Slow Slip Events (SSEs) play an important role in the seismic cycle, participating in the moment budget of active faults. SSEs can be monitored via space geodesy (e.g., Global Navigation Satellite System, GNSS). One of the major challenges when studying geodetic data is that they record the deformation due to many active sources (e.g., tectonic, hydrological, volcanic, and anthropogenic). We present a procedure to automatically reconstruct the spatio-temporal history of SSEs in the Cascadia subduction region in near real-time. The solution is updated daily, and the last update refers to the day before yesterday because of latency time to retrieve the rapid solutions. The experiment has been running since August 2024. Given the duration of days/weeks of slip episodes, these results open the door to prospective forecasting experiments rather than retrospective ones. First forecasting results will be presented and discussed.
How to cite: Gualandi, A. and Darcy, M.: Near Real-Time Cascadia Slow Slip Events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13637, https://doi.org/10.5194/egusphere-egu25-13637, 2025.
Tectonic stresses at convergent plate boundaries are primarily managed through seismic ruptures and aseismic slips, both vital for maintaining lithospheric balance. Stress drops in fault nucleation zones can occur at varying rates during fast and slow earthquakes, leading to permanent stress loads and nearby structural disruptions. This stress transfer often correlates with earthquake swarms triggered by slow slips and changes in principal stress axes following seismic events. Recent studies emphasize the intricate interactions between slow and fast earthquakes, highlighting the need to understand these dynamics for crustal stress management. 2018 we commenced a decade-long research plan after observing slow slip events (SSEs) in the Ryukyu subduction zone, offshore northeastern Taiwan. These SSEs likely initiate the downdip of a seismogenic locked zone, with potential Mw ≥ 8.0 megathrust earthquake and tsunami threats. The SSE source zone is separate from the coseismic slip area of the 2002 Mw 7.1 Hualien earthquake and overlaps with the afterslip, contributing to irregular SSE recurrence patterns. A significant correlation exists between SSEs and earthquake swarms, which typically occur every few years, featuring Mw ≥ 4.0 events above the SSE source zone. Notable swarms occurred in 2005, 2009, and 2015, intensifying occurrences of these significant earthquakes. We analyzed the Central Weather Administration (CWA) events in northeastern Taiwan from 2000 to 2016, calculating temporal variations in b-values and stress orientations using data from the Broadband Array for Seismology in Taiwan (BATS). The b-values showed a marked drop during the 2009 SSE, while stress orientations exhibited a rotation in σ1. This stress rotation likely initiated the SSEs, facilitating a cyclic transfer of stress to the earthquake swarms.
How to cite: Chen, S. K., Tang, C.-H., Peng, W., Chen, K. H., Lai, Y.-T., Shyu, J. B. H., and Wu, Y.-M.: Stress transfer cycle from slow to fast earthquakes across the southernmost Ryukyu subduction thrust during deep slow slip, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3387, https://doi.org/10.5194/egusphere-egu25-3387, 2025.
JAMSTEC have been monitoring changes in underground fluid pressure, or "pore pressure," from boreholes near the site of the 1944 Tonankai earthquake in southwestern Japan. These changes are linked to Slow Slip Events (SSEs), which occur on the boundary between the Eurasian plate and the subducting Philippine Sea plate beneath the Nankai Trough. By connecting their borehole observatory (LTBMS) to a seafloor monitoring network (DONET), they now collect real-time pore pressure data, allowing them to update their SSE catalog.
This updated catalog revealed something unusual: the SSE in February 2012 lasted significantly longer than similar events. Researchers studied pore pressure and seafloor pressure data to understand why. We found that the February SSE moved more slowly and lasted longer because of two key factors: internal and external forces.
Internally, the SSE occurred in a region where little stress had built up on the fault, causing it to slip more slowly, consistent with frictional behavior on faults. Externally, we found that changes in seafloor pressure, driven by shifts in the Kuroshio Current (a major ocean current), coincided with the end of the February SSE. This suggests that the Kuroshio Current's meander may influence the duration of SSEs.
