Slip observed within subduction zones falls within a broad range, with non-volcanic tremor (NVT), slow slip, repeating small earthquakes, to great earthquakes all contained within this spectrum.  The diversity of observed slip suggests diversity in fault zone conditions, which can be affected by a variety of factors, such as fluids, sediment inputs, upper plate structure, and topography on the subducting plate.  The role of subducting topography on seismicity appears to be regionally variable, with some bathymetric features leading to high slip during earthquakes, and others leading to plate creep and small earthquakes associated with upper plate deformation.  Seismicity characteristics in regions of subducting topography can be difficult to assess in many areas however, because traditional land-based seismic networks can easily miss offshore smaller earthquakes associated with this process. We use data from an amphibious seismic network deployed along the Cascadia subduction zone to examine seismic characteristics of thousands of newly detected and located earthquakes along the margin.  Over 5000 earthquakes in the catalog generally agree with asperity locations modeled from geodetic data and the 1700 M9 rupture, along with several clusters of upper plate earthquakes in areas of subducting topography.  Stress drop estimates, based on a spectral ratio technique, vary depending on earthquake location, with higher stress drop earthquakes occurring on the upper plate faults relative to the lower stress drop earthquakes occurring along the megathrust.  These variations may reflect important changes in fault conditions between upper plate splay faults and the megathrust.

How to cite: Bilek, S. and Morton, E.: Heterogeneous earthquake distributions and stress drop estimates along the Cascadia subduction megathrust and upper plate faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3600, https://doi.org/10.5194/egusphere-egu23-3600, 2023.

Aftershocks are a time-dependent (exponential decay) phenomenon in the aftermath of large earthquakes. In subduction zones, those occurring in the upper plate are of special concern given their potential seismic hazard, as they may produce substantial surface shaking close to highly populated cities. Therefore, the understanding of the mechanisms that drive upper-plate aftershocks is of utmost importance to improving seismic hazard assessment. Transfer of static coseismic stresses has been commonly proposed to explain this; however, they fail to explain their exponential decay over time. This time-dependency is observed in postseismic geodetic measurements, suggesting that the processes that control the postseismic surface deformation also govern or at least are involved in the generation of upper-plate aftershocks. Here, the postseismic surface deformation is dominated by aseismic slip along the fault interface (afterslip), non-linear viscoelastic relaxation in the lower crust and upper mantle, and pore-pressure diffusion in the crust. Despite great research efforts, however, the key driver remains elusive.

In this study, we investigate which postseismic mechanism mainly controls the occurrence of aftershocks in the upper plate in subduction zones using the 2014 Mw=8.1 Iquique earthquake, northern Chile, as a study case. We employ a 4D numerical forward model to simulate the transient poroelastic and non-linear viscoelastic relaxation, whose contributions are subtracted from the cumulative Global Navigation Satellite System (GNSS) measurements to then invert for afterslip. Using realistic rock material properties, we first show that this approach explains the surface displacements during the first nine months of postseismic deformation recorded by continuous GNSS. For the same period, we then compute the spatiotemporal Coulomb Failure Stress changes (ΔCFS) that result from individual postseismic processes and compare them with the upper-plate aftershocks using a high-resolution seismicity catalog and focal mechanisms. We show for the first time that the ΔCFS produced by pore-pressure diffusion induced by the mainshock are unambiguously better correlated in space and time with the increase in upper-plate aftershocks than those from afterslip or non-linear viscous relaxation. In addition, pore-pressure diffusion lowers the effective normal stress of the stress tensor more effectively, while its resulting ΔCFS are relatively independent of the fault orientation. The latter would also explain the diversity of faulting styles in the upper plate exhibited by focal mechanisms following the 2014 Iquique earthquake and other subduction zone earthquakes. Our findings provide new insights into the link between pore-pressure diffusion and upper-plate deformation in subduction zones with implications for time-dependent seismic hazard.

How to cite: Peña, C., Heidbach, O., Schurr, B., Metzger, S., Moreno, M., Oncken, O., and Faccenna, C.: Does transient pore-pressure diffusion drive upper-plate aftershocks following megathrust earthquakes?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8201, https://doi.org/10.5194/egusphere-egu23-8201, 2023.

Bathymetric highs resist subduction producing large- to small-scale change in the morphology of the subduction zone: Increase of the outer-rise curvature, trench indentation and large-scale slides and slumps in the fore-arc region. At the plate interface, bathymetric highs induce geometrical and frictional changes that can produce increase or decrease of the local coupling, and thus having an effect on the likelihood of occurrence of large megathrust earthquakes. Numerical models predict complex strain and stress patterns arising from subduction of such relief mainly driven by the size and relative geometry of trench and subducted high. However, the collision and subduction of bathymetric highs is investigated mainly via geophysical and geological surveys since seismic sequences have rarely illuminated the subduction of seafloor relief. Here, we report of a year-long and very energetic earthquake activity (10 Mw 6.5-7.5) at the Loyalty Ridge – Vanuatu trench at both the plate interface and in the outer-rise region. The spatio-temporal and magnitude of the earthquakes revealed complex release of the accrued flexural strain along the outer-rise and a pronounced segmentation of the interface with repeating M7 earthquakes, low aftershock activity and a large “aseismic” zone. The collision and subduction of the Loyalty Ridge along the Vanuatu trench seem to indicate a frictionally segmented interface where large megathrust earthquakes are unlikely to occur.

