The deformation of accretionary prisms, which is governed by different boundary geometries and critically influenced by megathrust properties, plays a central role in seismic hazard assessment [1] because the associated fault evolution and displacement are closely linked to subduction dynamics and seismic activity [2,3]. In this study, we use the discrete element method (DEM) [4] to investigate how variations in basal frictional properties and surface roughness (especially Horst-graben structures) affect the formation and evolution of accretionary prisms.

Our numerical sandbox approach [5] based on DEM simulates the collective behaviors of brittle rocks under tectonic loading [3,5,6] by representing the crust as a collection of rigid grains interacting according to grain-scale laws. This method allows for large grain displacements without prescribing fault locations or geometries, allowing fault systems to emerge automatically in response to tectonic forces. Through such numerical sandbox modelings, we explore how frictional properties and Horst-graben structures drive thrust vergence and wedge deformation in these different tectonic settings. We apply our model to the Sumatra and Japan trenches, both highly active subduction zones, and compare simulation results with observations to understand the subduction dynamics.

Although we focus on the Sumatra and Japan Trench, the lessons learned regarding fault geometry, thrust vergence, and wedge deformation have broader implications for subduction zones worldwide, including those that host both slow and fast earthquakes. By integrating observed geophysical data with our simulation results, we aim to advance the understanding of how frictional properties and upper plate structures modulate seismic and aseismic processes in tectonic environments.

Reference:

[1]. Cubas, N., Souloumiac, P., & Singh, S. C. (2016). Relationship link between landward vergence in accretionary prisms and tsunami generation. Geology, 44(10), 787–790. https://doi.org/10.1130/g38019.1

[2]. Qin, Y., Chen, J., Singh, S. C., Hananto, N., Carton, H., & Tapponnier, P. (2024). Assessing the risk of potential tsunamigenic earthquakes in the Mentawai region by seismic imaging, Central Sumatra. Geochemistry, Geophysics, Geosystems, 25, e2023GC011149. https:// doi.org/10.1029/2023GC011149

[3]. Furuichi, M., Chen, J., Nishiura, D., Arai, R., Yamamoto, Y., & Ide, S. (2024). Virtual earthquakes in a numerical granular rock box experiment, Tectonophysics, 874 (230230), https://doi.org/10.1016/j.tecto.2024.230230.

[4]. Matuttis, H.–G., & Chen, J. (2014). Understanding the discrete element method: Simulation of non‐spherical particles for granular and multi-body systems. John Wiley & Sons.

[5]. Furuichi, M., Nishiura, D., Kuwano, O., Bauville, A., Hori, T., & Sakaguchi, H. (2018). Arcuate stress state in accretionary prisms from real‐scale numerical sandbox experiments. Scientific Reports, 8(1), 8685. https://doi.org/10.1038/s41598–018–26534–x

[6]. Scholtès, L., & Donzé, F.–V. (2013). A DEM model for soft and hard rocks: Role of grain interlocking on strength. Journal of the Mechanics and Physics of Solids, 61(2), 352–369. https://doi.org/10.1016/j.jmps.2012.10.005

How to cite: Chen, J., Qin, Y., Nishiura, D., and Furuichi, M.: Subduction Deformation Under Frictional and Structural Controls: A DEM-Based Analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8078, https://doi.org/10.5194/egusphere-egu25-8078, 2025.

The subduction of active spreading centers is an unusual phenomenon along subduction zones. In southern Chile, the Nazca-Antarctic spreading system (Chile Rise) subducts beneath the South American plate at the Chile Triple Junction (CTJ), forming the Patagonian slab window. The onset of the slab window has been estimated based on plate kinematic reconstructions, but direct observations remain insufficient. To study this tectonic feature in detail, an Ocean Bottom Seismometer (OBS) array was deployed south of the CTJ between 2019 and 2021, and many earthquakes were detected and located around the CTJ.  Using these continuous data and the envelope correlation method, we searched for tectonic tremors to complement the seismic observations and detected more than 500 events in this period. The tremors detected are mainly located beneath the Taitao Ridge, where no fast earthquakes were observed. The tremors exhibit burst and episodic activity, reaching depths less than 20 km. A notable separation between fast seismicity and tremors is observed at the current location of the subducted Chile Rise segment. We interpret this seismic gap as evidence of the Patagonian slab window formation within the last 0.3 Myr. The shallow tremor activity is likely triggered by the migration of fluids, introduced by the subduction of the spreading ridge, into the accretionary prism preserved along the Taitao Ridge.

How to cite: Azúa, K., Ide, S., Yano, S., Ruiz, S., Sugioka, H., Shiobara, H., Ito, A., Miller, M., and Iwamori, H.: Revealing the beginning of Slab Windows at the Chilean Triple Junction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-747, https://doi.org/10.5194/egusphere-egu25-747, 2025.

Very low frequency earthquakes (VLFE) are seismic events whose sources emit significantly more energy at low frequencies than at high frequencies, when compared to standard earthquakes. Such original seismicity activity requires specific methods to be exhaustively detected and located. 