Our study highlights that SSEs are not only shaped by fault interactions but also by environmental factors like ocean currents and atmospheric pressure. Understanding these influences is key to better predicting such events. These findings are based on a paper accepted by Tectonophysics (https://doi.org/10.1016/j.tecto.2024.230439), and we plans to share additional insights and recent practical analysis in our presentation.
How to cite: Ariyoshi, K., Nagano, A., Hasegawa, T., Iinuma, T., Nakano, M., Saffer, D., Matsumoto, H., Yada, S., Araki, E., Takahashi, N., Hori, T., and Kodaira, S.: Understanding the Physical Process of Unusually Long-duration Slow Slip Events: Insights from Stress Interaction and Environmental Influences in the Nankai Trough, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1735, https://doi.org/10.5194/egusphere-egu25-1735, 2025.
Slip transients cover a wide range of scales in length, duration, moment, slip among others. They also exhibit a rich spectrum of behaviors like slow aseismic slip in subduction zones or transform faults, or faster and potentially devastating ruptures in the case of earthquakes. Earthquake swarms, either occurring in natural tectonic context or due to anthropogenic fluid injections at depth, have been found to exhibit another peculiar behavior: they show a global migration, sometimes accompanied by faster bursts, and could result from the interplay between fluid processes, aseismic slip and seismicity.
In this study, we synthesize findings from the literature on slow slip events and earthquakes, integrating insights from our research on earthquake swarms. We examine how swarms conform to or deviate from established scaling relations for seismic phenomena. Specifically, we compare earthquake swarms, slow slip events, and earthquakes in terms of moment and duration, and analyze the migration or rupture velocities of swarms and slow slip events relative to moment.
We highlight two different but parallel behaviors among these sequences: one linked to slow-slips, with elevated migration velocities and moments, and the other related to fluid-induced processes, featuring lower velocities and moments. These results provide metrics for distinguishing between the drivers of earthquake swarms —both natural and injection-induced— and their connections to fast seismic transients and foreshocks. This work also highlights promising directions for instrumentation and the study of slow and aseismic slip transients.
How to cite: Danre, P., De Barros, L., Cappa, F., and Passarelli, L.: Distinct yet Comparable Scaling Relations for Slow Slips and Fluid-Induced Seismic Swarms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9381, https://doi.org/10.5194/egusphere-egu25-9381, 2025.
Tremor is a weak seismic signal accompanying slow fault slip at plate boundaries. The relationship between tremor and slow slip, and the tremor source mechanism have been widely debated, owing largely to the challenge of accurately locating tremor in depth. We assemble catalogs of tremor seismicity beneath Vancouver Island during three slow slip episodes between 2003 and 2005 using a cross-station detection method adapted from previous studies to recover accurate depths. Improved tremor locations provide key constraints on i) thickness of the tremorgenic zone, ii) the relative location of tremor to key structural features in the subduction complex, and iii) the geologic context and mechanism of tremor. Tremor occurs in quasi-planar clusters < 500 m thick, beneath a high-reflectivity layer and within a zone of elevated Poisson’s ratio with P-wave velocities of 7 km/s. We interpret tremor as originating in the erosion of the upper few hundred meters of basaltic oceanic crust consistent with magnitude-frequency relations suggesting tremor generation through "block-in-matrix" granular jamming. Comminuted basalt with increasingly anisotropic fabric is underplated onto overriding lithosphere to generate high reflectivity. Tremor thus manifests areas of material transfer across the plate boundary during slow slip.
How to cite: Bostock, M., Sammis, C., Littel, G., Peacock, S., and Calvert, A.: Tectonic tremor: the chatter of mafic underplating, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7179, https://doi.org/10.5194/egusphere-egu25-7179, 2025.
Active faults exhibit a broad spectrum of slip modes, primarily governed by depth-dependent pressure and temperature conditions. These transitions manifest as fast earthquake ruptures at shallow, seismogenic depths, gradually evolving into transient slow slip and steady creep with increasing depth. In most subduction zones, the transition zone—situated between the updip seismogenic zone and the downdip steadily creeping region—is the locus of slow slip events, tectonic tremors, and Low-Frequency Earthquakes (LFEs). Within this transition zone, the recurrence patterns of tremors and LFEs display depth-dependent variations. Recurrence times decrease with depth, transitioning from low-recurrence, long-lasting bursts near the seismogenic zone to high-recurrence, short-duration bursts near the steadily creeping limit (e.g. Wech & Creager, 2011).
