Cross-station analysis of tectonic tremor, as introduced by John Armbruster for long (150 s) windows and extended by Allan Rubin to short (4 s) windows, has afforded the highest precision mapping of tremor locations achieved to date. Cross-station locations have been used to a) reveal tremor epicentral distributions over broad (to ~10⁴ km²) areas that are significantly sparser than those portrayed by more commonly used envelope-correlation methods; b) document a variety of tremor (and by inference slow-slip) propagation modes, and c) place tremor in a structural context leading to insights into its generation and that of slow slip more generally. In particular, our work on c) suggests that tremor occurs within the upper layer of oceanic metabasalt as the expression of disaggregation and comminution associated with underplating. The success of cross-station analysis in tremor characterization to date warrants further investigation and development of the methodology. In this presentation, we detail two improvements.  The first, applicable to both long and short windows, concerns the judicious analysis of traveltime circuits and binomial coefficients ("n choose 3") corresponding to triples of waveforms formed from many (n ≥ 4) stations. This leads to effective quality-control measures for balancing location precision versus accuracy and the leveraging of cross-event information. The second improvement concerns short windows and relies on low frequency earthquake template waveforms that characterize propagation characteristics between a localized tremor source region and surface stations. Past efforts at cross-station tremor detection have relied upon the similarity of tremor waveforms across stations. This condition can be relaxed through the use of phase normalization afforded by template waveforms thereby enabling inclusion of larger station complements and resulting in increased number and quality of detections. We demonstrate these improvements on deep tremor recordings from southern Vancouver Island between 2003-2006 and 2022-2025.

How to cite: Bostock, M., Sammis, C., and Perez Estay, N.: Improvements to Cross-Station Analysis of Tectonic Tremor , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4076, https://doi.org/10.5194/egusphere-egu26-4076, 2026.

California, as a transform plate boundary, provides a distinctive tectonic setting and an ideal natural laboratory for investigating tectonic tremor and the slow deformation associated with plate motion. By analyzing continuous seismic records across multiple stations with the envelope correlation method, we identified ∼66,000 tremor events from 2000 to 2024. These events exhibit waveform characteristics consistent with tectonic tremors observed elsewhere. Beyond the previously documented central section of the San Andreas fault, we identify several new tremor clusters, primarily concentrated near the Mendocino Triple Junction and within the Big Bend segment. Our results suggest that tremor events near the Mendocino Triple Junction may mark the southern edge of the Cascadia subduction zone, while tremor events in the Big Bend region, located within the rupture zone of the 1857 M7.9 Fort Tejon earthquake, could have implications for regional seismic hazard.

How to cite: Ide, S. and Lu, W.: Spatiotemporal Characteristics of Tectonic Tremor in California, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4734, https://doi.org/10.5194/egusphere-egu26-4734, 2026.

Along the central section of North Anatolian Fault Zone, which marks the boundary between the Eurasian and Anatolian plates, subsurface creep has been detected since the 1970s. This creep localizes within a ~60-70 km-long segment along-strike, known as the Ismetpasa segment. Measurements from creepmeters, GNSS and InSAR, show that aseismic slip occurs as episodic events, or slow slip events (SSEs), that last a few weeks, occur approx. every 2.5 years, and extend to a depth of 5 – 6 km. Notably, the location of these SSEs coincides with a region of shallow locking depth.  Several mechanisms have proposed to explain their occurrence, including elevated pore-fluid pressure, variation in fault-zone composition, and changes in stressing rates associated with the shallower locking depth. To understand the mechanisms that govern these events, we carry out 3D numerical simulations using rate and state friction. In our model, we explore the effect of effective normal stress, long-term slip rate distribution and friction parameters on the resulting slip behavior. We consider a range of model setups and identify scenarios that reproduce the first-order characteristics of Ismetpasa SSEs. Our results provide insights into the conditions that promote shallow and deep slow-slip on continental strike-slip faults.

How to cite: Perez-Silva, A., Martínez-Garzón, P., and Ozawa, S.: Probing the mechanism of slow slip events along the central North Anatolian Fault Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6825, https://doi.org/10.5194/egusphere-egu26-6825, 2026.

Short-term slow slip events (S-SSEs) occur at depths greater than ~30 km along the Nankai subduction zone in southwest Japan. Previous studies have successfully detected these events using GNSS data and characterized their spatial distribution along the subduction interface. However, fundamental questions remain regarding their role in the slip budget: what fraction of accumulated interseismic strain do S-SSEs release, and does their behavior remain stationary over time? Addressing these questions requires resolving the temporal evolution of S-SSE slip patterns — an aspect that has remained largely unexplored.

