The Accumulation of Slip Deficit in Subduction Zones in the Absence of Mechanical Coupling: Implications for the Behavior of Megathrust Earthquakes

Abstract

The distribution of slip during subduction megathrust earthquakes depends on the slip deficit that accumulates on the plate interface prior to the event. We develop 3‐D finite element models of subduction zones to investigate how locked zones restrict surrounding regions on the plate boundary from sliding. What is new is that we quantify the slip around asperities on the megathrust. The models show plate interface slip increasing from zero at the edge of a locked zone to the relative plate motion over a distance of ~200 km along the megathrust. This area of reduced slip accumulates a seismic moment deficit up to 10 times larger than the moment deficit in the asperity alone. Updip of locked areas, slip at the trench can be reduced by more than 50% of the plate motion. Despite large displacements of the upper plate near the trench, this region moves as a semirigid block. Rupture models of the 2011 Tohoku earthquake, its tsunami characteristics, and geophysical observations near the trench can be interpreted to reflect the consequences of slip deficit accumulated on a low friction interface updip of the seismogenic zone. Neighboring asperities affect plate interface slip in a nonlinear way. Multiple asperities have overlapping pseudo‐coupled regions that may restrict the magnitude of coseismic slip in single‐asperity ruptures. Once an earthquake has a rupture length greater than ~250 km, it may recover the entire accumulated slip deficit. This is consistent with the magnitude of coseismic slip in several recent great megathrust earthquakes.

Locating Fully Locked Asperities Along the South America Subduction Megathrust: A New Physical Interseismic Inversion Approach in a Bayesian Framework

“…Although binary locking may be a simplification of the plate interface rheology, it produces similar results as more complex representations of the megathrust (e.g., Corbi et al, 2017;Herman et al, 2018;Hetland & Simons, 2010;Kaneko et al, 2010). This is because the continuum mechanics underlying Equation 1 are valid independent of the specific megathrust rheology.…”

“…In this study, we assume that every location on the plate interface is in one of two possible states: fully locked with zero slip or fully unlocked with no resistance to slip (Figures 1c and 1d). Previous studies used finite element models (FEMs) to determine the distribution of fault slip rates under these binary 10.1029/2020GC009063 locking conditions (Gans et al, 2003;Herman et al, 2018;Malservisi et al, 2003), but FEMs are too computationally expensive to run many times in a statistical search. For example, the converged 3-D FEM of Herman et al (2018) takes up to 60 s to run on a single processor.…”

“…Previous studies used finite element models (FEMs) to determine the distribution of fault slip rates under these binary 10.1029/2020GC009063 locking conditions (Gans et al, 2003;Herman et al, 2018;Malservisi et al, 2003), but FEMs are too computationally expensive to run many times in a statistical search. For example, the converged 3-D FEM of Herman et al (2018) takes up to 60 s to run on a single processor. If instead the Green's functions relating fault slip to shear tractions are precomputed (using an FEM or analytical solutions), the fault slip can be calculated much more rapidly (~1 s for a comparable model).…”

“…Outside of the high-probability locked areas, the mean slip deficit rate is greater than zero because of proximity to the locked regions. This effect has a geographic footprint of several hundreds of km (Almeida et al, 2018;Herman et al, 2018); even in center of the 600 km long unlocked section offshore of northern Peru, the mean slip deficit rate is still 2 mm/year. We also show the probability distributions of slip deficit rates for individual sub-faults ( Figure 5, insets).…”

“…The smoothing inherent to our mechanical forward model reduces the uncertainty in the magnitude of the slip deficit accumulation rate ( Figure 5). In South America, the slip deficit accumulation rate in sub-faults adjacent to the trench is at least 20% of the local convergence rate almost everywhere along the length of the subduction zone (Almeida et al, 2018;Herman et al, 2018). The only trench-adjacent sub-faults with lower slip deficit rates are located in the long stretches with no locked sections, such as northern Peru and central Chile.…”