A secondary zone of surface uplift (SZU), located from 200 to 400 kilometers landward of the trench, has been measured after several megathrust earthquakes. The SZU reached a few centimeters hours to days after the 2011 Mw 9.1 Tohoku (Japan) and 2010 Mw 8.8 Maule (Chile) earthquakes. One interpretation is that this SZU is universal, driven by volume deformation around the slab interface (van Dinther et al. 2019). Indeed, published coseismic finite-fault models for these events do not reproduce the measured SZU.

Here, we build on the case of the SZU to understand if, and how, our prior assumptions on the forward model can prevent us (or allow us) to make the most out of our dataset. In particular, we investigate under which assumptions the SZU can, or cannot, be predicted with fault slip. We show the SZU cannot be reproduced with coseismic finite-fault models that neglect 3D elastic heterogeneities in lithospheric structure. In contrast, we can recover the SZU with fault slip if elastic heterogeneities associated with the subducting slab are accounted for, as opposed to assuming homogeneous or layered elastic lithospheric structures. The SZU may therefore result from slip on the slab interface, downdip of the main coseismic patch. We suggest SZU might be caused by rapid afterslip, but a deformation of the volume around the fault cannot be ruled out.

Reference: van Dinther, Y., Preiswerk, L. E., & Gerya, T. V. (2019). A Secondary Zone of Uplift Due to Megathrust Earthquakes. Pure and Applied Geophysics, 176(9), 4043–4068. https://doi.org/10.1007/s00024-019-02250-z

How to cite: Ragon, T. and Simons, M.: Assumptions on elastic structure in finite-fault models: the case of the secondary zone of uplift measured after megathrust earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19335, https://doi.org/10.5194/egusphere-egu24-19335, 2024.

For more than a decade, earthquakes induced by natural gas extraction have been a significant societal concern in Groningen, the Netherlands. Their occurence underlines the importance of understanding earthquake source characteristics and the development of suitable and accurate characterization methods. In this study we (i) estimate the source characteristics of ten relatively strong induced events (magnitude > 2 M_L) and (ii) analyse the rupture propagation of these events. The determination of the source characteristics (aspect i) involves the estimation of centroid-moment tensors (CMT) by means of an iterative workflow based on the Hamiltonian Monte Carlo algorithm. Importantly, this approach allows us to quantify the uncertainties of the model parameters (hypocenter, origin time, and moment tensor components) as the Markov Chain asymptotically approaches the posterior probability of these model parameters. The Bayesian inference problem is paired with geological prior knowledge of the Groningen subsurface (i.e., a detailed 3D velocity model and the know fault geometry). For the rupture propagation analysis (aspect ii), we employ the Empirical Green's Function method and exploit the dense sampling of the wavefield in Groningen resulting from the extensive seismic network in the region. This analysis allows us to estimate the directivity and speed of the ruptures, as such giving insight into the kinematics of the ten selected earthquakes.

We find that the lateral coordinates of the estimated centroids (posterior means) are consistent with the available Groningen fault map. Furthermore, the depths are mainly distributed in the vicinity of either the top or bottom of the gas reservoir. In terms of source mechanisms, the earthquakes are predominantly explained by double-couple sources featuring normal faulting. After conversion of the mean of the ensemble of moment tensors to strike, dip, and rake, we obtain values consistent with the known fault geometry. As for the rupture propagation analysis, our results indicate that these earthquakes display a relatively minor directivity effect. In spite of the minimal effect, however, the rupture directions are mostly consistent with the strike derived from both the MTs and the available fault map. 

How to cite: Weemstra, C. and Masfara, L. O. M.: Probabilistic centroid moment tensor inversion and rupture analysis of incuded seismic events in the Groningen gas reservoir, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18600, https://doi.org/10.5194/egusphere-egu24-18600, 2024.

On 22 March 2020 at 5:24 UTC, a M_W 5.4 earthquake occurred on the outskirts of Zagreb and was followed by a M_W 5.0 aftershock at 6:01 UTC on the same day. These moderate-magnitude events resulted in ground motion at the edge of detectability with satellite data but caused considerable damage in Zagreb’s historic centre. To illuminate the seismic sources, we compute the ground motion and invert seismic data using a state-of-the-art Bayesian inversion method that accounts for uncertainties in the data and in the Earth structure model. We compare the results to a well-established technique that uses a large number of hand-picked first-motion polarities.