In this study, we perform a comparative analysis across multiple plate boundaries, focusing on the along-dip spatio-temporal clustering of tremors and LFEs in Cascadia, Nankai, Mexico, and the San Andreas Fault. We developed a robust method to systematically analyze LFE catalogs from Mexico (Frank et al., 2014), Nankai (Kato et al., 2020), Cascadia (Sweet et al., 2019), and Parkfield (Shelly, 2017).
Our method consists in examining the autocorrelation function of LFE occurrence time series to estimate the periodicity and duration of LFE bursts at various depths. Across all studied regions, we observe a consistent trend: recurrence intervals and burst durations of LFE activity decrease with increasing depth. Finally, we connect these depth-dependent behaviors to the thermodynamic conditions specific to each region, and to the plate convergence rates, providing insights into the rheological properties governing LFE activity within the transition zone.
References
Wech, A.G., Creager, K.C.: A continuum of stress, strength and slip in the Cascadia subduction zone. Nature Geoscience 4(9), 624–628 (2011) https://doi.org/10.1038/ngeo1215
Frank, W.B., Shapiro, N.M.: Automatic detection of low-frequency earthquakes (LFEs) based on a beamformed network response. Geophysical Journal International 197(2), 1215–1223 (2014) https://doi.org/10.1093/gji/ggu058
Kato, A., Nakagawa, S.: Detection of deep low-frequency earthquakes in the Nankai subduction zone over 11 years using a matched filter technique. Earth, Planets and Space 72(1), 128 (2020) https://doi.org/10.1186/s40623-020-01257-4
Shelly, D.R.: A 15 year catalog of more than 1 million low-frequency earthquakes: Tracking tremor and slip along the deep san andreas fault. Journal of Geophysical Research: Solid Earth 122(5), 3739–3753 (2017) https://doi.org/10.1002/2017JB014047
How to cite: Radiguet, M., El Yousfi, Z., Rousset, B., and Frank, W.: Insights on the rheology of the transition zone and its along-dip variation using low-frequency earthquakes clustering , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19260, https://doi.org/10.5194/egusphere-egu25-19260, 2025.
Laboratory studies of granular friction have emerged as a powerful tool for investigating seismic fault slip [1], including dynamic triggering of earthquakes and landslides [2,3]. However, physical origins of triggering by small strain amplitude from a large remote earthquake still remain unclear.
Here we report the experimental investigation of quasi-static sliding in dry and wet granular gouges layers under constant pressure, monitored with passive (acoustic emission: AE) and active acoustic detections (wave velocity and coda correlation). Both avalanche-like dynamics and quasi-periodic stick-slip behaviour are observed, illustrating a ductile-brittle like transition induced by the cohesion. These phenomena are associated with by distinct statistics of AEs (labquakes) and specific granular flow patterns. A decrease of the acoustic velocity and an increase of AE rate (precursors) are also detected before mainshocks or mainslips.
Moreover, we have investigated the dynamic triggering of the mainslip associated with strong stress drop by applying relatively high-amplitude ultrasound (of the order of 10 nm) in the steady sliding state. This dynamically triggered stress drops appear as slower (lab) earthquakes than the (quasistatic) shear-induced fault slip. We show that such acoustic triggering of macroscopic shear instability originates from the reduction of shear contact stiffness and interparticle friction between grains by the acoustic lubrication [4,5], via microslips.