The VLFE_DRL (VLFE Detection and Relative Location) method has been developed in this respect and consists of two main steps. Its first step aims at detecting events that share similar low frequency waveforms with the ones of known earthquakes. Recorded waveforms of catalogued earthquakes are taken as templates for several stations. These templates are match-filtered on a low-frequency bandwidth to the continuous records of the corresponding stations. Among the stations, one is chosen as a reference, and the others are paired with it. For each pair, a time delay is allowed to maximize the correlation at the non-reference station. It enables the detection of events that are not collocated with the chosen templates. Moreover, thanks to the delays obtained with the different pairs, the events can be located relatively to their templates. The second step of the method aims at identifying VLFEs among the detected events, based on their high-frequency contents. Events detected by the same template are gathered in so-called “families” of similar earthquakes (both in location and mechanism). Within each family, the relative stress drop between each detected earthquake and the event with the largest stress drop is then computed. Events with abnormally low relative stress drops are identified as VLFEs.

The VLFE_DRL method can be applied iteratively : waveforms of detected events can then be used as templates. It allows the detection of events that are further from the catalogued earthquakes, increasing the likelihood of identifying abnormal events. The method has been applied iteratively in the Southern Ryukyu subduction zone, known for its VLFE activity. Between 1996 and 2024, its application detected and located several hundreds of VLFEs. This database of VLFEs waveforms is now used to extract their moment rate functions, in order to explore their magnitude-duration scaling law.

How to cite: Delaporte, T. and Vallée, M.: Very low frequency earthquakes detection and characterization in the Southern Ryukyu subduction zone using a new time-differential template matching method (VLFE_DRL), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10040, https://doi.org/10.5194/egusphere-egu25-10040, 2025.

Frequent slow slip events (SSEs) occur along the shallow (< 15 km depth) Hikurangi margin, which marks the subduction of the Pacific plate beneath the Australian plate in the North Island of New Zealand. Geodetic observations suggest that these events exhibit varying recurrence intervals and slip patterns along the margin (e.g., Wallace et al. 2012, Wallace 2020). However, the recurrence patterns of these SSEs have not been well characterized. To address this knowledge gap, in this work we statistically model the occurrence of shallow Hikurangi SSEs in space and time. For that purpose, we first construct a catalog of these SSEs using the method developed by Ducellier et al. (2022), which uses wavelet analysis to identify SSEs in GPS time series. We complement the method with manual picking of GPS time series to determine the start and end times of SSEs at each GPS station. We identify 92 SSEs along the shallow Hikurangi margin between 2006 and 2024. To investigate the recurrence patterns of SSEs in the catalog, we fit a renewal process using Bayesian inference to obtain the posterior distribution of the parameters. These posterior estimates are then used to infer SSEs’ inter-arrival time and periodicity. Our results show that SSE inter-arrival time distribution vary along the margin, less frequent SSEs occur in the southern part of the margin (offshore Cape Turnagain) and more frequent events occur in the northern part (offshore Tolaga Bay and Gisborne areas). These results are consistent with previous observations (Wallace 2020). The periodicity of SSEs also changes along strike. SSEs in the northern and southern parts of the margin occur more regularly than those at the central part of the margin. We also compare the recurrence patterns of SSEs before and after the 2016 M_w7.8 Kaikoura earthquake, which ruptured New Zealand’s northeastern South Island and triggered widespread slow slip in the Hikurangi subduction zone. Our findings show that after the earthquake SSE occurrence is more periodic in some parts of the margin, while SSE mean length increases in the central part of the margin (offshore Gisborne). Our results highlight the patterns of SSE behavior along the Hikurangi margin and their sensitivity to external stress perturbations.

How to cite: Perez-Silva, A., Wang, T., Wallace, L., Bebbington, M., and Denys, P.: Assessing occurrence patterns of shallow Hikurangi slow slip events using renewal processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13009, https://doi.org/10.5194/egusphere-egu25-13009, 2025.

Subduction zones are among the most seismically active regions in the world, producing more than half of the global seismicity, releasing most of the total seismic energy, and hosting the largest known earthquakes and slow slip events (SSEs). SSEs and “fast” earthquakes are observed to coexist, interact, and complement each other at subduction margins, raising seismological questions with significant implications for earthquake and tsunami hazard assessments. Over the past two decades, almost 50 M_w ≥ 5.0 SSEs have been recorded in Mexico, and at least six of them began shortly after M_w ≥ 7.0 fast earthquakes. Here, we statistically quantify the interaction between regular earthquakes and SSEs along the Mexican subduction zone by analysing variations in seismological (e.g., Gutenberg-Richter a- and b-values), geodetic (e.g., seismic coupling), and kinematic (e.g., surface velocities) parameters before, during, and after the occurrence of SSEs. To do this, we use a catalogue of M_w ≥ 4.0 declustered seismicity, long-term estimates of interseismic strain rates based on GNSS data, and detailed SSE source models. Preliminary results indicate that the largest SSEs in Mexico tend to be shallow (d ≤ 30 km), spatially coinciding with relatively large crustal deformation rates (above 3.0 x 10^-7 / year)  and nucleating at a distance of approximately 20 km from historical M_w ≥ 7.0 interface earthquake ruptures.