Sentinel-1 Interferometric Wide data both from the ascending and descending orbit is obtained and processed using the European Space Agency SNAP toolbox. The resulting images show a phase difference of about 2π corresponding to ground displacement of 3 cm. The hypocentre and centroid locations of the events suggest that both were initiated at a depth of around 10 km, the rupture propagated upward, and most of the energy was released at a depth of 4 to 5 km. The shallow depth of the earthquakes possibly led to measurable surface displacement and resulted in considerable damage in the epicentral area and the centre of Zagreb.

How to cite: Mustać Brčić, M., Hu, J., Herak, M., Tkalčić, H., and Pham, T.-S.: Surface displacement and source parameters of the 2020 Zagreb, Croatia earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15566, https://doi.org/10.5194/egusphere-egu24-15566, 2024.

The geometry of long strike-slip faults often includes local deviations from the general fault orientation, commonly known as restraining and releasing bends. As earthquake ruptures propagate along the fault, these bends experience either an increase or decrease of stress, influencing the rupture dynamics. While it is well established that these geometric features can dramatically affect rupture parameters such as propagation velocity, style, and extent, the exact dynamics and mechanics of the rupture-bend interaction are not yet clear. Here, we present direct experimental observations of the interaction of shear ruptures with restraining and releasing bends of different angles, deviating from a planar interface by up to 30 degrees. We trigger dynamic ruptures that spontaneously propagate along matching interfaces between two loaded PMMA plates and image ruptures at the area of the bends with an ultra-high-speed camera, operating at a rate of million frames per second. By applying Digital Image Correlation on the imaged frames, we produce high-resolution, full-field maps of the evolution of displacements, particle velocities, and stresses as the ruptures propagate through the bends. While all ruptures reach the bends as self-healing slip pulses propagating at sub-Rayleigh velocities, different bend geometries have different effects on the ruptures. These include arresting the ruptures, transitioning them into supershear and crack-like ruptures, and triggering secondary ruptures and back-propagations. These results can illuminate the complex dynamics that evolve around restraining and releasing bends during earthquakes and can be used for calibrating earthquake models and refining earthquake hazard assessments.

How to cite: Gabrieli, T. and Tal, Y.: The effects of restraining and releasing fault bends on the propagation of shear ruptures: direct experimental measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4586, https://doi.org/10.5194/egusphere-egu24-4586, 2024.

Earthquake is one of the greatest natural hazards and a better understanding of the physical processes causing earthquake ruptures is required for appropriate seismic hazard assessment. Kinematic modelling is a standard tool for providing paramount information about the complexity of the earthquake rupture process and for making inferences on earthquake mechanics. Despite recent advances, kinematic models are characterized by uncertainties and trade-offs among parameters (related to the non-uniqueness of the problem). It has been shown that, for the same earthquake, source models obtained with different methodologies can exhibit significant discrepancies in terms of slip distribution, fault planes geometry and rupture time evolution.

One of the crucial assumptions in kinematic modelling on causative faults is the source time function, because it contains key information about the dynamics. Such function is nonetheless one of the most poorly observationally constrained characteristics of faulting. Recently, slip velocity time histories have been studied with laboratory earthquakes, and it has been observed that a systematic change of mechanical properties and traction evolution corresponds to a change in the shape of slip velocity. However, in kinematic inversions this function is a-priori assumed using simplified shapes, although functions compatible with rupture dynamics should be preferred.

In this work, we run a series of forward and inverse modelling to investigate the effect of the assumed slip velocity function on the ground motion and on the inverted slip history on the fault plane. We generate spontaneous dynamic models and use their ground motion as real events, inverting these data with kinematic models. Kinematic inversions have been conducted using both single-time and multi-time windowing. Uncertainties have been investigated assuming four different source time functions (i.e., triangular, box, regularized-yoffe and exponential functions). In this way we examine how the slip velocity function influences the slip distribution on the fault plane, and test if the inferred kinematic parameters (rise time and rupture velocity) are consistent with the dynamic models. Also, for a dense grid of phantom receivers, we examine the variability of the peak ground velocity (PGV) of synthetic seismograms up to 1 Hz. The latter have been obtained with forward models assuming the same slip distribution, rise time and rupture velocity but changing the source time functions.

