# Seamount Subduction and Megathrust Seismicity: The Interplay Between Geometry and Friction

I. Menichelli, F. Corbi, S. Brizzi, E. van Rijsingen, Serge Lallemand, F.

Funiciello

► To cite this version:

I. Menichelli, F. Corbi, S. Brizzi, E. van Rijsingen, Serge Lallemand, et al. Seamount Subduction and Megathrust Seismicity: The Interplay Between Geometry and Friction. Geophysical Research Letters, 2023, 50 (9), 10.1029/2022GL102191. [https://doi.org/10.1029/2022GL102191](https://doi.org/10.1029/2022GL102191)

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

## RESEARCH LETTER

10.1029/2022GL102191

**Key Points:**

* We investigated the effect of seamount subduction on megathrust earthquakes by isolating the role of geometry from that of friction

* Seamounts act primarily as barriers to earthquake propagation, promoting small ruptures and decreasing seismic coupling

* Reduced friction displays maximum barrier efficiency, a configuration likely associated with fluids or fracture systems in nature

**Supporting Information:**

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

**Correspondence to:**

I. Menichelli,
irene.menichelli@uniroma3.it

**Citation:**

Menichelli, I., Corbi, F., Brizzi, S., van Rijsingen, E., Lallemand, S., & Funiciello, F. (2023). Seamount subduction and megathrust seismicity: The interplay between geometry and friction. *Geophysical Research Letters*, *50*, e2022GL102191. https://doi.org/10.1029/2022GL102191

Received 19 JAN 2023

Accepted 18 APR 2023

**Author Contributions:**

**Conceptualization:** I. Menichelli, F.

Corbi, S. Brizzi, S. Lallemand

**Data curation:** I. Menichelli

**Formal analysis:** I. Menichelli, S. Brizzi

**Investigation:** I. Menichelli, F. Corbi, S. Brizzi

**Methodology:** I. Menichelli, F. Corbi

**Project Administration:** F. Funiciello

**Supervision:** F. Corbi, S. Lallemand, F. Funiciello

**Validation:** I. Menichelli

**Visualization:** E. van Rijsingen, F.

Funiciello

**Writing – original draft:** I. Menichelli

© 2023. The Authors.

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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## Seamount Subduction and Megathrust Seismicity: The Interplay Between Geometry and Friction

I. Menichelli¹ ID, F. Corbi² ID, S. Brizzi¹ ID, E. van Rijsingen³ ID, S. Lallemand⁴ ID, and F. Funiciello¹

¹Dip Scienze, Laboratory of Experimental Tectonics, Università Roma TRE, Rome, Italy, ²Istituto di Geologia Ambientale e Geoeingegneria – CNR c/o Dipartimento di Scienze della Terra, Sapienza Università di Roma, Rome, Italy, ³Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands, ⁴Géosciences Montpellier, CNRS, Montpellier University, Montpellier, France

**Abstract** Subducting seamounts are recognized as one of the key features influencing megathrust earthquakes. However, whether they trigger or arrest ruptures remains debated. Here, we use analog models to study the influence of a single seamount on megathrust earthquakes, separating the effect of topography from that of friction. Four different model configurations have been developed (i.e., flat interface, high and low friction seamount, low friction patch). In our models, the seamount reduces recurrence time, interseismic coupling, and fault strength, suggesting that it acts as a barrier: 80% of the ruptures concentrate in flat regions that surround the seamount and only smaller magnitude earthquakes nucleate above it. The low-friction zone, which mimics the fluid accumulation or the establishment of fracture systems in natural cases, seems to be the most efficient in arresting rupture propagation in our experimental setting.

**Plain Language Summary** Seamounts – extinct volcanoes – are ubiquitous features of the seafloor of subducting plates. During their descent toward the mantle, seamounts are squeezed between the overriding- and subducting plates, creating a geometrical and mechanical discontinuity that is thought to control the largest earthquakes that occur on Earth. It is not known, however, whether they trigger or arrest earthquakes, as a variety of observations have been used to support one or the other hypothesis. Here, we tackle this subject using scaled laboratory models that allow reproduction of large earthquakes. Our models support a scenario where subducting seamounts primarily arrest rupture propagation, limiting therefore expected maximum magnitudes. Our models also suggest that subducting seamounts might influence the recurrence time of large earthquakes by decreasing their frequency.

