid pressure migrations (Ozawa et al., 2024; Warren-Smith et al., 2019). Slow deformation interacts with fluid migrations, as the deformation causes microfractures and increases permeability, leading to a positive feedback loop (Bhattacharya & Viesca, 2019). In this process, slow deformation can extend past the fluid pressure pulse (Romanet et al., 2024). Nonetheless, monitoring these processes and their interaction with fine-scale structures is challenging due to insufficient resolution.

Here, we use surface deformation and swarm seismicity associated with an SSE in 2023 in the subduction Copiapó ridge, Northern Chile, to investigate the processes and structures driving the SSE initiation and propagation. Between November 2020 and February 2024, the Copiapó ridge has been monitored with a dense network of GNSS (Socquet et al., 2023) and seismic stations (Figures S1 and S2 in Supporting Information S1). Based on the seismic records, Münchmeyer, Molina-Ormazabal et al. (2025) inferred a dense earthquake catalog using machine learning techniques, with precise hypocenters based on a local 3D velocity model and differential travel times from cross-correlation (Text S1 in Supporting Information S1). In the time and area of the 2023 SSE, the magnitude of completeness is below 1.5 (Figure S2 in Supporting Information S1), the absolute location uncertainties are typically below 5 km (Figure S3 in Supporting Information S1), and the relative location uncertainties are below 50 m for interevent distances of 500 m and below 100 m for events at 5 km distance (Figures S4 and S5 in Supporting Information S1). These observations allow to mitigate the observational gap obstructing our understanding of the mechanisms driving the slow slip instability and its interactions with seismicity. We are able to monitor with unprecedented detail how the SSE initiated, and what phenomenon triggered the SSE expansion and propagation. Our study sheds light on the role of structural complexities in trapping pressurized fluids, and on the interplay between slow slip, seismicity, and fluid pressure migrations.

## 2. The Subducted Copiapó Ridge and the 2023 Shallow SSE

At the latitude of Copiapó, an oceanic ridge enters into subduction, as marked by a series of aligned seamounts, corresponding to bathymetric anomalies of 2–3 km compared to the surrounding seafloor (Maksymowicz et al., 2024) (Figure 1). Above the megathrust, a local positive topographic anomaly is most likely the surface expression of a subducted seamount (Figure 1), as already shown in other settings (Bangs et al., 2023). The megathrust around the subducted Copiapó ridge is characterized by a low locking degree (Figure 1) (González-Vidal et al., 2023) and has repeatedly served as a barrier to seismic ruptures (Comte et al., 2002; Ruiz & Madariaga, 2018; Vigny et al., 2024). In this region, a large earthquake is pending, as adjacent locked patches have not been ruptured by a large earthquake ($M_w > 8$) since at least 1922 (González-Vidal et al., 2023; Kanamori et al., 2019; Vigny et al., 2024). The region is known to host seismic swarms with reports dating back to 1973 (Comte et al., 2002; Marsan et al., 2023; Ojeda et al., 2023). Previous studies have reported deep SSEs (40–60 km) (Klein et al., 2018, 2022) with a duration of ~1.5 years and a 5 years return period, but no shallow slow slip had previously been observed. A rare occurrence of shallow tremors has been proposed (Pastén-Araya et al., 2022), but the observation is disputed (Münchmeyer, Frank, et al., 2025).

In September 2023, we recorded a month-long transient motion on 18 GNSS stations between 26.1°S and 28.4°S with surface displacements up to 8 mm (Figures S6 and S7 in Supporting Information S1). We measure static displacements at each station and perform a regularized linear least square inversion (Tarantola, 2005) for the slip as a dislocation in an elastic medium based on these measurements (Texts S2–S4 in Supporting Information S1). The inferred shallow SSE in the Copiapó ridge has geodetic $M_w$ of 6.6 (Figures 2 and 3). The associated slip patch extends over ~100 km along strike between 20 and 35 km depths. The peak slip of ~5 cm is located on the edge of the inferred subducted seamount. The 2023 SSE is located updip of the deep SSEs with a band of interface seismicity separating their slip patches (Klein et al., 2018, 2022; Münchmeyer, Molina-Ormazabal et al., 2025).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Overview of the study region with the Nazca plate subducting below the South American plate. Copiapó ridge highlighted by bathymetry map. Background warm colors show the locking degree (González-Vidal et al., 2023). Dark green triangles denote permanent seismic stations, light green triangles temporary seismic stations, red squares GNSS stations. Colored contours show the slow slip event in 2014 (pink, Klein et al., 2018) and 2023 (maroon, this paper). Yellow stars indicate non-volcanic tremor on 20 September 2019 (Pastén-Araya et al., 2022). On the left, the along strike extent of historic earthquakes is shown (Ruiz & Madariaga, 2018; Vigny et al., 2024).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Slip distribution and seismicity during the 2023 slow slip event. The slip distribution is shown in yellow to red with additional gray 1 cm contours. The 5 mm slip contour for the initial period (until 10 September) is shown as a black, dotted line. Earthquakes during the time period are colored by time in the sequence and scaled by magnitude. For reference, all seismicity cataloged between November 2020 and February 2024 is shown as gray dots. (b) Zoom in on the onset cluster. (c) Cross section of the events in panel (b) along longitude. The black dashed circle in panels (a, b) indicates the approximate position of the subducted seamount inferred from bathymetric features and gravity data.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Temporal development of latitude distribution, magnitudes, and detrended GNSS series. The top two plots include all events shown in Figure 2a. The black line in the magnitude plot shows the total cumulative seismic moment release. GNSS time series have been selected for stations close to the main slip patch of the slow slip event. We highlight the approximate GNSS velocities at the station BING with blue dashed lines.</figcaption></figure>

Although our inversion revealed a small, deep, secondary slip patch with peak slip below 2 cm in the area of the deep 2020 SSE (Figure S6 in Supporting Information S1), it is unclear if this results from actual slip or insufficient resolution. The SSE starts around 1 September, increases in slip rate in the second half of September, before arresting in early October (Figure 3). Individual slip inversions for the early ($M_w$ = 6.16, until 10 September, Figure S6a in Supporting Information S1) and late periods ($M_w$ = 6.52, Figure S6b in Supporting Information S1) of the SSE indicate localized slip around the edge of the subducted seamount in the first phase and a substantially larger slip afterward, extending both toward the north and toward the south of the initial slip region.

