the Creative Commons Attribution 4.0 License.

the Creative Commons Attribution 4.0 License.

# A global, spherical finite-element model for post-seismic deformation using *Abaqus*

### Grace A. Nield

### Matt A. King

### Rebekka Steffen

### Bas Blank

We present a finite-element model of post-seismic solid
Earth deformation built in the software package *Abaqus* (version 2018). The model
is global and spherical, includes self-gravitation and is built for the
purpose of calculating post-seismic deformation in the far field
($>\sim \mathrm{300}$ km) of major earthquakes. An earthquake is
simulated by prescribing slip on a fault plane in the mesh and the model
relaxes under the resulting change in stress. Both linear Maxwell and
biviscous (Burgers) rheological models have been implemented and the model
can be easily adapted to include different rheological models and lateral
variations in Earth structure, a particular advantage over existing models.
We benchmark the model against an analytical coseismic solution and an
existing open-source post-seismic model code, demonstrating good agreement
for all fault geometries tested. Due to the inclusion of self-gravity, the
model has the potential for predicting deformation in response to multiple
sources of stress change, for example, changing ice thickness in
tectonically active regions.

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*Abaqus*, Geosci. Model Dev., 15, 2489–2503, https://doi.org/10.5194/gmd-15-2489-2022, 2022.

Earthquakes cause deformation at the Earth's surface from the immediate coseismic fault slip and, thereafter, from several processes. Afterslip on the fault on or near the rupture zone (e.g. Huang et al., 2014) and poroelastic relaxation due to changes in fluid pressure (e.g. Masterlark and Wang, 2002) result in deformation of near-field locations (typically <300 km from the fault). As the Earth responds to stress changes associated with the earthquake, the lower crust and upper mantle undergo prolonged post-seismic viscoelastic deformation. This is the dominant mode of displacement in the far field and, for very large earthquakes (> magnitude 9), this can be observed geodetically over 1000 km from the earthquake source (Shao et al., 2016) and be sustained over decades. For example, deformation from the 1960 magnitude 9.5 Chile earthquake was still being observed ∼40 years after the event (Khazaradze and Klotz, 2003). Post-seismic deformation can be observed by increasingly dense networks of geodetic measurements such as Global Positioning System (GPS) (e.g. Freed et al., 2012) and Interferometric Synthetic Aperture Radar (InSAR) (e.g. Wang and Fialko, 2018) motivating the development of models to help interpret geodetically observed deformation. The study of post-seismic deformation can provide useful information about the Earth and can be used to place constraints on the inferred Earth structure (Pollitz, 2005) or rheology (Freed et al., 2012). It is important to be able to accurately quantify post-seismic deformation in regions where deformation occurs as a result of multiple sources, for example, in Alaska where the Earth is also deforming in response to a changing ice load (Suito and Freymueller, 2009).

Post-seismic deformation has been studied using a variety of modelling
techniques, from simple layered half-space (also known as “flat-Earth”)
models to models that consider Earth's sphericity and three-dimensional
material properties. For studying the far-field effects of earthquakes
consideration of sphericity is required (Pollitz, 1992).
Pollitz (1997) developed a software (*VISCO1D*, freely available from the
USGS at https://www.usgs.gov/node/279413 (last access: March 2002), which contains
a link to download the software) to calculate post-seismic deformation on a
spherical layered Earth incorporating an Earth model that varies in the
radial direction only. It uses viscoelastic normal mode theory and
calculates deformation from spherical harmonic expansion of global modes of
relaxation. Deformation can be calculated both with and without the effect
of gravity, that is, the restoring forces within the Earth allowing it to
regain gravitational equilibrium. The effect of including gravity is that
deformation ceases once the Earth has adjusted to the change in stress,
whereas without gravity, deformation continues. This model has been employed
for a wide range of earthquake studies, such as constraining Earth
properties and rheological behaviour in response to the magnitude 7.9 2002
Denali earthquake in Alaska (Pollitz, 2005), computing post-seismic
gravity change following the magnitude 9.1 2011 Tōhoku earthquake
gravity change (Han et al., 2014) and modelling post-seismic
deformation of Antarctica (King and Santamaría-Gómez, 2016).
Furthermore, *VISCO1D* has been used to benchmark other codes of post-seismic
deformation calculation (Wang et al., 2006). Whilst *VISCO1D* is a
powerful tool, it is limited to a one-dimensional Earth model which may not
be appropriate for some studies where lateral variations in Earth structure
may be required to fully explain geodetic observations (King and
Santamaría-Gómez, 2016).