Finally, we use the retrieved kinematic history on the pseudo-dynamic models to examine how different kinematic assumptions lead to a variability in the shear stress evolution. We focus on dynamic parameters such as breakdown work, stress drop, and Dc parameter. Our results provide a glimpse of the variability that kinematic source time functions (dynamically consistent or not) might have when used as a constraint to model the earthquake dynamics.

How to cite: Locchi, M. E., Mosconi, F., Supino, M., Casarotti, E., and Tinti, E.: Studying the Viability of Kinematic Rupture Models and Source Time Functions with Dynamic Constraints, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7887, https://doi.org/10.5194/egusphere-egu24-7887, 2024.

The study of earthquake rupture complexity plays a crucial role in assessing and mitigating seismic hazards.
Among the earthquake source parameters, the rupture velocity is generally unknown and assumed as a fixed percentage (60%-90%) of the shear wave velocity, although it controls the rupture duration, directivity, amplitude, and frequency content of the radiated wavefield. The rupture velocity is indeed the key parameter that determines the earthquake rupture length and indirectly the stress release, through the seismic moment. The observed large variability of stress drop estimates (over three orders of magnitude) can be partly related to the uncertain estimate of the rupture velocity and its possible heterogeneity or scaling with magnitude.

In this study, we tackle the specific problem of estimating earthquake rupture velocity, together with other characteristic source parameters, using a time-domain technique, applied to a 24-event dataset of micro-seismic events occurring in The Geysers area in California, USA. We propose a methodology that combines P and S half-pulse durations to infer independent estimates of the rupture radius and speed of microearthquakes assuming a circular fracture surface. For this aim a previous technique, that uses the P- and S-wave log-displacement curves as a function of the time along the seismogram has been used to measure attenuation-corrected, average P and S half-durations and plateau levels from a set of recorded earthquake waveforms. Refined estimations of seismic moment, rupture radius, and velocity allowed to determine accurate estimations of the stress release. Results show that rupture velocity is not constant but increases with magnitude in the explored range, reaching in 16 cases supershear speed values. Noteworthy, a self-similar, constant stress drop scaling is obtained only if a variable rupture velocity with magnitude is used for rupture radius and stress release determinations. The possibility of extremely fast ruptures (super-shear) occurring in the considered geothermal area can be attributed to the effect of lubrication and/or pore pressure increase due to massive volumes fluids injected under high pressure in the subsoil, to exploit the energy resources of the geothermal reservoir.

This work paves the way for a deeper comprehension of the physical and geological conditions determining the nucleation, propagation, and arrest of the fracture in crustal rock volumes with different faulting mechanisms, and offers a new and interesting approach for a more complete and accurate earthquake source parameters estimation through a time domain technique.

How to cite: Lambiase, A., Nazeri, S., Longobardi, V., and Zollo, A.: Rupture Velocity Estimation and Source Parameter Analysis of Micro-Seismic Events at The Geysers, California, USA, Applying A Time-Domain Technique, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16185, https://doi.org/10.5194/egusphere-egu24-16185, 2024.

Understanding the dynamics of microearthquakes is a timely challenge to solve current paradoxes in earthquake mechanics, such as the stress drop and fracture energy scaling with seismic moment. Dynamic modelling of microearthquakes induced by fluid injection is also relevant for studying rupture propagation following a stimulated nucleation. The ERC-Synergy project FEAR (Fault Activation and Earthquake Ruptures) in the Bedretto Underground Laboratory (Swiss Alps) at approximately 1500m depth offers a unique opportunity to investigate fluid-induced micro-events on broadband seismic arrays. In this study, we leverage this opportunity to perform dynamic ruptures caused by fluid injection on a target pre-existing fault (50m x 50m), generating a Mw ≤ 1 seismic event. We conduct fully dynamic rupture simulations coupled with seismic wave propagation in 3D using a linear slip-weakening constitutive law, implemented on the supercomputer Leonardo (CINECA) with a multi-GPU distributed system.

