How Sediment Thickness Influences Subduction Dynamics and Seismicity

Abstract

It has long been recognized that sediments subducting along the megathrust influence the occurrence of giant (Mw ≥ 8.5) megathrust earthquakes. However, the limited observation span and the concurrent influence of multiple parameters on megathrust behavior prevent us from understanding how sediments affect earthquake size and frequency. Here, we address these limitations by using two‐dimensional, visco‐elasto‐plastic, seismo‐thermo‐mechanical numerical models to isolate how sediment thickness affects subduction geometry and seismicity. Our results show that increasing sediment thickness on the incoming plate results in a decrease of the slab dip, as the trench retreats due to the seaward growth of the sedimentary wedge that also unbends the slab. This decrease in megathrust dip results in a wider seismogenic zone, so that the maximum magnitude of megathrust earthquakes increases. Concurrently, the recurrence time of characteristic events increases and partial ruptures are introduced. The maximum magnitude estimated for subduction segments with the thickest sediment input (Makran, West‐Aegean, and Calabria) is distinctly higher than the instrumentally recorded magnitude. These segments may thus experience larger than as of yet observed earthquakes, albeit infrequently. Increasing sediment thickness also decreases megathrust normal stresses, as the seismogenic zone is more shallow and overlain by a lighter forearc structure. Thicker incoming plate sediments also favor more splay fault activity, whereas we observe more outer rise events for low sediment thickness. Finally, we demonstrate that modeling long‐term subduction dynamics and sediment subduction is crucial for understanding and quantifying megathrust seismicity and seismic potential of subduction zones.

What Controls Maximum Magnitudes of Giant Subduction Earthquakes?

“…This conclusion fully supports the concept by Kelleher et al (1974) with the addition of the concept by Ruff (1989), which suggests that the key factor for a high magnitude event to occur is a large and coherent contact zone between the downgoing slab and overriding plate that could be ruptured in a single event. Note that similar results were also obtained by Herrendörfer et al (2015), Corbi et al (2017), and Brizzi et al (2020) using simplified numerical and analog modeling. This means that under the assumption that rupture length scales with rupture width, the models demonstrate that maximum magnitudes of the earthquakes are mostly controlled by the factors that increase rupture width.…”

“…These are high velocity of the overriding plate forcing fast trench rollback (e.g., Christensen, 2000), mantle flow tending to flatten the slab (e.g., Ficini et al, 2017; Hager & O'Connell, 1978), and a low negative buoyancy of the slab due to its young age or incorporated oceanic plateau in combination with fast overriding by the upper plate (e.g., van Hunen et al, 2000, 2002). Recently, modeling study by Brizzi et al (2020) suggested that a decrease of the slab dip may also result from the increasing sediment thickness on the incoming plate. Note, however, that this is not a case in Central and South Andes where the dipping angles of the slabs are similar despite that the thickness of sediments is very different (Hoffmann‐Rothe et al, 2006).…”

Empirical Analysis of Global-Scale Natural Data and Analogue Seismotectonic Modelling Data to Unravel the Seismic Behaviour of the Subduction Megathrust

“…The efforts to highlight which are the conditions enhancing the "productivity" of mega-earthquakes have recognized the potential role of several subduction-related parameters, but no consensus has been reached so far. For instance, empirical results point out at different parameters such as plates curvature (Bletery et al, 2016), roughness (e.g., van Rijsingen et al, 2019 or trench-parallel length of the subduction zone (Brizzi et al, 2018) and amount of trench sediments (Brizzi et al, 2020). Convergent margins are complex tectonic settings, whose diversity results from the interplay of several parameters operating at different spatial and temporal scales and whose relative importance is thus hard to assess (e.g., Figure 1B).…”

Influence of Subduction Zone Dynamics on Interface Shear Stress and Potential Relationship With Seismogenic Behavior

“…For example, subduction of buoyant material promotes slab flattening, while also reducing unbending as the slab enters the asthenosphere (Van Hunen et al, 2002). Brizzi et al (2020) showed that an increase in incoming sediment thickness in a subduction zone with imposed convergence velocity increases sediment accretion, trench retreat, and megathrust length, promoting large earthquakes. Increasing megathrust length may therefore offset decreasing shear stress during trench retreat.…”

“…These end‐members are conceptualized as seismic and aseismic margins respectively (Heuret et al., 2011; Pacheco et al., 1993; Scholz & Campos, 1995; Uyeda & Kanamori, 1979). The controls on seismogenic behavior remain uncertain, with the leading hypotheses invoking key roles for stress state, pore‐pressure, sediment thickness, megathrust roughness or curvature, and frictional properties (Bletery et al., 2016; Brizzi et al., 2020; Pacheco et al., 1993; Scholz & Campos, 1995; Scholl et al., 2015; van Rijsingen et al., 2018; Wang & Bilek, 2011). The hypothesis that stress state influences seismogenic behavior is based primarily on experimental data (Goebel et al., 2013; Scholz, 1968) and intraplate seismicity (Schorlemmer et al., 2005; Spada et al., 2013).…”