TS7.1

Inter- and intraplate seismicity in subduction zones

Since approximately 90% of the seismic moment released by earthquakes worldwide occurs near subduction zones, it is crucial to improve our understanding of seismicity and the associated seismic hazard in these regions. Seismicity in subduction zones takes many forms, ranging from relatively shallow seismicity on outer-rise and splay faults and the megathrust to intermediate-depth (70-300 km) and deep events (>300 km). While most research on subduction earthquakes focuses on the megathrust, all these different seismic events contribute to the seismic hazard of a subduction zone.

This session aims to integrate our knowledge on different aspects of subduction zone seismicity to improve our understanding of the interplay between such events and their relationship to subduction dynamics. We particularly invite abstracts that use geophysical and geological observations, laboratory experiments and/or numerical models to address questions such as: (1) What are the mechanisms behind intraplate seismicity? (2) How do outer-rise and splay fault seismicity relate to the seismogenic behaviour of the megathrust? (3) How do slab dynamics influence both shallow and deep seismicity?

Co-organized by GD5/SM4
Convener: Silvia Brizzi | Co-conveners: Elenora van RijsingenECSECS, Iris van ZelstECSECS, Stephen Hicks
Presentations
| Tue, 24 May, 10:20–11:50 (CEST)
 
Room K2

Presentations: Tue, 24 May | Room K2

10:20–10:25
Deep seismicity
10:25–10:35
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EGU22-8846
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solicited
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On-site presentation
Magali Billen, Rebecca Fildes, Marcel Thielmann, and Menno Fraters

The occurrence of deep earthquakes within subducting lithosphere (slabs) remains enigmatic because these earthquakes have many similarities to shallow earthquakes, yet frictional failure is strongly inhibited at high pressure. Regardless of depth, earthquakes occur where the temperature is cold enough that elastic deformation is accumulated over time: for frictionally controlled earthquakes at shallow depth, the rate of seismic moment release is correlated with the strain-rate. Comparison of spatial variation in strain-rate magnitude from 2D simulations of subduction to observed seismicity versus depth profiles suggest that strain-rate may also be a determining factor in the occurrence of deep slab seismicity (1). In addition, proposed mechanisms for deep earthquakes, including transformational faulting of metastable olivine and thermal shear instability, are known to depend directly on strain-rate. To test the hypothesis that strain-rate is a determining factor in the spatial distribution of deep earthquakes, we are creating 2D models of subduction with visco-elasto-plastic (VEP) rheology and a free surface in the software ASPECT (2). The 2D slab structure is constructed for specific locations in which the slab geometry is extracted from Slab 2.0 (3) and the plate age and convergence rate are used to define the thermal structure using a new mass-conserving slab temperature model (4) implemented in the Geodynamic WorldBuilder (5). The resulting strain-rate and stress, together with the pressure and temperature along multiple transects of the slab are used as input values for a 1D thermal shear instability model (6) using the same VEP rheology as the slab deformation models.  Using this approach we can test whether the conditions in the slab favor failure through thermal shear instability and compare the spatial distibution to obsered seismicity. Initial results of this workflow will be presented, including how we have overcome some of the challenges in running VEP models for comparison to present-day slab seismicity. References: 1. Billen, M. I. , Sci. Advances, 2020. 2. Bangerth, W. et al., https://doi.org/10.5281/ZENODO.5131909, 2021. 3. Hayes, G.P. et al., Science, 2018. 4. Billen, M. I. and Fraters, M. R. T., EGU Abstract, 2022. 5. Fraters, M. R. T. et al., Solid earth, 2019. 6. Thielmann, M. Tectonophysics, 2018. 

 

How to cite: Billen, M., Fildes, R., Thielmann, M., and Fraters, M.: Testing the Strain-rate Hypothesis for Deep Slab Seismicity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8846, https://doi.org/10.5194/egusphere-egu22-8846, 2022.