Stress field and fault geometry are constrained by in-situ characterization, allowing us to minimize the a priori imposed parameters. We investigate the dynamics of rupture propagation and its arrest for a target M_w < 1 induced earthquake with spatially heterogeneous stress drops caused by pore pressure changes and different constitutive parameters (i.e., critical slip-weakening distance, D_c, dynamic friction). We explore different homogenous conditions of frictional parameters, and we show that the spontaneous arrest of a propagating rupture following a dynamic instability is possible in the modeled stress regime by assuming a high fault strength parameter S, that is high ratio between strength excess and dynamic stress drop characterizing the fault before injection. The arrest of rupture propagation in our modeled induced earthquakes depends on the heterogeneity of dynamic parameters caused by the spatially variable effective normal stress, which controls the on-fault spatial increment of fracture energy G_c. Furthermore, in faults with high S values (i.e., low rupturing potential), we find that even minor variations in D_c (from0.45 to 0.6 mm) have a substantial effect on the rupture propagation and on the ultimate size of the earthquakes. Our results show that modest variations of dynamic stress drop determine the rupture mode, distinguishing self-arresting from run-away ruptures. Studying dynamic interactions (stress transfer) among slipping points on the rupturing fault provides insights on the dynamic load and shear stress evolution at the crack tip. The inferred spatial dimension of the cohesive zone in our crack models is roughly ~0.3-0.4m, with a maximum slip of ~0.6cm. Finally, analyzing the radiated synthetic waveforms, we examine the differences in the high-frequency content of simulated waveforms between self-arresting and run-away earthquakes and provide an estimation of the source parameters obtained through the spectral inversion. This estimation is then compared with source parameters of the dynamic forward models.

Our results suggest that several features inferred for accelerating dynamic ruptures differ from those observed during rupture deceleration in a self-arresting earthquake caused by the spatial gradients of normal stress and pore-pressure. These results related to rupture arrest integrate those obtained with spatial variations of the initial stress, highlighting the role of the heterogeneities of stress drop and G_c.

How to cite: Mosconi, F., Tinti, E., Casarotti, E., Gabriel, A.-A., Dal Zilio, L., Rinaldi, A. P., Dorozhinskii, R., and Cocco, M.: Modeling 3D dynamic rupture and arrest of spontaneous fluid-induced microearthquake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11935, https://doi.org/10.5194/egusphere-egu24-11935, 2024.

The subduction zone seismic-cycle is a complex phenomena with individual earthquakes as clearer manifestations. Although earthquakes are fundamentally space-extended, they are inferred to be material ruptures mostly considered as points in space. Nevertheless, in the temporal dimension, because they are plate-velocity dependent, it is less clear that they can be considered as point processes. Therefore, when considering this plate velocity in the balance analysis and assuming a locally homogeneous stochastic process hypothesis along coarse-graining upscaling it is possible to get a picture that makes sense of the whole seismic-cycle. This picture has emergent properties not available from purely seismic events, but that are more and more frequently recognized from geodetic and satellite observations, such as distant interactions and slow slip events. Taking advantage of the instrumentation installed at northern Chile, which makes use of both temporary and permanent stations from the National Seismological Center and IPOC it has been possible to obtain a well detailed picture of the seismic cycle between 2007 and 2023, that is consistent with representations obtained from geodesical measurements. We also obtain this representation for 49-year ISC catalog. We discuss possible applications of this seismic cycle representation.

How to cite: Siegel, C. E., Toledo, P., Riquelme, S., Madariaga, R., and Campos, J.: The Balance Process in Subduction Zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20904, https://doi.org/10.5194/egusphere-egu24-20904, 2024.

The Blanco transform fault system (BTFS) in the northwest off the coast of Oregon is highly segmented and one of the newly evolving transform faults. While for most transform systems, no high-resolution seismological data is available, the BTFS was instrumented with a dense network of 54 ocean-bottom-seismometer stations in 2012-2013.  We use one year of ocean-bottom-seismometer data from the Blanco Transform OBS Experiment (network code X9) to compare the seismicity with high-resolution multibeam bathymetry, aeromagnetic, and gravity datasets to study the seismotectonics of the BTFS. 

We determine P and S-phase arrivals using a new machine learning picker trained on OBS data to create a high-resolution local seismicity catalogue. We compare seismicity catalogues based on different picking algorithms and event associators, including an automated phase picker for OBS data (PICKBLUE) using the hydrophone and seismometer channels. 