Several finite-element models of post-seismic deformation have also been developed (Freed et al., 2006; Masterlark et al., 2001; Takeuchi and Fialko, 2013), although the majority have been restricted to a half-space geometry. Hu and Wang (2012) updated their finite-element model (Hu et al., 2004) to include spherical geometry and used it to study the 2004 magnitude 9.2 Sumatra earthquake, where a spherical geometry allowed for more realistic slab and fault geometry to be included; however, they limited the spatial extent of the model to the case study region. Agata et al. (2019) modelled post-seismic deformation following the 2011 magnitude 9.0 Tōhoku earthquake with a finite-element model incorporating spherical (but not global) geometry, restricting the model domain to 2500 km by 2500 km to allow for a high-resolution mesh. Whilst limiting the spatial extent is computationally efficient and does not cause a problem for the estimation of post-seismic deformation, to compute a fully self-gravitational solution using spherical harmonics a full global model is required. Self-gravitation takes account of the change in gravity field caused by deformation and the redistribution of mass within the Earth. The effect of self-gravitation due to earthquake-related deformation is likely to be small for the majority of earthquakes, and not on the same scale as that caused by glacial isostatic adjustment since the magnitude of deformation is much smaller, but including it allows consistent modelling of deformation due to multiple sources (e.g. glacial cycles). Furthermore, for large earthquakes, the change in gravity field resulting from deformation and redistribution of mass within the Earth could be significant, and, if the earthquake occurs under the ocean, redistribution of water also contributes to the change in gravity and hence affects sea levels (Broerse et al., 2011).

The purpose of the model presented in this paper is to estimate far-field
post-seismic deformation primarily from large earthquakes. The model is a
global, spherical finite-element model constructed with the commercial
software *Abaqus* (https://www.3ds.com/products-services/simulia/products/abaqus/ (last access: February 2019); Hibbitt et
al., 2016). It is an improvement on existing models since it includes both
global spherical geometry and the capability to use 3-D variations in Earth
properties which can significantly affect viscoelastic deformation (e.g. Latychev et al., 2005; Zhong et al., 2003; Wu et al., 2013). Furthermore,
the model has the potential to simultaneously include different sources of
Earth deformation, for example, post-seismic deformation in a region
undergoing, or which has previously undergone, surface-mass change (e.g.
ice-mass change). This is needed, for instance, to separate present-day
surface deformation signals observed by GPS.

This paper describes the model setup and the methods implemented to estimate coseismic and post-seismic deformation (Sects. 2 and 3). Since afterslip and poroelastic relaxation are considered only to affect deformation in the near field of the fault for all but the largest earthquakes (Peña et al., 2019), they are not included in our model. We benchmark model results for several simple scenarios against existing codes (Sect. 4) and discuss the advantages and limitations of the model (Sect. 5). The input files are available to allow other users to replicate results and use the methods to set up their own post-seismic deformation models.

## 2.1 Model geometry and mesh

The model is a global, spherical representation of the Earth, developed in
*Abaqus* version 2018 but is also compatible with older versions of the software as
well. It is based on the finite-element model for computing glacial
isostatic adjustment following the methods described by Wu (2004),
whereby we use the same setup of a viscoelastic Earth relaxing in response
to a stress change, but instead of applying a changing ice-surface load we
implement a fault and associated slip. We model a spherical Earth rather
than taking a layered half-space approach so a more realistic geometry can
be included and gravity perturbations due to deformation can be accounted
for by means of a spherical harmonic expansion. The model has layers from
the surface to the core–mantle boundary with computational feasibility being
the only limit on the number of layers. Each layer is assigned material
properties according to user requirements and would typically consist of an
elastic outer layer representing the uppermost lithosphere, with
viscoelastic lower lithosphere and mantle layers below.

