[CIG-SHORT] Mesh interpenetration

[Li, Teng]

Hi Brad,
 
I am sorry for the late reply.

Please see the pictures for model setup and slip values in the HDF5 files.  (The red lines means the positions of the secondary faults).

Here I will give a description of my problem. The model is an 18000*25130 rectangular domain. In the model, there are a main fault and secondary faults. The main fault in the middle split the whole body into two equal rectangular. While in some places, there are 60-degree secondary faults link directly to the main fault. The friction model for all the faults is slip-weakening friction. And this is a dynamic problem. For the initial shear stress on the main fault, I apply overstress in a certain length in the bottom of the main fault and apply smaller stress in later part of the main fault. And I apply a certain compressive normal stress along the main fault. For the secondary fault, I apply same normal stress and smaller shear stress (same as the stress on the main fault where no overstress applied). The stresses in the secondary faults are decomposed according to their angles. The total time is 9.5s to make sure the rupture will go to the other end of the main fault, and dt is 0.0004s. The output is 200, which means 0.08s a step.

The mesh size is 12.5m. All the elements are quad elements. In the junction area where the main fault and lowest secondary fault link together, the mesh shapes are irregular.  And I also find the main fault open in this part. Now the only thing I am not sure is the negative normal slip in this region.


Best,
Teng


Teng Li

Master Candidate in Structures

Department of Civil and Environmental Engineering

University of Illinois at Urbana-Champaign

205 North Mathews Ave, Urbana, IL. 61801

Phone:(217)8196210, Email: tengli2 at illinois.edu



________________________________________
From: CIG-SHORT [cig-short-bounces at geodynamics.org] on behalf of Brad Aagaard [baagaard at usgs.gov]
Sent: Wednesday, November 01, 2017 3:39 PM
To: cig-short at geodynamics.org
Subject: Re: [CIG-SHORT] Mesh interpenetration

On 11/01/2017 12:59 PM, Li, Teng wrote:
> Hi Brad,
>
> Thank you. Actually, I asked the similar questions in the last month. I find normal traction become zero in the junction area. And also shear traction become zero in this area. After analyzing HDF5 file, I find there are both positive and negative normal slip occur in different time steps. Last time, we thought it was because the inappropriate parameters chosen for my problem. For my problem, the mesh size is 12.5m. And I first choose nordimensinalizer: normalizer.shear_wave_speed = 1*km/s,  normalizer.wave_period = 1*s. Now I change them to 0.1*km/s and 0.1s. Furthermore, I add formulation.norm_viscosity = 0.4. And I still see the both positive and negative normal slips. The magnitudes for both are about 0.005m.
>
> Is it because the rounding error brought by Matlab? Or is it because the mesh? I use Trellis to mesh all the domain.

Teng,

I went back to our email exchange in Aug and looked at your JSON
parameter file that you provided then. It looks like you are running a
dynamic simulation with multiple intersecting faults with a friction
interface.

The JSON parameter file doesn't contain your spatial databases so I
don't know what sort of tractions you have on the fault or what your
material properties are. What information are you using to select your
new nondimensionalization scales? The shear wave speed should be close
to your minimum shear wave speed and the wave period should be close to
the minimum wave period you can resolve with your discretization (a
wavelength of usually 10-12 times your cell size). You updated
nondimensionalization scales correspond to a wavelength (length scale)
of 0.01 km or 10m. I suspect that the wavespeed you use for
nondimensionalization should be on the order of 1 km/s not 0.1 km/s.

With a geometric incompatibility, usually you can keep the fault closed
with an appropriate normal traction. That is, if you have a small amount
of slip relative to your cell size, then the local transient normal
tractions are usually much smaller than the compressive normal tractions
arising from the overburden pressure.

If you need further help, I suggest sending a diagram of what you are
trying to do along with a description (include approximate values for
your materials and BC) and the JSON parameter file.

Regards,
Brad
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