The SSE is accompanied by seismic swarm activity with complex migration patterns (Figures 2 and 3). The cumulative seismic moment release is $M_w$ = 5.4, only about 1.6% of the geodetic moment release. Swarm seismicity starts on 26 August within a well-confined, low-magnitude cluster (total $M_w$ < 3.0) on the northern edge of a subducted seamount. On 1 September a vigorous swarm is activated downdip, in the area of highest geodetic slip. After a 5-day quiescence, an interface earthquake ~5 km further downdip ($M_L$ = 5.3) marks the acceleration of the geodetic displacement. It also marks the onset of a large-scale, bilateral migration of the seismicity at a rate of ~3.5 km/day (Figure 3), a typical migration rate for month-long SSEs (Danré et al., 2022, 2024). The short northward migration terminates after 20 km with three magnitude 3.5 earthquakes within 2 days. The southward migration terminates around 27.9°S with the last large event ($M_L$ = 4.3) on 6 October. The migration activates several seismicity clusters surrounding the seamount, among which the largest occurs updip of the inferred slip area, including one $M_L$ = 4.7 event. Within the area of the seamount, only few isolated events occur (Figure 2), yet these fit the surrounding seismic migration patterns. While there are clear seismic swarms accompanying the SSE, no evidence of tectonic tremors or LFEs around it has been found (Münchmeyer, Frank, et al., 2025).

The seismo-geodetic joint monitoring suggests two different phases during the SSE (Figure 3). In the initiation phase, up to the $M_L$ = 5.3 event and the geodetic acceleration, seismicity, and slip colocation in a small area. In the second phase, the slow slip and the seismicity are migrating over larger distances at a constant velocity, with the seismic activity primarily surrounding the main slip patch. Such two-phase SSE behavior has previously been
observed, for example, in Cascadia (Gomberg et al., 2016; Itoh et al., 2022). To understand the SSE's initiation mechanisms, we now focus on the seismic activity before the expansion of geodetic slip.

### 3. Seismic Swarms Outlining the SSE Initiation

The full initiation sequence of the SSE occurs along the northern flank of the subducted seamount (Figures 2b and 2c). It commences with an initial weak burst of seismicity on 26 August (blue in Figure S8 in Supporting Information S1). From 1 to 6 September a second cluster is activated downdip (cyan), confined to a narrow area of approximately 10 by 3 km. Within this swarm, a double migration occurs: the main activity migrates southwards with a slowly decaying rate between 2 and 5 km/day. It is intersperse with bursts migrating with velocities above 15 km/day (Figure S9 in Supporting Information S1). Such double migrations and interactions between slow slip and seismicity are indicative of pressure pulses triggering asperities on creeping faults infiltrated by fluid (Dublanchet & De Barros, 2021).

This second swarm is clearly delineated downdip by a gap in the seismicity. Activity beyond this gap only occurs after a 5 days quiescence and initiates with an $M_L$ = 5.3 earthquake on 11 September. Within the first 2 days, the aftershocks of this event (green) expand within an area of around 75 km². The expansion rate fits the afterslip model by Perfettini et al. (2018) with the aftershock area scaling approximately with the square of the logarithm of time (Figure S10 in Supporting Information S1). The three onset clusters show qualitatively different magnitude distributions (Figure S8 in Supporting Information S1). The third cluster, containing the largest event, shows approximately Gutenberg-Richter distributions. In contrast, the second cluster, with by far the highest event count, shows a clear scale break around $M_L$ = 3.2. This scale break suggests spatially confined ruptures, for example, due to limited size of the seismic asperities (Behr et al., 2021; Seno, 2003).

Within this early sequence, almost all events locate on the narrow plate interface (Figure S9 in Supporting Information S1). In addition, we are able to resolve earthquakes on splay fractures trending westward in the upper plate. The seismic activity shows that the splay fractures extend over several kilometers. Timing of upper plate seismicity coincides with the activation of interface seismicity and is most active above the second cluster (cyan in Figure S8 in Supporting Information S1). This activity of splay fractures might be indicating the drainage of fluid overpressure from the interface (Chesley et al., 2021; Perez-Silva et al., 2023).

### 4. Revisiting Historic Swarm Activity

In order to investigate whether such events are recurrent and if any repetitive pattern is found, we compare the swarm activity in 2023 to nine historic swarms around the Copiapó ridge. These swarms encompass newly detected sequences and known swarms using four seismic catalogs (Figure 4, Figure S11 in Supporting Information S1) (Comte et al., 2002; Marsan et al., 2023; Ojeda et al., 2023). Due to limited detection capabilities, especially prior to 2014, we likely miss additional swarms in between the early swarms. The five swarms with sufficient data (2015, 2016, 2018, 2020, and 2023) have been located using a common 3D velocity model (Münchmeyer, Molina-Ormazabal et al., 2025). These swarms consistently activate the same interface patches, in particular, in the initiation phase around the seamount and in the arrest region of the 2023 SSE (Figure 4). The active patches match at least down to the kilometer scale, similar to the location uncertainties of the longer-term catalog used for the swarms from 2015 to 2020. This suggests that the location of the swarm seismicity is controlled by persistent interface structures that are repeatedly activated. This observation is consistent with the previous observation that the 2023 initial swarm is structurally confined.