Here, we analyze nearly two decades of GNSS data to quantify the slip contribution of S-SSEs and to investigate their temporal evolution. To resolve these small-amplitude, short-lived signals, we leverage the exceptional density of the GNSS network (>700 stations) by stacking time series from triplets of nearby stations, enhancing the signal-to-noise ratio. Using tremor and LFE timing to synchronize the detection, we measure offsets before and after each episode and infer local slip rates. By applying this approach across multiple successive time windows, which has not been done in Nankai or other subduction zones, we track how slip patterns evolve through time.

We find that S-SSEs release a significant fraction of accumulated slip within the tremor/LFEs zone (between 40 and 30%, which is consistent with the long-term coupling of 60%). Still, this contribution and the associated spatial slip pattern vary across different time periods. Our results reveal that S-SSE behavior is not stationary: the along-strike slip distribution and slip rates show systematic changes over multi-year timescales. We also observe that S-SSE slip occasionally extends to shallower depths, approaching the base of the seismogenic zone.

These spatio-temporal variations in slow slip provide new constraints on the evolution of interplate coupling and on how stress accumulation in the seismogenic zone may be modulated by deeper slow slip processes.

How to cite: Maubant, L., Itoh, Y., and Kato, A.:  Tracking short-term slow slip along Nankai with GNSS: temporal evolution of slip rate and interaction with the seismogenic zone , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3646, https://doi.org/10.5194/egusphere-egu26-3646, 2026.

Over the last two decades, the importance of Slow Slip Events (SSEs) in the deformation and seismic cycle of subduction zones has become more widely recognized. Knowledge of the evolution and slip distribution during SSEs can provide key insights into processes that influence SSE occurrence and their relationship to seismic slip. However, the offshore nature of many SSEs makes them difficult to observe with onshore geodetic methods alone, necessitating the deployment of seafloor-geodetic instruments to detect seabed deformation.

The Hikurangi subduction zone offshore New Zealand is characterized by frequent, large SSEs, and previous experiments have shown that such events regularly produce up to a few centimetres of seabed uplift that is detectable using seafloor pressure data. We are presenting a first look on ocean bottom pressure data recovered from the most recent GONDOR deployment across the northern Hikurangi subduction zone. The 2022-2025 GONDOR project is the largest seafloor geodetic experiment to date at Hikurangi, with over 50 seafloor instruments deployed in a dense array with several kilometres spacing, of which 39 were fitted with Absolute Pressure Gauges (APG) to detect vertical displacement of the seabed. 13 of these instruments have self-calibrating A-0-A sensors, enabling mitigation of instrument drift from the pressure record. The deployment is collocated with an IODP CORK observatory, allowing for future ground truthing of the pressure data and includes two arrays of Direct-Path-Acoustic sensors, giving horizontal deformation information of SSEs. During the 2024/2025 period, at least three large SSEs have occurred beneath the array, one beneath the centre and one each beneath the northern and southern subarrays.

I will present preliminary results from our analysis of seafloor pressure data, using depth-matched reference sites to mitigate oceanographic noise. I will also outline a processing workflow for pressure data analysis, including improved drift-removal techniques that do not depend on prolonged periods of oceanographic calm and are robust to sensor vertical displacement during the initial deployment phase when drift is most rapid. Further, I will explore the advantages of interspersing A-0-A instruments with conventional APGs for more robust drift mitigation to enable resolution of SSE vertical displacement during the early deployment phase.

How to cite: Gehrig, J., Ku, A., Wallace, L., Webb, S., Watts, D. R., Hino, R., Ito, Y., Wei, M., Palmer, N., and Jacobs, K.: First look at seafloor geodetical pressure data acquired during a large 2024 Slow-Slip-Event at the Hikurangi Margin offshore New Zealand, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10951, https://doi.org/10.5194/egusphere-egu26-10951, 2026.

Shallow slow slip events (SSEs) within seismogenic zones have been increasingly reported to be related to subducted seamounts (Wang and Bilek, 2014; Vallée et al., 2013; Wallace et al., 2016; Yokota and Ishikawa, 2016). However, the underlying mechanisms of this phenomenon is unclear. Slow slip events are commonly inferred to occur under high pore-pressure conditions, based on analyses of low-frequency seismic spectra and numerical simulations (Rogers and Dragert, 2003; Shelly et al., 2006; Liu and Rice, 2009; Li and Liu, 2016). Investigating the role of seamounts in alternating shallow SSEs and coseismic rupture propagation will provide important insights into long-term fault slip budgets. Here we conduct numerical simulations in the framework of rate-and-state dependent friction with the “aging” evolution law, in which a curved interface representing the subducted seamount is set in a velocity weakening zone on a two-dimensional subducted fault model. Our preliminary results suggest: 1) Slow slip events and slow aseismic creep can appear in the seamount leading area, where coseismic slip is suppressed; 2) the seamount can play a crucial role in stopping large rupture propagation when it is located at intermediate depths within the velocity-weakening zone; 3) irregular geometry will introduce diversity in long-term slip partitioning on the subduction fault regardless of constant velocity-weakening friction and consistently effective normal stress. This study will provide invaluable insights on understanding the interactions between large earthquakes and aseismic slip, as well as the influence of fault geometry such as a subducted seamount.

