Natural earthquakes occur in clusters or sequences that arise from complex triggering mechanisms, but direct measurement of the mechanisms responsible for complex temporal sequences and delayed triggering is rarely possible. A central question involved whether delayed triggering is due to slow slip and stress transfer or local weakening/fatigue processes such as stress corrosion. We investigate the origins of this complexity and its relationship to fault heterogeneity using a biaxial loading apparatus with an experimental fault that has two dominant seismic asperities. The fault is composed of a 5 mm layer of quartz powder, a velocity weakening material common to natural faults, sandwiched between 760 mm long polymer blocks that deform similar to the way 10 meters of rock would behave. Due to the higher local normal stress and the free surface boundary condition on the sample ends, the sample behaves like two asperities, one at each end, that can fail independently. As the quartz powder was continuously sheared, the friction properties changed, and we observed a transition from steady sliding to periodic repeating earthquakes that transitioned into aperiodic and complex sequences of fast and slow events. There is also reason to believe that friction properties evolved differently on the higher normal stress asperities and made them more unstable than the center part of the laboratory sample. Sequential ruptures on the two different asperities were linked via migrating slow slip which resembles creep fronts observed in numerical simulations and on tectonic faults. The propagation velocity of the creep fronts ranged from 0.1 to 10 m/s, which is broadly consistent with the velocity of slow slip fronts inferred from migrating tectonic tremor sources. Utilizing both local stress measurements and numerical simulations, we observe that the speed and strength of creep fronts are highly sensitive to fault stress levels left behind by previous earthquakes and may serve as on-fault stress meters.

How to cite: Cebry, S. B., Ke, C.-Y., Shreedharan, S., Marone, C., Kammer, D., and McLaskey, G.: Complex laboratory earthquake sequences show asperity interactions through creep fronts and illuminate the mechanics of delayed earthquake triggering, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10664, https://doi.org/10.5194/egusphere-egu23-10664, 2023.

Large earthquakes take place on mature faults with hundreds of meters to kilometres of cumulative slip. At shallow depths, the fault zone is generally composed of non-cohesive rock wear products, often referred to as gouge. Seismic and aseismic slip occur in this fault gouge and fracture/brecciation of the wall rock and damage zone can add to the fault gouge as part of the wear process. Gouge thickness generally increases linearly with the cumulative fault shear displacement and laboratory work shows that gouge tends to stabilize fault frictional stability. Previous works show that frictional stability of simulated fault gouge varies as a function of shear displacement. The stability evolution is interpreted as a consequence of the degree of shear localisation within the simulated fault gouge: the more the deformation is localized, the more the fault slip is unstable. This implies that for bare rock surfaces, unstable behaviour is expected as the deformations are forced to be localized at the interface between the two sheared surfaces.

On natural faults at large shear displacement (or for faults having a high gouge production rate), a competition must take place between 1) the localization of the deformation at rock-on-rock surfaces, 2) the delocalization of deformation due to gouge production and wall rock brecciation, 3) fault zone lithification and frictional healing and 4) shear localization within the gouge and wear material. The competition and interaction between these phenomena are modulated by cumulative fault slip, temperature and fluid chemistry. In turn, this competition may influence the frictional stability of faults with increasing shear displacement, and thus, their potential seismic activity.

To characterise the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to large displacement. Two initially intact rocks were chosen as starting material: a high porosity Fontainebleau sandstone and a low porosity quartzite. These samples represent very different resistances to abrasion (i.e., wear production with slip) for the same initial mineral composition (< 95% quartz), which allows us to investigate wear and wear rate on fault stability. Additionally, simulated quartz gouge was tested for comparison. Mechanical data are analysed within the rate-and-state framework, and post-mortem microscopic analyses of the sample were performed. For initially bare surface experiments a threshold shear displacement is required to transition from stable to unstable sliding. Stick-slip events (laboratory earthquakes) evolve systematically as a function of fault zone shear displacement. The inversion of the rate-and-state parameters shows that shear displacement has a dominant influence on both (a-b) and D_c. For all the faults tested, (a-b) decreases with increasing shear displacement. For high wear rates and simulated gouge, D_c decreases with increasing shear displacement. However, for low wear rate faults, D_c is constant within the tested shear displacement. These results demonstrate that, under the tested boundary conditions, fault stability varies systematically with fault maturity and in particular that shear displacement and strain localization are the dominant parameters controlling fault slip stability.