How to cite: Passarelli, L., Cesca, S., Nooshiri, N., and Jónsson, S.: Earthquakes illuminate the incipient collision and subduction of Loyalty Ridge at Vanuatu subduction zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4944, https://doi.org/10.5194/egusphere-egu23-4944, 2023.

It has been widely recognized that the presence of seamounts can profoundly affect megathrust seismicity. With their outstanding topography, seamounts can tune interplate stress and favor the development of a fracture network in the overriding plate. Subducting seamounts can also control fluid accumulation and sediment porosity. However, their role as barriers or triggers for rupture propagation remains a matter of debate.

In this work, we used analog models to study how geometric and frictional heterogeneities associated with a single subducting seamount influence the seismogenic behavior of the megathrust. We used four different model configurations (i.e., a flat interface, a high-friction and low-friction seamount, and a low-friction patch) to investigate both the combined and individual effect of geometry and friction.

Our results show that low friction areas, either flat or with a seamount relief, reduce interplate coupling. Also the presence of a geometric feature tends to decrease seismic coupling and segment ruptures promoting earthquakes enucleation on the flat region. The maximum barrier efficiency is achieved with the low-friction patch model, where the accumulated stress is preferentially released by the occurrence of small earthquakes. This behavior is well suited to natural cases where seamounts are supposed to lower interplate friction due to fluid release or by the development of fracture systems development, causing microseismicity and slow slip events.

How to cite: Menichelli, I., Corbi, F., Brizzi, S., van Rijsingen, E., Funiciello, F., and Lallemand, S.: Seamount subduction and megathrust seismicity: the interplay between geometry and friction., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4981, https://doi.org/10.5194/egusphere-egu23-4981, 2023.

Postseismic deformation following subduction earthquakes includes the combined effects of afterslip surrounding the coseismic rupture areas and viscoelastic relaxation in the asthenosphere and provides unique and valuable information for understanding the rheological structure. Because the two postseismic mechanisms are usually spatiotemporally intertwined, we developed an integrated model combining their contributions, based on 5 years of observations following the 2016 Pedernales (Ecuador) earthquake. The results show that the early, near-field postseismic deformation is dominated by afterslip, both updip and downdip of the coseismic rupture, and requires heterogeneous interface frictional properties. Viscoelastic relaxation contributes more to far-field displacements at later time periods. The best-fit integrated model favors a 45-km thick lithosphere overlying a Burgers body viscoelastic asthenosphere with a Maxwell viscosity of 3 × 10¹⁹ Pa s (0.9 - 5 × 10¹⁹ Pa s at 95% confidence), assuming the Kelvin viscosity equal to 10% of that value. In addition to the postseismic afterslip, the coastal displacements of sites north and south of the rupture clearly require extra slip in the plate motion direction due to slow slip events that may be triggered by the coseismic stress changes (CSC), but are not purely driven by the CSC. Spatially variable afterslip following the Pedernales event, combined with the SSEs during the interseismic period, demonstrate that spatial frictional variability persists throughout the whole earthquake cycle. The interaction of adjacent fault patches with heterogeneous properties may contribute to the clustered large earthquakes in this area.

How to cite: tian, Z., Freymueller, J., Yang, Z., Li, Z., and Sun, H.: Frictional properties and rheological structure at the Ecuadorian subduction zone revealed by the postseismic deformation due to the 2016 Mw 7.8 Pedernales (Ecuador) earthquake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10957, https://doi.org/10.5194/egusphere-egu23-10957, 2023.