In total, we locate ~9.000 local events, which reveal lateral deformation along the BTFS in very high detail, including smaller step-overs, transtensional structures, and focused seismicity along the fault trace and several ~15 km long aseismic segments along the BTFS. At segments where the BTFS is linear, most seismicity is very concentrated to the transform fault, suggesting that the deformation is within +/-7 km of the fault trace and, hence, very focused. Furthermore, we will present vp and vp/vs velocity models, which reveal the three-dimensional structure of the BTFS and compare the seismicity with seismic velocities.  

The local seismicity indicates substantial along-strike variations, indicating different slip modes in the eastern and western BTFS. Seismic slip vectors suggest that the eastern BTFS is a mature transform fault system accommodating the plate motion. At the same time, the western BTFS is immature as its re-organization is still ongoing.

The available datasets provide no evidence of either transform faults or fracture zones around the BTFS before 2 Ma, supporting that there were no pre-existing transform faults before the initiation of the BTFS. Therefore, we suggest that the BTFS developed from two broad transfer zones instead of pre-existing transform faults. 

Furthermore, we applied the deep learning phase picker, to analyse an extensive OBS dataset over three years, including a total of 225 OBS deployments (network codes: 7D, X9, and Z5) deployed offshore the south-central Cascadia subduction zone, south of the BTFS. As a result, ~7000 well-constrained local events were derived, providing new insights into offshore fault dynamics and the segmentation of the Gorda ridge  South of the BTFS.

How to cite: Lange, D., Ren, Y. R., and Grevemeyer, I.: Seismicity, Segmentation and Structure of the Blanco Transform Fault System in the Northeast Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5657, https://doi.org/10.5194/egusphere-egu24-5657, 2024.

Quantifying the stress field variability at different wavelengths and how it evolves over time has important implications for the development of earthquake sequences. As small earthquakes occur more frequently than larger ones, their focal mechanisms are essential to create larger catalogs, that allow to statistically quantify the kinematics of earthquake sequences within a region and the preferential orientations of co-seismic strain release. Inverting earthquake focal mechanisms is widely used to infer a number of stress-related parameters characterizing a crustal volume, including the orientation of the principal stress axes, maximum horizontal stress, and the relative magnitudes of the principal (stress ratio R) and horizontal stresses (A_PHI). If resolution allows, these stress parameters can be employed to quantify stress variability and heterogeneity in different rock volumes. Here we studied the 2016-2017 Central Italy seismic sequence that is characterized by the occurrence of three mainshocks: the M_w 6.0 Amatrice, M_w 5.9 Visso and M_w 6.5 Norcia. These major events nucleated on normal faults with kinematics consistent with the regional stress field. Taking advantage of a high-resolution catalog generated with deep learning and containing ~56.000 focal mechanisms, we calculated the distribution of stress parameters over small crustal volumes activated during the sequence and resolved the variability of the stress field during three different time periods punctuated by the three largest events. We found a change in the local trend of S_HMax with better defined orientations consistent with the regional stress field towards the SE of the Amatrice mainshock, and larger variability to the NW where also Visso and Norcia mainshocks ruptured. These changes in S_HMax trend also coincide with a contrast in the A_PHI parameter quantifying the relative magnitude of the horizontal stresses. The area between the Amatrice, Visso and Norcia epicenters displayed the lowest A_PHI (i.e. representing a normal faulting stress regime where S_V >> S_HMAX) and the lowest relative magnitude of the S₂ (S_HMax in the case of normal faulting). Furthermore, an increase in the A_PHI parameter is observed at depths below 8 km, reaching transtensional and strike-slip stress regimes in some local volumes. The area surrounding the rupture of the Amatrice mainshock displays the largest deviations from the regional stress over the entire analysed time period, indicating a stress anomaly driven by the properties of the medium or stress heterogeneities caused by static stress transfer that are persisting over the one-year time span of the catalogue. The observed local stress deviations from the regional stress field can help illuminating dominant local loadings affecting the deformation pattern.

How to cite: Martínez-Garzón, P., Meier, M.-A., Lanza, F., Collettini, C., and Dresen, G.: Stress heterogeneity and its relation to geology during the 2016-2017 central Italy earthquake sequence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8773, https://doi.org/10.5194/egusphere-egu24-8773, 2024.