Without the continuous geodetic monitoring installed starting 2021, it is impossible to identify slow deformation associated to the swarms. An event with the size of the 2023 SSE would not be observable. Nonetheless, several indicators point at slow slip associated with the historic swarm activity. Most prominently, all swarms exhibit migration patterns (Figure S11 in Supporting Information S1). This is particularly evident for the 1973 swarm (northward migration) and the 2006 swarm (bilateral). Furthermore, for the 2006 sequence, one continuous GNSS station running during the swarm detected seaward motion of the upper plate, consistent with a shallow SSE (Ojeda et al., 2023). For the 1973 event, moment tensor solutions show that the events features thrust mechanisms, indicating that the seismicity is mostly located on the subduction interface (Comte et al., 2002). Notably, the region shows almost no seismic activity in between the seismic swarms. In combination with the observed
<figure><img src="image_6.png" /></figure>

**Figure 4.** Zoom in on the central part of the 2015, 2016, 2018, 2020, and 2023 swarms using catalogs from Münchmeyer, Molina-Ormazabal et al. (2025). The black circle highlights the approximate location of the subducted seamount. Gray lines show the 1 cm slip contours of the 2023 slow slip event. All swarms have been located with the same 3D seismic velocity model. The 2015–2020 swarms use similar station coverage, consisting of permanent seismic networks, while for the 2023 swarm a collection of much denser temporary networks was used. Therefore, catalog completeness and location accuracy are best for the 2023 swarm. The 2015–2020 swarms have been jointly relocated using GrowClust3D, the 2023 model individually.

migration patterns, this suggests that swarm seismicity is driven by an underlying aseismic transient, such as recurrent SSEs.

The detected swarms vary in seismic moment by almost three orders of magnitude. As geodetic moment typically scales with seismic moment, this suggest that the region possibly hosts repeated SSEs with highly varying moment release (Nishikawa & Ide, 2018; Passarelli et al., 2021; Wang & Barbot, 2023). Given this observations, it seems possible that the low locking in this area is not the result of continuous creep, but that the slip pattern is highly intermittent, with episods of large slip detectable as SSEs with accompanying seismic swarms. Such intermittence on short time-scales is not resolvable in on-shore geodetic data, and will require offshore deployments to verify.

**5. The Structure of the Subduction Interface**

Based on our observations, we propose a schematic model of the SSE area (Figure 5). The high resolution of the earthquake catalog shows that the subduction interface is only seismically active on small patches (Figure 5 inset), each of them extending up to 50 m vertically and up to 2 km horizontally. We observe rapid migrations within these patches. The individual patches are disconnected from each other and distributed within a thin, 3D fault interface volume of less than 250 m thickness, overall forming the subduction interface on a macroscopic scale (Figure S9 right in Supporting Information S1). This is consistent with estimates of the interface thickness in other subduction zones (Chalumeau et al., 2024). The patches show a fractal behavior from a length scale of 5 km down to the location uncertainty around 40 m, meaning that the seismic patches cluster and are themselves subdivided into smaller seismic patches (Figure 5 inset, Figure S4 in Supporting Information S1). Our comparison to historic swarms shows that this segmentation into seismic and aseismic patches is stable at least on the decade scale. As the total seismic moment is insufficient to accommodate the geodetic moment, the seismic patches need to be embedded within a slowly deforming
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Schematic overview of the slow slip event initiation sequence and an interpretation of its potential drivers. Each earthquake is represented by a dot colored according to the time within the sequence. Blue arrows show fluid pressure migrations. The insets show a schematic cross section of the fault interface (bottom left) and the interface structure in map view (top right). The cross section covers approximately 1 by 10 km and is exaggerated vertically. The map view covers approximately 3 by 3 km.</figcaption></figure>

matrix (Behr et al., 2021; Lavier et al., 2021; Perfettini et al., 2018; Seno, 2003). This observation is consistent with geological observations of fossil subduction interfaces, a shear zone with frictional lenses embedded in a viscous and fluid-rich matrix (Behr et al., 2021). This segmentation provides evidence for a strong heterogeneity of stess and frictional conditions on the interface.

To explain the episodic, swarm-type seismic behavior and the transient slow deformation, we suggest an inter-
action between pressure pulses migrating on a fluid-rich interface and shear stress propagation (Figures 5I–5III).
Recurrent fluid pressure increases are evidenced by swarm seismicity, the characteristic double-migration pat-
terns (Figure S9 in Supporting Information S1) (Dublanchet & De Barros, 2021), the activation of splay fractures
for fluid pressure drainage (Figure 5 cross-section) (Chesley et al., 2021; Fasola et al., 2019), and the typical
migration velocities (Figure 3) (Danré et al., 2022). Furthermore, the seismic quiescence between 6 and 11
September suggests the existence of a structural permeability seal (Figure 5IV) (Gosselin et al., 2020; Manga
et al., 2012). We note that the existence of such a seal can not be definitely confirmed without in situ mea-
surements. Before its rupture, the overpressured fluids, structurally confined by the seal, lower the effective
normal stress and thereby trigger the most active seismicity swarm in the sequence. This fluid overpressure
infiltrating the size-constrained asperities explains the scale-break observed in the magnitude distribution of the
onset cluster (Figure S8 in Supporting Information S1). Once shear stress on the asperities is released, the cluster
goes dormant, even though fluid pressure stays high and maintaining low effective normal stress. During this
cluster and the subsequent quiescence, the aseismic slip remains confined to the initiation area. Only once the
permeability seal is broken, as marked by the *M*<sub>L</sub> = 5.3 event, the fluid pressure pulse and the shear failure can
migrate further along the interface. They interact in a feedback loop between increased permeability from shear
failure and decreased effective normal stress from pressure migration (Figure 5V). During that propagation stage,
the propagation of the slow slip front ruptures seismic asperity through shear loading visible in the seismicity. The
propagation of the slow slip is likely arrested by stronger seismic asperities (Yabe & Ide, 2014), as shown by the
peripheral location of the seismic swarms with the largest magnitudes compared to the area slipping slowly and
inferred by GPS (Figure 2).
The initiation of the SSE, as imaged by the seismic swarm activity, occurs along the northern flank of a subducted seamount (Figure 2). Abundant evidence suggests that subducted ridges stay intact after subduction, leading to zones of low coupling in its wake, and enhance increased normal stress on its leading edge and weakening of the surrounding medium through fracturing (Barnes et al., 2020; Contreras-Reyes & Carrizo, 2011; Singh et al., 2011; Sun et al., 2020). In addition to these geometric and stress heterogeneities, seamount flanks are typically observed to feature overpressurized sediments, caused by the subducting topography and have been shown to supply fluids to the interface (Bangs et al., 2023; Chesley et al., 2021; Gase et al., 2023; Shaddox & Schwartz, 2019; Sun et al., 2020). We hypothesize that this high fluid pressure on the interface enables the migrations of pressure pulses as described above (Cruz-Atienza et al., 2018). Further evidence for a fluid-rich environment is provided by tomographic studies, showing high vp/vs ratios in this region (Pastén-Araya et al., 2022). Notably, while in our observation the highest activity occurs on the northern flank of the seamount, we see further swarms on its western and south-eastern edges, suggesting similar interactions of interface segmentation and fluid overpressure. Combining these aspects, the seamount together with the inherited oceanic fractures likely controls the interface segmentation and creates a fluid-rich environment, enabling seismic swarm activity and slow deformation.