How to cite: Noël, C., Giorgetti, C., Scuderi, M. M., Collettini, C., and Marone, C.: Fault stability transition with slip and wear production: laboratory constraints, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7118, https://doi.org/10.5194/egusphere-egu23-7118, 2023.

The multi-scale roughness of a fault interface is responsible for multiple asperities that establish a complex and discrete set of real contacts. Since asperities control the initiation and evolution of the fault slip, it is important to explore the intrinsic relationships between the collective behavior of local asperities and the frictional stability of the global fault system. However, such a mechanism is still elusive due to the difficulty of imaging an exhaustive spatiotemporal variability of a fault interface at depth, and the limited computational efficiency of the numerical models with heterogeneity over a large time and space domain. Here we propose a novel analog experimental approach, which allows us to capture the temporal evolution of the slip of each asperity on a faulting interface. We link the collective behavior of asperities with the mechanical response of the whole fault interface. We find that many destabilizing events at the local asperity scale occurred in the frictional strengthening stage which is conventionally considered as the stable regime of a fault. We compute the interseismic coupling to evaluate the slipping behaviors of asperities during the fault strengthening stage. Based on a high-resolution topographical map of the fault surface, we evidence that the interseismic coupling is not only dependent on the normal load and the peak height of asperity but also can be affected by the interactions between asperities through the embedding soft matrix. Furthermore, we quantify the spatiotemporal interactions of asperities as slip episodes. The significant characteristics and scaling-laws observed in natural earthquakes, such as the magnitude-frequency distribution and the moment-duration scaling, are reproduced through the catalog of slip episodes to demonstrate the effective upscaling. We give geophysical implications for the physics and mechanics of natural faults and discuss some limitations of our experimental setup.

How to cite: Shu, W., Lengliné, O., and Schmittbuhl, J.: Collective behavior of asperities before large stick-slip events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4151, https://doi.org/10.5194/egusphere-egu23-4151, 2023.

Since the relatively recent discovery of slow slip events (SSEs), the nature of the relationship between SSEs and ordinary earthquakes has become one of the most important questions in earthquake science.  Specifically, questions as to whether SSEs decrease or increase the likelihood of large-magnitude earthquakes, whether or how slow and fast earthquakes can occur on the same fault patch, and whether SSEs are potential earthquake precursors have important implications for earthquake hazards.

Here, laboratory friction experiments on simulated fault gouges are used to gain insight into the relationship between SSEs and ordinary earthquakes.  The experiments are conducted water-saturated, at room temperature and at low pressure (10 MPa effective normal stress) to simulate the shallow, near-surface portions of major fault zones.  A key feature of these experiments is employing driving velocities as low as 5 cm/yr (1.6 nm/s) to simulate natural far-field tectonic driving rates.  From a larger dataset which includes a wide range of simulated fault gouges, four gouge types exhibited consistent stick-slip and these are analyzed further.  These materials are pyrite, hematite, gypsum, and Carrara marble powders.

Preliminary results show that the pyrite and hematite gouges exhibit small stress drops and increases in sample sliding velocity, interpreted to be SSEs, prior to stick-slips.  The SSEs occur near the peak in friction before the large stick-slip stress drop, suggesting that they are precursors.  In hematite at 5 cm/yr, the precursory SSEs exhibit stress drops on the order of 10’s of kPa and peak slip velocities within an order of magnitude of the driving rate, whereas the stick-slips exhibit stress drops of about 1 MPa and peak slip velocities of up to ~1 mm/s.  The peak stress at which the SSEs occur is within 1% of the peak stress prior to the stick-slip events.  Gypsum and Carrara marble, however, did not exhibit SSEs prior to stick-slips.  The results suggest that both slow and fast slip can occur on the same fault patch under the same conditions, and indicate the possibility that SSEs can be used as earthquake precursors in some cases.  However, the lack of precursory SSEs in the gypsum and marble gouges suggests that precursory SSE behavior is not universal and requires further investigation.

