Content

# Pseudo Dynamic Rupture - *A stress-based self-similar finite source model*¶

## Introduction¶

Physics-based, dynamic rupture models, which rely on few parameters only, are needed for realistic forward modelling and to reduce the effort and non-uniqueness of the inversion. The `PseudoDynamicRupture`

is a simplified, quasi-dynamic, semi-analytical rupture model suited for wavefield and static displacement simulation and earthquake source inversion. The rupture model builds on the class of self-similar crack models. On one hand it is approximative as it neglects inertia and so far the details of frictional effects and treats the rupture front growth in a simplified way. On the other hand, it is complete as the slip fulfils the boundary condition on the broken plane for every instantaneous rupture front geometry and applied stress.

### Theoretical foundation¶

Note

Dahm, T., Heimann, S., Metz, M., Isken, M. P. (2021). A self-similar dynamic rupture model based on the simplified wave-rupture analogy. Geophysical Journal International. Volume 225, Issue 3, June 2021, Pages 1586–1604, https://doi.org/10.1093/gji/ggab045

## Source implementation details¶

The implementation of the pseudo dynamic rupture model into the Pyrocko and Pyrocko-GF framework is based on [Dahm2021]. Essential building blocks are the class `PseudoDynamicRupture`

and in the `AbstractTractionField`

subclasses. Additionally the model is constrained by the subsurface underground model as provided by the metadata in the Pyrocko GF `Store`

. See [Dahm2021] which describes the rupture source model in detail.

## Forward modelling a pseudo dynamic rupture¶

The `PseudoDynamicRupture`

model is fully integrated into Pyrocko-GF. The model can be used to forward model synthetic waveforms, surface displacements and any quantity that is delivered by the store. Various utility functions are available to analyse and visualize parameters of the rupture model.

In this section we will show the parametrisation, introspection and resulting seismological forward calculations using the `PseudoDynamicRupture`

.

### Forward calculation of waveforms and static displacement¶

Parametrisation of the source model is straight forward, as for any other Pyrocko-GF source. In the below code example we parametrize a shallow bidirectional strike-slip source.

More details on dynamic and static Green’s function databases and other source models are laid out in section Pyrocko-GF - Geophysical forward modelling with pre-calculated Green’s functions.

#### Simple pseudo dynamic rupture forward model¶

We create a simple forward model and calculate waveforms for one seismic station (`Target`

) at about 14 km distance - The tractions will be aligned to force the defined slip rake angle. The modeled waveform is displayed in the Snuffler application.

**Important:** the spatial sampling of the GF store used in the example must be dense enough to prevent aliasing artifacts.

Download `gf_forward_pseudo_rupture_basic.py`

```
import os.path as op
from pyrocko import gf
km = 1e3
# The store we are going extract data from:
store_id = 'iceland_reg_v2'
# First, download a Green's functions store. If you already have one that you
# would like to use, you can skip this step and point the *store_superdirs* to
# the containing directory.
if not op.exists(store_id):
gf.ws.download_gf_store(site='kinherd', store_id=store_id)
# We need a pyrocko.gf.Engine object which provides us with the traces
# extracted from the store. In this case we are going to use a local
# engine since we are going to query a local store.
engine = gf.LocalEngine(store_superdirs=['.'])
# The dynamic parameters used for discretization of the PseudoDynamicRupture
# are extracted from the store's config file.
store = engine.get_store(store_id)
# Let's define the source now with its extension, orientation etc.
rupture = gf.PseudoDynamicRupture(
lat=0.,
lon=0.,
north_shift=2.*km,
east_shift=2.*km,
depth=3.*km,
strike=43.,
dip=89.,
rake=88.,
length=15*km,
width=5*km,
nx=10,
ny=5,
# Relative nucleation between -1. and 1.
nucleation_x=-.6,
nucleation_y=.3,
slip=1.,
anchor='top',
# Threads used for modelling
nthreads=1,
# Force pure shear rupture
pure_shear=True)
# Recalculate slip, so that the rupture's magnitude fits the given value
rupture.rescale_slip(magnitude=7.0, store=store)
# Create waveform target, where synthetic waveforms are calculated for
waveform_target = gf.Target(
lat=0.,
lon=0.,
east_shift=10*km,
north_shift=10.*km,
interpolation='multilinear',
store_id=store_id)
# Get synthetic waveforms and display them in snuffler
response = engine.process(rupture, waveform_target)
response.snuffle()
```

### Traction and stress field parametrisation¶

The rupture plane can be exposed to different stress/traction field models which drive and interact with the rupture process.

