Understanding how earthquakes initiate lies at the heart of earthquake physics and analyzing foreshock sequences is one way to probe the initiation process of large earthquakes. A few oceanic transform fault (OTF) earthquakes have been observed to have more foreshocks compared to continental earthquakes. It has also been proposed that OTFs accommodate plate motion primarily by slow creep instead of rapid seismic slip, though with significant along-fault variability. These characteristics make OTFs unique laboratories for probing the physical processes underlying foreshocks and their relations with slow slip events. However, in the past, detailed studies at OTFs have been limited due to their distance from land-based seismic stations. Since 2015, small arrays of ocean-bottom seismometers and hydrophones have been permanently deployed on cabled seafloor observatories in the northeast Pacific Ocean, allowing for monitoring of seismicity on the Blanco Transform Fault (BTF) using the earthquake’s radiated hydroacoustic energy (T-phase). T-phases propagate through the SOFAR channel in the world’s oceans with little attenuation, allowing for the detection of low-magnitude earthquakes at large distances. In this study, we apply a suite of techniques to detect, associate, and locate foreshocks and aftershocks of large mainshocks occurring along the BTF since 2015. We define a mainshock as the largest event occurring within two weeks and a radius of 50 km. 19 Mw  5.0 mainshocks are selected from the GCMT catalogue. However, we are only able to analyze 12 mainshocks due to data availability issues. We use the STA/LTA algorithm to detect T-phase arrivals one week before and after each mainshock through the continuous waveforms recorded on both OBSs and hydrophones. For each detection, we then use the relative station arrival times compared to the mainshock to make sure we only retain events close to the mainshock, i.e., its foreshocks and aftershocks. We then employ the GLOBAL mode Non-Linear Location (NLLoc) program for event localization using a 3D ocean sound velocity model. Compared to the IRIS catalogue which only has a minimum detection level of magnitude 3, lower-magnitude T-phase events are successfully detected by our method. Using our T-phase catalogue, we quantify how the BTF sequences compare with earthquake sequences observed at continental transform faults and test the various proposed models to explain foreshock spatiotemporal behavior and the partitioning of seismic and slow slips along the BTF.

How to cite: Liu, H., Tan, Y. J., and Dziak, R.: Hydroacoustic Monitoring of Earthquake Sequences on the Blanco Oceanic Transform Fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6821, https://doi.org/10.5194/egusphere-egu23-6821, 2023.

The stress that accumulates on faults due to tectonic plate motion can be released seismically and aseismically. The seismic release of stress takes place during earthquakes at short time scales (seconds to minutes), and can be identified on seismic records. The aseismic part of this stress release occurs during Slow Slip Events (SSEs), that last from days to years and do not radiate energetic seismic waves. SSEs are monitored with dense Global Navigation Satellite System (GNSS) networks that record the deformation induced at the surface. A precise identification of slow slip events is key to better understand the mechanics of active faults and to better describe the role of aseismic slip in the seismic cycle. Yet, the characterization of SSEs of various sizes from existing GNSS networks is challenging, and extensive SSEs catalogs remains sparse and incomplete: for example, 64 events for SSEs in Cascadia (Michel et al., 2018), 24 long-term SSEs (Takagi et al., 2019) and 284 short-term SSEs (Okada et al., 2022) in Nankai, Japan. Traditional SSE characterization either focus on specific events, identified visually with high signal to noise ratio (e.g. Radiguet et al., 2016), use time series decomposition approaches such as ICAIM (independent Component analysis inversion method) (Michel et al., 2018; Radiguet et al., 2020), or, for small events, geodetic template matching (Okada et al., 2022; Rousset et al., 2017).

We focus on the Cascadia subduction zone, where a link between slow slip and bursts of tremors has been established (Rogers & Dragert, 2003). In this direction, tremor catalogues can be used to validate potential SSEs detections against the spatiotemporal distribution of tremors. Moreover, a catalogue of SSEs has been recently assessed by (Michel et al., 2019), providing additional benchmark to our analyses. We generate synthetic SSEs from synthetic dislocations (Okada, 1985) using the slab2 model (Hayes et al., 2018). Each SSE template, assumed as a sigmoidal-shaped transient, is further added to a window of noise obtained from real GNSS data (Costantino et al., in prep.).