10:35–10:40
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EGU22-11762
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Presentation form not yet defined
Marcel Thielmann and Thibault Duretz

Since their discovery in 1928, deep earthquakes have been the subject of extensive research to unravel their nucleation and rupture mechanisms. Due to the elevated pressures and temperatures at depths below 50 km, brittle failure becomes less likely and ductile deformation is favored. To date, there is no consensus on the mechanisms resulting in deep earthquake generation. Three main mechanisms (dehydration embrittlement, transformational faulting and thermal runaway) have been proposed to cause deep earthquakes, but neither of them has been sufficiently quantified to yield a definite answer under which conditions they are active.

Here, we explore the feasibility of the thermal runaway hypothesis using 1D and 2D thermo-mechanical models. In particular, we investigate the impact of grain size reduction in conjunction with shear heating to see whether grain size reduction and shear heating are competitive mechanisms (which would prevent thermal runaway) or whether they are collaborative. Our results show that the combination of both mechanisms facilitates thermal runaway and significantly reduces the stress required for the occurrence of thermal runaway. We then investigate whether this combined failure mechanism may explain the seismicity observed in regions of detaching lithosphere, such as the Hindu Kush and the Vrancea earthquake nests. 

How to cite: Thielmann, M. and Duretz, T.: Crushed and fried: ductile rupture at depth due to grain size reduction and shear heating, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11762, https://doi.org/10.5194/egusphere-egu22-11762, 2022.

10:40–10:45
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EGU22-6564
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Virtual presentation
Julien Gasc, Clémence Daigre, Damien Deldicque, Arefeh Moarefvand, Blandine Gardonio, Julien Fauconnier, Claudio Madonna, Pamela Burnley, and Alexandre Schubnel

     This year marks the 100th anniversary of the discovery of Deep Focus Earthquakes (DFEs). Despite the elaboration of several hypotheses, the mechanisms responsible for their occurrence at depths where rocks flow in a viscous way are not entirely elucidated. DFEs are far from ubiquitous and only occur in certain subducting slabs as they descend through the mantle transition zone, where olivine transforms to wadsleyite and ringwoodite. This has led to associating DFEs to the transformation of metastable olivine. Faulting induced by the olivine transformation was proven to cause brittle behavior under conditions where ductile deformation otherwise prevails [Burnley et al., 1991]. It can also explain the anomalously high DFE activity in Tonga, which has been attributed to the thermal state of the subducting slab, colder slabs allowing for more metastable olivine.

     However, there are limited data regarding the conditions required for transformational faulting in terms of reaction kinetics, as well as regarding its possible propagation in ringwoodite peridotites. The seminal work of Burnley, Green and co-authors regarding transformational faulting used a Ge-olivine analogue, a material that undergoes the transition to the ringwoodite structure (Ge-spinel) at much lower pressures than the silicate counterpart [Burnley et al., 1991]. Here we continue to build upon this work by combining high pressure and temperature deformation experiments with Acoustic Emission (AE) monitoring. The experiments investigate lower temperatures and strain rates to assess the extrapolation of transformational faulting towards natural conditions. Ge-olivine samples were deformed in the Ge-spinel field at 1.5 GPa and various temperatures in a modified Griggs apparatus.

     We demonstrate that transformational faulting can initiate in metastable olivine, and then continue to propagate via shear-enhanced melting in the stable high-pressure phase, which is a paramount finding since transformational faulting has been contested as the origin of DFEs on the basis that large DFEs cannot be contained within a metastable olivine wedge. The experiments yielded a range of mechanical behaviors and acoustic signals depending on the kinetics of the olivine-ringwoodite transformation. The b-values associated with the obtained AEs range from 0.6-1.5, consistent with those of DFEs. In addition, we evidence that transformational faulting is controlled by the ratio between strain rate and reaction kinetics and extrapolate this relationship to the natural conditions of DFEs. Counterintuitively, these results imply that cold slabs induce transformational faulting at higher temperatures as a result of faster descent rates. This produces more numerous small DFEs and explains the higher b-values observed.

Burnley, P. C., H. W. Green, and D. J. Prior (1991), Faulting Associated With The Olivine To Spinel Transformation In Mg2geo4 And Its Implications For Deep-Focus Earthquakes, Journal of Geophysical Research-Solid Earth and Planets, 96(B1), 425-443.

