Table of Contents

• Noise Cross-correlation Simulations
  • New Requirements on ‘Old’ Input Parameter Files
  • Noise Simulations: Step by Step
    • Pre-simulation
    • Simulations
    • Post-simulation
  • Examples
  • References

Noise Cross-correlation Simulations

The new version of SPECFEM3D_GLOBE includes functionality for seismic noise tomography. Users are recommended to familiarize themselves first with the procedures for running regular earthquake simulations (Chapters [cha:Running-the-Mesher]–[cha:Regional-Simulations]) and adjoint simulations (Chapter [cha:Adjoint-Simulations]). Also, make sure you read the paper ‘Noise cross-correlation sensitivity kernels’ (Tromp et al. 2010) in order to understand noise simulations from a theoretical perspective.

New Requirements on ‘Old’ Input Parameter Files

As usual, the three main input files are crucial: DATA/Par_file, DATA/CMTSOLUTION and DATA/STATIONS.

DATA/CMTSOLUTION is required for all simulations. However, it may seem unexpected to have it listed here, since the noise simulations should have nothing to do with the earthquake – hence the DATA/CMTSOLUTION. For noise simulations, it is critical to have no earthquakes. In other words, the moment tensor specified in DATA/CMTSOLUTION must be set to ZERO!

DATA/STATIONS remains the same as in previous earthquake simulations, except that the order of stations listed in DATA/STATIONS is now important. The order will be used later to identify the ‘main’ receiver, i.e., the one that simultaneously cross correlates with the others. Please be noted that the actual station file used in the simulation is OUTPUT_FILES/STATIONS_FILTERED, which is generated when you run your simulations. (e.g., in regional simulations we may have included stations out of the region of our interests in DATA/STATIONS, so we have to get rid of them.)

DATA/Par_file also requires careful attention. New to this version of SPECFEM3D_GLOBE, a parameter called NOISE_TOMOGRAPHY has been added that specifies the type of simulation to be run. NOISE_TOMOGRAPHY is an integer with possible values 0, 1, 2 and 3. For example, when NOISE_TOMOGRAPHY equals 0, a regular earthquake simulation will be run. When NOISE_TOMOGRAPHY is equal to 1/2/3, you are about to run step 1/2/3 of the noise simulations respectively. Should you be confused by the three steps, refer to Tromp et al. (2010) for details.

Another change to DATA/Par_file involves the parameter RECORD_LENGTH_IN_MINUTES. While for regular earthquake simulations this parameter specifies the length of synthetic seismograms generated, for noise simulations it specifies the length of the seismograms used to compute cross correlations. The actual cross correlations are thus twice this length. The code automatically makes modification accordingly, if NOISE_TOMOGRAPHY is not zero.

There are other parameters in DATA/Par_file which should be given specific values. For instance, NUMBER_OF_RUNS and NUMBER_OF_THIS_RUN must be 1; ROTATE_SEISMOGRAMS_RT, SAVE_ALL_SEISMOGRAMS_IN_ONE_FILES, USE_BINARY_FOR_LARGE_FILE and MOVIE_COARSE should be .false.. Moreover, since the first two steps for calculating noise cross-correlation kernels correspond to forward simulations, SIMULATION_TYPE must be 1 when NOISE_TOMOGRAPHY equals 1 or 2. Also, we have to reconstruct the ensemble forward wavefields in adjoint simulations, therefore we need to set SAVE_FORWARD to .true. for the second step, i.e., when NOISE_TOMOGRAPHY equals 2. The third step is for kernel constructions. Hence SIMULATION_TYPE should be 3, whereas SAVE_FORWARD must be .false..

Noise Simulations: Step by Step

Proper parameters in those ‘old’ input files are not enough for noise simulations to run. We have a lot more ‘new’ input parameter files to specify: for example, the ensemble-averaged noise spectrum, the noise distribution etc. However, since there are a few ‘new’ files, it is better to introduce them sequentially. Read through this section even if you don’t understand some parts temporarily, since some examples you can go through are provided in this package.