Postseismic deformation following large megathrust earthquakes is widely observed with geodetic measurements and coastal geological investigations and contains important information on subduction zone rheology and megathrust slip behaviour. Short-term (a few years) horizontal deformation exhibits a consistent pattern common to almost all large megathrust earthquakes. For example, the coastal area moves consistently in the seaward direction, but the trench area moves in the landward direction. The horizontal deformation is readily explained by the effects of viscoelastic relaxation (VER) of earthquake-induced stress and afterslip. However, the vertical deformation exhibits greater complexity. For example, the sense of coastal deformation varies not only between different earthquakes but also along strike for the same earthquake. In this work, by separately modelling the VER and afterslip processes of synthetic and real subduction earthquakes, we show that the vertical motion can be explained in a simple manner in the same conceptual framework as for the horizontal motion, although the vertical motion is more sensitive to details of the rheological structure and afterslip. VER results in a long-wavelength, tri-segment deformation pattern consisting of near-trench uplift, midway subsidence, and near-arc uplift. The near-trench uplift and midway subsidence follow coseismic uplift and subsidence, respectively, and are both controlled by the viscosity of the sub-slab oceanic mantle. The near-arc uplift results from viscoelastic relaxation in the presence of a cold and elastic forearc mantle wedge corner (the cold nose) and is controlled by the viscosity of the hot part of the mantle wedge beneath the arc and back arc (Luo & Wang, 2021). In contrast to VER, each afterslip patch results in a local bi-modal pattern of uplift and subsidence dominated by elastic deformation, with variable wavelengths depending on the location and size of the afterslip. The complexity in postseismic vertical motion arises mainly from the heterogeneity and site-specific nature of afterslip (Luo & Wang, 2022). If observations are made near the rupture area, the observed vertical postseismic motion can be very complex, because the effects of heterogeneous afterslip around or within the rupture zone can obscure or conceal the pattern of near-trench uplift and midway subsidence due to VER. Farther away from the rupture area, the observed postseismic deformation mainly reflects the contribution of VER, and near-arc uplift appears to be ubiquitous. Separating the common VER process and the site-specific afterslip effect helps to constrain mantle rheology and illuminate fault slip behaviour. Our work also has strong implications for deciphering paleoseismic estimates of coastal motion associated with ancient earthquakes to understand coseismic vs. postseismic contribution.

References:

Luo, H., & Wang, K. (2021). Postseismic geodetic signature of cold forearc mantle in subduction zones. Nature Geoscience, 14(2), 104-109. https://doi.org/10.1038/s41561-020-00679-9

Luo, H., & Wang, K. (2022). Finding simplicity in the complexity of postseismic coastal uplift and subsidence following great subduction earthquakes. Journal of Geophysical Research: Solid Earth, 127(10), e2022JB024471. https://doi.org/10.1029/2022JB024471

How to cite: Luo, H. and Wang, K.: Complex postseismic vertical motion following megathrust earthquakes explained by simple mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3007, https://doi.org/10.5194/egusphere-egu23-3007, 2023.

How to cite: Ninivaggi, T., Selvaggi, G., Mazza, S., Filippucci, M., Tursi, F., and Czuba, W.: Evidence of water transport in the Earth’s mantle from an Undetected Seismic Phase in Waveforms from Southern Tyrrhenian (Italy) intermediate-depth and Deep Earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5748, https://doi.org/10.5194/egusphere-egu23-5748, 2023.

We report preliminary results based on the construction of a new 3D model for the geometry of the subducted lithosphere of the Nazca Plate. This new 3D model differs from Slab2 in that it enables capture of the slab geometry in greater detail and allows the identification of previously unrecognised potential slab tears, both down-dip and along strike, as well as slab gaping as tears open. These differences in the inferred 3D structure emerge because previous models were excessively smoothed. Here we use the interpolation of line strings derived from the interpretation of individual cross-sections: a method that has the capacity to capture detail, and to accurately drape and thus derive a 3D geometry consistent with the observed hypocenter patterns, by using Delaunay triangulation alongside with 3D grid interpolation. The 3D slab morphology obtained provides insight into the interplay between subduction earthquakes at different depths. The variation in slab morphology reinforces the concept that the megathrust comprises distinct rupture segments that behave differently in terms of their overall seismotectonic behaviour. The slab morphology also links to changes in the broad geodynamics of the subduction zone, with links between shallow, intermediate, and deep seismicity that are consistent with variation in 3D slab morphology. It is also possible to explain the variation in surficial crustal tectonic processes in the context of geodynamic processes inferred to be taking place at depth in specific segments of the descending slab.

Surficial structures in the form of megathrust ruptures can be seen to be both a consequence of the highly segmented nature of the overriding plate, particularly in the Peruvian Andes, as well as the deeper variation in 3D slab morphology. For example, the inferred slab tears at the northern (Puna) edge and the southern (Payenia) edge of the Pampean segment may be explained as due to significant variation of the slab morphology and are reflected in changes in the characteristics of earthquake ruptures, and different tectonic modes in the supra-subduction zone lithosphere. Lineaments that separate the segments of the frontal megathrust also coincide with inherited surface fault systems on the South American plate. Clusters of intermediate extensional earthquakes weaken the lithosphere south of the Taltal Ridge (TR) and north of the Juan Fernandez Ridge (JFR). These structures mark the edge of the magmatically inactive plate interface due to the termination of magmatism in shallow dipping slab segments associated with the JFR, or in the flat slab of the Pampean segment.

How to cite: Nakrong, N., Forster, M., Jelsma, H., Spakman, W., and Lister, G.: Slab segmentation of the Nazca plate across the Juan Fernández Ridge, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9869, https://doi.org/10.5194/egusphere-egu23-9869, 2023.