How to cite: Gasc, J., Daigre, C., Deldicque, D., Moarefvand, A., Gardonio, B., Fauconnier, J., Madonna, C., Burnley, P., and Schubnel, A.: Transformational Faulting in Metastable Olivine, from Lab to Slab, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6564, https://doi.org/10.5194/egusphere-egu22-6564, 2022.

10:45–10:50
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EGU22-9820
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ECS
|
Virtual presentation
Gilberto Leite Neto and Jordi Julià

The occurrence of deep-focus earthquakes (h > 300 km) is restricted to a handful of regions worldwide, generally associated with subduction zones. In particular, the South American subduction zone hosts two narrow belts of deep-focus seismicity with depths greater than ~500 km along the Peru-Brazil border and Bolivia/northern Argentina. This subduction zone has a thermal parameter of Φ < 2500 km and is regarded as a warm end-member. Only in 2015, the USGS catalog listed up to 25 deep-focus events in the Peru-Brazil belt, with magnitudes and depths ranging from 4.0 to 7.6 Mw and 515 to 655 km, respectively. Notably, this sequence included a well-investigated doublet of two 7.6 Mw events occurring 5 min apart trailed by a number of aftershocks of magnitude 4.0 Mw or larger. Published focal mechanisms for the main doublet display predominantly double-couple components that closely agree with the GCMT solution (E1: 350°, 39°, -80° and E2: 350°, 30°, -81°), suggesting shear failure at those depths. Mechanisms capable of shear instability at those large depths traditionally include dehydration embrittlement, transformational faulting, thermal runaway or a combination of those. Aiming at investigating the physical mechanism responsible for these deep-focus events, we are using a combination of regional and teleseismic recordings from the Brazilian Seismographic Network (RSBR) and other regional and national networks in the continent to determine focal mechanisms for deep-focus earthquakes (M > 4) that occurred between 2014 and 2022. The mechanisms are being determined through a Cut and Paste approach, which compensates for inaccuracies in the velocity model through independent relative time shifts between observations and predictions for P, SV and SH wave trains sampling both the upper and lower hemispheres of the focal sphere. The results on the 2015 doublet, using the full dataset (regional and teleseismic stations), indicated two very similar normal faults fully consistent with the GCMT solutions, at the preferred depths of 616 (E1) and 621 (E2) km. Preliminary inversions using only regional networks (RSBR) for 15 smaller earthquakes (4.3 < M < 7.1) also yield normal mechanisms with T axes oriented roughly E-W. This apparent uniformity of the focal mechanisms for the South-American deep-focus earthquakes, with near-vertical P axes and near-horizontal (east-west-oriented) T axes, strongly suggests vertical compression along the subducting plate is the main source of stress driving deep-focus seismicity. Down-dip compression is expected from either buoyancy forces, equilibrium phase transformations or a metastable olivine wedge (MOW); however, how earthquakes are nucleated at those depths is harder to explain. Transformational faulting within the MOW has been the preferred mechanism in cold slabs, while in warm slabs its presence has been more debated due to wedge size being expected to decrease with temperature. Transformational faulting in other metastable minerals such as enstatite is our preferred alternative, as dehydration embrittlement and thermal runaway seem to lack the capacity of triggering earthquakes at those large depths.

How to cite: Leite Neto, G. and Julià, J.: Investigating Source Mechanisms of Deep-Focus Earthquakes at the Peru-Brazil Border with Regional and Teleseismic Data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9820, https://doi.org/10.5194/egusphere-egu22-9820, 2022.

Intermediate-depth seismicity
10:50–10:55
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EGU22-11670
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ECS
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Presentation form not yet defined
Blandine Gardonio, David Marsan, Stéphanie Durand, and Alexandre Schubnel

The last twenty years have seen a number of large, devastating earthquakes on subduction zones. In many ways, the M9.0 Tohoku-oki earthquake was bewildering for the seismological community. It occurred on a previously identified coupled area but ruptured a larger zone than expected and, above all, the large amount of near-trench coseismic slip was a surprise.