How to cite: Ikari, M.: Slow slip events as stick-slip precursors in laboratory friction experiments on simulated fault gouges, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2602, https://doi.org/10.5194/egusphere-egu23-2602, 2023.

The nucleation of earthquakes relies on an interfacial instability that facilitates the transition of a slow fault slip to a faster dynamic rupture. Here, we highlight the scenarios when (rate-weakening) interfaces exhibit a phase of slow aseismic rupture—before slip instability—that propagate large distances compared to the usual nucleation sizes. This aseismic rupture propagation results from rate-weakening interfaces' response to reach a state of steady sliding when forced to slip below steady-state frictional conditions. We numerically simulate the slip cycle—aseismic rupture, instability, and dynamic rupture—driven by prototype loading configurations:

• Slip dislocation accrues at a constant rate at one end of a finite fault with the other end (a) at the free surface of an elastic half-space and (b) completely locked (buried) in an elastic full-space.
• Imposed slip dislocation accruing at a constant rate on both ends of a finite fault
• A localized distribution of shear traction that increases at a constant rate.

All the above loading conditions can permit a slow aseismic rupture along the fault when the fault is initially locked: a state of interfacial slip for which the frictional strength, at the current slip rate, is significantly less than the steady-state frictional strength at the same slip rate. The slow rupture occurs in all the above loading configurations when the fault is initially locked; the subsequent transition to instability, or not, shows a fault-size dependence for configurations 1a and 1b, even when the fault exceeds the usual nucleation sizes. The cut-off fault size that permits instability after aseismic rupture also depends on the friction parameters, the extent of initial contrast from steady-state sliding, and slip conditions towards which aseismic rupture progresses. The remaining loading configurations exhibit instability whenever the fault size exceeds the usual nucleation sizes. Further, we find that slow rupture’s transition to early-stage instability happens through an intermediate breathing (spatiotemporal oscillation) type evolution of slip rate.

How to cite: Ray, S. and Garagash, D. I.: Aseismic rupture on rate-weakening faults before slip instability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15559, https://doi.org/10.5194/egusphere-egu23-15559, 2023.

Fault systems have geometrically complex structures in nature, such as stepovers, branches, and roughness. Both geological and geophysical studies indicate that the fault geometry complexities can have a first order effect on spatio-temporally complex slip dynamics. However, a vast majority of models of slip dynamics are conducted on planar faults due to algorithmic limitations. We develop a 3D quasi-dynamic slip dynamics model with Hierarchical matrices to overcome this restriction. The calculation of elastic response due to slip is a matrix-vector multiplication, which can be accelerated by using hierarchical matrices and easily multi-threaded. The computational complexity is reduced from the order of O(N²) to O(NlogN). We cross-validate our code with the SCEC run SEAS benchmark/validation exercise. With this approach, we then explore the role of fundamental geometry complexities and realistic fault geometry on slip dynamics. We also plan to analyse synthetic signals and compare with seismological and geodetic observations.

How to cite: Cheng, J., Almakari, M., Peruzzo, C., Lecampion, B., and Bhat, H.: 3D Quasidynamic cycles accelerated using Hierarchical Matrices: Role of complex fault geometry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7807, https://doi.org/10.5194/egusphere-egu23-7807, 2023.

During fast slip, fault strength may decrease due to weakening mechanisms linked to constitutive properties of the deformed material (e.g., flash heating, thermal pressurisation), but also due to structural effects driven by changes in strain distribution within the shear zone. Extensive theoretical work on thermally activated weakening mechanisms, such as thermal pressurisation of pore fluids, has shown that strain can spontaneously localise in very narrow zones during rapid shear, which promotes further macroscopic weakening of faults. Here, we develop a multiscale fault model which combines a detailed description of thermal pressurisation of fault gouges within large scale elastodynamic rupture simulations. We show that spontaneous strain localisation inside the fault gouge dramatically changes the dynamics of ruptures, and makes the faults more brittle, i.e., decreases the fracture energy and thus produces faster ruptures. We provide closed-form approximations for the resulting localised width and fracture energy as functions of rupture speed. Our work provides a link between structural observations and earthquake dynamics.