We develop a deep learning-based method for the systematic detection and characterization of SSEs using a Convolutional Neural Network (CNN) in combination with a Graph Neural Network (GNN). We test our method both on synthetic and real position time series. Results on synthetic data are consistent and show a detection trade-off between the SSEs location, magnitude and the density of the GNSS network. Results on real GNSS positional time series show a good agreement with existing catalogues (cf. Michel et al., 2019). Moreover, new detections have been carried out, which correlate well with the temporal distribution of tremors, suggesting that those events could be new SSE detections, which will be further validated by assessing their spatial-temporal consistency through scaling laws output by the deep learning model.

How to cite: Costantino, G., Giffard-Roisin, S., Dalla Mura, M., Radiguet, M., Marsan, D., and Socquet, A.: Detection and characterization of slow deformation from GNSS data by deep learning in the Cascadia subduction zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16838, https://doi.org/10.5194/egusphere-egu23-16838, 2023.

    An ongoing triggered slow slip event (SSE) on the inland Hengchun fault after the 2006 M_L 7.0 Pingtung offshore earthquake in Taiwan is proposed in this study by analyzing the coordinate time series of 13 continuous GNSS stations, 37 campaign-mode GNSS stations and 3 precise leveling routes in Hengchun Peninsula from 2002 to 2022. Four surface velocity patterns have been determined based on these geodetic data: (1) the interseismic period from 2002 to the 2006 Pingtung offshore earthquake; (2) the 2^nd period after the earthquake to April 2010; (3) the 3^rd period from April 2010 to 2016; (4) the 4^th period from 2016 till 2022. In general, a velocity discontinuity is discovered approximately located at 1-2 km east of the currently known Hengchun fault trace. Then we evaluate the slip deficit rate and slip rate distributions of the Hengchun fault through baseline inversion model and coseismic fault model, respectively. The modeling results shows that Hengchun fault is a reverse fault with a minor left-lateral component. Two asperities are shown in the southern and northern segments, respectively. Furthermore, the energy on the asperities has been gradually released from south to north after the 2006 earthquake, even though the postseismic deformation has faded. On the other hand, the geological investigation results also indicate that surface ruptures were generated on the Hengchun fault until 2017. Therefore, we infer that (1) the temporal pattern changes of surface velocity in Hengchun Peninsula are driven by the 2006 M_L 7.0 Pingtung offshore earthquake; (2) the Hengchun fault ought to be relocated at 1-2 km to the east; (3) a SSE occurs on the Hengchun fault after the 2006 M_L 7.0 Pingtung offshore earthquake. (4) the energy keeps releasing through the SSE after the earthquake and decrease the earthquake potential in Hengchun Peninsula.

How to cite: Hsiao, S.-H., Ching, K.-E., Chang, W.-L., Tsai, P.-C., Giletycz, S., and Chen, C.-L.: An ongoing triggered slow slip event after the 2006 Pingtung offshore earthquake in Hengchun Peninsula, Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4667, https://doi.org/10.5194/egusphere-egu23-4667, 2023.

The Nankai subduction zone is considered as a region with a high seismic risk. Large earthquakes with magnitudes larger than 8 have occurred and will recur in the future. Additionally, several observations of slow earthquakes at the shallow and deep plate interface have continuously been being happening. Large and slow earthquakes originate from different types of frictional characteristics, mostly driven by an accumulation of interplate slip deficit. Here, we present results from geodetic observations to estimate the slip deficit rate at the Nankai subduction zone based on displacement rate data of the dense onshore GNSS array as well as that of the offshore GNSS-Acoustic stations during the period 2002 to 2016 (Nishimura et al. 2018). After removing co-seismic and post-seismic deformations due to earthquakes in other regions, we estimated the slip deficit rate at the plate interface by using a trans-dimensional reversible jump Markov chain Monte Carlo algorithm (Tomita et al. 2021). To obtain a smooth slip distribution without imposing smoothing constraints we used a Voronoi partitioning for slip parameters, in which the number of cells is automatically set during the inversion, based on the spatial resolution of data. In contrast to previous slip deficit inversions at the Nankai region, we included scaling weight factors for different types of data. The scaling factors were parameterized to be adjusted in the inversion procedure. Furthermore, we used elastic Green Functions estimated from a three-dimensional heterogeneous structure of the Nankai regions (Hori et al. 2021), instead of a homogeneous medium as previously done. We accomplished a new model of slip deficit rate for the Nankai subduction zone that complements previous models. Regions with high slip deficits agree with the rupture areas of historic large earthquakes. Lastly, the location of deep and shallow slow earthquakes agrees with estimated intermediate values of slip deficit. The slow earthquake region defines the transition between a locked and unlocked plate interface, while shallow slow earthquakes can also be generated by shallow heterogeneous patches or subsurface structures at the subducting plate.