Because Japan is one of the best area instrumented in the world, the 2011 Mw 9.0 Tohoku-oki earthquake is one of the world's best-recorded ruptures. Many studies have analyzed with great details the pre-seismic, co-seismic and post-seismic phases of the Tohoku earthquake. Researchers also focused on the triggering of on-land seismicity following the mega-thrust earthquake. However, no study zoom out and considered the consequences of this earthquake on the Pacific plate in this area.

 

In this study, we analyzed the Japanese Meteorological Agency seismic catalog over ten years of data to assess the consequences of such large mega-thrust earthquake over the Pacific plate from the Izu-Bonin area to the north of Hokkaido island. We studied the seismicity from 0 to 700km depth, taking advantage of one of the most complete subduction zone catalogue.

Our results show that the seismic rate south of Japan experienced a decrease at the time of Tohoku about 30% and an increase of 20% underneath the Hokkaido island. The subduction zone that is downdip Tohoku doesn’t seem affected by the megathrust earthquake. While it is difficult to understand and to model such large scale effects of the Tohoku earthquake on the Pacific plate, we think it is primordial to observe and detail them with precision.

How to cite: Gardonio, B., Marsan, D., Durand, S., and Schubnel, A.: From the Izu-Bonin to the north of Hokkaido : how did the M9.0 Tohoku earthquake affect the Pacific plate seismicity ?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11670, https://doi.org/10.5194/egusphere-egu22-11670, 2022.

10:55–11:00
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EGU22-12565
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ECS
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Presentation form not yet defined
Audrey Chouli, David Marsan, Sophie Giffard-Roisin, Michel Bouchon, and Anne Socquet

An increase of both shallow and intraslab intermediate-depth seismicity has been observed days to years before some great subduction earthquakes, as before Tohoku-oki (Mw 9.0, 2011), Maule (Mw 8.8, 2010) or Iquique (Mw 8.2, 2014) earthquakes (Bouchon et al., 2016, Jara et al,. 2017). These observations suggest that a link exists between these deep and shallow foreshocks, but it is still poorly understood and not characterized in a systematic manner. Some studies have attempted to address this lack of systematic characterization by using a statistical approach (Delbridge et al., 2017).

The aim of this study is to systematically and statistically identify and characterize the potential correlations between deep and shallow seismicity. We want to assess whether or not such interactions exist. If they exist, we plan to characterize when and where they occur, at what frequency, their characteristic duration, and with what spatial pattern.  

For this purpose, we develop a statistical method to assess the relevance of deep-shallow interactions, that allows to identify statistically significant correlations between deep and shallow seismicity. We focused on the seismicity of the Japan trench subduction zone during the decade prior to the Tohoku-oki earthquake, because deep-shallow interactions were identified there, and because we can test the events picked by our method against the correlations highlighted in published papers (Bouchon et al., 2016). The correlation values between the deep and shallow events from the Japan Meteorological Agency catalog are calculated on various different sliding-windows with durations from month to week. These correlation values are then compared to the ones obtained using synthetic series of shallow events that meet the spectral properties of the real series, and the significance of the correlation is calculated.

Some windows show a strong correlation. The dependence of our results to different parameters, such as the completeness magnitude, the length of the window, the lag, the smoothing etc… are evaluated. The spatio-temporal analysis of the seismicity on maps for these windows is also explored. While the results are still preliminary, we believe that this method has the potential to systematically and quantitatively assess the current presumptions on the link between deep and shallow seismicity, that would lead to a better understanding of the mechanisms leading to megathrust earthquakes.

 

Bouchon, M., Marsan, D., Durand, V., Campillo, M., Perfettini, H., Madariaga, R., & Gardonio, B. (2016). Potential slab deformation and plunge prior to the Tohoku, Iquique and Maule earthquakes. Nature Geoscience, 9(5), 380.

Delbridge, B. G., Kita, S., Uchida, N., Johnson, C. W., Matsuzawa, T., & Bürgmann, R. (2017). Temporal variation of intermediate‐depth earthquakes around the time of the M9. 0 Tohoku‐oki earthquake. Geophysical Research Letters, 44(8), 3580-3590.