How to cite: Brantut, N. and Barras, F.: Rupture dynamics driven by strain localisation within fault gouges, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13002, https://doi.org/10.5194/egusphere-egu23-13002, 2023.

Transient aseismic deformation is observed using dense geodetic measurements across the northern Jordan Valley Fault segment of the Dead Sea Fault. The fault was creeping until 2013 at a rate of 2.7±0.4 mm/yr. It stopped creeping between 2013 and 2018 and then started creeping again at a similar rate. These transitions between the creep and locked modes of deformation correlate well with the 2013 and 2018 seismic sequences that occurred near the tip of the northern Jordan Valley creeping segment. The creep caused the accumulation of Coulomb stresses near the fault tip, which promoted earthquake nucleation in this region. The 2013 seismic sequence was probably too small to release these stresses, and they were released during the 2018 seismic sequence, which allowed the fault to creep again. We suggest that seismic activity will continue to occur near the tip of this creeping segment.

How to cite: Hamiel, Y. and Piatibratova, O.: Interplay between seismic and aseismic deformation near the tip of a creeping segment: Insights from the northern Jordan Valley segment of the Dead Sea Fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4225, https://doi.org/10.5194/egusphere-egu23-4225, 2023.

Subduction zones host some of the greatest diversity in seismic and aseismic fault slip behaviors, such as recurrent slow slip, non-volcanic tremors and repeating earthquakes, that are large enough to be measurable at the surface. Our understanding of the mechanisms leading to fault rupture, especially the role of aseismic slip is limited by the sparsity of instrumentation near the nucleation zone, which is predominantly located offshore away from permanent onland seismic networks.

Fiber-optic Distributed Acoustic Sensing (DAS) offers a new opportunity for long-term seismic observation of off-shore active faults by turning existing fiber-optic seafloor telecom cables into dense arrays of seismic and acoustic sensors. We conducted a one-month long DAS experiment on the northern leg of the Concón landing site of the Prat cable belonging to the GTD company. The longitudinal strain rate was recorded every 4m over a 150km-long fiber section at a temporal sampling rate of 125 Hz, which enabled us to measure low magnitude earthquakes and to locate them precisely. The earthquake catalog generated from the DAS data comprises more than 900 seismic events, which greatly extends the existing regional catalog. A preliminary analysis indicates that several seismic sequences are clustered in time and space, which include numerous events that cannot be detected by the onland seismological network. The ABYSS project will deploy this new observation tool continuously over several years, which will offer a new opportunity to better characterize the distribution of the seismicity in time and space, and will provide new constraints to the models of fault behavior during the seismic cycle. Combined with other types of analysis, such as seismic wave velocity changes monitoring at depth, these data will also provide additional constraints on the aseismic deformation of the fault zone.

How to cite: Rivet, D., Baillet, M., Trabattoni, A., van den Ende, M., Vernet, C., Lior, I., Barrientos, S., Sladen, A., and Ampuero, J.-P.: Improving active faults monitoring leveraging submarine telecom fiber optic cables : first results from central Chile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13207, https://doi.org/10.5194/egusphere-egu23-13207, 2023.

Nearby faults interact with each other through stress fluctuation incurred by seismic rupture, aseismic slip, and viscoelastic flow in the lithosphere. Understanding fault interactions and their temporal variation under different geometry are critical to regional seismic hazard and risk assessments. However, the complex interplay between adjacent faults is often unclear due to insufficient observations of large earthquakes with prolonged recurrence intervals. The 2022 Chihshang earthquake sequence in eastern Taiwan provides unprecedented insights into the interaction between two head-to-head thrust faults during and after a major earthquake. The Chihshang sequence was initiated by an M_w 6.5 foreshock on 17 September, followed by an M_w 7.0 mainshock 7 km to the north and 17 hours later. Based on the coseismic displacements constrained by field survey, optical satellite images, interferometric synthetic aperture radar (InSAR) data, and a dense network of Global Navigation Satellite System (GNSS) measurements, we map the major coseismic rupture on the east-verging Central Range fault (CRF), and the secondary induced slip on the west-verging Longitudinal Valley fault (LVF). The induced slip on the LVF accounts for 9-15% of the total moment release (Mw 7.1). Before the Chihshang earthquake sequence, the seismic hazard along the CRF was much overlooked due to the high seismic activity of the LVF. The 2022 Chihshang earthquake sequence demonstrates for the first time that the CRF is capable of generating earthquakes of M_w 7. The early afterslip primarily took place on the downdip extension of the CRF at great depth, indicating a contribution of ductile deformation there. Incorporating historical earthquake records over the past 120 years, we demonstrate that a rupture on the CRF or LVF reduces the stress level on the other, causing periods of seismic quiescence and an out-of-phase moment release pattern over time between the two faults. These results not only illuminate the fault geometry at the plate suture zone of eastern Taiwan, but also revise the conventional view of the nearby fault interaction. Integrating geometric complexity and fault slip history among adjacent faults in future modeling is essential for assessing realistic seismic hazards in similar structural settings.