[1] C. Marone, Laboratory-derived friction laws and their application to seismic faulting, Ann. Revs. Earth & Plan. Sci. 26, 643 (1998); C.H. Scholz, The Mechanics of Earthquake and Faulting (3rd edition, Cambridge University Press, 2018)
[2] P. Johnson and X. Jia, Nonlinear dynamics, granular media and dynamic earthquake triggering, Nature 437, 871 (2005)
[3] V. Durand et al, Repetitive small seismicity coupled with rainfall can trigger large slope instabilities on metastable volcanic edifices, Communications Earth & Environment 4, 383 (2023)
[4] X. Jia, T. Brunet, and J. Laurent, Elastic weakening of a dense granular pack by acoustic fluidization: Slipping, compaction, and agingPhys. Rev E 84, 020301(R) (2011)
[5] J. Léopoldès, X. Jia, A. Tourin, and A. Mangeney, Triggering granular avalanches with ultrasound, Phys. Rev. E 102, 042901 (2020)
How to cite: Jia, X., Zhou, G., Derand, P., and Tourin, A.: Acoustic triggering of shear instabilities in dry and wet granular fault gouges, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13974, https://doi.org/10.5194/egusphere-egu25-13974, 2025.
Seismic faults release the stress accumulated during tectonic movement through a spectrum of events ranging from rapid ruptures to slow slip events. Slow slip plays a crucial role in the seismic cycle impacting the occurrence of earthquakes. However, the interplay mechanisms between a slow-slip region and seismogenic zones are not well understood. In addition, the conditions required for a fault to experience slow slip have not yet been established, and the question of whether the same fault can experience different slip behavior is still under debate.
In this experimental study, we highlight a system where a slow slip region acts as a nucleation center for seismic ruptures, increasing the frequency of earthquakes (Faure and Bayart, 2024). Furthermore, we observe that along the same interface, zones can rupture seismically or experience slow slip depending on the loading conditions.
In our experiments, we emulate slow slip regions by introducing a granular material inclusion along part of a laboratory fault. By measuring the response of the fault to shear and performing interfacial slip measurements, we show that the slow-slip region acts as an initial rupture that destabilizes into a dynamic rupture, leading to a seismic event. By varying the loading conditions of the granular inclusion, we show that the earthquake frequency is related to the initial rupture characteristics, i.e., length and loading at the tip, as predicted by initiation criteria for rupture destabilization. We also find that the region of slow slip extends beyond the compositional heterogeneity, along regions that otherwise rupture seismically, demonstrating that fault composition is not the only requirement for slow slip. Our results pave the way for the construction of novel models that account for the evolution of the slow slip region under varying loading conditions, in order to improve fault monitoring and seismic hazard mitigation.
Faure, Y., Bayart, E. Experimental evidence of seismic ruptures initiated by aseismic slip. Nat Commun 15, 8217 (2024). doi:10.1038/s41467-024-52492-2.
How to cite: Bayart, E. and Faure, Y.: Interplay between slow slip and seismic ruptures: an experimental study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18355, https://doi.org/10.5194/egusphere-egu25-18355, 2025.
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.
Subduction zone megathrust faults host about 75% of the large earthquakes (M≥8) worldwide. Geodetic and seismological observations indicate that the subduction zone also exhibits various types of slow earthquakes including slow slip events (SSEs), low-frequency earthquakes (LFEs), very-low-frequency earthquakes (VLFEs), and tectonic tremor. Developing the spatial variations of 3D fault geometries and seismic slip behavior along subduction plate slab are important and contributes to the understanding of the mechanisms of large earthquakes in subduction zones, as well as to the improved forecasting of earthquakes and tsunamis in subduction zones. The Hikurangi subduction zone is located on the eastern margin of North Island, New Zealand, where the Pacific Plate subducts beneath the eastern North Island at a rate of 55 mm/yr. In north Hikurangi margin, the shallow plate boundary megathrust hosted two large magnitude earthquakes (Mw 7.0-7.2) in 1947 that produced 8 to 10 m tsunami along the coast of the North Island. Geodetic observations indicate that slow slip events (SSEs) vary along the margin: in the northern and central segments, they are shallow (<15 km), short (<1 month), and frequent (every 1-2 years), whereas in the southern segment, they are deeper (25-40 km), longer (>1 year), and less frequent (occurring every 5 years).