Jara, J., Socquet, A., Marsan, D., & Bouchon, M. (2017). Long-Term Interactions Between Intermediate Depth and Shallow Seismicity in North Chile Subduction Zone. Geophysical Research Letters, 44(18), 9283-9292.

How to cite: Chouli, A., Marsan, D., Giffard-Roisin, S., Bouchon, M., and Socquet, A.: Analysis of the potential correlation between intraslab intermediate-depth and shallow earthquakes in the Japan trench subduction zone prior to the Mw 9.0 Tohoku-oki earthquake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12565, https://doi.org/10.5194/egusphere-egu22-12565, 2022.

Megathrust & shallow seismicity
11:00–11:05
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EGU22-1613
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ECS
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Virtual presentation
Constanza Rodriguez Piceda, Magdalena Scheck-Wenderoth, Mauro Cacace, Judith Bott, Ya-Jian Gao, Frederik Tilmann, and Manfred Strecker

The southern Central Andes (SCA, 29°S—39°S) orogen is one of the seismically most active regions along the length of the South-American convergent margin, where past earthquakes (e.g., San Juan in 1944, Valdivia M9.5 in 1960 and M8.8 Maule in 2010) have had devastating effects on the population. Past research has extensively focused on linking the occurrence of seismic activity with the stress regime on individual faults at a local scale.  In order to more systematically address the relationship between the long-term rheological configuration of the whole lithosphere and the spatial patterns of seismic deformation in the SCA, we computed a 3D model of the expected mechanical strength and rheology (brittle, ductile) of the SCA and adjacent forearc and foreland regions based on an existing 3D model describing the first-order variations of thickness, composition and temperature of geological units forming the upper and subducting plates. We found that the spatial variation in the predicted rheology correlates well with the distribution of seismic deformation in the upper plate, with seismicity bounded to the modelled brittle deformation domain. Moreover, seismic events localize at the transition between mechanically strong and weak domains. This ultimately indicates that the strength of the lithosphere exerts a first-order control on the mechanical stability of the region.

In contrast, the results from the rheological model fail to reconcile the observed slab seismicity at depths > 50—70 km, where ductile rheological conditions are expected. In this case, we evaluated possible additional mechanisms triggering these earthquakes, including compaction of sediments at the interface, metamorphic reactions within the oceanic crust and mantle, and slab flexural stresses. To characterize the state of hydration of the mantle related to dehydration reactions and/or sediment compaction, we made use of the Vp/Vs ratio from a seismic tomography model. The majority of the slab seismicity was found to spatially correlate with hydrated areas of the slab and overlying continental mantle, apart from a cluster where the slab attains a sub-horizontal angle. In this region, the correlation between the focal mechanisms of these earthquakes and the slab orientation, suggests that seismicity here is driven by enhanced flexural stresses within the oceanic plate.

This contribution showcases the importance of a quantitative characterization of the rheological state of the lithosphere to elucidate the causative dynamics of the spatial distribution of seismicity in the area.

How to cite: Rodriguez Piceda, C., Scheck-Wenderoth, M., Cacace, M., Bott, J., Gao, Y.-J., Tilmann, F., and Strecker, M.: How does lithospheric strength, mantle hydration and slab flexure relate to seismicity in the southern Central Andes?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1613, https://doi.org/10.5194/egusphere-egu22-1613, 2022.

11:05–11:10
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EGU22-2924
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ECS
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On-site presentation
Malte Metz, Angela Carillo Ponce, Felipe Vera, Simone Cesca, Frederik Tilmann, and Joachim Saul

On August 12, 2021, an earthquake doublet with a cumulative magnitude MW 8.0 – 8.3 hit the South Sandwich Trench in the South Atlantic where the South American plate is subducted beneath the Sandwich microplate. Significant differences in location, depth, and magnitude are reported by international agencies. Discrepant results might be due to the short inter-event time of ~150 s between both subevents and the lack of local and regional data.