Reference:

• Li, D., and Y. Liu (2016), Spatiotemporal evolution of slow slip events in a nonplanar fault model for northern Cascadia subduction zone, Journal of Geophysical Research: Solid Earth, 121, 6828–6845.
• Liu, Y., and J. R. Rice (2009), Slow slip predictions based on granite and gabbro friction data compared to GPS measurements in northern Cascadia, Journal of Geophysical Research: Solid Earth, 114(B9).
• Rogers, G., and H. Dragert (2003), Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip. Science, 300, 1942-1943.
• Shelly, D., Beroza, G., Ide, S., et al. (2006), Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip. Nature, 442, 188–191.
• Vallée, M., Nocquet, J. M., Battaglia, J., et al. (2013), Intense interface seismicity triggered by a shallow slow slip event in the Central Ecuador subduction zone. Journal of Geophysical Research: Solid Earth, 118(6), 2965-2981.
• Wallace, L. M., Webb, S. C., Ito, Y., et al. (2016), Slow slip near the trench at the Hikurangi subduction zone, New Zealand. Science, 352(6286), 701-704.
• Wang, K., and S. L. Bilek (2014), Invited review paper: Fault creep caused by subduction of rough seafloor relief. Tectonophysics, 610, 1-24.
• Yokota, Y., and T. Ishikawa (2020), Shallow slow slip events along the Nankai Trough detected by GNSS-A. Science Advance, 6 (3): eaay5786.

How to cite: Liu, Y., Li, D., Yang, H., Williams, C., and Shao, Z.: Role of Subducted Seamounts in Earthquake Rupture and Aseismic Slip: Insights from Multi-Cycle Simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11523, https://doi.org/10.5194/egusphere-egu26-11523, 2026.

The subduction zone in the Mexican Pacific comprises a complex tectonic environment of interacting convergent active margin plates. Along the Middle American Trench (MAT), the Cocos and Rivera plates subduct beneath the North American and Caribbean plates, generating an irregular distribution of seismicity due to variations in the subduction angle of the slab. The Guerrero Seismic Gap (GGap) is a ~140 km segment at the Cocos-North America plate boundary. Since 1911 there has been no record of a large subduction thrust earthquake in the NW portion of the GGap, and taking into account the seismic evolution and subduction dynamics, specialists see a possible scenario of a Mw ~8.2 earthquake in the area. Therefore, understanding the nature of the rupture process in the crust is a fundamental question of this study.

Gravity techniques are accurate methods to investigate the crustal configuration and define the structure in the subducting slab. In this project, data from the global satellite model of Sandwell et al., 2014, we intend to generate a model of the density distribution in the shallow crust in the GGap area. The shallow crust is of particular interest because lateral heterogeneity in the slab zone modifies subduction dynamics. These heterogeneities comprise seafloor structures (e.g., seamounts) and may be key to studying the seismogenic zone of the Mexican subduction and better assessing its risk. Some studies show how the bathymetric relief on the seafloor, when it enters subduction, modifies the mechanical properties at the interface between the subducting plate and the overriding plate. This arrangement can be an important factor because it can affect the distribution of large earthquakes.

Gravimetric inversion methods can solve subsurface mapping problems by determining the density and/or depth of the layers that comprise it. Here, we will use statistical methods of gravity inversion to relate the parameters (density) to the observed data. To do this, we will use a Bayesian approach, defining our likelihood functions, evaluating the forward map, and our prior function, smoothed using Markov Random Fields, which restricts the field values to their neighboring dependencies.

The objective of this work is to conduct a detailed analysis of the surface crust, its configuration, and its relationship with seismicity in the area, in order to improve understanding of subduction dynamics and seismic risk assessment.

How to cite: Diaz de leon, A., Bañales, I., and Ito, Y.: Modelling the shallow crustal structure of the Guerrero seismic gap from gravity data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16342, https://doi.org/10.5194/egusphere-egu26-16342, 2026.