Pre-simulation

• As usual, we first configure the software package using:

  ./configure FC=ifort MPIFC=mpif90
  

• Next, we need to compile the source code using:

  make xmeshfem3D
  make xspecfem3D
  

  Before compilation, the DATA/Par_file must be specified correctly, e.g., NOISE_TOMOGRAPHY shouldn’t be zero; RECORD_LENGTH_IN_MINUTES, NEX_XI and NEX_ETA must be what you want in your real simulations. Otherwise you may get wrong informations which will cause problems later. (it is good to always re-complie the code before you run simulations)

• After compiling, you will find two important numbers besides the needed executables:

  number of time steps = 31599
  time_stepping of the solver will be: 0.19000
  

  The first number will be denoted as NSTEP from now on, and the second one as dt. The two numbers are essential to calculate the ensemble-averaged noise spectrum from either Peterson’s noise model or just a simple flat power spectrum (corresponding to 1-bit preprocessing). Should you miss the two numbers, you can run ./xcreate_header_file to bring them up again (with correct DATA/Par_file!). FYI, NSTEP is determined by RECORD_LENGTH_IN_MINUTES in DATA/Par_file, which is automatically doubled in noise simulations; whereas dt is derived from NEX_XI and NEX_ETA, or in other words your element sizes.

• A Matlab script is provided to generate the ensemble-averaged noise spectrum.

  EXAMPLES/noise_examples/NOISE_TOMOGRAPHY.m  (main program)
  EXAMPLES/noise_examples/PetersonNoiseModel.m
  

  In Matlab, simply run:

  NOISE_TOMOGRAPHY(NSTEP, dt, Tmin, Tmax, NOISE_MODEL)
  

  NSTEP and dt have been given when compiling the specfem3D source code; Tmin and Tmax correspond to the period range you are interested in; NOISE_MODEL denotes the noise model you will be using. Details can be found in the Matlab script.

  After running the Matlab script, you will be given the following information (plus a figure in Matlab):

  *************************************************************
  the source time function has been saved in:
  /data2/yangl/3D_NOISE/S_squared         (note this path must be different)
  S_squared should be put into directory:
  ./NOISE_TOMOGRAPHY/ in the SPECFEM3D_GLOBE package
  

  In other words, the Matlab script creates a file called S_squared, which is the first ‘new’ input file we encounter for noise simulations.

  One may choose a flat noise spectrum rather than Peterson’s noise model. This can be done easily by modifying the Matlab script a little bit.

• Create a new directory in the SPECFEM3D_GLOBE package, name it as NOISE_TOMOGRAPHY. In fact, this new directory should have been created already when checking out the package. We will add/replace some information needed in this folder.

• Put the Matlab-generated-file S_squared in NOISE_TOMOGRAPHY.

  That’s to say, you will have a file NOISE_TOMOGRAPHY/S_squared in the SPECFEM3D_GLOBE package.

• Create a file called NOISE_TOMOGRAPHY/irec_main_noise. Note that this file should be put in directory NOISE_TOMOGRAPHY as well. This file contains only one integer, which is the ID of the ‘main’ receiver. For example, if in this file shows 5, it means that the fifth receiver listed in DATA/STATIONS becomes the ‘main’. That’s why we mentioned previously that the order of receivers in DATA/STATIONS is important.

  Note that in regional (1- or 2-chunk) simulations, the DATA/STATIONS may contain receivers not within the selected chunk(s). Therefore, the integer in NOISE_TOMOGRAPHY/irec_main_noise is actually the ID in DATA/STATIONS_FILTERED (which is generated by xspecfem3D).

• Create a file called NOISE_TOMOGRAPHY/nu_main. This file holds three numbers, forming a (unit) vector. It describes which component we are cross-correlating at the ‘main’ receiver, i.e., ${\hat{\bf \nu}}^{\alpha}$ in Tromp et al. (2010). The three numbers correspond to N/E/Z components respectively.