We apply a multi-disciplinary approach to clarify the source processes and characterize different features of the doublet. Our centroid solutions of the mainshocks, separated by ~290 km, confirm the overall southward rupture directivity. The predominant thrust mechanisms, with different strike directions, suggest the activation of a bent portion of the slab. We estimate a cumulative magnitude of Mwc 7.65 inverted from body waves in the frequency band 0.01 – 0.03 Hz. Our magnitude estimate is substantially smaller than the one reported, e.g., by Global CMT, suggesting that a significant part of the moment has been released at lower frequency as a slow slip process. It is verified by a W-phase inversion in the frequency band 0.005 - 0.01 Hz with a resulting magnitude Mww of 8.0. The iterative deconvolution and stacking method (IDS) resolves high slip patches located in the area of the two mainshock centroids. High-frequency back-projection results confirm the unilateral southward rupture propagation. Complex fault and slab geometries do not significantly improve the fit, providing no clear evidence for the activation of secondary faults. Centroid moment tensors, estimated for 87 aftershocks between August 12, 2021 and August 31, 2021, support the identification and characterization of activated fault segments.

How to cite: Metz, M., Carillo Ponce, A., Vera, F., Cesca, S., Tilmann, F., and Saul, J.: Multi-disciplinary assessment of the August 12, 2021, South Sandwich earthquake doublet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2924, https://doi.org/10.5194/egusphere-egu22-2924, 2022.

11:10–11:15
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EGU22-11060
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ECS
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Virtual presentation
Juan Carlos Graciosa, Fabio Antonio Capitanio, Mitchell Hargreaves, Thyagarajulu Gollapalli, and Mohd Zuhair

Understanding the controls on large magnitude seismicity occurrence remains an open challenge, yet a pressing one, for the exceptional hazard associated with earthquakes. Different parameters are proposed to exert control on the generation and propagation of megathrust earthquakes and untangling their complex interactions across scales remains challenging. Here, we use explainable artificial intelligence to unravel the interactions between different parameters and elucidate the underlying mechanisms. We use three types of datasets from a number of convergent margins: a) a catalogue of earthquake hypocentre and rupture, b) geophysical observations of subduction zones properties (e.g., gravity, bathymetric roughness, sediment thickness), and c) the distribution of stress within the slab due to slab pull calculated from flexure models. These constitute the three types of nodes in the input layer of a Fully Connected Network (FCN) trained to classify earthquake magnitude embedding the state of the system (b), the driving mechanism (c) and the resulting seismicity (a). We then analyse the trained network using Layer-wise Relevance Propagation (LRP) to determine the relative weights of the input nodes, providing relevant constraints on the mechanisms that dominate the seismicity in a region, their scale and likelihood.

How to cite: Graciosa, J. C., Capitanio, F. A., Hargreaves, M., Gollapalli, T., and Zuhair, M.: Megathrust Seismicity Through the Lens of Explainable Artificial Intelligence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11060, https://doi.org/10.5194/egusphere-egu22-11060, 2022.

11:15–11:20
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EGU22-8542
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ECS
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On-site presentation
Sonny Aribowo, Laurent Husson, Christophe Basile, Danny H. Natawidjaja, Christine Authemayou, Mudrik R. Daryono, and Manon Lorcery

The Java subduction megathrust is undoubtedly the source of high magnitude, extremely damaging earthquakes. In the back-arc of the subduction zone, severe earthquakes also affect the northern part of Java. The Jakarta basin lies at the western end of the Java back-arc thrust, which stems on the seismogenic Flores thrust in the east and propagates westward across Java. The tectonic activity of the Java Back-arc Thrust in the Jakarta basin has been overlooked because of its low recurrence time. Yet, historical records reveal that it was destructive, resulting in severe destruction in Bogor and Jakarta. Tracking fault activity in large cities is problematic because the original landscape is often profoundly anthropized and has little to do with its pre-industrial physiography. In the Jakarta basin, this is even more complex owing to the fast Plio-Quaternary sedimentation that conceals the morphotectonic features associated with the fault. We combine geomorphic observations and subsurface data using DEMs and optical imagery, seismic reflection and biostratigraphic well data. At depth, seismic data reveal a partitioned fault network of compressive fault-propagation folds and transpressive flower structures that deform the Plio-Quaternary sedimentary layers of the Jakarta basin and interplay with volcanoes. At the surface, morphological observations in the rims of the basin reveal that several river meanders were abandoned and uplifted hundreds of meters above the current riverbeds above the fault network. In the basin, multiple meter scale waterfalls that we interpret as knickpoints above active faults scar the flat surface of the basin. We conclude that the western end of the Java back-arc thrust fault bears a potentially high risk for the infrastructures of the densely populated province of Jakarta.

