[CIG-SHORT] update on benchmarks

[Marc Spiegelman]

Hi Brad/Charles,
    just lurking on the CIG-SHORT list but this is very interesting.  
There was a very similar issue in the Subduction zone benchmark (e.g. 
http://www.geo.lsa.umich.edu/~keken/subduction02.html) which was 
initially ambiguous about how to describe the discontinuity in  
velocity across the fault.  Same issue: small differences in the 
assumptions of how to describe discontinuities in contiuum models 
completely dominated the solutions near the wedge corner.

In retrospect, I suppose we shouldn't be surprised, but we would never 
catch these things without solid benchmarking efforts.  They consume an 
enormous amount of time but it's well worth it in the end.

Keep up the good work
cheers
marc





On Jan 25, 2007, at 11:17 PM, Brad Aagaard wrote:

> Hi all:
>
> Charles Williams and I have been spending some time scrutinizing the 
> results from the strike-slip benchmark. We think we have finally 
> zeroed in on a couple of problems. The most significant issue is that 
> the benchmark description does not specify what to do where the two 
> taper regions overlap. The supplied Matlab scripts for computing the 
> analytical solution uses the minimum of the two tapers (maintaining a 
> linear taper with respect to either y or z). Charles has been using a 
> quadratic taper (product of the two linear tapers) which is why the 
> GeoFEST results computed in late Nov have not been agreeing with the 
> PyLith results. In the next few days, we should have new PyLith 
> results with much better agreement for the strike-slip results.
>
> Unfortunately, this taper issue also affects the reverse-slip 
> benchmark results for BOTH GeoFEST and PyLith (and perhaps 
> COMSOL/Femlab as well?). Greg Lyzenga used the PyLith input files in 
> setting up the GeoFEST runs for this benchmark, and since Charles used 
> a quadratic taper, the taper in the simulations doesn't match the 
> taper in the analytical solution. So we should be able to reduce the 
> misfit with respect to the analytical solution by redoing the 
> benchmarks with the correct taper.
>
> Charles is working on fixing the PyLith input files for the two 
> benchmarks. Once we run the strike-slip benchmark and verify 
> everything is as we expect, we will need someone to fix the GeoFEST 
> reverse-slip input files and rerun the reverse-slip benchmark for each 
> of the three resolutions. Any volunteers?
>
> We all need to thanks Charles for staring at a bunch of numbers for 
> more hours than he would have liked in order to get to the bottom of 
> this!
>
> Brad
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Marc Spiegelman
Lamont-Doherty Earth Observatory &
Dept. of Applied Physics/Applied Mathematics
Columbia University
SKYPE:   Tel: +1 (845) 363 4747
http://www.ldeo.columbia.edu/~mspieg
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