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1. Interseismic Coupling and Slow Slip Events on the Cascadia Megathrust

   https://doi.org/10.1007/s00024-018-1991-x  

   Michel, Sylvain ; Gualandi, Adriano ; Avouac, Jean-Philippe ( September 2018 , Pure and Applied Geophysics)

   Abstract—In this study, we model geodetic strain accumulation along the Cascadia subduction zone between 2007.0 and 2017.632 using position time series from 352 continuous GPS stations. First, we use the secular linear motion to determine interseismic locking along the megathrust. We determine two end member models, assuming that the megathrust is either a priori locked or creeping, which differ essentially along the trench where the inversion is poorly constrained by the data. In either case, significant locking of the megathrust updip of the coastline is needed. The downdip limit of the locked portion lies * 20–80 km updip from the coast assuming a locked a priori, but very close to the coast for a creeping a priori. Second, we use a variational Bayesian Independent Component Analysis (vbICA) decomposition to model geodetic strain time variations, an approach which is effective to separate the geodetic strain signal due to non-tectonic and tectonic sources. The Slow Slip Events (SSEs) kinematics is retrieved by linearly inverting for slip on the megathrust the Independent Components related to these transient phenomena. The procedure allows the detection and modelling of 64 SSEs which spatially and temporally match with the tremors activity. SEEs and tremors occur well inland from the coastline and follow closely the estimated location of the mantle wedge corner. The transition zone, between the locked portion of the megathrust and the zone of tremors, is creeping rather steadily at the long-term slip rate and probably buffers the effect of SSEs on the megathrust seismogenic portion. 

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2. Dynamic Modeling of Interactions between Shallow Slow-Slip Events and Subduction Earthquakes

   https://doi.org/10.1785/0220220138  

   Meng, Qingjun ; Duan, Benchun ( October 2022 , Seismological Research Letters)

   Abstract Shallow slow-slip events (SSEs) contribute to strain release near the shallow portions of subduction interfaces and may contribute to promoting shallow subduction earthquakes. Recent efforts in offshore monitoring of shallow SSEs have provided evidence of possible interactions between shallow SSEs and megathrust earthquakes. In this study, we use a dynamic earthquake simulator that captures both quasi-static (for SSEs) and dynamic (for megathrust earthquakes) slip to explore their interactions and implications for seismic and tsunami hazards. We model slip behaviors of a shallow-dipping subduction interface on which two locally locked patches (asperities) with different strengths are embedded within a conditionally stable zone. We find that both SSEs and earthquakes can occur, and they interact over multiple earthquake cycles in the model. Dynamic ruptures can nucleate on the asperities and propagate into the surrounding conditionally stable zone at slow speeds, generating tsunami earthquakes. A clear correlation emerges between the size of an earthquake and SSE activities preceding it. Small earthquakes rupture only the low-strength asperity, whereas large earthquakes rupture both. Before a large earthquake, periodic SSEs occur around the high-strength asperity, gradually loading stress into its interior. The critically stressed high-strength asperity can be ruptured together with the low-strength one in the large earthquake, followed by a relatively quiet interseismic period with very few SSEs and then a small earthquake. An SSE may or may not directly lead to nucleation of an earthquake, depending on whether a nearby asperity is ready for spontaneously dynamic failure. In addition, because of different SSE activities, the coupling degree may change dramatically between different interseismic periods, suggesting that its estimate based on a short period of observation may be biased. 

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3. Subduction Zone Geometry Modulates the Megathrust Earthquake Cycle: Magnitude, Recurrence, and Variability

   https://doi.org/10.1029/2024JB029191  

   Biemiller, J ; Gabriel, A‐A ; May, D A ; Staisch, L ( August 2024 , Journal of Geophysical Research: Solid Earth)

   Megathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate codetandemto account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults.

    

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4. Linking 3D Long‐Term Slow‐Slip Cycle Models With Rupture Dynamics: The Nucleation of the 2014 M w 7.3 Guerrero, Mexico Earthquake

   https://doi.org/10.1029/2023AV000979  

   Li, Duo ; Gabriel, Alice‐Agnes ( March 2024 , AGU Advances)

   Slow slip events (SSEs) have been observed in spatial and temporal proximity to megathrust earthquakes in various subduction zones, including the 2014M_w7.3 Guerrero, Mexico earthquake which was preceded by aM_w7.6 SSE. However, the underlying physics connecting SSEs to earthquakes remains elusive. Here, we link 3D slow‐slip cycle models with dynamic rupture simulations across the geometrically complex flat‐slab Cocos plate boundary. Our physics‐based models reproduce key regional geodetic and teleseismic fault slip observations on timescales from decades to seconds. We find that accelerating SSE fronts transiently increase shear stress at the down‐dip end of the seismogenic zone, modulated by the complex geometry beneath the Guerrero segment. The shear stresses cast by the migrating fronts of the 2014M_w7.6 SSE are significantly larger than those during the three previous episodic SSEs that occurred along the same portion of the megathrust. We show that the SSE transient stresses are large enough to nucleate earthquake dynamic rupture and affect rupture dynamics. However, additional frictional asperities in the seismogenic part of the megathrust are required to explain the observed complexities in the coseismic energy release and static surface displacements of the Guerrero earthquake. We conclude that it is crucial to jointly analyze the long‐ and short‐term interactions and complexities of SSEs and megathrust earthquakes across several (a)seismic cycles accounting for megathrust geometry. Our study has important implications for identifying earthquake precursors and understanding the link between transient and sudden megathrust faulting processes.

    

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5. Earthquake swarms and slow slip on a sliver fault in the Mexican subduction zone

   https://doi.org/10.1073/pnas.1814205116  

   Fasola, Shannon L. ; Brudzinski, Michael R. ; Holtkamp, Stephen G. ; Graham, Shannon E. ; Cabral-Cano, Enrique ( March 2019 , Proceedings of the National Academy of Sciences)

   The Mexican subduction zone is an ideal location for studying subduction processes due to the short trench-to-coast distances that bring broad portions of the seismogenic and transition zones of the plate interface inland. Using a recently generated seismicity catalog from a local network in Oaxaca, we identified 20 swarms of earthquakes (M < 5) from 2006 to 2012. Swarms outline what appears to be a steeply dipping structure in the overriding plate, indicative of an origin other than the plate interface. This steeply dipping structure corresponds to the northern boundary of the Xolapa terrane. In addition, we observed an interesting characteristic of slow slip events (SSEs) where they showed a shift from trenchward motion toward an along-strike direction at coastal GPS sites. A majority of the swarms were found to correspond in time to the along-strike shift. We propose that swarms and SSEs are occurring on a sliver fault that allows the oblique convergence to be partitioned into trench-perpendicular motion on the subduction interface and trench-parallel motion on the sliver fault. The resistivity structure surrounding the sliver fault suggests that SSEs and swarms of earthquakes occur due to high fluid content in the fault zone. We propose that the sliver fault provides a natural pathway for buoyant fluids attempting to migrate upward after being released from the downgoing plate. Thus, sliver faults could be responsible for the downdip end of the seismogenic zone by creating drier conditions on the subduction interface trenchward of the sliver fault, promoting fast-slip seismogenic rupture behavior.

    

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