A few peculiarities should be noted regarding the swarm behavior and its relation to fluid pressure: the downdip migration, the roughly constant migration velocity, and the high value of the migration velocity. All three aspect would be unexpected for fluid diffusion, but are fully compatible with pressure pulses on a fluid-rich interface during slow slip (Cruz-Atienza et al., 2018; Farge et al., 2021, 2023; Perez-Silva et al., 2023). This is caused by transient changes in permeability, due to changes in effective pressure and dilatancy from the slow slip, and by anisotropy of the permeability (Kawano et al., 2011). We note that this is not the only interaction mode of slow slip with seismic swarms. Alternative modes are intraplate swarms in the subducting plate, related to the fluid supply to the interface (Nishikawa et al., 2021), and swarms on upper plate faults caused by fluid drainage (Fasola et al., 2019).

The micro-structures and pressure pulse migrations define the macro-structure of the subduction megathrust and its seismic potential. Differential stresses built up by repeated SSEs increase the likelihood of triggering large earthquakes in adjacent segments (Obara & Kato, 2016; Radiguet et al., 2016). At the same time, the predominantly aseismic interface acts as an energy sink for seismic rupture propagation (Kaneko et al., 2010; Molina et al., 2021), explaining the barrier behavior of this region (Ruiz & Madariaga, 2018; Vigny et al., 2024). The seismic patches encircling the seamount indicate that, although the Copiapó ridge generally acts as a barrier, it might still break during a megathrust rupture with sufficient along-dip extent (Rice & Ben-Zion, 1996; Yáñez-Cuadra et al., 2022).

## 6. Conclusion

Our findings combine dense seismo-geodetic observations to study the 2023 Copiapó SSE and its seismic swarm activities. Using a high-resolution catalog, we gain a detailed view of the mechanisms of shallow slow slip and the subduction interface structure. Our observations suggests that the interplay of a fine-scale interface segmentation and stress heterogeneities with migrating fluid pressure pulses controls both slow slip and the associated swarm seismicity. Notably, our study describes the initiation phase of a SSE with unprecedented detail. We suggest that the slow slip instability initiates with a structurally controlled seismic swarm driven by pressurized fluids interacting with shear failure, eventually triggering the acceleration and propagation of the slow slip after the rupture of a permeability seal. The link between slow slip and seismicity significantly differs between the initiation and the propagation phases of the SSE, indicating that the driving mechanisms evolve from the slow slip initiation to its propagation. Our results also provide a detailed perspective on the functioning of a seismic barrier, and thereby the understanding of fast and slow megathrust earthquakes.

## Data Availability Statement

The seismicity catalogs from Münchmeyer, Molina-Ormazabal et al. (2025) are available at https://doi.org/10.5281/zenodo.15083297. GNSS time series used in this manuscript are available at https://doi.osug.fr/public/GNSS_products/GNSS.products.SouthAmerica_GIPSYX.daily.html.
Acknowledgments

We thank everyone involved in the
deployment of the seismic and geodetic
networks and the data management. Some
of the data used are provided by the Epos-
France research infrastructure (www.epos-
france.fr—Résif until 2023), and some of
the seismological instruments used to
collect data are those of Epos-France's
Sismob mobile seismological park
(sismob.epos-france.fr) (INSU-CNRS).
The computations presented in this paper
were performed using the GRICAD
infrastructure (https://gricad.univ-
grenoble-alpes.fr), which is supported by
Grenoble research communities. This work
has been partially funded by the European
Union under the ERC CoG 865963 DEEP-
trigger and the Grant 101104996
("DECODE"). Views and opinions
expressed are however those of the authors
only and do not necessarily reflect those
of the European Union or REA. Neither the
European Union nor the granting authority
can be held responsible for them. D.M.'s
salary was covered by a Grant from CNES.
J-C.B., F.O-C., and M.M. acknowledge
funding from ANID PIA-ACT192169
Anillo PRECURSOR project. F.O-C.
acknowledge support from FONDECYT
1231684 ANID project. M.M.
acknowledge support from FONDECYT
1221507 ANID project. J-C-B.
acknowledge support from FONDECYT
1240501 ANID project. Stations from the
9C network were funded by CNRS-Tellus
"COP2020: SlowSlip Trigger" and ANR-
S5 (ANR-19-CE31-0003) projects.

References

Bangs, N. L., Morgan, J. K., Bell, R. E., Han, S., Arai, R., Kodaira, S., et al. (2023). Slow slip along the Hikurangi margin linked to fluid-rich sediments trailing subducting seamounts. *Nature Geoscience*, **16**(6), 505–512. https://doi.org/10.1038/s41561-023-01186-3

Barnes, P. M., Wallace, L. M., Saffer, D. M., Bell, R. E., Underwood, M. B., Fagereng, A., et al. (2020). Slow slip source characterized by lithological and geometric heterogeneity. *Science Advances*, **6**(13), eaay3314. https://doi.org/10.1126/sciadv.aay3314

Behr, W. M., Gerya, T. V., Cannizzaro, C., & Blass, R. (2021). Transient slow slip characteristics of frictional-viscous subduction megathrust shear zones. *AGU Advances*, **2**(3), e2021AV000416. https://doi.org/10.1029/2021AV000416

Bhattacharya, P., & Viesca, R. C. (2019). Fluid-induced aseismic fault slip outpaces pore-fluid migration. *Science*, **364**(6439), 464–468. https://doi.org/10.1126/science.aaw7354

Chalumeau, C., Agurto-Detzel, H., Rietbrock, A., Frietsch, M., Oncken, O., Segovia, M., & Galve, A. (2024). Seismological evidence for a multifault network at the subduction interface. *Nature*, **628**(8008), 558–562. https://doi.org/10.1038/s41586-024-07245-y

