Temporal Alignment of Seismic Traces in SplitLab

This material addresses an error source in the code of the MATLAB based shear wave splitting software package SplitLab (Wüstefeld et al., 2008). The error causes a wrong temporal alignment of the single traces (Z, N, E components) of one earthquake relative to each other. The resulting wrong horizontal particle motion and wrong waveforms in the ray (LQT) coordinate system lead to a wrong shear wave splitting measurement.

Here we provide a test with seismological data for your personal SplitLab version and a suggested correction of the SplitLab function getFileAndEQseconds.m for the publicly available SplitLab versions.

This modified SplitLab function is also introduced by StackSplit (Grund, 2017) version 3.0 during the installation process.

Citation

If you make use of this material, please cite the relating publication in which this issue is described in detail:

• Fröhlich Y., Grund M. & Ritter J. R. R. (2022). On the effects of wrongly aligned seismogram components for shear wave splitting analysis. Annals of Geophysics, volume 66, issue 2. https://doi.org/10.4401/ag-8781.

Furthermore you can cite the Zenodo DOI given above.

Content

• 00_SAC_files

  • Subfolders corresponding to some of the filename formats supported by SplitLab
  • Vertical (Z), North (N), East (E) components as SAC-files
  • Sampling interval: 0.05 s

• 01_your_results

  • Until now empty
  • Output folder for your own shear wave splitting measurement results

• 02_SL_diagnosticplots

  • SplitLab diagnostic plots for comparison (PDF and PNG formats)
  • Without (*_wrong) and with (*_correct) consideration of the milliseconds (msecs)

• 03_SL_getFileAndEQseconds

  • Modified SplitLab function getFileAndEQseconds.m
  • Publicly available SplitLab versions (*_SLxyz)

Test

How to do

0) Choose one of the three provided examples (see subsection "Details on earthquakes and traces")

• Stuttgart (STU), 2001/06/29 (2001.180) 18:35 (UTC)
• Stuttgart (STU), 2009/11/14 (2009.318) 19:44 (UTC)
• Échery (ECH), 2018/08/28 (2018.240) 22:35 (UTC)

1) Set up SplitLab project

Click for single steps

• General
  • Seismic data directory
    • Go to folder 00_SAC_files
    • Select subfolder with preferred filename format
  • Output directory
    • Select folder 01_your_results
• Station
  • Stuttgart
    • Station code: STU
    • Network code: GE
    • Latitude in deg North: 48.771
    • Longitude in deg East: 9.194
  • Échery
    • Station code: ECH
    • Network code: G
    • Latitude in deg North: 48.216
    • Longitude in deg East: 7.159
• Event window
  • Moment magnitude: 6.00 to 9.75
  • Epicentral distance in deg: 90 to 140
  • Hypocentral depth in km: 0 to 1000
  • Start and end date: corresponding to the date of the chosen example / earthquake
• Phases
  • Earth model: IASP91
  • Phases: (at least) SKS, SKKS, PKS
• Find files
  • File search string: corresponding to the chosen filename format
  • Offset: 0 s
  • Tolerance: 420 s

2) Perform shear wave splitting measurement

• Bandpass filter: 0.020 Hz (lower corner), 0.20 Hz or 0.15 Hz (upper corner)
• Coordinate system: LQT
• Phase: SKS

3) Compare your result / SplitLab diagnostic plot with the provided diagnostic plots

• Folder 02_SL_diagnosticplots: diagnostic plots for wrong and correct relative temporal alignment
• Shape of the E-N particle motion: elliptic or linear?
• SKS phase-related signal on the transverse (T) component: yes or no?

Details on earthquakes and traces

Click for more information

Stuttgart (STU), 2001/06/29 (2001.180)

• Earthquake
  • Date: 2001/06/29 (2001.180)
  • Time: 18:35:51 (UTC)
  • Moment magnitude: 6.1
  • Source region: Southern Bolivia
  • Hypocentral depth: 274 km
  • Backazimuth: 246.5 deg
  • Epicentral distance: 95.29 deg
• Traces
  • msecs of start times: North = 0027, East = 0927, Vertical = 0627
  • Relative msec difference: |E-N| = |900|, i.e., |18| samples

Stuttgart (STU), 2009/11/14 (2009.318)

• Earthquake
  • Date: 2009/11/14 (2009.318)
  • Time: 19:44:29 (UTC)
  • Moment magnitude: 6.2
  • Source region: Jujuy province, Argentina
  • Hypocentral depth: 220 km
  • Backazimuth: 244.5 deg
  • Epicentral distance: 98.15 deg
• Traces
  • msecs of start times: North = 0145, East = 0895, Vertical = 0945
  • Relative msec difference: |E-N| = |750|, i.e., |15| samples

Échery (ECH), 2018/08/28 (2018.240)

• Earthquake
  • Date: 2018/08/28 (2018.240)
  • Time: 22:35:13 (UTC)
  • Moment magnitude: 6.5
  • Source region: Mariana Islands
  • Hypocentral depth: 60 km
  • Backazimuth: 40.1 deg
  • Epicentral depth: 106.00 deg
• Traces
  • msecs of start times: North = 0950, East = 0000, Vertical = 0950
  • Relative msec difference: |E-N| = |950|, i.e., |19| samples