Here we used an implicit approach to combine multi-sourced data, including seismic reflection profiles, relocated seismicity, focal mechanism solutions and topography profiles to develop a new slab model for Hikurangi subduction margin. The Hikurangi slab model (HSM-1.0) provide the detailed 3d geometry of a ~750 km subducting slab with ~8 km resolution. The geometry of shallow slab varies along-strike, from a steep (5°-10°) southern part, to a gentle (~2°) central segment; and then an irregular (1°-5°) northern margin. The southern margin has the deepest (~110 km) transition zone, longest (~200 km) distance from transition zone to trench, shallowest (~250 km) seismogenic zone, and steepest (~77°) deep slab. The modeling results indicate that the curvature of the Hikurangi slab (10-4) is two orders of magnitude higher than that of the global slab (10-6), and it displays a more irregular slab morphology. The slow-slip event (SSEs) source area at the northern margin of the Hikurangi slab exhibits a wide range of curvature values, while the locked region at the southern margin shows relatively less variation. In the SSEs region, the maximum principal stresses (σ1) to the fault plane are oriented at a high angle (>50–60°), whereas in the southern locked region, the maximum principal stresses (σ1) are oriented at a lower to moderate angle (30–40°). The shear strength gradients along the subducted slab suggest that the northern SSEs source region is relatively spatially heterogeneous, while the southern locked region demonstrates greater spatial homogeneity. The HSM-1.0 will facilitate fault system analysis, subducted slab reconstructions, and dynamic rupture modeling in the Hikurangi margin.
How to cite: Wang, M., Ma, H., and Wang, F.: Three-dimensional modeling of slab geometry and shear strength along Hikurangi subduction interface, New Zealand: implications for slow slip events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8153, https://doi.org/10.5194/egusphere-egu25-8153, 2025.
Slow earthquakes, distinct from regular earthquakes in their gradual release of seismic energy, play a key role in stress accumulation and release at subduction plate interfaces and have been proposed as potential precursors to large earthquakes. In the northern Ryukyu Trench, the Philippine Sea Plate subducts beneath the Eurasian Plate, and the region is characterized by a few large earthquakes over the past several centuries, including the magnitude 8.0 event in 1911 near Kikai Island. Although interplate coupling and crustal heterogeneity in this area remain debated, the occurrence of shallow, very low-frequency earthquakes (VLFEs), often associated with slow-slip events (SSEs), highlights the need to clarify the current state of seismicity.
From September 2018 to June 2019, a broadband ocean-bottom seismometers (OBSs) network, supplemented by land-based stations, was deployed to detect and locate VLFEs in the northern Ryukyu Trench. We used the envelope correlation method (ECM) to determine VLFE epicenters. Rayleigh-wave group velocity was estimated at approximately 2.5 km/s for 0.05–0.1 Hz frequencies. Our results reveal that VLFEs predominantly cluster northeast of Amami Island and east of Okinoerabu Island, with minimal spatial overlap with areas exhibiting regular earthquakes. Recurring VLFE activity occurred in distinct locations along the trench, sometimes coinciding with earthquake swarms.
These findings indicate that VLFEs and regular earthquake seismicity in the northern Ryukyu Trench may be influenced by high-pressure fluids migrating along the subducting plate interface—similar to observations in other subduction zones such as Hikurangi. In particular, earthquake swarms that precede or follow VLFE swarms suggest a delayed triggering mechanism, potentially driven by fluid migration or stress changes related to SSEs. Our observations imply weak interplate coupling in the low-seismicity area (LSA) northeast of Amami Island. Although SSEs remain difficult to detect at offshore distances, studying the spatiotemporal distribution of VLFEs alongside regular seismicity offers valuable insight into the dynamic processes governing stress release, fluid migration, and fault mechanics in this subduction environment.
How to cite: Nakamura, M., Kuo, B.-Y., Lin, P.-Y. P., Kodaira, S., and Ishihara, Y.: Very Low-Frequency Earthquakes and Earthquake Swarms in the Northern Ryukyu Trench, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4649, https://doi.org/10.5194/egusphere-egu25-4649, 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 Mw7.8 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 Mw ≥ 5.0 SSEs have been recorded in Mexico, and at least six of them began shortly after Mw ≥ 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 Mw ≥ 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 Mw ≥ 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.