An earthquake is simulated in the model by movement on a fault plane within
the mesh (see Sect. 3 for details) and in order to compute deformation to a
sufficient accuracy a high-resolution mesh is required in the vicinity of
the fault. To balance the need for a high-resolution mesh against a model
with a computationally feasible number of elements, we take the approach of
constructing two separate parts (*Abaqus* keyword *PART): one for the
high-resolution fault region and one for the lower resolution surrounding
Earth (Fig. 1). The two parts are then tied together (*Abaqus* keyword *Tie) using
surface-to-surface tie constraints. This means that although the two
separate meshes have non-conforming elements relative to each other, the tie
constraints ensure there is no relative movement between the surfaces and
that displacement and stress are continuous through the boundaries. Tie
coefficients are generated and used to interpolate quantities from nodes on
one side of the mesh to nodes on the other side of the mesh. The size of the
part containing the fault and high resolution mesh can be changed according
to the requirements of the individual case study, and in the simulations
presented in this study, is large enough to contain the far-field area of
interest. The high-resolution mesh block extends to 670 km depth (i.e. the
base of the upper mantle), so that any deformation that may be caused by the
fault movement is within the block, and hence there is minimal stress to
transfer across the tied surfaces. The element type used is an eight-node linear
brick element (C3D8 in *Abaqus*).

## 2.2 Rheology and Earth parameters

All layers within the model are assigned a density (*ρ*), Poisson's
ratio (*ν*) and Young's modulus (*E*) (*Abaqus* key words *Density *Elastic). For
viscoelastic layers below the purely elastic outer shell, a relaxation time
is specified which is based on a Prony time series (*Abaqus* key words
*Viscoelastic). In this study, we limit our benchmarking examples to a 1-D
linear viscoelastic rheology with one (Maxwell) or two (Burgers) relaxation
times and include one example implementing the elastic properties of the
widely used Preliminary Reference Earth Model (PREM, Dziewonski and
Anderson, 1981). However, *Abaqus* has the capability to implement a variety of
rheological models, including user-specified constitutive equations. For
example, Freed et al. (2012) combined power-law rheology with a transient
phase to model post-seismic deformation following the 1999 magnitude 7.1
Hector Mine earthquake, and van der Wal et al. (2010) used a composite
rheology based on laboratory-derived flow laws for diffusion and dislocation
creep (Hirth and Kohlstedt, 2003; Karato and Wu, 1993) to model global
glacial isostatic adjustment. Furthermore, variations of our model could be
constructed using a 3-D Earth structure (e.g. van der Wal et
al., 2015) (more information on how this can be done is included in the
the supporting material; Nield, 2022).

## 2.3 Boundary conditions

We follow the approach described in Sect. 4.1 of Wu (2004) and
apply elastic foundations (*Abaqus* keyword *Foundation) to each layer boundary with
a material density contrast occurring across it (including the surface and
core–mantle boundary). This means that advection of pre-stress is included
and takes care of the restoring forces of buoyancy neglected in a
conventional finite-element model (Wu, 2004, Eq. 3). The
elastic foundations have a stiffness equal to the difference in density
multiplied by gravitational acceleration (see Wu, 2004, Eq. 12a–c).
Schmidt et al. (2012) show that the use of foundations at
surfaces not perpendicular to the direction of gravity produces errors in
results, but we ensure that density contrasts occur only at spherical layer
boundaries in our model which satisfies this requirement. The foundations
could be also replaced by spring elements when an inclined density contrast
is used (Schmidt et al., 2012).

## 2.4 Time steps

Once the model has been set up with the geometry, rheology, Earth parameters, boundary conditions and fault slip (see Sect. 3); time steps are created to run the model. The first step of the run is a static step to allow the fault to slip. In this step, all material properties are treated as elastic and the fault displaces immediately; hence, no actual time needs to be assigned. All subsequent steps are for the simulation of post-seismic relaxation and consequently must have a time assigned to them. One full run of all the time steps is one iteration.

