Text S1. DEM Modeling
In our implementation of the discrete-element-method (DEM), we simulate
an assemblage of discrete spherical particles that interact with each
other according to elastic-frictional (Hertz-Mindlin) contact law. We
introduce cohesion by adding a mechanical bond at interparticle contacts
(Morgan, 2015). This property can allow us to simulate cohesive
materials as may occur within the deeper accretionary prism. The more
frontal region of a prism is approximated as non-cohesive, i.e., lacking
interparticle bonds. The combination of pre-assigned interparticle
contact parameters, in combination with the mechanical properties of the
particles themselves, define the overall behavior of the particle
assemblage. This study uses a version of DEM implemented in RICEBAL.
Details about the method are provided in Morgan and Boettcher (1999),
Guo and Morgan (2004; 2006), and Morgan (2015). Continuum approximations
of the bulk properties and behavior of the numerical model are derived
using the contact force distribution and displacement gradients. By
averaging continuum properties over finite volumes, stress and strain
fields can be calculated for the domain (Thornton and Barnes, 1986;
Morgan and Boettcher, 1999; Morgan and McGovern, 2005b; a; Morgan,
2015).
Figure S1 shows the general model setup. The initial length of the
simulated wedge is set to 200 km, comparable to the dimensions of the
rupture areas in Chile Margin (Moreno et al. , 2010;
Contreras-Reyes et al. , 2017). To best balance model run time and
model resolution, the upper wedge is constructed of approximately
200,000 discrete particles with radii of 100, 120, 160 and 200 m.
Particles are randomly generated within a two-dimensional box
(400\(\times\)60 km) and allowed to settle under gravity.
The mechanical properties of the simulated system are defined by the
assigned particle properties and interparticle friction coefficients
(Table S3). Particles within the wedge, indicated by the black and blue
layers in Figure S1, are free to translate, rotate and interact as the
wedge evolves. The gray particles defining the basal sliding surface are
fixed in space, their small radii (10 m) ensure a relatively smooth
sliding surface, unimpeded by particle roughness.
The coefficients of internal and basal sliding friction are determined
by the interparticle friction coefficients assigned between wedge
particles (\(\mu_{\text{int}}^{\text{part}}\)) and wedge and basal
particles (\(\mu_{\text{bas}}^{\text{part}}\)) respectively. The
calibrated relationships between the interparticle and bulk friction
coefficients are presented in a previous study (Wang and Morgan, 2019).
The assigned internal friction coefficient within the outer wedge and
basal friction coefficient beneath are further annotated as\(\mu_{int\_outer}^{\text{part}}\) and\(\mu_{bas\_outer}^{\text{part}}\), respectively. Similarly, the
internal and basal friction coefficients for the inner wedge are written
as \(\mu_{int\_inner}^{\text{part}}\) and\(\mu_{bas\_inner}^{\text{part}}\). We tested multiple combinations of
internal and basal friction values to obtain first order fits to
observed earthquake displacements along the Chile margin, and selected
the following interparticle friction coefficient as initial friction
conditions for our simulations: \(\mu_{int\_outer}^{\text{part}}\) =\(\mu_{int\_inner}^{\text{part}}\)= 0.100 (friction coefficients
between wedge particles), which remains fixed throughout the simulation;
initial \(\mu_{bas\_outer}^{\text{part}}\) =\(\mu_{bas\_inner}^{\text{part}}\) = 0.040 (friction coefficients
between wedge and basal particles), which is comparable to the effective
basal friction coefficient used by Wang and He (2008). Basal friction
coefficients are subsequently varied during the simulations as described
below and in the text.