How to cite: Tang, C.-H., Lin, Y. N., Tung, H., Wang, Y., Lee, S.-J., Hsu, Y.-J., Shyu, J. B. H., Kuo, Y.-T., and Chen, H.-Y.: Interactions between the dual-verging thrust faults in eastern Taiwan revealed by the 2022 Chihshang earthquake sequence and centennial historical seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4292, https://doi.org/10.5194/egusphere-egu23-4292, 2023.

Earthquake is produced by shear dislocation of rocks across the fault, the frictional status and the area of locked/creeping patches on the fault thus govern the size and occurrence of damaging earthquakes. To better understand these fundamental earthquake physics issues, we deployed over 130 short period seismic nodal stations along the plate boundary type Sumatran fault in Aceh region to cover a segment that was reported to be creeping at various depths. We maintained the nodal array deployment from Jan 2020 to July 2021 by recharging the nodes every 35 days. A machine learning based earthquake detection algorithm was applied to the acquired dataset, which results in a high-resolution seismic catalog that has more than 8000 micro-seismic events. These events clearly delineate the subvertical creeping segment of the Sumatran fault and its Seulimeum branch to the northwest. The seismicity on the creeping segment is almost uniformly distributed from 3 to 12 km in depth, confirming the creeping nature of the fault segment as revealed by geodetic observations, but providing a much more accurate depth constraint. In contrast, the Seulimeum fault branch shows a much deeper seismicity at the depth range of 18 to 25 km, indicating the entire upper crust is fully locked. Sharp stepovers are observed along both strike (~10km) and strike-normal (~4km) directions between the seismicity on these fault segments. The creeping segment of the Sumatran fault, as defined by similar earthquake families, agrees well with the lenses of serpentinite, which has much smaller frictional coefficient that facilitates fault creep. Similar earthquake families show ~ km scale lineation along strike of the fault, where repeating earthquake pairs are identified. However, two shallow Mw6 earthquakes occurred on the creeping segment in the last 25 years. In particular, finite fault inversion of the 2013 Mw6.1 earthquake shows the rupture from 12 km to the surface. These observations suggest a partially creeping/locking or conditionally stable frictional status on the serpentinite-rich segment of the Sumatran fault, that should be considered in both single event and earthquake cycle simulations, as well as seismic hazard assessment.

How to cite: Wei, S., Choong, Z.-Y., Li, C., Chen, Y., Umar, M., Lythgoe, K., Arifullah, A., and Simanjuntak, A.: Creep and seismic rupture of a serpentinite-rich Sumatran fault segment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4704, https://doi.org/10.5194/egusphere-egu23-4704, 2023.

In Ecuador, on some areas of the subduction interface, accumulated stress is released aseismically through slow slip events (SSE) synchronous to seismic swarms (S5). In the La Plata island region in the Central Ecuadorian forearc, recurrent and shallow S5 occur near a portion of the plate interface highly coupled by the subduction of a massive oceanic relief. This study shows a sequence of seismicity and SSE organization propitious to investigate the cause and effect relation-ship between both phenomena.