## 1. Introduction

The most destructive earthquakes occur along subduction megathrusts. Previous studies have shown that megathrust seismicity is influenced by different geometrical and physical parameters including interplate roughness (Bletery et al., 2016; Kelleher & McCann, 1976; Lallemand et al., 2018; Scholz & Small, 1997; van Rijsingen et al., 2018). van Rijsingen et al. (2018) and van Rijsingen et al. (2019) showed that large earthquakes preferentially propagate on smooth megathrusts, while rough interfaces host seismic events with shorter duration, smaller magnitude and mean slip. Seafloor roughness is indeed thought to influence the topography and friction of the plate interface, hence seismic behavior. However, it is still not clear whether seamounts act as asperities, promoting ruptures (Bilek et al., 2003; Heuret et al., 2011; Scholz & Small, 1997; Thatcher, 1990) or barriers, inhibiting large earthquakes propagation (e.g., Collot et al., 2017; Kodaira et al., 2000; Marcaillou et al., 2016; Mochizuki et al., 2008; Morton et al., 2018; Wang & Bilek, 2011; Xia et al., 2021). Recent studies suggest that seamount subduction creates a fracture network along its path, promoting smaller earthquakes and creeping (Mochizuki et al., 2008; Passarelli et al., 2022; Wang & Bilek, 2011, 2014), but opposite scenarios have also been proposed in which seamounts indent the overlying plate, increasing normal stress and promoting large earthquakes (Bilek et al., 2003; Cloos & Shreve, 1996; Husen et al., 2002; Scholz & Small, 1997). Seamounts are also thought to influence pore fluid pressure and sediment consolidation and, in turn, megathrust strength and slip behavior. Sun et al. (2020) showed that stronger consolidation develops on the leading flank of the seamount, promoting fault instability and the nucleation of small earthquakes, while on the top and behind the seamount, high-porosity sediments and the presence of fluids promote low fault strength and slow, aseismic slip. These observations suggest a dualistic control of the seamount on rupture propagation.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Oblique view of the experimental apparatus and experimental configurations (models 1–6). Red triangles and light blue rectangles highlight the trench and the seismogenic zone, respectively. The seamount location is represented by the dark blue (with sandpaper, i.e., high friction) and white (without sandpaper, i.e., low friction) squares.</figcaption></figure>
detailed in Corbi et al. (2017) and van Rijsingen et al. (2019). PIV measurements of relevant parameters are
performed at the model surface, but we assume them to mirror their equivalents along the analog megathrust.
This assumption implies a minor (i.e., <10%) underestimation of average displacement and negligible impact on
the recurrence time, lateral extent and location of individual events (Mastella et al., 2022).

**3. Results**

**3.1. General Model Behavior**

All models show an initial four to nine min-long loading phase, characterized by landward motion of the trench and shortening (~2.0–3.5 cm) of the gelatin wedge. This phase represents the time needed for the system to overcome the strength threshold before entering into the stick-slip frictional dynamics. Each stick-slip cycle is equivalent to an analog seismic cycle consisting of an interseismic phase in which stress is accumulated, and a coseismic phase during which stress is suddenly released as slip episodes that propagate at the wedge-basal plate interface. Stick-slip dynamics is illuminated by PIV as a sequence of velocity peaks (i.e., quasi-dynamic coseismic phases; Figure 2a) that alternate with phases of slow velocities (i.e., interseismic periods). Figure 2a shows the distribution of velocity peaks over time. Two phases can be distinguished, based on the amplitude and interval between velocity peaks. During the initial ~380–1,200 s, peak velocities reach a maximum of 0.8 cm/s with an average value of 0.6 cm/s. After this stage, the top of the seamount passed beyond the downdip limit of the seismogenic zone and the maximum peak velocity decreases to ~0.15–0.2 cm/s. Rt is shorter here than in the initial phase, where it reaches a maximum of 48 s. Analog earthquakes are quasi-periodic, as proven by a coefficient of variation Cv = 0.48. Figure 2b shows the rupture propagation along a section parallel to the trench. Ruptures strike at different locations along the seismogenic zone, filling “seismic gaps” or repeating in the same region (Figure 2c). Again, two phases can be distinguished: between ~380 and 1,200 s, ruptures tend to alternate, extending along the entire seismogenic zone or concentrating within or outside the seamount. Between ~1,200 s and the end of the experimental run, ruptures tend to be smaller both in terms of extent and displacement and occur preferentially outside the seamount path. Figures 2c and 2d show surface displacement associated with four characteristic analog earthquakes with ruptures (a) interrupted by the seamount; (b) involving the entire seismo-genic zone; (c) confined within the seamount; (d) propagating outside the seamount. Larger ruptures correspond to larger Mw (>8.2), Cd (~0.25 cm), Rt (>30–35 s) and longer duration (>1.4 s). Ruptures involving only the seamount or falling outside it are smaller, with Mw ~7.5, shorter Rt (<20 s), and lower Cd (~0.05–0.1 cm).