## 2.5 Self-gravitation

Movement of mass due to deformation of the Earth perturbs the gravity field which in turn affects the Earth's deformation. The effect of changes in the gravity field needs to be taken into account to make the model self-gravitating. This is done iteratively as described in Sect. 4.3 of Wu (2004), first using a non-self-gravitating model and computing the resulting gravitational potential from the radial displacement using the equations presented in Sect. 4 of Wu (2004). Applying this as a new load at the interfaces of the model where a density contrast occurs across them, the displacement is recalculated (i.e. another iteration is run). Wu (2004, Sect. 4.3) suggests running the model for four to five iterations to achieve convergence of the solution.

## 2.6 Outputs

*Abaqus* can compute many different output variables depending on the model setup.
For our model, the main output of interest is a global grid of deformation in
the east, north and radial directions. It is also possible to output stress,
strain and perturbation to the geoid which could be used to correct
satellite gravimetry data for the gravity change associated with very large
earthquakes (e.g. Han et al., 2014).

## 3.1 Fault geometry and slip

In our model, an earthquake is simulated by prescribing slip on a fault plane. In order to implement this in a finite-element mesh, we require two separate surfaces that can move relative to each other. We take the approach of Steffen et al. (2014), whose model simulates fault slip due to changes in stress during glacial cycles, whereby the fault plane geometry is created prior to meshing and then, once generated, the mesh is subsequently altered to produce two surfaces. This is accomplished by duplicating the nodes that lie on the fault surface and then reassigning the duplicated nodes to the elements on one side of the fault plane, thereby creating a “cut” in the mesh. Although the elements on each side of the fault are defined by different nodes, the node pairs initially have the same coordinates (Fig. 2).

The model of Steffen et al. (2014) incorporates a complex stress history comprising tectonic stresses and stresses relating to glacial cycles allowing faults to slip in response to the changing conditions. Our model differs from this approach because the amount of slip that occurs on the fault plane is prescribed. This negates the need to specify any fault surface parameters such as coefficient of friction or cohesion but does require detailed knowledge of the earthquake event. Prescribing an amount of slip on the fault plane is a reasonable approach as fault properties, such as length, width, strike, dip, rake and slip are often solved through independent means such as fault inversion studies (Hayes, 2017) and detailed fault slip information is not required when studying far-field deformation. Furthermore, we do not include any pre-stress in the model; that is, there is no tectonic or background stress applied before the fault slips. This has no effect for models that use Maxwell or Burgers rheology as they are independent of stress.

## 3.2 Coseismic slip

Once the finite-element mesh has been adjusted to accommodate the fault
plane, coseismic displacement is prescribed following the approach of
Masterlark (2003). For every node pair on the fault plane (i.e. nodes on either side of the fault that have the same coordinates), a third
“dummy” node is created at the same location but not connected to any
element in the mesh (Fig. 2). Dummy nodes are assigned a displacement
boundary condition (*Abaqus* keyword *Boundary) in the *x* and *y* directions of a local
coordinate system aligned with the fault plane (see Fig. 2) with the amount
of displacement in each direction depending on the slip and rake. For
example, using basic trigonometry, a slip of 5 m at a rake of −60^{∘}
would equate to slip of 2.5 m in the *x* direction and 4.3 m in the *y* direction.
Kinematic constraint equations (*Abaqus* keyword *Equation) are then constructed for
each node pair and its corresponding dummy node which specifies the relative
displacement between the node pair in the *x* and *y* direction. There is no
relative displacement in the *z* direction of the local coordinate system
(i.e. normal to the fault plane), as the fault is not allowed to open.
Because the constraint equations are specified separately for each node, the
fault plane can accommodate a spatially variable slip distribution. The
first step of the model run is a static step (*Abaqus* keyword *Static) in which the
nodes (and hence the fault plane) displace according to the kinematic
constraint equations. During the static step, all materials are treated as
elastic.