GPS data show that an SSE (Mw 6.3) initiated at the end of November 2012 and ruptured 2 shallow aseismic patches 25 km apart (~10 km along the vertical direction). The first patch (P1), located southeastward of the island on a moderately coupled portion of the plate interface at the leading edge of the subducting oceanic mount, has a rupture area of about 80 km² and a maximum cumulated slip of 15 cm. Its slipping behavior is pulse-like for about a month and a half. Mid-January, the slip of the SSE suddenly accelerates. A day later, a second aseismic patch (P2), updip from P1, ruptured a highly coupled area of about 250 km² with a maximum slip of 35 cm. This second rupture lasted 8 days and accounted for 80% of total aseismic moment. Both SSE patches stopped slipping by the end of January 2013.

The spatial-temporal distribution of 2,000 micro-earthquakes between November 2012 and February 2013 provides clues about the interface processes and highlights that faulting occurred on secondary faults during an S5. An outer rise seismic cluster with an M_L 4.8 earthquake occurred on a bending fault of the Nazca Plate, 10 days before P1 started. The cluster is collinear with P1 with respect to the relative plate convergence direction, suggesting a possible causal relationship. Almost no seismicity affects the plate interface during the pulse-like development of P1 until mid-January. As the P1-SSE’s slip accelerates, an intense seismic swarm developed updip of P1 along a narrow NNE-SSW trending direction, organized in sub-vertically active structures within the subducting plate. The Coulomb stress variation computed from the cumulative slip of P1 as well as the velocity migration of the cluster (about 10 km/day) suggests that the intraplate swarm is triggered and developed at the P1-SSE’s rupture front. Synchronously to P2, seismicity developed at the Northern edge of the oceanic relief.

The seismicity swarm witnesses the reactivation of oceanic bending faults within the Nazca plate. We hypothesize that this reactivation is likely responsible of a fluid release on the plate interface, that contributed to overpressuring the highly coupled area near P2, priory saturated with fluids, which ruptures aseismically afterwards. 

How to cite: Wickham-Piotrowski, A., Font, Y., Regnier, M., Bletery, Q., Segovia, M., Nocquet, J.-M., and Delouis, B.: Synchronous Slow Slip Event and Seismic Swarm in Central Ecuadorian forearc, 2013, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6896, https://doi.org/10.5194/egusphere-egu23-6896, 2023.

The Xianshuihe Fault Zone (XSHF) is one of the most active strike-slip faults on the eastern Tibetan Plateau. Along the NW-striking, left-lateral XSHF, as many as 8 M >7 and 29 M >6.5 earthquakes have occurred since 1700 CE. The Kangding segment is a special part of the XSHF that has four active faults and can exhibit large earthquakes. From north to south, they are the Yalahe Fault, Selaha Fault, Mugecuo South Fault, and Zheduotang Fault. However, the activity and paleoearthquake sequence of branch faults in Kangding segment remain controversial. Our detailed research is focus on the Yalahe Fault and Zheduotang Fault in Kangding segment. We mapped accurate fault traces and deformed landforms based on detailed interpretations of high-resolution imagery and aerial photographs combined with field observations. Geological and geomorphological evidence was obtained for the Holocene activities. Paleoearthquake sequence was built based on the trench work. We discussed the recurrence characteristics and slip behavior.

The Yalahe Fault follows a quasiperiodic recurrence model and Zheduotang Fault displays uniform slip behavior. From the result of paleoearthquake, the Yalahe Fault, Selaha Fault, and Zheduotang Fault experienced cascading ruptures. Therefore, the branch faults in Kangding segment have ability to generate large earthquakes in the future.

How to cite: Ma, J., Zhou, B., and Wang, M.: Latest quaternary active faulting and paleoearthquakes on the Kangding segment of the Xianshuihe Fault Zone, Eastern Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7491, https://doi.org/10.5194/egusphere-egu23-7491, 2023.

Quantitative stress data is crucial to understanding the mechanical behaviour of faults and the variation of  interface slip behaviours at subduction zones. The Hikurangi Subduction Margin (HSM), New Zealand is characterized by along-strike variations in subduction interface and fault slip behaviour, changing from shallow slow slip events (SSEs) and creep to interseismic locking and stress accumulation moving south. We quantify the shallow (<3km) HSM stress magnitudes and orientations and utilise this new data to determine tectonic variation along the HSM and discuss how this may relate to the large-scale observation in HSM subduction dynamics. For depths below ~650 mTVD results show σ₃: S_v ratios of 0.92-1 along the entire HSM, and S_Hmax: S_v ratios of 0.95-1.81 in the central HSM, and 0.95-2.15 in the southern HSM. Such ratios infer that below ~650 mTVD a prevalent thrust to strike-slip (σ₁=S_Hmax) faulting regime exists along the entire HSM. Our results also reveal a NE-SW (margin-parallel) S_Hmax orientation in the shallow central HSM, which rotates to a WNW- ESE/NW-SE (margin-perpendicular) S_Hmax orientation in the shallow southern HSM.