How to cite: Aribowo, S., Husson, L., Basile, C., Natawidjaja, D. H., Authemayou, C., Daryono, M. R., and Lorcery, M.: Back-arc thrusting in the Jakarta basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8542, https://doi.org/10.5194/egusphere-egu22-8542, 2022.

Slow slip, tremors, transient deformation, and subduction structure
11:20–11:25
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EGU22-6099
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ECS
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Presentation form not yet defined
Nicolai Nijholt, Wim Simons, Taco Broerse, Joni Efendi, Dina Sarsito, and Riccardo Riva

The Celebes Sea subducts beneath the North Arm of Sulawesi, Indonesia, at the Minahassa trench. Over the past three decades, only a few Mw>7 earthquakes ruptured this plate interface, despite a 40 mm/yr convergence rate. The left-lateral Palu-Koro fault delineates the extent of the overriding plate at the western termination of the Minahassa subduction zone and hosted a Mw7.5 earthquake in September 2018. Observations of post-seismic surface motion following the 2018 event were interpreted in a previous study to result from afterslip that extended underneath the co-seismic rupture plane. A mismatch between observed post-seismic surface motions and predictions from afterslip distributions remained at the North Arm of Sulawesi.

In this study we revisit and reprocess the GNSS observations in NW Sulawesi. We analyse the post-2018 time series to determine whether the post-seismic signal can be ascribed to a single source. This is not the case, as we detect another, yet smaller amplitude signal. We take a Bayesian approach and find that this smaller magnitude signal corresponds to slow slip on the Minahassa subduction interface. This delayed-triggered, (apparently aseismic) slow slip event occurred just east of the 1996 Mw7.9 megathrust rupture.

The 20-year long time series is characterized by four additional periods of transient surface motion. Three of these periods are likely the result of distinct slow slip events and one is a post-seismic signal from the 2008 subduction Mw7.4 earthquake. The presumed slow slip events generally take more than 300 days to quiet down again with a recurrence interval of about five years.

How to cite: Nijholt, N., Simons, W., Broerse, T., Efendi, J., Sarsito, D., and Riva, R.: Recurrent episodes of transient deformation in NW Sulawesi, Indonesia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6099, https://doi.org/10.5194/egusphere-egu22-6099, 2022.

11:25–11:30
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EGU22-6888
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ECS
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Presentation form not yet defined
Raymundo Plata-Martinez and Yoshihiro Ito

The Guerrero seismic gap, at the Mexican subduction zone, has been a region of great seismological interest because of the absence of a large earthquake in more than 110 years. If an earthquake were to rupture the entire Guerrero seismic gap the resulting earthquake could be disastrous to major Mexican cities. Additionally, the Guerrero subduction zone has plenty of slow earthquake activity with large slow slip events and tectonic tremors, located at the deep plate interface. To obtain a new and unique observation point of seismicity in the Guerrero seismic gap and continue evaluating its seismic risk, we deployed an array of ocean bottom seismometers (OBS) offshore the Guerrero seismic gap. We were able to detect and locate shallow tremors near the trench and deduce that a portion of the shallow plate interface undergoes stable slip. We used data from the OBS to analyse the new catalogue of shallow tremors and describe their source. Focal mechanisms of shallow tremors were estimated using S wave polarisation. We found that slip azimuth tends to follow the subduction plate motion, suggesting that tremors rupture at the plate interface. We also estimated shallow tremor radiated seismic energy. We found a heterogeneous energy release of shallow tremors along strike. Our observations of a heterogeneous shallow tremor energy release can be explained with the different mechanical properties, inside and outside the Guerrero seismic gap, and help to characterise the seismogenic zone at the shallow plate interface.