How to cite: Bayona, J. A., Villafuerte, C., Plata-Martínez, R., Passarelli, L., Husker, A., and Werner, M. J.: Quantifying the interplay between fast and slow earthquakes along the Mexican subduction zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13627, https://doi.org/10.5194/egusphere-egu25-13627, 2025.

The Japan Agency for Marine-Earth Science (JAMSTEC) is operating some long-term borehole monitoring systems (LTBMSs) in the Nankai Trough seismogenic zone, by which slow slip events (SSEs) had been detected using pore fluid pressure monitoring (e.g., Araki et al. 2017 and Ariyoshi et al., 2021). The latest LTBMS named as C9038B has been installed in 2023, and the in-situ dataset have been available in the early 2024 by connecting with the Dense Ocean-floor Network System for Earthquakes and Tsunamis (DONET). The in-situ pore fluid pressure is measured by two absolute pressure gauges deployed at the seafloor, which is connected to the borehole and processed by calculation difference between the borehole pressure and the seafloor pressure to cancel the effect of ocean tide. Three pressure gauges, i.e., two pressure gauges and one pressure gauge respectively measuring borehole pressure and seafloor pressure are deployed at the seafloor, making it possible to replace the sensor itself in the case of degradation. Prior to deployment at the seafloor, we pressurized 20 MPa, almost equivalent to 2,000 m depth to three pressure gauges by using a pressure balance with ambient temperature of 2 °C for one month to evaluate the sensor’s drift, and determined which sensors are suitable to measure the borehole pressure and the seafloor pressure based on the experimental results. The pressure gauges showing the first and the second smallest sensor drift were used for seafloor and the borehole pressure measurements, respectively. Comparing the sensor drift of the experiment with that of the in-situ measurement for the initial one month, our laboratory evaluation could support the in-situ observation. The recent pressure measurement suggests that the pore fluid pressure is rising up at a rate of 0.3 hPa per day in the borehole over time. Although the permeability of the pore fluid pressure section in the borehole can be changing, further discussions may be needed to clarify the reason.

How to cite: Matsumoto, H., Araki, E., Ariyoshi, K., and Machida, Y.: Initial evaluation of pore fluid pressure of the new long-term borehole monitoring system in the Nankai Trough, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4953, https://doi.org/10.5194/egusphere-egu25-4953, 2025.

Deep slow earthquakes are generally recognized where subducting oceanic plates are in contact with the shallow mantle. High fluid pressure is commonly invoked as an important factor in the generation of slow earthquakes and the associated fluids are thought to be derived from the breakdown of hydrous minerals such as chlorite and serpentine. When considering how such fluids may be related to earthquake generation it is important to consider both the quantities and fluid paths ways. Buoyancy will tend to drive the fluids vertically upwards. But serpentinite developed along the base of the mantle wedge may act as an effective seal and cause flow to be channelled along the subduction boundary. Numerous serpentinite bodies are exposed throughout the Cretaceous subduction-type Sanbagawa belt of SW Japan. These serpentinite bodies are derived from the wedge mantle and the adjacent metamorphic units, consisting primarily of mafic, quartz, and  pelitic schists are derived from basalt, chert and mudstone of the crust of the subducting slab. The boundary between the serpentinite bodies and the schists therefore represents the paleo subduction boundary and the rocks along this boundary are a potentially important record of the way in which subduction fluids move. We highlight the characteristics of two separate kilometer-scale bodies, the Shiragama Yama body (SY) which rose from depths of ~35 km and the Kamabuse Yama (KY) body which rose from depths of ~25 km. The mineralogy of SY suggests SiO2 transported by hydrous fluids is restricted to a ~70 m thick shear zone at the base consisting of high-T serpentine, antigorite. In contrast, KY shows evidence for pervasive SiO2 enrichment with a more limited zone sheared zone consisting dominantly of low-T serpentine, chrysotile. Analyses of noble gas and halogen of the boundary domain lithologies were performed to help identify the source of fluids related to hydration and material transport. In SY these data support the idea that far-travelled fluids were channeled along the subduction boundary. The results for KY are more complex with distinct fluids responsible for serpentinization (1) and metasomatism (2). The metasomatism can be further divided into a chlorite-forming stage (2-1) and a later talc-forming stage (2-2). The sources for the fluids involved in each of these stages were likely derived from (1) altered oceanic crust, (2-1) altered oceanic crust + sedimentary porosity, and (2-2) sedimentary porosity + serpentinized slab.

Combining the results from SY and KY suggests that channeling of subduction fluids in the Sanbagawa subduction zone was important at depths of around 35 km but was less effective at shallower levels. The change in the source region of the fluids with time shown in KY, suggests fluid flow may become more channelized as a shear zone develops along the subuction interface.

How to cite: Wallis, S., Aida, K., and Sumino, H.: Noble gas and halogen records of subduction fluids along the paleo subduction boundary at the base of the shallow mantle wedge: example of the Cretaceous Sanbagawa belt, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18557, https://doi.org/10.5194/egusphere-egu25-18557, 2025.