Tectonic faults exhibit a wide spectrum of processes releasing stress: from fast earthquakes to slow deformation. Mapping smaller-scale slow deformation directly is challenging because of the limited signal-to-noise ratio of geodetic recordings. Instead, we need to rely on other telltale signs of slow deformation. Low-frequency earthquakes (LFEs) are such signs: a class of seismically observable signals that coincide with slow deformation.

However, detecting LFEs is challenging due to their emergent nature and generally low magnitude. The most common method for LFE detection is template matching. Due to the need for waveform templates, this method is specific to a set of LFE sources and seismic stations. This limitation renders the template matching unable to discover uncataloged sources or detect LFEs in regions without prior known LFE activity.

Here, we present a novel, deep learning method for detecting LFEs. Our method detects phase arrivals from LFEs, allowing to detect them with a workflow closely modeled after a standard earthquake detection workflow. In contrast to template matching, the deep learning method is substantially more flexible and can generalise to unknown LFEs and even across world regions. We apply our method to a diverse set of regions, including regions with previously known LFE activity, such as Cascadia and Nankai, and without known LFE activity, such as Northern Chile.

How to cite: Münchmeyer, J., Giffard-Roisin, S., Malfante, M., Frank, W., Poli, P., Marsan, D., and Socquet, A.: Identifying uncataloged low-frequency earthquake sources with deep learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10195, https://doi.org/10.5194/egusphere-egu24-10195, 2024.

Active faults present a wide spectrum of slip behaviors, from fast earthquakes, transient slow slip events to steady creep. Although each fault has its own specificities, the distribution of these behaviors is dictated by the evolution of depth-dependent pressure and temperature conditions to first order. In most of the subduction zones, transient slow slip events, accompanied by tectonic tremors and Low Frequency Earthquakes (LFEs), are located in the transition zone, between the updip seismogenic zone and the downdip steadily creeping zone. 

When present, tremors and LFEs are unique tools to monitor in detail the slip behavior in the transition zone. Previous studies analyzed the spatio-temporal clustering of tremors and LFEs (Wech et Creager, 2011, Rubin et Armbruster, 2013, Frank et al., 2013), and revealed that the properties of tremors/LFE bursts evolve with increasing depth: long recurrence intervals and long-lasting bursts occur close to the seismogenic zone while short recurrence intervals and short-lasting bursts happen deeper, close to the continuously creeping zone. These observations were made in various regions and tectonic contexts, including Cascadia (Wech and Creager, 2011), Nankai (Obara et al., 2011) and Mexico (Frank et al., 2013) for subduction zones, and the San Andreas strike-slip fault (Shelly et al., 2017).

In this study, we aim to gain insight on the rheological properties of the transition zone by a systematic analysis of the LFEs clustering properties in different regions and tectonic contexts. To do so, we analyze with similar methods LFE catalogs obtained with template matching, for Mexico (Frank et al., 2013), Nankai (Kato et al., 2020), Cascadia (Sweet et al., 2019) and Parkfield (Shelly et al., 2017).

We analyze the clustering properties of the LFE families (asperities), by computing the auto-correlation spectra of LFE catalogs for single families, and stacking them for bins along depth. This spectrum shows the density of recurrence of LFE bursts, and we observe a clear variation of maximum recurrence intervals between 100 and 10 days along depth for all regions. Additionally, we calculated and compared LFE burst recurrences using the cumulative LFE time series derivative, from which we retrieved prominent bursts, and using other clustering methods such as DBSCAN. Finally, we compile the results from different regions and compare them with the along depth pressure-temperature  conditions of these faults (Behr and Bürgmann, 2021) to have a rheological insight on these observations made using LFEs.

How to cite: El Yousfi, Z., Rousset, B., Radiguet, M., and Frank, W. B.: Insight on along-dip fault transition zone rheology through LFE clustering , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20029, https://doi.org/10.5194/egusphere-egu24-20029, 2024.