Chesley, C., Naif, S., Key, K., & Bassett, D. (2021). Fluid-rich subducting topography generates anomalous forearc porosity. *Nature*, **595**(7866), 255–260. https://doi.org/10.1038/s41586-021-03619-8

Comte, D., Haessler, H., Dorbath, L., Pardo, M., Monfret, T., Lavenu, A., et al. (2002). Seismicity and stress distribution in the Copiapo, Northern Chile subduction zone using combined on- and off-shore seismic observations. *Physics of the Earth and Planetary Interiors*, **132**(1), 197–217. https://doi.org/10.1016/S0031-9201(02)00052-3

Contreras-Reyes, E., & Carrizo, D. (2011). Control of high oceanic features and subduction channel on earthquake ruptures along the Chile–Peru subduction zone. *Physics of the Earth and Planetary Interiors*, **186**(1), 49–58. https://doi.org/10.1016/j.pepi.2011.03.002

Cruz-Atienza, V. M., Villafuerte, C., & Bhat, H. S. (2018). Rapid tremor migration and pore-pressure waves in subduction zones. *Nature Communications*, **9**(1), 2900. https://doi.org/10.1038/s41467-018-05150-3

Drané, P., De Barros, L., Cappa, F., & Ampuero, J.-P. (2022). Prevalence of aseismic slip linking fluid injection to natural and anthropogenic seismic swarms. *Journal of Geophysical Research: Solid Earth*, **127**(12), e2022JB025571. https://doi.org/10.1029/2022JB025571

Drané, P., De Barros, L., Cappa, F., & Passarelli, L. (2024). Parallel dynamics of slow slips and fluid-induced seismic swarms. *Nature Communications*, **15**(1), 8943. https://doi.org/10.1038/s41467-024-53285-3

Delahaye, E. J., Townend, J., Reyners, M. E., & Rogers, G. (2009). Microseismicity but no tremor accompanying slow slip in the Hikurangi subduction zone, New Zealand. *Earth and Planetary Science Letters*, **277**(1), 21–28. https://doi.org/10.1016/j.epsl.2008.09.038

Dublanchet, P., & De Barros, L. (2021). Dual seismic migration velocities in seismic swarms. *Geophysical Research Letters*, **48**(1), e2020GL090025. https://doi.org/10.1029/2020GL090025

Farge, G., Jaupart, C., Frank, W. B., & Shapiro, N. M. (2023). Along-strike segmentation of seismic tremor and its relationship with the hydraulic structure of the subduction fault zone. *Journal of Geophysical Research: Solid Earth*, **128**(12), e2023JB027584. https://doi.org/10.1029/2023JB027584

Farge, G., Jaupart, C., & Shapiro, N. M. (2021). Episodicity and migration of low frequency earthquakes modeled with fast fluid pressure transients in the permeable subduction interface. *Journal of Geophysical Research: Solid Earth*, **126**(9), e2021JB021894. https://doi.org/10.1029/2021JB021894

Fasola, S. L., Brudzinski, M. R., Holtkamp, S. G., Graham, S. E., & Cabral-Cano, E. (2019). Earthquake swarms and slow slip on a sliver fault in the Mexican subduction zone. *Proceedings of the National Academy of Sciences of the United States of America*, **116**(15), 7198–7206. https://doi.org/10.1073/pnas.1814205116

Garagash, D. I. (2012). Seismic and aseismic slip pulses driven by thermal pressurization of pore fluid. *Journal of Geophysical Research*, **117**(B4), B04314. https://doi.org/10.1029/2011JB008889

Gase, A. C., Bangs, N. L., Saffer, D. M., Han, S., Miller, P. K., Bell, R. E., et al. (2023). Subducting volcaniclastic-rich upper crust supplies fluids for shallow megathrust and slow slip. *Science Advances*, **9**(33), eadh0150. https://doi.org/10.1126/sciadv.adh0150

Gomberg, J., Wech, A., Creager, K., Obara, K., & Agnew, D. (2016). Reconsidering earthquake scaling. *Geophysical Research Letters*, **43**(12), 6243–6251. https://doi.org/10.1002/2016GL069967

González-Vidal, D., Moreno, M., Sappl, C., Baez, J. C., Ortega-Culaciati, F., Lange, D., et al. (2023). Relation between oceanic plate structure, patterns of interplate locking and microseismicity in the 1922 atacama seismic gap. *Geophysical Research Letters*, **50**(15), e2023GL0103565. https://doi.org/10.1029/2023GL0103565

Gosselin, J. M., Audet, P., Estève, C., McLellan, M., Mosher, S. G., & Schaeffer, A. J. (2020). Seismic evidence for megathrust fault-valve behavior during episodic tremor and slip. *Science Advances*, **6**(4), eaay5174. https://doi.org/10.1126/sciadv.aay5174

Ide, S., Beroza, G. C., Shelly, D. R., & Uchide, T. (2007). A scaling law for slow earthquakes. *Nature*, **447**(7140), 76–79. https://doi.org/10.1038/nature05780

Itoh, Y., Aoki, Y., & Fukuda, J. (2022). Imaging evolution of Cascadia slow-slip event using high-rate GPS. *Scientific Reports*, **12**(1), 7179. https://doi.org/10.1038/s41598-022-10957-8

Itoh, Y., Socquet, A., & Radiguet, M. (2023). Largest aftershock nucleation driven by afterslip during the 2014 Iquique sequence. *Geophysical Research Letters*, **50**(24), e2023GL104852. https://doi.org/10.1029/2023gl104852

Kanamori, H., Rivera, L., Ye, L., Lay, T., Murotani, S., & Tsumura, K. (2019). New constraints on the 1922 Atacama, Chile, earthquake from historical seismograms. *Geophysical Journal International*, **219**(1), 645–661. https://doi.org/10.1093/gji/ggz302

Kaneko, Y., Avouac, J.-P., & Lapusta, N. (2010). Towards inferring earthquake patterns from geodetic observations of interseismic coupling. *Nature Geoscience*, **3**(5), 363–369. https://doi.org/10.1038/ngeo0843

Kato, A., & Ben-Zion, Y. (2021). The generation of large earthquakes. *Nature Reviews Earth and Environment*, **2**(1), 26–39. https://doi.org/10.1038/s43017-020-00108-w