In the central HSM, we determine the  NE-SW orientation of S_Hmax= σ₁, which is inconsistent with  NNE/NE striking reverse faults (inferring a NW-SE oriented S_Hmax= σ₁) in the region. This suggests that the stress state evolved over time from a contractional to a strike/oblique-slip state. This temporal change in stress state in the central HSM is likely driven by development of clockwise rotation of the Hikurangi forearc and upper plate overpressures. A contemporary NW-SE oriented S_Hmax in the southern HSM, associated with NNE/NE striking faults, suggests the stress regime here remains contractional over time, and is less effected by forearc rotation. The variation in stress state along the HSM spatially correlates with reported along-strike variation in subduction interface slip behaviour. This spatial correlation suggests that contemporary stresses in the overriding plate above the subduction interface may reflect contemporary elastic strain accumulation processes related to subduction megathrust locking.

How to cite: Behboudi, E., McNamara, D., and Lokmer, I.: Quantifying stress fields to better understand shallow tectonics of the Hikurangi Subduction Margin, NZ, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8512, https://doi.org/10.5194/egusphere-egu23-8512, 2023.

The East Anatolian Fault (EAF) is one of the continental transform systems in the Eastern Mediterranean, with a length of about 420 km between Karlıova (Bingöl) in the northeast and Türkoğlu (Kahramanmaraş). The 24 January 2020 Sivrice earthquake with Mw 6.8 once again demonstrated the seismic potential of this sinistral strike-slip fault.

The recent earthquakes and discoveries creeping along the Palu Segment (~100km far to the NE of epicenter of Mw6.8 earthquake) by geodesy- and seismology-based studies increase scientific attention. The spatio-temporal characteristics of the creep observed along the Palu Segment were investigated using nearfield GPS, InSAR, and creepmeter data. After Mw6.8 earthquake, we expanded our study area towards the Pütürge Segment in the south to determine the postseismic effects of the 24 January 2020 earthquake and the kinematics of the Pütürge Segment using multidisciplinary methods (GNSS, creepmeter, InSAR, seismology, paeloseismology) and to investigate the effects on the surrounding faults.

During the first year of the project work, a new GNSS network was established in the region and these networks will be regularly measured every six months. In addition, two permanent GNSS stations were installed. Additionally, using Sentinel-1 data sets, surface deformations mapped by PSINSAR analysis. Generally, post-earthquake effects continue and deformations moved to the SW part of unbroken part of Pütürge segment and based on the creepmeter data, surface deformations still continue at the epicenter locations, following the logarithmic afterslip responce.

This work is supported by TUBITAK project number 121Y400.

Keywords: East Anatolian Fault, Earthquake, GNSS, InSAR

How to cite: Özarpacı, S., Doğan, U., Ergintav, S., Çakır, Z., Zabcı, C., Özdemir, A., Ayruk, E. T., Farımaz, İ., and Köküm, M.: Earthquake Cycle of the East Anatolian Fault Between Palu-Erkenek (Eastern Türkiye): Insights of Tectonic Geodesy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12162, https://doi.org/10.5194/egusphere-egu23-12162, 2023.

Serpentinites are poly-mineralic rocks distributed almost ubiquitously in active tectonic regions worldwide. They are composed of rheologically weak (lizardite and crysotile) and strong (e.g., magnetite and pyroxene) phases. In particular, lizardite typically shows low friction coefficients and is supposed to localise deformation along weak shear zones characterized by aseismic behaviour. Major faults hosting serpentinite lithologies are characterised by seismic activity, tremors, and other slip modes. We advance the hypothesis that low strain domains, which are enriched in rheologically strong phases, can act as potential site of nucleation of unstable slip as the result of the velocity-dependent rheology of magnetite-rich serpentinites. Through an experimental and microstructural approach, we explore the different mechanisms whose interplay controls the complex behaviour of these lithologies.