In the central to western Nankai Trough. a series of dense seismic reflection surveys were conducted in 2018-2020 (Nakamura et al., 2022 GRL). They show impressive topographic features of the subducting plate boundary, including a subducting seamount in the Hyuga-Nada region and off Cap Muroto. Compiled seismic dataset was used for picking BSRs (bottom-simulating reflectors), which define a boundary between the hydrate-rich formation above and a gas-bearing layer below. The heat flow values are calculated from these BSR depths and the average thermal conductivity between the seafloor and BSR. The short wavelength variations are filtered out and the obtained heat flow values are regionally averaged ones. The result was then merged with the existing heat flow data (surface, borehole and BSR-derived) in this area.

Heat flow is highest near the trough axis off Cape Muroto (near the central Nankai Trough), which is interpreted as the fluid seepage along the decollement. In the forearc region, heat flow varies between 50-70 mW/m^2, On the forearc area off Muroto, the bathymetry is characterized by a large landward embayment including the trough axis and deformation front. Within 20 km landward from the deformation front, heat flow is ~80 mW/m^2 in this embayed area, whereas it is 40-60 mW/m^2 on either side of embayment. Further landward, we found a low heat flow (~30mW/m^2) region above the subducted seamount. We propose that the heat flow is affected by the subduction of seamount.

In Hyuga-Nada forearc, the westernmost portion of the Nankai Trough region off eastern Kyushu, the Kyushu-Palau Ridge (KPR) is obliquely subducting toward N30W since several Ma B.P. Heat flow marks a sharp contrast at KPR; to the east it is 50-100mW/m2 to the east and 25-40mW/m2 to the west. The transition from high to low heat flow occurs in only 20 km across KPR. Higher heat flows of 100 mW/m2 to the east are located near the axis of Nankai Trough, similar to those reported off Muroto. Lower heat flow to the west is attributed to the subduction of older West Philippine Basin. Near the KPR we observed a ‘bowl-shape’ negative heat flow anomaly; heat flow outside is ~45mW/m2, whereas it is ~25 mW/m2 above the subducted KPR.

We hypothesize these local low heat flow close to subducted seamounts in 2 models. The first is that the subducted seamount was already cooled down by the downward fluid flow (recharge) after it was formed as the opening of Shikoku Backarc basin (15-25 Ma). The second is that the seamount subduction caused a stress contrast between the leading and trailing sides, encouraging a poroelastic fluid flow in the sediment above seamount. Through numerical simulations we found either model is possible to explain the observation. However, considering a frequent occurrence of low-frequency tremors around the seamount in Hyuga-Nada, we favor the latter model, because the fluid flow can reduce the effective stress, leading to the occurrence of seismic activities. We further discuss these mechanisms.

How to cite: Kinoshita, M., Nakata, R., Shiraishi, K., Hamada, Y., and Hashimoto, Y.: Heat flow in the Nankai forearc, SW Japan, derived from BSR and drilling: Possible effect ofseamount subduction on earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20899, https://doi.org/10.5194/egusphere-egu24-20899, 2024.

The fault zone process that creates slow earthquakes remains unclear, but one proposed mechanism to limit slip speeds depends on shear-induced dilatancy and the resulting pore pressure changes.  This process would imply that the fault zone dilates during slow earthquakes.  So in this study, we search for the signature of dilation in the focal mechanisms of tremor’s low frequency earthquakes (LFEs).

It is, however, difficult to directly observe dilation in LFE waveforms.  The paths travelled by LFEs’ 1-10 Hz seismic waves are complex, and Earth structure is poorly known at the short wavelengths of interest.   This complexity makes it difficult to disentangle path effects from source properties.  Thus we look for differences in the seismic waves created by two groups of LFEs: groups of events that are in the same location but occur at different times.  The earlier events are smaller and thought to rupture through more solid, low-permeability fault rock and thus may have large dilation, while the later events rupture areas that have recently slipped and thus may have smaller dilation.

In our initial analysis, we stack and analyse waveforms of LFEs identified in Cascadia by Bostock et al (2015).  However, we identify no significant difference in the waveforms or any significant trends in the polarisation of the difference.  Preliminary results suggest that the early and late LFEs have waveforms that differ by less than 1-2%.  