Kawano, S., Katayama, I., & Okazaki, K. (2011). Permeability anisotropy of serpentinite and fluid pathways in a subduction zone. *Geology*, **39**(10), 939–942. https://doi.org/10.1130/G32173.1

Klein, E., Duputel, Z., Zigone, D., Vigny, C., Boy, J.-P., Doubre, C., & Meneses, G. (2018). Deep transient slow slip detected by survey GPS in the region of Atacama, Chile. *Geophysical Research Letters*, **45**(22), 12263–12273. https://doi.org/10.1029/2018GL080613

Klein, E., Vigny, C., Duputel, Z., Zigone, D., Rivera, L., Ruiz, S., & Potin, B. (2022). Return of the Atacama deep slow slip event: The 5-year recurrence confirmed by continuous GPS. *Physics of the Earth and Planetary Interiors*, **334**, 106970. https://doi.org/10.1016/j.pepi.2022.106970

Lavier, L. L., Tong, X., & Biemiller, J. (2021). The mechanics of creep, slow slip events, and earthquakes in mixed brittle-ductile fault zones. *Journal of Geophysical Research: Solid Earth*, **126**(2), e2023JB020325. https://doi.org/10.1029/2023JB020325

Liu, Y., & Rice, J. R. (2007). Spontaneous and triggered aseismic deformation transients in a subduction fault model. *Journal of Geophysical Research*, **112**(B9), B09404. https://doi.org/10.1029/2007JB004930
Maksymowicz, A., Contreras-Reyes, E., & Lara, L. E. (2024). Joint flexural-density modeling of the Taltal, Copiapó, and Iquique hotspot ridges and the surrounding oceanic plate, offshore Chile. *Geosphere*, 20(4), 1066–1084. https://doi.org/10.1130/ges02733.1

Manga, M., Beresnev, I., Brodsky, E. E., Elkhoury, J. E., Elsworth, D., Ingebritsen, S. E., et al. (2012). Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms. *Reviews of Geophysics*, 50(2), RG2004. https://doi.org/10.1029/2011RG000382

Marsan, D., Reverso, T., & Socquet, A. (2023). Earthquake swarms along the Chilean subduction zone, 2003–2020. *Geophysical Journal International*, 235(3), 2758–2777. https://doi.org/10.1093/gji/ggad359

Molina, D., Tassara, A., Abarca, R., Melnick, D., & Madella, A. (2021). Frictional segmentation of the Chilean megathrust from a multivariate analysis of geophysical, geological, and geodetic data. *Journal of Geophysical Research: Solid Earth*, 126(6), e2020JB020647. https://doi.org/10.1029/2020jb020647

Molina-Ormazabal, D., Ampuero, J.-P., & Tassara, A. (2023). Diverse slip behaviour of velocity-weakening fault barriers. *Nature Geoscience*, 16(12), 1200–1207. https://doi.org/10.1038/s41561-023-01312-1

Münchmeyer, J., Frank, W. B., Giffard-Roisin, S., Marsan, D., & Socquet, A. (2025). A systematic search for tectonic tremor and low-frequency earthquakes in the atacama segment of the Chilean subduction zone (24°S-31°S) turns up empty. arXiv preprint arXiv:2501.16934.

Münchmeyer, J., Molina-Ormazabal, D., Marsan, D., Langlais, M., Baez, J.-C., Heit, B., et al. (2025). Characterising the Atacama segment of the Chile subduction margin (24°S-31°S) with > 165,000 earthquakes. arXiv preprint arXiv:2501.14396.

Nakajima, J., & Uchida, N. (2018). Repeated drainage from megathrusts during episodic slow slip. *Nature Geoscience*, 11(5), 351–356. https://doi.org/10.1038/s41561-018-0090-z

Nielsen, S. (2017). *From slow to fast faulting: Recent challenges in earthquake fault mechanics* (Vol. 375, No. (2103)). The Royal Society Publishing.

Nishikawa, T., & Ide, S. (2018). Recurring slow slip events and earthquake nucleation in the source region of the M 7 Ibaraki-Oki earthquakes revealed by earthquake swarm and foreshock activity. *Journal of Geophysical Research: Solid Earth*, 123(9), 7950–7968. https://doi.org/10.1029/2018JB015642

Nishikawa, T., Nishimura, T., & Okada, Y. (2021). Earthquake swarm detection along the Hikurangi Trench, New Zealand: Insights into the relationship between seismicity and slow slip events. *Journal of Geophysical Research: Solid Earth*, 126(4), e2020JB020618. https://doi.org/10.1029/2020JB020618

Obara, K., & Kato, A. (2016). Connecting slow earthquakes to huge earthquakes. *Science*, 353(6296), 253–257. https://doi.org/10.1126/science.aaf1512

Ojeda, J., Morales-Yáñez, C., Ducret, G., Ruiz, S., Grandin, R., Doin, M.-P., et al. (2023). Seismic and aseismic slip during the 2006 Copiapó swarm in North-Central Chile. *Journal of South American Earth Sciences*, 123, 104198. https://doi.org/10.1016/j.jsames.2023.104198

Ozawa, S., Yang, Y., & Dunham, E. M. (2024). Fault-valve instability: A mechanism for slow slip events. *Journal of Geophysical Research: Solid Earth*, 129(10), e2024JB029165. https://doi.org/10.1029/2024JB029165

Passarelli, L., Selvadurai, P. A., Rivalta, E., & Jónsson, S. (2021). The source scaling and seismic productivity of slow slip transients. *Science Advances*, 7(32), eabg9718. https://doi.org/10.1126/sciadv.abg9718

Pastén-Araya, F., Potin, B., Azúa, K., Sáez, M., Aden-Antoniów, F., Ruiz, S., et al. (2022). Along-dip segmentation of the slip behavior and rheology of the Copiapó ridge subducted in North-Central Chile. *Geophysical Research Letters*, 49(4), e2021GL095471. https://doi.org/10.1029/2021GL095471