A `TractionField`

defines the absolute stress release on the fault plane:

`UniformTractions`

An `AbstractTractionField`

can modify an existing `TractionField`

:

These fields can be used independently or be combined into a `TractionComposition`

, where `TractionField`

are stacked and `AbstractTractionField`

are multiplied with the stack. See the reference and code for implementation details.

Pure tractions can be visualised using the utility function `plot_tractions()`

.

### Plotting and rupture model insights¶

Convenience functions for plotting and introspection of the dynamic rupture model are offered by the `pyrocko.plot.dynamic_rupture`

module.

#### Traction and rupture evolution¶

Initialize a simple dynamic rupture with uniform rake tractions and visualize the tractions and rupture propagation.

Download `gf_forward_pseudo_rupture_basic_plot.py`

```
import os.path as op
from pyrocko import gf
from pyrocko.plot.dynamic_rupture import RuptureView
km = 1e3
# The store we are going extract data from:
store_id = 'iceland_reg_v2'
# First, download a Green's functions store. If you already have one that you
# would like to use, you can skip this step and point the *store_superdirs* to
# the containing directory.
if not op.exists(store_id):
gf.ws.download_gf_store(site='kinherd', store_id=store_id)
# We need a pyrocko.gf.Engine object which provides us with the traces
# extracted from the store. In this case we are going to use a local
# engine since we are going to query a local store.
engine = gf.LocalEngine(store_superdirs=['.'])
# The dynamic parameters used for discretization of the PseudoDynamicRupture
# are extracted from the store's config file.
store = engine.get_store(store_id)
# Let's define the source now with its extension, orientation etc.
rupture = gf.PseudoDynamicRupture(
# At lat 0. and lon 0. (default)
north_shift=2.*km,
east_shift=2.*km,
depth=3.*km,
strike=43.,
dip=89.,
rake=88.,
length=15*km,
width=5*km,
nx=10,
ny=5,
# Relative nucleation between -1. and 1.
nucleation_x=-.6,
nucleation_y=.3,
slip=1.,
anchor='top',
# Threads used for modelling
nthreads=1,
# Force pure shear rupture
pure_shear=True)
# Recalculate slip, that rupture magnitude fits given magnitude
rupture.rescale_slip(magnitude=7.0, store=store)
plot = RuptureView(rupture, figsize=(8, 4))
plot.draw_patch_parameter('traction')
plot.draw_time_contour(store)
plot.draw_nucleation_point()
plot.save('dynamic_basic_tractions.png')
# Alternatively plot on screen
# plot.show_plot()
```

#### Rupture propagation¶

We can investigate the rupture propagation speed with `draw_patch_parameter()`

.
Rupture speed is proportional to the S-wave velocity in the Earth model and scaled with the attribute `gamma`

.

```
# rupture is a PseudoDynamicRupture object
plot = RuptureView(rupture)
plot.draw_patch_parameter('vr')
plot.draw_time_contour(store)
plot.draw_nucleation_point()
plot.show_plot()
```

#### Slip distribution¶

Dislocations of the dynamic rupture source can be plotted with `draw_dislocation()`

:

```
# rupture is a PseudoDynamicRupture object
plot = RuptureView(rupture)
plot.draw_dislocation()
plot.draw_time_contour(store)
plot.draw_nucleation_point()
plot.show_plot()
```

#### Rupture evolution¶

We can animate the rupture evolution using the `rupture_movie()`

function.

```
from pyrocko.plot.dynamic_rupture import rupture_movie
rupture_movie(
rupture, store, 'dislocation',
plot_type='view')
```

#### Combined complex traction fields¶

In this example we will combine different traction fields: `DirectedTractions`

, `FractalTractions`

and taper them using `RectangularTaper`

.

After plotting the tractions and final dislocations, we will forward model the waveforms.