• Describe the noise direction and distributions in src/specfem3d/noise_tomography.f90. Search for a subroutine called noise_distribution_direction in noise_tomography.f90. It is actually located at the very beginning of noise_tomography.f90. The default assumes vertical noises and a uniform distribution across the whole physical domain. It should be quite self-explanatory for modifications. Should you modify this part, you have to re-compile the source code. (again, that’s why we recommend that you always re-compile the code before you run simulations)

Simulations

As discussed in Tromp et al. (2010), it takes three simulations to obtain one contribution of the ensemble sensitivity kernels:

• Step 1: simulation for generating wavefield

  SIMULATION_TYPE = 1
  NOISE_TOMOGRAPHY = 1
  SAVE_FORWARD  not used, can be either .true. or .false.
  

• Step 2: simulation for ensemble forward wavefield

  SIMULATION_TYPE = 1
  NOISE_TOMOGRAPHY = 2
  SAVE_FORWARD = .true.
  

• Step 3: simulation for ensemble adjoint wavefield and sensitivity kernels

  SIMULATION_TYPE = 3
  NOISE_TOMOGRAPHY = 3
  SAVE_FORWARD = .false.
  

  Note Step 3 is an adjoint simulation, please refer to previous chapters on how to prepare adjoint sources and other necessary files, as well as how adjoint simulations work.

It’s better to run the three steps continuously within the same job, otherwise you have to collect the saved surface movies from the old nodes to the new nodes.

Post-simulation

After those simulations, you have all stuff you need, either in the OUTPUT_FILES or in the directory specified by LOCAL_PATH in DATA/Par_file (most probably on local nodes). Collect whatever you want from the local nodes to your workstation, and then visualize them. This process is the same as what you may have done for regular earthquake simulations. Refer Chapter [cha:graphics] if you have problems.

Simply speaking, two outputs are the most interesting: the simulated ensemble cross correlations and one contribution of the ensemble sensitivity kernels.

The simulated ensemble cross correlations are obtained after the second simulation (Step 2). Seismograms in OUTPUT_FILES are actually the simulated ensemble cross correlations. Collect them immediately after Step 2, or the Step 3 will overwrite them. Note that we have a ‘main’ receiver specified by NOISE_TOMOGRAPHY/irec_main_noise, the seismogram at one station corresponds to the cross correlation between that station and the ‘main’. Since the seismograms have three components, we may obtain cross correlations for different components as well, not necessarily the cross correlations between vertical components.

One contribution of the ensemble cross-correlation sensitivity kernels are obtained after Step 3, stored in the LOCAL_PATH on local nodes. The ensemble kernel files are named the same as regular earthquake kernels.

You need to run another three simulations to get the other contribution of the ensemble kernels, using different forward and adjoint sources given in Tromp et al. (2010).

Examples

In order to illustrate noise simulations in an easy way, three examples are provided in EXAMPLES/noise_examples/. Note however that they are created for a specific cluster (SESAME@PRINCETON). You have to modify them to fit your own cluster.

The three examples can be executed using (in directory EXAMPLES/noise_examples/):

./run_this_example.sh regional
./run_this_example.sh global_short
./run_this_example.sh global_long

Each corresponds to one example, but they are pretty similar.

Although the job submission only works on SESAME@PRINCETON, the procedure itself is universal. You may review the whole process described in the last section by following commands in those examples.

Finally, note again that those examples show only one contribution of the ensemble kernels!

References

Tromp, Jeroen, Yang Luo, Shravan Hanasoge, and Daniel Peter. 2010. “Noise Cross-Correlation Sensitivity Kernels.” Geophys. J. Int. 183: 791–819. https://doi.org/10.1111/j.1365-246X.2010.04721.x.

This documentation has been automatically generated by pandoc based on the User manual (LaTeX version) in folder doc/USER_MANUAL/ (Dec 20, 2023)