How to cite: Plata Martinez, R. O., Iinuma, T., Tomita, F., Nishimura3, T., and Hori, T.: The slip deficit rate, slow and fast earthquake at the Nankai subduction zone., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15859, https://doi.org/10.5194/egusphere-egu23-15859, 2023.

A central goal of this work is to understand the extent to which fault friction vs. pore fluids and other factors is the cause of slow earthquakes and the spectrum of fault slip behaviors. Slow earthquakes and quasi-dynamic modes of fault slip such as tremor and LFEs have now been observed in essentially every tectonic setting, which suggests that the underlying mechanism(s) are generic rather than specific to a particular fault setting, rock type, or tectonic regime. Here, I discuss lab data that illuminate the mechanics of slow slip. I focus on frictional stick-slip failure events, the lab equivalent of earthquakes, that reproduce slow slip and the full range of slip behaviors observed on tectonic faults.  These studies document repetitive slip events and the complete lab seismic cycle for the full spectrum of slip behaviors from aseismic creep to slow slip and aperiodic elastodynamic failure. Working with data for repetitive slip events is critical for understanding the underlying mechanics. In the lab, we also document the rate of frictional weakening with slip k_c = σ_n (b-a)/D_c  ––the so-called critical stiffness–– where σ_n is fault normal stress, (b-a) is the friction rate parameter and D_c is the critical slip distance. We measure k_c for the same conditions of the slow slip events by altering the machine loading stiffness. These works assess directly frictional instability theory, which predicts the slip stability transition when the elastic stiffness of the fault zone k equals the frictional weakening rate k_c. The lab work confirms friction theory in relation to the transition from stable to unstable slip but it also reveals additional complexity showing that k_c varies with slip rate. Several works now document the velocity dependence of k_c(V) and its role in dictating slow slip in the lab. These works show complex behavior near k/k_c ≈ 1 including slow slip, aperiodic failure and chaotic motion. I discuss these results in the context of basic questions that remain regarding how slow ruptures can propagate quasi-dynamically, at speeds far below the Rayleigh wave speed, and how tectonic faults can host both slow slip and dynamic earthquake rupture.

How to cite: Marone, C.: Slow Earthquakes and the Spectrum of Fault Slip Modes: A View From the Lab, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5886, https://doi.org/10.5194/egusphere-egu23-5886, 2023.

Over the last decades, new observations of complex slip dynamics have emerged. We now observe a continuum of transients energy release happening on fault systems, such as slow slip events, LFEs and tremors. Present quasi-dynamic numerical models are capable of producing such complex slip events on fault planes by considering more realistic complex fault geometries. We aim in this study to bridge the gap between source modeling and observations by generating synthetic surface records. 

For this, we consider a fault system which consists of a main self-similar rough fault, surrounded by a dense network of off-fault fractures. We embed our 2D quasi-dynamic fault zone in a 3D elastic half-space and cover the free surface with a wide array of colocated broadband accelerometers and high rate GPS stations. Over multiple seismic cycles, we record broadband signals at 50 Hz and high rate GPS at 1 Hz. We aim to understand how the different sequences of complex behavior that we observe on the fault plane are recorded on the stations. What are the different contributions of the main fault and off-fault fractures to the radiated signals recorded on the stations? 

How to cite: Almakari, M., Bhat, H. S., Kheirdast, N., Villafuerte, C., and Thomas, M. Y.: How is geometrical complexity in fault zone recorded by geodesy and seismology?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7998, https://doi.org/10.5194/egusphere-egu23-7998, 2023.

Slow slip and tremor (SST) has now been observed along many subduction margins worldwide, and the phenomenon is commonly linked to fluid and/or fluid pressure variations and migration. Crucial to understanding and modeling how fluids and seismicity might interact are estimates of porosity (Φ) and permeability (k) in and around the deforming subduction megathrust shear zone. Constraints on k from deeply buried metamorphic rocks are difficult to obtain, however. Experiments and some small-scale field observations indicate very low k for subduction-related lithologies, on the order of 10^-18m² or less. However, thus far no attempts have been made to quantify the large-scale (or transient) permeability of subduction shear zones at deep metamorphic conditions.