Download `gf_forward_pseudo_rupture_complex.py`

```
import os.path as op
from pyrocko import gf
from pyrocko.plot.dynamic_rupture import RuptureView
km = 1e3
# The store we are going extract data from:
store_id = 'iceland_reg_v2'
# First, download a Green's functions store. If you already have one that you
# would like to use, you can skip this step and point the *store_superdirs* to
# the containing directory.
if not op.exists(store_id):
gf.ws.download_gf_store(site='kinherd', store_id=store_id)
# We need a pyrocko.gf.Engine object which provides us with the traces
# extracted from the store. In this case we are going to use a local
# engine since we are going to query a local store.
engine = gf.LocalEngine(store_superdirs=['.'])
# The dynamic parameters used for discretization of the PseudoDynamicRupture
# are extracted from the store's config file.
store = engine.get_store(store_id)
# Let's define the source now with its extension, orientation etc.
rupture = gf.PseudoDynamicRupture(
lat=0.,
lon=0.,
north_shift=2.*km,
east_shift=2.*km,
depth=3.*km,
strike=43.,
dip=89.,
length=15*km,
width=5*km,
nx=10,
ny=5,
# Relative nucleation between -1. and 1.
nucleation_x=-.6,
nucleation_y=.3,
# Relation between subsurface model s-wave velocity vs
# and rupture velocity vr
gamma=0.7,
slip=1.,
anchor='top',
# Threads used for modelling
nthreads=1,
# Force pure shear rupture
pure_shear=True,
# Tractions are in [Pa]. However here we are using relative tractions,
# Resulting waveforms will be scaled to the maximumslip [m]
tractions=gf.tractions.TractionComposition(
components=[
gf.tractions.DirectedTractions(rake=56., traction=1.e6),
gf.tractions.FractalTractions(rake=56., traction_max=.4e6),
gf.tractions.RectangularTaper()
])
)
rupture.discretize_patches(store)
# Plot the absolute tractions from strike, dip, normal
plot = RuptureView(rupture, figsize=(8, 4))
plot.draw_patch_parameter('traction')
plot.draw_nucleation_point()
plot.save('dynamic_complex_tractions.png')
# Plot the modelled dislocations
plot = RuptureView(rupture, figsize=(8, 4))
plot.draw_dislocation()
# We can also define a time for the snapshot:
# plot.draw_dislocation(time=1.5)
plot.draw_time_contour(store)
plot.draw_nucleation_point()
plot.save('dynamic_complex_dislocations.png')
# Forward model waveforms for one station
engine = gf.LocalEngine(store_superdirs=['.'], use_config=True)
store = engine.get_store(store_id)
waveform_target = gf.Target(
lat=0.,
lon=0.,
east_shift=10*km,
north_shift=30.*km,
interpolation='multilinear',
store_id=store_id)
response = engine.process(rupture, waveform_target)
response.snuffle()
```

#### Moment rate function / source time function¶

With this example we demonstrate, how the moment rate or source time function (STF) of a rupture can be simulated using the slip rate on each subfault , the average shear modulus and the subfault areas :

Use the method `draw_source_dynamics()`

:

```
plot = RuptureView(rupture)
# variable can be:
# - 'stf', 'moment_rate': moment rate function
# - 'cumulative_moment', 'moment': cumulative seismic moment function
# of the rupture
plot.draw_source_dynamics(variable='stf', store=store)
plot.show_plot()
```

#### Individual time-dependent subfault characteristics¶

Sometimes it might be also interesting to check the time-dependent behaviour of an individual subfault.

Use the method `draw_patch_dynamics()`

:

```
plot = RuptureView(rupture)
# nx and ny are the indices of the subfault along strike (nx) and down dip (ny)
# variable can be:
# - 'dislocation': length of subfault dislocation vector [m]
# - 'dislocation_<x, y, z>': subfault dislocation vector component
# in strike, updip or normal direction in [m]
# - 'slip_rate': subfault slip rate in [m/s]
# - 'moment_rate': subfault moment rate function
# - 'cumulative_moment', 'moment': subfault summed moment function
# of the rupture
plot.draw_patch_dynamics(variable='slip_rate', nx=6, ny=3, store=store)
plot.show_plot()
```

#### Radiated seismic energy¶

For rather complex ruptures also directivity effects in the waveforms are of interest. Using the function `plot_directivity()`

allows to plot synthetic waveforms or its envelopes at a certain distance from the source in a circular plot. It provides an easy way of visual directivity effect imaging.

```
from pyrocko.plot.directivity import plot_directivity
# many more possible arguments are provided in the help of plot_directivity
resp = plot_directivity(
engine,
rupture,
store_id,
phases={
'P': '{cake:p|cake:P}-10%',
'S': '{cake:s|cake:S}+50'},
# distance and azimuthal density of modelled waveforms
distance=300*km,
dazi=5.,
# waveform settings
component='R',
quantity='displacement',
envelope=True,
plot_mt='full')
```

## References¶

- Dahm2021
Dahm, T., Heimann, S., Metz, M., Isken, M. P. (2021). A self-similar dynamic rupture model based on the simplified wave-rupture analogy. Geophysical Journal International. Volume 225, Issue 3, June 2021, Pages 1586–1604, https://doi.org/10.1093/gji/ggab045