Oceanic transform faults (OTFs) accommodate a significant portion of global plate motions, yet the physical mechanisms governing their characteristically low seismic coupling and the initiation of earthquake swarms remain poorly understood. Here, we provide the first direct geodetic evidence of shallow slow slip events (SSEs) on an OTF. By leveraging the increase of seismic activity during earthquake swarms as temporal constraints on high-resolution, land-based continuous GNSS data from near the just-offshore Húsavík-Flatey Fault (HFF) in North Iceland, we utilized a signal-stacking strategy to isolate ultra-slow transients from stochastic noise. This approach detected SSEs that are several weeks in duration and with an average moment magnitude of Mw 5.34, systematically preceding the seismic swarms. The marked spatial complementarity between SSEs and swarms, combined with their temporal synchronicity, seem to indicate that the aseismic transients act as a mechanical trigger for the swarm-like activity along the western portion of HFF. The pronounced contrast between the weakly coupled western segment of the HFF—characterized by concurrent SSEs and earthquake swarms—and the strongly coupled eastern segment, which lacks moderate to large earthquake clusters, reveals a fundamental along-strike heterogeneity in fault behavior. The evidence of systematic aseismic slip release along the HFF accompanying swarm activity indicates that the seismic moment deficit of OTFs can be reconciled by aseismic slip transients. Our results corroborate that OTFs are complex faults where rheological and geometrical segmentation result in a complex interplay of slow and fast slip release.

Acknowledgements: We gratefully acknowledge Baptiste Rousset and Estelle Neyrinck for generously sharing their geodetic matched filter code for this analysis.

How to cite: Liu, X., Luigi, P., Alejandra, B., Ófeigsson, B., Xu, Q., and Jónsson*, S.: First direct geodetic evidence of precursory shallow slow-slip associated with seismic swarms on oceanic transform faults, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21784, https://doi.org/10.5194/egusphere-egu26-21784, 2026.

Understanding the physical controls on the transition between slow and fast earthquakes remains a fundamental challenge in earthquake physics. Here we show, through laboratory experiments on granular quartz gouge simulating natural fault zones, that both slow and fast slip can emerge on the same fault under identical stress conditions. The transition between slip modes is governed by the elastodynamic interaction between the fault and its surroundings. By systematically varying system stiffness at constant normal stress, we observe a continuous spectrum of slip behavior from stable sliding to slow events and ultimately fast rupture. Continuous acoustic monitoring reveals distinct seismic signatures: slow slip produces swarms of small events, while fast slip generates high-amplitude energy bursts. Continuous scaling of breakdown work with seismic moment supports a unified physical mechanism. Moment-duration scaling highlights a key transition in energy partitioning: in slow events, acoustic energy accounts for a minor portion of slip duration, whereas in fast events it contributes a much larger portion, indicating a shift in how seismic energy is radiated across slip modes. These findings suggest that slow and fast earthquakes are not distinct phenomena but reflect end-members of a fault slip continuum.

How to cite: Scuderi, M. M., Pignalberi, F., Mastella, G., Giorgetti, C., and Marone, C.: A Continuum of Laboratory Fault Slip Reveals Distinct Seismic Energy Partitioning from Slow to Fast Slip, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16699, https://doi.org/10.5194/egusphere-egu26-16699, 2026.

The Atacama Fault System is located in the northern Chilean subduction forearc region and hosts a complex system of trench-parallel faults with mapped surface ruptures. Previous work based on continuous monitoring of aseismic fault slip by the IPOC Creepmeter Array over the last ~15 years has shown that the Chomache, Cerro Fortuna, Salar del Carmen, and Mejillones Faults host Sudden Displacement Events (SDEs) that are often triggered by passing seismic waves, showing a clear temporal correlation between SDE signals and local, as well as teleseismic, earthquakes. Here we present a new study using data from two creepmeter sites that are instrumented with two collocated broadband seismometers to investigate the correlation between SDEs, the ground motions of preceding earthquakes, and the consistency of transient stress changes with observed deformation inferred from the creepmeter time series.

Our analysis reveals two primary observations. First, we identify a seasonal trend in the polarity of SDEs, suggesting modulation of the system response over the annual cycle. Second, we observe a dependency between the peak ground velocity (PGV) of the preceding earthquakes recorded by the collocated seismometers and SDE occurrence. We observe an absence of SDEs below a PGV threshold of approximately 0.07–0.15 cm/s that suggests that the triggering mechanism is at least partly amplitude-controlled.

Based on the apparent seasonal polarity changes and dependence on ground shaking, we will present results that test whether SDEs reflect tectonic fault slip processes or a non-tectonic, seasonally modulated response of near-surface sediments to dynamic triggering. We will test the hypothesis using the back azimuth of triggering seismic waves to resolve the dynamic stresses imposed on the well-constrained geometry of the monitored fault planes in comparison to the volumetric changes in the surrounding fault zone. This study will contribute to a deeper understanding of dynamic triggering and the underlying processes that control fault behavior in forearc settings.

How to cite: Wache, R. M., Bocchini, G. M., Harrington, R. M., Victor, P., Liu, Y., and Wei, M. ".: Constraining mechanisms for dynamically triggered Sudden Displacement Events (SDEs) in the northern Chilean Subduction Forearc using collocated creepmeter and broadband seismic data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10077, https://doi.org/10.5194/egusphere-egu26-10077, 2026.