Here we use structural, microstructural and geochemical observations on an exhumed sliver of metamafic rocks, with thermal conditions comparable to the Cascadia subduction zone, to quantify the hydrological properties of the deep SST source region. The study locality (Megas Gialos, Syros Island, Greece) records structures consistent with ductile deformation during subduction, underplating, and subsequent partial exhumation under high pressure greenschist facies conditions within the subduction shear zone. 

The 100-m-length outcrop we studied consists of mafic greenschists with a strong ductile foliation and several generations of syn- to late-kinematic dilational faults and veins. Evidence for both along- and across-dip fluid flow is preserved in the form of metasomatic selvages parallel to the foliation, foliation-parallel quartz veins with foliation-perpendicular growth fibers, and dilational faults oriented at high angles to the foliation. The orientations and cross-cutting relationships between the foliation and multiple vein generations indicate the veins acted as transient fluid-flow conduits opened cyclically during background distributed viscous flow under extremely low differential stresses. In thin section, most of the veins exhibit crack-seal textures, consistent with episodic hydrofracturing. 

To estimate the porosity and 2D permeability tensor from outcrops, we mapped the youngest generation of veins using high resolution drone models, then used Matlab-based software FracPaq to calculate permeability. Our assumptions include a) the latest generation of veins were at some stages opened simultaneously or in close succession (consistent with evidence for very low differential stress magnitudes), and b) the characteristic opening aperture was assumed to be an average of measured crack-seal widths in thin section. This approach yields an estimate of Φ of ~0.8 to 8% and an anisotropic k of 6.0x10^-15 to 1.4x10^-14 m² in the along-dip and across-dip orientations, respectively. These values are 3+ orders of magnitude greater than would be inferred for the background unfractured rock. They are broadly consistent with estimates of k from geophysical observations of tremor migration patterns, and with models of the permeability contrasts (background/transient) required for viscous compaction of fluid pressure to lead to unstable slip. The method we demonstrate can be applied to other outcrops with subduction contexts and can provide essential ‘ground-truthed’ data to test assumptions of fluid flow in the deep tremor source region.

How to cite: Muñoz-Montecinos, J. and Behr, W.: Quantifying paleo-permeability on the deep subduction interface from exhumed rocks: a case study from Syros Island, Greece, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2888, https://doi.org/10.5194/egusphere-egu23-2888, 2023.

The interface between a subducting and overriding plate usually exhibits low seismic velocities within a thin (<6 km thick) layer. The hydrologic and petrologic conditions surrounding this layer control the behavior of the plate interface fault, which shows a wide range of rupture behavior from the megathrust through the down-dip transition to tremor and slow-slip. Many analyses of the properties of this plate interface low-velocity layer (LVL) use receiver functions (RFs), which sample sharp seismic velocity gradients and depend primarily on the time separation between RF phases from the top and bottom of the layer, to provide diagnostic estimates of thickness and Vp/Vs. Previous hypotheses for plate interface rupture behavior have invoked high pore-fluid pressure to explain inferences of apparently high Vp/Vs (exceeding 2.2) along the seismogenic zone and the adjacent down-dip slow-slip region determined from RF analyses. However, new higher-resolution analyses of scattered teleseismic P-wave coda that sample this region in two different subduction zones reveal Vp/Vs within the range of normal lithologies (1.6-2.0) and remove the observational requirement for a thick region of high pore-fluid pressure. These results agree with recent laboratory analyses of the properties of exhumed megathrust rocks. Instead, the mechanical properties of a thick damage zone surrounding the interface or entrained sediments explain both the scattered-wave observations and observed fault rupture behavior in the seismogenic zone and deeper. Pore pressure could play a role, but it may operate more locally or intermittently than conventionally thought. Additionally, from this analysis we find that the frequency content of the scattered phases generated at the delimiting boundaries of the LVL are limited and do not provide enough resolution to include direct (up-going P-to-S converted waves) conversions in LVL RF analyses without biasing the results to high values of Vp/Vs and thickness.

How to cite: Mann, M. E., Abers, G., and Fulton, P.: The environment surrounding the subduction zone plate interface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16875, https://doi.org/10.5194/egusphere-egu23-16875, 2023.