**3.2. Insights From the Experimental Series**

The impact of the seamount on seismic behavior was studied through the different model configurations. Figure 3 summarizes how the presence, location, and friction of the seamount affect the investigated source parameters. Figure 3a illustrates the variation of Rt in the different models. In the models with a high friction seamount (models 1 and 2), the average Rt is about 16 s and the maximum Rt is lower than 32 s. Models with a low friction seamount (models 3 and 4) display relatively longer recurrence times (average Rt = 23.8 s) than the high-friction ones. Mw (Figure 3b) reaches higher values (7.8–7.7) in the high-friction seamount models (models 1–2) and decreases to 7.5–7.4 in the low-friction seamount models (models 3–4), regardless of the configuration (central, lateral seamount, and base). The highest median Mw was always recorded for the flat experiment (~Mw 8; model 6). Figure 3c shows that the trench-parallel distribution of the normalized cumulative displacement follows a symmetrical pattern in the flat model and when the seamount is placed at the center of the seismogenic zone. On the other hand, with a lateral seamount, the slip distribution is asymmetrical with respect to the model center and skewed toward the flat area. This pattern reflects the distribution of ruptures during the experimental run. Figure 4 confirms this behavior, summarizing the relative proportion of different types of ruptures for models with central seamount with both high and low friction seamount and base configuration. In all models, the seamount acts primarily as a barrier, with the vast majority (i.e., 80%) of ruptures occurring in the flat region at the sides of the seamount (Figure 4a). Those ruptures have generally small magnitude and small peak slip (around 0.01 cm) and dominate the second half of the experiment (Figure S5 in Supporting Information S1). The remaining 20% of ruptures are equally distributed between full ruptures and ruptures above the seamount in the high friction seamount model. The low friction model favors full ruptures instead of ruptures above the seamount, while an opposite behavior is observed in the base configuration. This rupture distribution in four types is also clearly visible in the maps of coseismic displacement cumulated over all events (Figures 4b–4e): higher friction concentrates the highest displacement above the seamount, while lower friction distributes the cumulative displacement along
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Time series reporting the maximum velocity (trench orthogonal component) measured within the seismogenic zone surface projection for model 1 (high friction central seamount) that exemplifies the overall model behavior. The solid red line highlights the velocity threshold used for event detection. Yellow stars highlight the four earthquakes represented in panels (c-d). (b) Line-time rupture history sampled along a trench parallel section highlighted by the white line in panels d. Red lines indicate the location of the seamount. (c) Distribution of the cumulative displacement of the four characteristic ruptures. The blue rectangle marks the location of the seamount. (d) Map view of the cumulative slip of the 4 types of ruptures. The white dashed lines represent the updip and downdip limits of the seismogenic zone. The solid white line represents the section of the profiles shown in panel c. The white transparent rectangle is the seamount.</figcaption></figure>

the entire seismogenic zone with two peaks at the sides (i.e., in trench parallel direction) of the seamount. This distribution can also be observed in Figure 3c where the normalized Cd is almost flat with two minor peaks at ~13 and 26 cm. The base configuration promotes a cumulative displacement with the same spatial pattern as in the high friction model but with around 66% smaller amplitude. The relative proportion between rupture types could be affected by the arbitrary minimum displacement threshold used to define the rupture area (see Supporting Information S1). To check the sensitivity of our results, we tested different thresholds to constrain the rupture areas. Considering 20% higher- and lower-than-reference thresholds, we observe stable classifications for full and above the seamount rupture types, while “around” and “away” the seamount ruptures” are subject to a <15% classification variation (in both cases the largest variations are observed in base configuration). In addition, to avoid uncertainties related to selecting a threshold, we used the seismic asperity distribution, that is, the area where 50% of the maximum slip has been recorded (Lay et al., 1982). Also in this case, rupture classification appears stable, with a maximum variation of 20% observed for around seamount rupture types in model 5.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Source parameters for the six model configurations. (a) recurrence time, (b) Mw, and (c) normalized cumulative displacement (i.e., the displacement value is summed for all ruptures and normalized with respect to the normalized maximum cumulative slip of the flat model). Solid and dashed lines indicate the high and low seamount friction models, respectively.</figcaption></figure>