Perez-Silva, A., Kaneko, Y., Savage, M., Wallace, L., & Warren-Smith, E. (2023). Characteristics of slow slip events explained by rate-strengthening faults subject to periodic pore fluid pressure changes. *Journal of Geophysical Research: Solid Earth*, 128(6), e2022JB026332. https://doi.org/10.1029/2022JB026332

Perfettini, H., Frank, W. B., Marsan, D., & Bouchon, M. (2018). A model of aftershock migration driven by afterslip. *Geophysical Research Letters*, 45(5), 2283–2293. https://doi.org/10.1002/2017GL076287

Radiguet, M., Perfettini, H., Cotte, N., Gualandi, A., Valette, B., Kostoglodov, V., et al. (2016). Triggering of the 2014 Mw7.3 Papanoa earthquake by a slow slip event in Guerrero, Mexico. *Nature Geoscience*, 9(11), 829–833. https://doi.org/10.1038/ngeo2817

Rice, J. R., & Ben-Zion, Y. (1996). Slip complexity in earthquake fault models. *Proceedings of the National Academy of Sciences of the United States of America*, 93(9), 3811–3818. https://doi.org/10.1073/pnas.93.9.3811

Rolandone, F., Nocquet, J.-M., Mothes, P. A., Jarrin, P., Vallée, M., Cubas, N., et al. (2018). Areas prone to slow slip events impede earthquake rupture propagation and promote afterslip. *Science Advances*, 4(1), eaa06596. https://doi.org/10.1126/sciadv.aao6596

Romanet, P., Bhat, H. S., Jolivet, R., & Madariaga, R. (2018). Fast and slow slip events emerge due to fault geometrical complexity. *Geophysical Research Letters*, 45(10), 4809–4819. https://doi.org/10.1029/2018gl077579

Romanet, P., Scuderi, M. M., Ampuero, J.-P., Chaillat, S., & Cappa, F. (2024). Coupled boundary element and finite volume methods for modeling fluid-induced seismicity in fault networks within low-permeability rocks. (No. arXiv:2412.03194). arXiv. https://doi.org/10.48550/arXiv.2412.03194

Ruiz, S., & Madariaga, R. (2018). Historical and recent large megathrust earthquakes in Chile. *Tectonophysics*, 733, 37–56. https://doi.org/10.1016/j.tecto.2018.01.015

Seno, T. (2003). Fractal asperities, invasion of barriers, and interplate earthquakes. *Earth Planets and Space*, 55(11), 649–665. https://doi.org/10.1186/BF03352472

Shaddox, H. R., & Schwartz, S. Y. (2019). Subducted seamount diverts shallow slow slip to the forearc of the northern Hikurangi subduction zone, New Zealand. *Geology*, 47(5), 415–418. https://doi.org/10.1130/G45810.1

Singh, S. C., Hananto, N., Mukti, M., Robinson, D. P., Das, S., Chauhan, A., et al. (2011). Aseismic zone and earthquake segmentation associated with a deep subducted seamount in Sumatra. *Nature Geoscience*, 4(5), 308–311. https://doi.org/10.1038/ngeo1119

Socquet, A., Tsapong-Tsague, A., Lovery, B., Janex, G., & Radiguet, M. (2023). *Metadata and GNSS daily position solutions for permanent GNSS stations in South America*. CNRS, OSUG, ISTERRE. https://doi.org/10.17178/GNSS.products.SouthAmerica_GIPSYX.daily

Sun, T., Saffer, D., & Ellis, S. (2020). Mechanical and hydrological effects of seamount subduction on megathrust stress and slip. *Nature Geoscience*, 13(3), 249–255. https://doi.org/10.1038/s41561-020-0542-0

Takemura, S., Hamada, Y., Okuda, H., Okada, Y., Okubo, K., Akuhara, T., et al. (2023). A review of shallow slow earthquakes along the Nankai Trough. *Earth Planets and Space*, 75(1), 164. https://doi.org/10.1186/s40623-023-01920-6

Tarantola, A. (2005). *Inverse problem theory and methods for model parameter estimation*. SIAM.

Uchida, N., Iinuma, T., Nadeau, R. M., Bürgmann, R., & Hino, R. (2016). Periodic slow slip triggers megathrust zone earthquakes in northeastern Japan. *Science*, 351(6272), 488–492. https://doi.org/10.1126/science.aad3108

Uchida, N., & Matsuzawa, T. (2013). Pre-and postseismic slow slip surrounding the 2011 Tohoku-Oki earthquake rupture. *Earth and Planetary Science Letters*, 374, 81–91. https://doi.org/10.1016/j.epsl.2013.05.021
Vigny, C., Klein, E., & Ojeda, J. (2024). In search for the lost truth about the 1922 and 1918 Atacama earthquakes in Chile. *Journal of South American Earth Sciences, 143*, 104983. https://doi.org/10.1016/j.jsames.2024.104983

Voss, N., Dixon, T. H., Liu, Z., Malservisi, R., Protti, M., & Schwartz, S. (2018). Do slow slip events trigger large and great megathrust earthquakes? *Science Advances*, 4(10), eaat8472. https://doi.org/10.1126/sciadv.aat8472

Wang, B., & Barbot, S. (2023). Pulse-like ruptures, seismic swarms, and tremorgenic slow-slip events with thermally activated friction. *Earth and Planetary Science Letters*, 603, 117983. https://doi.org/10.1016/j.epsl.2022.117983

Warren-Smith, E., Fry, B., Wallace, L., Chon, E., Henrys, S., Sheehan, A., et al. (2019). Episodic stress and fluid pressure cycling in subducting oceanic crust during slow slip. *Nature Geoscience*, 12(6), 475–481. https://doi.org/10.1038/s41561-019-0367-x

Yabe, S., & Ide, S. (2014). Spatial distribution of seismic energy rate of tectonic tremors in subduction zones. *Journal of Geophysical Research: Solid Earth*, 119(11), 8171–8185. https://doi.org/10.1002/2014jb011383

Yañez-Cuadra, V., Ortega-Culaciati, F., Moreno, M., Tassara, A., Krumm-Nualart, N., Ruiz, J., et al. (2022). Interplate coupling and seismic potential in the atacama seismic gap (Chile): Dismissing a rigid Andean Sliver. *Geophysical Research Letters*, 49(11), e2022GL098257. https://doi.org/10.1029/2022GL098257

