TS7.1

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.

     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.

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.

On August 12, 2021, an earthquake doublet with a cumulative magnitude M_W 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 M_wc 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 M_ww 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.

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.

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.

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.

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.