## 3.3 Post-seismic deformation

Following the first step of the model run (i.e. the earthquake), the
subsequent time steps simulate post-seismic deformation (*Abaqus* keyword *Visco) by
allowing the mesh to deform under the stresses caused by the displacement on
the fault plane. The kinematic constraint equations remain in place
throughout the model run which means that there is no further relative
displacement between the node pairs.

In order to verify the model, we benchmark the results against those produced
with the Okada analytical solution for coseismic displacement (Okada,
1985) and the *VISCO1D* code (version 3) for gravitational post-seismic relaxation
(Pollitz, 1997).

## 4.1 Test setup

We perform three benchmarking tests each with different geometry: a
strike-slip fault, a 30^{∘} dipping reverse fault and a
45^{∘} dipping fault with rake of −60^{∘}, as summarised
in Table 1. All faults outcrop at the surface of the model. The resolution
of the mesh on the fault plane is 10×5 km. In plan view, the
*Abaqus* mesh has 10 km elements in the vicinity of the fault, increasing to 50 km
at the edge of the fault region, with elements in the surrounding
low-resolution part, starting 1000 km away from the fault, being up to
500 km (Fig. 1). The element size increases with depth as shown in Table 2.
For the 30^{∘} dipping reverse fault, we tested a coarser and a finer
mesh and found that our chosen mesh resolution provided a satisfactory
trade-off between computation time and accuracy, with the finer mesh giving
only small differences in near- and far-field displacement compared to our
chosen mesh, for a 10-fold increase in computation time. The results of this
sensitivity test are discussed in more detail in Appendix A.

The model is run for 500 years to verify both the short- and long-term
post-seismic deformation and for four iterations to ensure convergence of the
self-gravitational solution. The *Abaqus* input files for the benchmarking tests are
included in the supporting material (Nield, 2022). *VISCO1D* is run for the same
time period and with maximum spherical harmonic degree of 2000, equivalent
to ∼10 km resolution.

In all benchmarking tests, the same simple Earth structure is used (details
for the *Abaqus* model are in Table 2), which is based on the Earth structure used
by Pollitz (1997) for the uppermost 670 km, and we include a
uniform lower mantle layer below. We found that the vertical deformation
results for *VISCO1D* were sensitive to the number of layers in the input Earth
structure and the presence of a lower mantle, so whilst the material
properties are the same for both models, the *VISCO1D* Earth model contains more
layers. We use this simple Earth structure to ensure our results are
consistent with the Okada analytical solution for a homogenous half-space
and the results presented by Pollitz (1997). We use a linear Maxwell
rheology for all three benchmarking tests. We additionally verify the
implementation of Burgers rheology against *VISCO1D* for the third test, with the
transient viscosity (*η*) given in Table 2. Furthermore, we run the
third test case in *Abaqus* using the elastic properties from PREM, keeping the same
three-layer Maxwell viscosity profile as the benchmarking Earth model (Table 2). We do not benchmark the results from this test case as the Okada
analytical solution is only valid for a homogeneous Earth. The input files
for each test case and the Earth models used in the *VISCO1D* modelling for both
Maxwell and Burgers rheology are given in the supporting material (Nield,
2022). All output displacements are shown normalised to the fault slip, in
other words as a percentage of fault slip. This is consistent with the
presentation of results by Pollitz (1997) and provides a useful
metric for comparison of results. Differences between the model results are
given as the difference between the percentages of fault slip.