For this study we collected natural samples of lizardite-magnetite rich serpentinites within the low strain domains of the Elba Island ophiolites (Italy). Rocks were characterised, powdered, and deformed in a set of shear experiments at four different normal stresses (25, 50, 75 and 100 MPa) in the biaxial apparatus BRAVA. The experiments consist of an initial phase of sliding at 10 μm/s, a slide-hold-slide test, and two series of velocity stepping (sliding velocity from 0.1 to 300 μm/s). Fundamental parameters to quantify the frictional properties of serpentinites are individuated in the (a-b) value, the critical slip distance Dc, and the critical stiffness kc, which is derived by their combination.

The material shows friction values of ~0.4 with velocity weakening behaviour and negative frictional healing. The module of the negative (a-b) parameter increases neatly with decreasing sliding velocity while Dc decreases, causing kc to rise. At low velocities (< 3 μm/s) sliding is unstable and the fault undergoes stick-slip behaviour. This is explained by the increase of the critical stiffness to values higher than the loading system stiffness. This systematic change of mechanical properties and fault slip behaviours with sliding velocity is interpreted to be the result of the time-dependent arrangement of grains in a heterogeneous experimental fault architecture.

Back-scattered SEM images of the principal slip zones of recovered samples support this hypothesis. Elasto-frictional behaviour is controlled by the build-up of a partial (granular) load-bearing framework of strong magnetite grains, while visco-frictional rheology is controlled by the (phyllosilicatic) anastomosing and foliated lizardite matrix. At low sliding velocities, the granular phase interacts creating force chains thus promoting frictional instabilities. At higher velocities, dilation promotes the activity of throughgoing weaker phyllosilicate planes thus favouring stable slip.

Our experiments shed light on the role of fault rock heterogeneity in nucleating dynamic slip in nature as well as in controlling the slip mode during earthquakes or slow-slip events in serpentinite terrains.

How to cite: Pozzi, G., Collettini, C., Scuderi, M., Tinti, E., Tesei, T., Viti, C., Marone, C., Amodio, A., and Cocco, M.: Slip velocity and fault stability in serpentine-rich experimental faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14610, https://doi.org/10.5194/egusphere-egu23-14610, 2023.

The Palghar Swarm in Western India is unique given its occurrence within the stable continental interior, its unusually long duration (having started in November 2018 it continues unabated), and extremely high seismicity rate (up to a few hundreds of earthquakes a day). Given the small spatial extent (around 100 km²) of the swarm and the dense seismic network deployed by Indian agencies to monitor it, the swarm offers a unique opportunity to understand the processes driving swarms within the stable interior of the Indian plate which, compared to continental interiors elsewhere in the world, is unusually seismically active. The swarm clusters along two lineaments not expressed on the earth surface. Our InSAR analysis, assuming the lineaments to be subsurface faults, reveals predominantly normal dip-slip motion along both faults during several time windows between March 2019 and January 2020. We find the geodetically inferred moment to be an order-of-magnitude larger than the cumulative seismic moment throughout this time window indicating the presence of substantial aseismic slip. The aseismically slipping patches on the two faults migrate spatially and seem well correlated with the migration of seismicity. We explore the interaction between aseismic slip and the swarm seismicity by calculating resolved Coulomb Stress changes due to migrating aseismic slip on each fault and at the hypocentres of earthquakes large enough for a reliable moment tensor to be inferred. Preliminary results suggest a complex relationship between aseismic and seismic slip and a possible involvement of fluids. These results raise the question whether aseismic slip is commonly associated with earthquake swarms within the Indian continental interior and if these might be associated with deep fluid sources within the Indian continental crust.

How to cite: Bhattacharya, P., Sreejith, K., Gahalaut, V., James, A. S., Mukherjee, S., Bhagat, R., and Agrawal, R.: Interplay between aseismic and seismic slip in an earthquake swarm in Western India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16840, https://doi.org/10.5194/egusphere-egu23-16840, 2023.