4. Discussion

Our models allowed us to study separately the effect of a single geometric feature with high (models 1–2) or
low friction (models 3–4) on earthquake rupture patterns. Our results show that a geometrical feature on a flat
megathrust reduces the seismic coupling (Figures 4d and 4b), in agreement with van Rijsingen et al. (2019). By
comparing models with the high friction seamount (model 1) and the flat interface (model 6) (Figures 4b and 4c),

<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) Histogram of rupture classification as illustrated on Figure 2d. (b)-(e) Top view map of the normalized cumulative displacement Cd for the four model configurations. The red square indicates the position of the seamount at the beginning of the experiment and the blue one at the end. Model 5 shows the higher barrier effect with the lowest long term seismic coupling (LTSC) and only 2% of total ruptures.</figcaption></figure>
we observe that the presence of the seamount reduces seismic coupling by 60% (i.e., long term seismic coupling LTSC from 60% to 24%). This decrease promotes the occurrence of smaller earthquakes on top of the seamount with the same recurrence time of model 6 (i.e., flat interface). These smaller earthquakes are the consequence of the primary role of the seamount acting as a barrier (i.e., 80% of the ruptures are arrested by the presence of the seamount). As for the geometrical protrusion, also a low friction patch (model 5) reduces seismic coupling by 38% (i.e., long term seismic coupling LTSC from 24% to 15%) as compared to the high-friction model (Figures 4b and 4e). The lower coupling of model 5 causes relatively longer recurrence times, although the percentage of arrested ruptures (Figure 4a) is roughly the same as for the other models (models 1 and 3). The combined effect of lower friction and the geometrical protrusion can be observed in the low friction seamount model (model 3; Figure 4d). The decreased seismic coupling and the percentage of ruptures arrested by the seamount are about the same as in model 1 (Figure 4a). The primary difference between models 1 and 3 is the distribution of seismic coupling (Figures 4b and 4d), which is focused above the seamount in model 1 and homogeneously distributed along the strike in model 3. In model 1, the higher cumulative displacement above the seamount (Figure 4b) is related to the higher number of events that propagate through this area (Figure 4a). Our results support a scenario where a subducting seamount acts primarily as a barrier as evidenced by the higher percentage of ruptures occurring outside and stopping at the seamount (Figure 4a). Model 5 (i.e., low friction patch) is the most efficient model configuration for segmenting ruptures along the seismogenic zone: total ruptures reach only ~2% (Figure 4a) and more stress is released through small earthquakes on the low-friction patch spaced by longer recurrence times. When geometry is added at the interface (model 3), the barrier efficiency is the lowest; the higher coupling (31%) causes the accumulated stress to be such that more ruptures (~17%) propagate along the entire seismogenic zone. Model 1 causes the percentage of total and above seamount to be roughly similar; the barrier effect is not as prominent as in model 5. Numerical models have shown how a seamount, with different geometry, location, and normal stress, can act as a barrier stopping earthquake propagation (Sun et al., 2020; Yang et al., 2013; Zielke et al., 2017). Yang et al. (2013) showed that not only seamounts with a higher height-to-width ratio and closer to the rupture nucleation zone but also those with negative normal stress (i.e., with the presence of fluids) at the interface can halt rupture propagation. Therefore, frictional heterogeneities are a key parameter for inhibiting megaearthquakes, which on the contrary are free to propagate above a flat interface. Similar behavior has been observed in numerical models, focusing on the hydrological effect of a seamount (Sun et al., 2020). These models highlighted that seamounts prevent the occurrence of mega earthquakes, favoring microseismicity in the downdip zone and aseismic or slow-slip events in the area from the top of the seamount upward. These numerical results match with our laboratory observations where the seamount plays a key role in inhibiting megathrust earthquakes, but friction has an important part in the barrier effect too. The seamount and uneven distribution of friction at the interface introduce heterogeneous stress distribution, confirmed by a variable recurrence time, ruptures characterized by different extensions and locations, and an asymmetrical Cd distribution along the trench (Figures 3c and 4). This heterogeneous stress distribution, as already evidenced by Wang and Bilek (2011), Wang and Bilek (2014), and van Rijssingen et al. (2019), defines a segmentation of the seismogenic zone and a lower coupling effect on the subduction interface.