## 4.2 Coseismic results

The results of the benchmarking exercises are shown in Figs. 3–5. We show
coseismic displacement in the east, north and vertical directions for a
profile perpendicular to the fault strike for each of the benchmarking
tests. Surface displacement is shown in Appendix B, Figs. B1–B3. The Okada
analytical solution is shown by the dark green line with the equivalent
*Abaqus* coseismic output shown by the light green dots. The difference between the
models is shown in panel (b) of Figs. 3–5. Overall, there is an excellent
agreement between the *Abaqus* model and the Okada analytical solution. Of the three
cases, the strike-slip fault agrees most closely (Fig. 3), with differences
peaking at 1 % of the fault slip in the north direction (Fig. 3b). There
are larger differences visible for the dipping fault cases (Figs. 4 and 5). Some
of the near-field finer details of displacement in the east and vertical
directions for the dipping faults (i.e. the directions with most
displacement) are not perfectly captured by *Abaqus*, for example, at 50 km from the
fault (see Fig. 4a, vertical direction), and we attribute this to the mesh
resolution and the distorted element shape that is required to mesh around a
dipping fault. However, these differences are a maximum of 6 % (Fig. 4b)
and at distances greater than 300 km from the fault this decreases to less
than 0.5 %. Since the aim of our model is to predict far-field deformation,
these differences are acceptable.

## 4.3 Post-seismic results

Figures 3–5 also show profiles of the post-seismic displacement in the east,
north and vertical directions for *VISCO1D* (light blue line) and the *Abaqus* model (dark
blue dashed line). We show displacement as a percentage of fault slip at
three times: 10, 50 and 100 years after the earthquake with corresponding
differences between the models shown in panel (b) of each figure. Surface
post-seismic displacement at each of these times is shown in Appendix B,
Figs. B1–B3. The *Abaqus* predictions closely agree with *VISCO1D* for the strike-slip fault
case (Fig. 3). Small differences can be observed for the dipping faults
particularly in the vertical direction (Figs. 4b and 5b). The mismatch in
the coseismic displacement due to limitations in mesh resolution is the
cause of mismatch for the post-seismic displacement, i.e. less coseismic
displacement from the *Abaqus* model would result in less stress and therefore less
relaxation. Slight improvements in the near-field displacement could be made
by increasing the mesh resolution in the vicinity of the fault but would
come at a computational cost (see Appendix A). However, the differences
remain small and are concentrated within 100 km of the fault with a maximum
difference of 5 % of the fault slip after 100 years of relaxation (Fig. 4b) and less than 0.5 % at distances greater than 300 km from the fault.
Therefore, we conclude that the model results are reliable for far-field
post-seismic deformation.

Figures 6 and 7 show displacement through time for the fault geometry in test 3
for two points at 100 km (Fig. 6) and 300 km (Fig. 7) from the fault using
both a Maxwell and Burgers rheology. There is clear consistency in the
evolution of post-seismic relaxation between our model and *VISCO1D*, for both
rheologies (compare solid with dashed lines in Figs. 6a and 7a). The insets
in Figs. 6 and 7 show the results for the initial 30 years of the model run,
where a rapid displacement can be observed from the transient viscosity of
the Burgers rheology (orange/red lines) before converging with the results
from the Maxwell model (blue lines) after approximately 30 years. Over the
500-year period, differences between our *Abaqus* model and *VISCO1D* for the Burgers
rheology are less than 0.6 % of the fault slip within 100 km of the fault
(Fig. 6b) or less than 0.1 % at distances of 300 km or more (Fig. 7b).
For the Maxwell rheology, differences peak at 0.5 % of the fault slip
within 100 km, reducing to 0.2 % at 300 km.

Figures 6a and 7a also show the displacement for test case 3 with the PREM elastic properties (green dotted lines). At 100 km distance from the fault (Fig. 6a), the different elastic structure results in slightly more coseismic displacement than the simple elastic structure used in the benchmarking tests, which results in greater post-seismic displacement with time. At 300 km distance from the fault (Fig. 7a), the depth-varying PREM elastic structure results in only very small differences. The surface coseismic and post-seismic displacements at 10, 50 and 100 years after the earthquake are shown in Fig. B4, Appendix B.

We have presented a finite-element model constructed in *Abaqus* for the purposes of
modelling far-field coseismic and post-seismic deformation. The model is
global, spherical and self-gravitating, and it allows for simple modification
to include three-dimensional Earth structure, non-linear rheologies and
alternative or multiple sources of stress change.