How to cite: Plata-Martinez, R. and Ito, Y.: First analysis of shallow tremors in the Guerrero seismic gap., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6888, https://doi.org/10.5194/egusphere-egu22-6888, 2022.

11:30–11:35
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EGU22-12386
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ECS
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Virtual presentation
Pousali Mukherjee and Yoshihiro Ito

Subduction zones host some of the greatest megathrust earthquakes in the world. Slow earthquakes have been also discovered around the subduction zones of the Pacific rim very close to megathrust earthquakes in several subduction zones in Chile, Cascadia, Mexico, Alaska, and New Zealand (Obara and Kato, 2016). Investigating the lithosphere of the slow earthquake area versus non slow-earthquake areas in subduction zones is crucial in understanding the role of the internal structure to control slow earthquakes. Deep transient slow slip had been detected in the Lower and Upper Cook Inlet in the Alaska subduction region(Fu et al. 2015; Li et al. 2016; Wei et al. 2012). In this study, we investigate the lithospheric structure beneath the stations in and around the slow earthquake area in Alaska. We also study the non slow-earthquake areas in the Alaska subduction zone using receiver function analysis and inversion method using teleseismic earthquakes. Here we focus on, especially the Vs and Vp/Vs ratios from both the slow and non-slow earthquake areas, because of the sensitivity  to the fluid distribution in the lithosphere; the fluid distribution possibly controls the potential occurrence of slow earthquakes.
Additionally, the nature of the slab can also play a crucial factor. The velocities around the plate interface region in the lower continental mantle, subducted oceanic crust and upper oceanic mantle has the potential to reveal information that the structural heterogeneity could be related to the slow slip.

How to cite: Mukherjee, P. and Ito, Y.: Lithospheric structure in and around Slow Slip in the Alaska Subduction Region, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12386, https://doi.org/10.5194/egusphere-egu22-12386, 2022.

11:35–11:40
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EGU22-6527
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On-site presentation
Marco Calò, Erika Alinne Solano Hernández, Karina Bernal Manzanilla, Luisa García Gomora, Xyoli Perez-Campos, and Arturo Iglesias Mendoza

Cocos plate assumes a peculiar flat subduction beneath Mexico. Oaxaca region is the part of Mexico where the trench is closest to the coastline and where a transition from flat to a more dipping subduction plane occurs.

The closest seismic broadband seismometers existing near the coast are managed by the Mexican National Seismological Service (SSN) and consist of three stations installed over a straight coastline of Oaxaca of more than 200 km. The limited number of stations makes it very      difficult to get a detailed study of the seismicity able to provide sufficient information to characterize the events of magnitude less than 4.0-4.5 in this portion of the subduction.

In this work we show the preliminary results of a temporary network of 11 stations (9 broadband and 2 Raspberry Shakes) installed since September 2021 on the Oaxaca coast and designed to complement the coverage of the SSN stations. The two networks are now able to provide enough information to obtain refined catalogs and carry out studies that can probe the structure of the crust and upper mantle of the region with unprecedented detail.

In particular we will show the first results of the refined event locations, focal mechanisms and 3D seismic velocity models. All this information is lighting several features unknown of this portion of the Cocos plate and the overlaying North America one, opening new questions about the tectonics and geodynamics of the region.

Work supported by the PASPA-DGAPA, UNAM program, as a sabbatical year at Universidad del Mar (UMAR), Puerto Angel, by the PAPIIT-DGAPA project: IN108221, and by the internal project of the UMAR: 2II2003 and PRODEP UMAR-PTC-181.

How to cite: Calò, M., Solano Hernández, E. A., Bernal Manzanilla, K., García Gomora, L., Perez-Campos, X., and Iglesias Mendoza, A.: Probing the structure of the flat subduction in Oaxaca, Mexico, using a temporal seismic array. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6527, https://doi.org/10.5194/egusphere-egu22-6527, 2022.

11:40–11:50