That zero to minimal difference could indicate that there is no dilation.  Perhaps early and late LFEs are exclusively shear slip, and shear-induced dilatancy does not limit LFE slip speeds.  However, it is also possible that dilation is simply small.  The early, potentially large-dilation LFEs could have a dilation-to-shear slip moment ratio of 0.05, and the later, potentially small-dilation LFEs have a dilation-to-shear ratio of 0.04.   Such dilation would be large enough to significantly affect LFE slip rates but would not be visible with our current data and techniques.

In our continuing work, then, we seek to decrease the uncertainty in our observations by identifying and stacking more LFEs and by extracting more information from seismograms including many LFEs.

How to cite: Hawthorne, J., Thomas, A., Papin, L., Huang, H., and Cavendish, S.: A search for dilation in low frequency earthquake waveforms, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9837, https://doi.org/10.5194/egusphere-egu24-9837, 2024.

Slow earthquakes are transient events detected with geodetic (slow slip) and/or seismic (low-frequency earthquakes and tremors) observations that release energy over a period of hours to months, i.e., much longer than regular earthquakes. These events are observed in several subduction at a specific range of depths and with their properties, such as strength, duration, recurrence interval, and predominance of either slip or seismic components varying along the subduction interface. In this study, we investigate slow earthquakes based on numerical modeling by applying a combined Maxwell viscoelastic and damaging rheology. Unlike conventional rate and state modeling practices utilizing infinitely thin fault assumptions with fault friction laws, our approach incorporates a finite fault thickness through a damage mechanism and suitable failure criteria. Our model directly predicts a geodetically observable slow displacement, while rock damaging rate is considered as a proxy to seismic tremors.

We run a set of numerical simulations to systematically explore the effects of model parameters such as viscosity, yield stress, and healing time on slow slip and tremors. We then incorporate along-fault variations in temperature, pore pressure, and permeability rate to infer changes in viscosity, yield stress, and healing time. The model successfully reproduces the observed synchronous episodes of slow slip and tremors in the initial stage. We show that decreasing strength and healing time results in reducing the recurrence intervals between these episodes and the amplitude of slow slip. Conversely, decreasing viscosity leads to a larger recurrence time and increased amplitude. After introducing the along-fault variability of main mechanical parameters, the model successfully predicts tremor concentration in the downdip zone which can be explained by the influence of high pore pressure in the yield stress profile as a function of depth. The capacity of the model to reproduce some patterns seen in observations underscores its capacity to capture the physics of slow earthquakes, emphasizing its potential to improve our understanding of seismic phenomena in subduction zones.

How to cite: Massoumi, S., Dansereau, V., Weiss, J., Campillo, M., and Shapiro, N. M.: Modeling the along-fault variability of slow earthquakes in subduction zones based on a combined Maxwell Viscoelastic and damaging rheology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9095, https://doi.org/10.5194/egusphere-egu24-9095, 2024.

Fast and slow earthquakes represent distinct modes of energy release during tectonic fault rupture. While laboratory stick-slip experiments have observed both fast and slow slips, limitations in sampling rates have obscured detailed insights into fault thickness variation. Specifically, the underlying reasons for a single fault exhibiting different slip modes have remained elusive. In this study, we conducted ring shear experiments employing an ultrahigh sampling rate (10 MHz) to shed light on the contrasting physical processes between fast and slow slip events. Our findings reveal that slip durations varied from dozens to hundreds of milliseconds. Fast slip events exhibit continuous large-amplitude Acoustic Emission (AE) signals alongside complex variations in sample thickness: a brief compaction pulse during rapid stress release, succeeded by sample dilation and thickness vibrations. As the slip concludes, the sample thickness initially undergoes slow compaction, followed by dilation preceding the nucleation of subsequent slip events. Conversely, during slow slip events, shear stress reduction coincides with intermittent bursts of low-amplitude AE and sample dilation. Fast and slow slips have similar AE spectra. Detailed observations of thickness variations during slips indicate that dilation occurs in both fast and slow slips, aligning with natural observations of coseismic dilatation. This study offers insights into the mechanisms governing fault slips during fast and slow earthquakes, potentially explaining their impact on stress redistribution and structural reorganization within faults.

How to cite: Ge, Y., Hu, W., and Gou, H.: Concordant Dilatancy in Stick-Slip Events and Fast/Slow Earthquakes: Insights from High-Resolution Studies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2509, https://doi.org/10.5194/egusphere-egu24-2509, 2024.