The model performs well when compared with the Okada coseismic analytical
solution and predictions from the post-seismic *VISCO1D* programme for all three fault
scenarios we have tested. For the coseismic displacement, differences are
less than 6 % of the fault slip, with the largest differences in the
vertical direction and near to the fault; in the fault far field, the
differences are negligible. For the post-seismic displacement, differences
are less than 5 % of the fault slip and at distances of 300 km from the
fault, i.e. the far field which is the focus of our model, this reduces to
differences of 0.5 %. Furthermore, we have verified that the evolution of
displacement through time is an excellent match with *VISCO1D* for both Maxwell and
Burgers rheologies. For interest, we also computed the displacement for test
case 3 using the PREM elastic properties, finding differences in results
within 100 km of the fault. However, in the far field, changing the elastic
structure has minimal impact on the resulting post-seismic displacement as we
used the same viscosity structure.

Inclusion of self-gravitation makes only a small difference for the displacement, peaking at 0.2 mm (less than 0.1 % of the fault slip) for the reverse fault (test 2) in the vertical displacement and is negligible in the horizontal directions. This demonstrates that it is not necessary to include self-gravitation when modelling post-seismic deformation in isolation, but it could become important when modelling post-seismic deformation alongside other larger sources of deformation such as changes in ice loading.

The main limitation of the model at present is that the geometry is
restricted to a single fault plane within the mesh and it cannot have
multiple segments of a fault plane with different strike or dip. This is due
to the difficulties of constructing a valid spherical mesh in *Abaqus* with brick
elements that conform to verification checks. For the same reason, including
more than one earthquake in the same model would be problematic. This issue
could potentially be resolved by employing external meshing software (e.g.
Gmsh; Geuzaine and Remacle, 2009). In the case of a fault inversion that
suggests multiple fault segments (e.g. Ye et al.,
2014), an approximation of all the fault planes into a single geometry could
still provide a realistic far-field estimate of post-seismic deformation
(e.g. Takeuchi and Fialko, 2013), particularly if the fault geometry and
slip are adjusted so that model output matches observations of coseismic
displacement (e.g. Sun et al., 2018). Far-field post-seismic deformation is
less sensitive to simplifications made to the fault geometry and slip
distribution than near-field deformation (Khazaradze et al., 2002; Tregoning
et al., 2013; Zhou et al., 2012). Alternatively, each fault segment could be
analysed in a separate model and resulting deformation be combined,
providing a linear rheology is used. The model can, however, account for
varying slip and rake along the single fault plane by specifying individual
values at each node pair. At present, the fault is not permitted to open, and
whilst this is a realistic scenario for a fault, it would have negligible
impact on the far-field post-seismic deformation.

We have demonstrated the validity of this model for far-field coseismic and
post-seismic deformations. It is an improvement on existing models as it
includes global spherical geometry, self-gravitation, and can be adapted to
include 3-D Earth structure. It will prove particularly useful for
investigating earthquakes in regions that may have large lateral variations
in Earth properties where a 1-D Earth model cannot reproduce geodetic
observations. Furthermore, the capability of *Abaqus* to model surface loading such
as a changing ice load has already been established (e.g. van der Wal et
al., 2010; Wu, 2004) and could easily be incorporated into this model.

In general, the higher the mesh resolution the more accurate the predictions
of coseismic and post-seismic displacement. However, increases in the mesh
resolution in the near field of the fault quickly result in a prohibitively
large number of elements in the model, and it becomes computationally too
expensive to run. To verify that our choice of mesh resolution is
sufficient, we set up the 30^{∘} reverse fault test with a coarser
and a finer mesh resolution and compared the model-predicted displacement in
the near and far fields. Since the aim of our model is to calculate
far-field post-seismic deformation, the convergence of displacement at
distances of 300 km from the fault is what determines the choice of mesh
resolution, although results are given for the near field for interest.

Table A1 shows the resolution of each mesh along with computation time for
one iteration of the model. Note that this computation time is for parallel
processing on eight cores on a standard Linux workstation. Due to the way
*Abaqus* licensing works, individual setups may be faster than the time quoted in
Table A1 if users have access to more cores and licenses. We term the mesh
used in the main body of the paper the “reference mesh”.