## References From the Supporting Information

Bertiger, W., Bar-Sever, Y., Dorsey, A., Haines, B., Harvey, N., Hemberger, D., et al. (2020). GIPSYX/RTGX, a new tool set for space geodetic operations and research. *Advances in Space Research*, 66(3), 469–489. https://doi.org/10.1016/j.asr.2020.04.015

Eberhart-Phillips, D., Thurber, C., Rietbrock, A., Fry, B., Reyners, M., & Lanza, F. (2021). *Simul2017: A flexible program for inversion of earthquake data for 3-D velocity and hypocenters or 3-D Q*. Zenodo. https://doi.org/10.5281/zenodo.5746047

Graeber, F. M., & Asch, G. (1999). Three-dimensional models of p wave velocity and p-to-s velocity ratio in the southern Central Andes by simultaneous inversion of local earthquake data. *Journal of Geophysical Research*, 104(B9), 20237–20256. https://doi.org/10.1029/1999JB900037

Hansen, P. C., & O'Leary, D. P. (1993). The use of the l-curve in the regularization of discrete ill-posed problems. *SIAM Journal on Scientific Computing*, 14(6), 1487–1503. https://doi.org/10.1137/0914086

Hayes, G. P., Moore, G. L., Portner, D. E., Hearne, M., Flamme, H., Furtney, M., & Smoczyk, G. M. (2018). Slab2, a comprehensive subduction zone geometry model. *Science*, 362(6410), 58–61. https://doi.org/10.1126/science.aat4723

Kissling, E., Ellsworth, W. L., Eberhart-Phillips, D., & Kradolfer, U. (1994). Initial reference models in local earthquake tomography. *Journal of Geophysical Research*, 99(B10), 19635–19646. https://doi.org/10.1029/93JB03138

Larson, K. M., Lowry, A. R., Kostoglakov, V., Hutton, W., S'ánchez, O., Hudnut, K., & Su'arez, G. (2004). Crustal deformation measurements in Guerrero, Mexico. *Journal of Geophysical Research*, 109(B4), B04409. https://doi.org/10.1029/2003jb002843

Lomax, A., Virieux, J., Volant, P., & Berge-Thierry, C. (2000). Probabilistic earthquake location in 3D and layered models. In C. H. Thurber & N. Rabinowitz (Eds.), *Advances in seismic event location* (pp. 101–134). Springer Netherlands. https://doi.org/10.1007/978-94-015-9536-0_5

Lovery, B., Chlieh, M., Norabuena, E., Villegas-Lanza, J., Radiguet, M., Cotte, N., et al. (2024). Heterogeneous locking and earthquake potential on the South Peru megathrust from dense GNSS network. *Journal of Geophysical Research: Solid Earth*, 129(2), e2023JB027114. https://doi.org/10.1029/2023jb027114

Marill, L., Marsan, D., Socquet, A., Radiguet, M., Cotte, N., & Rousset, B. (2021). Fourteen-year acceleration along the Japan trench. *Journal of Geophysical Research: Solid Earth*, 126(11), e2020JB021226. https://doi.org/10.1029/2020jb021226

Michelini, A., Cianetti, S., Gaviano, S., Giunchi, C., Jozinović, D., & Lauciani, V. (2021). Instance-the Italian seismic dataset for machine learning. *Earth System Science Data*, 13(12), 5509–5544. https://doi.org/10.5194/essd-13-5509-2021

Münchmeyer, J. (2024). PyOcto: A high-throughput seismic phase associator. *Seismica*, 3(1). https://doi.org/10.26443/seismica.v3i1.1130

Münchmeyer, J., Woollam, J., Rietbrock, A., Tilmann, F., Lange, D., Bornstein, T., et al. (2022). Which picker fits my data? A quantitative evaluation of deep learning based seismic pickers. *Journal of Geophysical Research: Solid Earth*, 127(1), e2021JB023499. https://doi.org/10.1029/2021JB023499

Okada, Y. (1992). Internal deformation due to shear and tensile faults in a half-space. *Bulletin of the Seismological Society of America*, 82(2), 1018–1040. https://doi.org/10.1785/bssa082002108

Ortega-Culaciati, F., Simons, M., Ruiz, J., Rivera, L., & D'iaz-Salazar, N. (2021). An epic Tikhonov regularization: Application to quasi-static fault slip inversion. *Journal of Geophysical Research: Solid Earth*, 126(7), e2020JB021141. https://doi.org/10.1029/2020JB021141

Schorlemmer, D., & Woessner, J. (2008). Probability of detecting an earthquake. *Bulletin of the Seismological Society of America*, 98(5), 2103–2117. https://doi.org/10.1785/0120070105

Shapiro, S. A., Patzig, R., Rothert, E., & Rindschwentner, J. (2003). Triggering of seismicity by pore-pressure perturbations: Permeability-related signatures of the phenomenon. In H.-J. Kümpel (Ed.), *Thermo-hydro-mechanical coupling in fractured rock* (pp. 1051–1066). Basel: Bir- khäuser. https://doi.org/10.1007/978-3-0348-8083-1_16

Trugman, D. T., Chamberlain, C. J., Savvaidis, A., & Lomax, A. (2022). GrowClust3D.jl: A julia package for the relative relocation of earthquake hypocenters using 3D velocity models. *Seismological Research Letters*, 94(1), 443–456. https://doi.org/10.1785/0220220193

Trugman, D. T., & Shearer, P. M. (2017). GrowClust: A hierarchical clustering algorithm for relative earthquake relocation, with application to the Spanish springs and Sheldon, Nevada, earthquake sequences. *Seismological Research Letters*, 88(2A), 379–391. https://doi.org/10.1785/0220160188

Woollam, J., Münchmeyer, J., Tilmann, F., Rietbrock, A., Lange, D., Bornstein, T., et al. (2022). Seisbench-A toolbox for machine learning in seismology. *Seismological Research Letters*, 93(3), 1695–1709. https://doi.org/10.1785/0220210324

Zhu, W., & Beroza, G. C. (2019). Phasenet: A deep-neural-network-based seismic arrival-time picking method. *Geophysical Journal International*, 216(1), 261–273. https://doi.org/10.1093/gji/ggy423