## 4.1. Comparison With Natural Cases

Observations from natural cases support contradictory theories on the relationship between seamounts and large megathrust earthquakes (Mochizuki et al., 2008; Scholz & Small, 1997; Wang & Bilek, 2011, 2014). Despite the simplicity of our models, our findings are consistent with the hypothesis that seamounts act as seismic barriers, decreasing both seismic coupling and maximum magnitude of earthquakes (von Huene, 2008; Mochizuki et al., 2008; Wang & Bilek, 2011, 2014; Todd et al., 2018; Passarelli et al., 2022). Seamounts likely act as a barrier to rupture propagation due to a fracture network forming in the upper plate, which decreases seismic coupling and increases creep (Wang & Bilek, 2011, 2014). In this theoretical model, stress is supposed to be released through fractures and smaller earthquakes, such that large stress cannot accumulate on the seamount. In addition, subduction of fluid-rich sediments generally promotes overpressure conditions that decrease friction. This results in heterogeneous seismic properties of the fault zone that disadvantage the occurrence of large earthquakes (Bassett et al., 2014; Bell et al., 2010; Ellis et al., 2015). Offshore geodetic measurements at the Nankai trough (Yokota et al., 2016) suggest that seamount subduction is associated with low-frequency earthquakes and low slip deficit rates. This behavior is thought to be promoted by increased pore fluid pressure, and the development of a fracture system within the upper plate due to the indentation of seamounts. These observations support the model of
Wang and Bilek (2011), which assumes that the roughness of the plate interface causes creep. In addition, Todd et al. (2018) linked the subduction of shallow seamounts in the northern margin of Hikuragi (New Zealand) to low-frequency seismic activity. In this area, the presence of seamounts enhances weak interplate coupling (von Huene, 2008; Mochizuki et al., 2008; Wang & Bilek, 2011, 2014) that promotes the nucleation of shallow slow slip events, tsunami earthquakes and microseismicity, and controls the location of tectonic tremors. Observations from the Loyalty Ridge confirm how the presence of bathymetric highs along the subduction zone leads to the development of a complex deformation pattern, and possibly inhibits the occurrence of earthquakes with Mw > 8 (Passarelli et al., 2022). Observations at natural subduction zones suggest that two main mechanisms (i.e., development of the upper plate fracture network and lower friction due to sediments rich in fluids) associated with subducting seamounts are responsible for stress heterogeneities and, in turn, promote earthquakes of relatively lower magnitude. Despite the absence of a network of fractures and fluid-rich sediments in the analog models, even in our study, the presence of geometrical and frictional heterogeneities results in narrow ruptures that rarely propagate through the entire lateral extent of the seismogenic zone. In our models, the decrease in coupling is likely related to nonuniform friction and stress conditions acting at the interface. The different features of each model configuration gave us the valuable opportunity to discern the effect of lower friction from that of geometric irregularity: the greatest segmenting power is achieved with the lower friction patch (model 5), which can be linked in nature to an accumulation of fluids at the interface, released during the subduction process, or of water-rich sediments dragged by the subducting plate.

## 5. Conclusions

We used analog models to investigate the role of a single subducting seamount on megathrust seismicity and separate the contribution of geometry from that of friction. Results show that the seamount decreases seismic coupling, favoring the occurrence of smaller earthquakes outside the seamount area and acting as a barrier to rupture propagation. The strongest segmenting power is achieved in the low-friction patch model, inducing stress release through smaller earthquakes. This behavior fits with natural cases in which the likely low friction, due to seamount fracture formation, fluid release, or the different porosity of subducting sediments, inhibits the occurrence of large earthquakes and promotes microseismicity, slow slip events, and seismic tremor instead.

## Data Availability Statement

PIV Data and codes underlying this study are published open access in Menichelli, Corbi, Brizzi, van Rijsingen, Lallemand and Funiciello (2023), https://doi.org/10.5880/fidgeo.2022.047.

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