SplitLab function getFileAndEQseconds.m

SplitLab versions

• 03_SL_getFileAndEQseconds
  • SplitLab 1.0.5 (*_SL105) (Wüstefeld et al., 2008)
  • SplitLab 1.2.1 (*_SL121) (Porritt, 2014)
  • SplitLab 1.3.0 (*_SL130) (Creasy, 2020; based on SplitLab 1.2.1)
  • SplitLab 1.9.0 (*_SL190) (based on SplitLab 1.0.5)

How to do

• The filename part xyz or x.y.z indicates the SplitLab version
• Go to the folder ~/SplitLabx.y.z/Tools/ on your computer
• Rename the existing function getFileAndEQseconds.m, e.g., getFileAndEQseconds_original.m in this folder
• Copy and past the modified function getFileAndEQseconds_SLxyz.m into this folder
• Remove the end of the filename _SLxyz
• Assign earthquake catalogue and seismological data in your SplitLab project

Releases

Release	Zenodo DOI
dev main branch
v2.0
v1.0

For details on the single releases see the changelog.

Contributing

For bug reports, suggestions, or recommendationsfeel free to open an issue or submit a pull request directly here on GitHub.

Related topics

In the table below, some other tools for measuring shear wave splitting (besides SplitLab) are listed. Please note, that these tools were not checked regarding the issue of an incorrect relative temporal alignment of the single component traces.

Software	Language	Description	Author
SplitLab 1.0.5	MATLAB	Original SplitLab version	Andreas Wüstefeld, Götz Bokelmann & Christophe Zaroli
SplitLab 1.2.1	MATLAB	Update of SplitLab 1.0.5	Rob Porritt
SplitLab 1.3.0	MATLAB	Update of SplitLab 1.2.1	Neala Creasy
SplitLab 1.9.0	MATLAB	Most recent SplitLab version	IPGP
StackSplit	MATLAB	Plugin for multi-event analyses in SplitLab	Michael Grund & Yvonne Fröhlich
SplitRacer	MATLAB	Semi-automated workflow	Miriam C. Reiss
SplitPy	Python	Based on SplitLab	Pascal Audet & Andrew Schaeffer
SWSPy	Python	Automated workflow	Tom Hudson, Andrew M. Walker & Joseph Asplet
STADIUM-Py	Python	Fully automated workflow	Utpal Kumar & Cédric Legendre
Pytheas	Python	Local shear-wave splitting studies	Ioannis Spingos & George Kaviris
Sheba	Fortran		James Wookey
SeisSplit.jl	Julia		Andy Nowacki

References

• Bowman, J. R. & Ando, M. (1987). Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone. Geophysical Journal International, volume 88, issue 1, pages 25-41. https://doi.org/10.1111/j.1365-246X.1987.tb01367.x.
• Creasy, N. M. (2020). SplitLab version 1.3.0. available at https://github.com/nmcreasy/SplitLab1.3.0.
• GEOFON Data Centre (1993). GEOFON (GeoForschungsNetz). Deutsches GeoForschungsZentrum (GFZ), Seismic Network. https://doi.org/10.14470/TR560404.
• GEOSCOPE (1982). French Global Network of broad band seismic stations. Institut de physique du globe de Paris (IPGP) & Ecole et Observatoire des Sciences de la Terre de Strasbourg (EOST). https://doi.org/10.18715/GEOSCOPE.G.
• Grund, M. (2017). StackSplit - a plugin for multi-event shear wave splitting analyses in SplitLab. Computers & Geosciences, volume 105, pages 43-50. https://doi.org/10.1016/j.cageo.2017.04.015. versions 1.0, 2.0, and 3.0 available at https://github.com/michaelgrund/stacksplit.
• Porritt, R. W. (2014). SplitLab version 1.2.1. available at https://robporritt.wordpress.com/software/.
• Silver, P. G. & Chan, W. W. (1991). Shear wave splitting and subcontinental mantle deformation. Journal of Geophysical Research, volume 96, issue B10, pages 16429-16454. https://doi.org/10.1029/91JB00899.
• Walsh, E., Arnold, R. & Savage, M. K. (2013). Silver and Chan revisited. Journal of Geophysical Research: Solid Earth, volume 118, issue 10, pages 5500-5515. https://doi.org/10.1002/jgrb.50386.
• Wüstefeld, A., Bokelmann, G., Zaroli, C. & Barruol, G. (2008). SplitLab: A shear-wave splitting environment in Matlab. Computers & Geosciences, volume 34, issue 5, pages 515-528. https://doi.org/10.1016/j.cageo.2007.08.002. version 1.0.5 was available at http://splitting.gm.univ-montp2.fr, version 1.9.0 is available at https://github.com/IPGP/splitlab.

Funding

The presented research and YF received support from:

• Scholarship of the Graduate Funding from the German States