We evaluate overall displacement results in terms of differences in percentage of fault slip to be consistent with Figs. 3–7. The key results are summarised in Table A2. Comparing results for the reference mesh with the coarse mesh shows that near-field coseismic differences peak at 4.5 % of the fault slip and the maximum difference in the post-seismic deformation after 100 years is 3.1 %. When comparing the reference mesh with the fine mesh, this reduces to 1.5 % for coseismic differences and 1.2 % for post-seismic differences.

In the far field, however, differences between the coarse mesh and reference mesh are small, peaking at 0.2 % of the fault slip after 100 years of post-seismic deformation. Far-field differences between the reference mesh and the fine mesh are negligible. Since the focus of our model is the far field, these results might suggest that any of the mesh resolutions tested would be sufficient. However, in terms of absolute displacement, we see an improvement from differences of 5–9 mm between the coarse mesh and the reference mesh, and to 0.1–1.6 mm for differences between the reference and fine meshes. This demonstrates that results have converged at the resolution of our chosen mesh to be less than the magnitude of uncertainty associated with GPS-observed post-seismic deformation (e.g. Freed et al., 2006; Wang and Fialko, 2018). We therefore conclude that the chosen mesh resolution is more than fit for purpose, with only small improvements to near-field displacement to be gained by using a finer mesh.

*Abaqus* is a commercial software and can be purchased from the developer
(https://www.3ds.com/products-services/simulia/products/abaqus/, last access: February 2019). We have
used version 2018 in this study, but the methods and functionality used are
available in older versions of the software as well. *VISCO1D* version 3 is available for
download at https://www.usgs.gov/node/279413 (last access: March 2022; Pollitz, 2007).

*Abaqus* input files for all
five models presented in this paper are available at
https://github.com/ganield/ABAQUS_Postseismic_model/releases/tag/v2.0 (last access: 8 March 2022) and archived on Zenodo:
https://doi.org/10.5281/zenodo.5897863 (Nield, 2022). Instructions on how to
run the files are also available at this link. The
*VISCO1D* input files and Earth model files used in the experiments presented in this
paper are included in the GitHub repository, along with some instructions on
how to run the code.

GAN developed the model. MAK conceived the study and consulted in the model
implementation. RS contributed meshing code for *Abaqus* and consulted in the model
development. BB contributed the self-gravitational part of the code. All
authors contributed to the manuscript.

The contact author has declared that neither they nor their co-authors have any competing interests.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

We are grateful to
Fred Pollitz for making *VISCO1D* freely available and for his assistance with using
the software. We also thank Tim Masterlark for his advice on implementing
fault slip in *Abaqus* and Wouter van der Wal for guidance on constructing *Abaqus* models.
The handling editor Lutz Gross and three anonymous reviewers are gratefully
acknowledged for their constructive comments.

Grace A. Nield and Matt A. King are supported by the Australian Research Council Discovery Project (grant no. DP170100224).

This paper was edited by Lutz Gross and reviewed by three anonymous referees.

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- Abstract
- Introduction
- Model setup
- Implementation of an earthquake
- Benchmarking
- Discussion and conclusions
- Appendix A: Mesh resolution test
- Appendix B: Surface displacement results
- Code availability
- Data availability
- Author contributions
- Competing interests
- Disclaimer
- Acknowledgements
- Financial support
- Review statement
- References

*Abaqus*for the purpose of calculating post-seismic deformation in the far field of major earthquakes. The model is benchmarked against an existing open-source post-seismic model demonstrating good agreement. The advantage over existing models is the potential for simple modification to include 3-D Earth structure, non-linear rheologies and alternative or multiple sources of stress change.

- Abstract
- Introduction
- Model setup
- Implementation of an earthquake
- Benchmarking
- Discussion and conclusions
- Appendix A: Mesh resolution test
- Appendix B: Surface displacement results
- Code availability
- Data availability
- Author contributions
- Competing interests
- Disclaimer
- Acknowledgements
- Financial support
- Review statement
- References