Unit 4: The phenomenology of earthquakes from InSAR data
How are different types of earthquakes represented in InSAR data? How can we obtain detailed information on the earthquake source from InSAR data? How well can we resolve those details? In this unit, students investigate how simple elastic dislocation models can be matched to interferograms of earthquakes, and the various geometrical and surficial factors that can affect that process.
Unit 4 Learning Outcomes
- Students will depict the relationship between earthquake source parameters and fault geometry.
- Students will differentiate between coseismic deformation from a single earthquake and the long-term signature of multiple earthquake cycles that is recorded in the landscape.
- Students will model coseismic deformation of an earthquake captured by InSAR data, in order to obtain earthquake source parameters.
- Students will relate different coseismic deformation patterns to different faulting styles and orientations.
- Students will relate different degrees of variability in source parameters from different contributed results to the intrinsic uncertainties in those parameters and the non-uniqueness of the results.
Unit 4 Teaching Objectives
- Cognitive: Promote student ability to understand the relationship between fault geometry/earthquake source parameters and surface displacement, as measured with InSAR. Enable exploration of uncertainty in model results and its possible causes.
- Behavioral: Facilitate development of skills in data-fitting and pattern matching.
Context for Use
The content in Unit 4 is appropriate for advanced geology/geoscience courses conducted at the junior and/or senior level in which geodesy data can be introduced in conjunction with traditional presentations of material on faults and faulting; this would typically be in a course on structural geology but could also be part of a course on tectonics, geomorphology, geophysics, or advanced geohazards. Unit 4 builds upon the content of Unit 3: How to see an earthquake from space (InSAR), by moving from interpretation of authentic InSAR data to modeling of those data. If desired, Units 3 and 4 can be used on their own with minimal alteration, as a mini-module on the study of earthquakes with InSAR. Alternatively, Unit 4 can instead be used as a stand-alone exercise in cases where the basics of InSAR theory and interpretation have already been covered in earlier classes. Some of the terminology of fault geometry and definitions of earthquake source parameters are covered in this exercise, but it would be more effective if these concepts had been introduced in an earlier class.
Description and Teaching Materials
This unit is a practical exercise that requires access to computers. After some reinforcement of the terminology of earthquake source parameters and some exploration of concepts of the earthquake cycle and elastic rebound, students use the Visible Earthquakes tool to model InSAR data of at least two earthquakes. The whole class will model the same event (a normal faulting earthquake from Turkey), and then a variety of other events of other faulting styles and orientations can be modeled by subsets of the class as a jigsaw exercise. This will facilitate student exploration and/or class discussion of two topics: 1) uncertainty in earthquake source parameters, and the potential causes of it; and 2) how faulting style and orientation affect the deformation pattern that InSAR records.
IF you are planning to use either of the two suggested case study earthquakes of El Major Cucapah and South Napa for the Unit 5: How do earthquakes affect society? summative assessment, than it is recommended that you steer students away from choosing those for their second earthquake in Unit 4.
Visible Earthquakes - this interactive web tool forms the basis of most of the activities in the unit.
Unit 4 First Motion Background Presentation (PowerPoint 2007 (.pptx) 6.4MB Dec11 15)
Teaching Notes and Tips
You may choose to use this file just for your own reference or to present portions of it to the students after they have already worked through the exercise.
1) Take a little time experimenting with and becoming familiar with the Visible Earthquakes tool before starting to teach Unit 4. Visible Earthquake has a good Getting Started page that overviews the online tool function as well as the basics of faulting and InSAR, thus serving as a useful reference for students.
2) When assessing a student's attempt at modeling a given earthquake interferogram, consider the following:
- Are the largest positive and negative deformation signals approximately matched in terms of their amplitudes?
- In the residual view, the amplitude of any remaining signal should be small.
- In the wrapped interferogram view, the numbers of fringes should be similar on both sides of the fault.
- Does the modeled deformation pattern have a similar spatial extent to that seen in the data?
- Use the ruler tool to measure lengths and widths, if necessary.
- Is the modeled deformation pattern in a similar location to that seen in the data?
- Again, the ruler tool can be helpful to make comparisons between positions.
- Gaps or holes in the data can also be used to assess position.
- Very large localized residuals can sometimes indicate a mislocated fault (mapping positive deformation on top of negative, for instance, would cause a large residual in the area of overlap).
Remember that there is often no single right answer. Different earthquake source parameters can give rise to similar deformation patterns. In other words, some source parameters are poorly constrained by the data and model results can be non-unique. So long as you are not too prescriptive about what an acceptable answer might be, these ideas should emerge when students start comparing their results with those of their classmates.
3) For the jigsaw portion of the exercise, pick a few different earthquakes from the selection available with different mechanisms and orientations, and divide them among the students.
- It will help to have students working on reverse faulting earthquakes and strike-slip faults with different strikes (N-S vs E-W).
- Reverse faulting earthquakes are very similar to model to normal faults (except for the reversed sense of motion and rake).
- All dip-slip earthquakes have reasonably simple deformation patterns in general, regardless of fault orientation, as they mostly cause vertical displacements of the surface, which InSAR is very sensitive to.
- N-S and E-W strike-slip faults are very different in their deformation patterns, as satellite-based InSAR is very insensitive to N-S displacements, but moderately sensitive to E-W displacements.
- These similarities and differences can be highlighted during the report out following the jigsaw exercise.
4) Comparing the source parameters across the class is a powerful way to bring up uncertainty in scientific findings. The plotting of histograms of the source parameters from students' approved models can work a couple of different ways:
- The values they write on their handouts can be collated as a class and plotted by the students themselves, either on their computers, or by hand. (This may be useful if you want to emphasize histogram plotting as a skill.)
- A quicker way to see the histograms is available within the Visible Earthquakes tool. The models they submit to the Visible Earthquakes database can be viewed within the web tool.
- In this case, it is imperative that all student submit their models under the same group name (otherwise, the results will not appear under the same group name within the tool). The names are case sensitive! A quick overview of the submission process is also given in the Visible Earthquake's Getting Started page.
- Make the group name unique and easy to spell (all one case, maybe one word, consider adding a date), and give it to the students in advance.
- Once an event has been modeled, a "Results" button will appear next to it on the main Visible Earthquakes page. Clicking on this will take you through to the histograms for each fault parameter. By default, all results from all submissions are shown.
- To see just the results from your group, click the "none" link toward the top right of the window, and then click on your group name.
- The buttons below the histogram allow you to choose which parameter (strike, dip, length, etc) is displayed on the histogram.
The rubric can be used to guide grading of the assignment. Unit 4 Grading Rubric (Microsoft Word 2007 (.docx) 107kB Dec1 15)
References and Resources
Primer on Focal Mechanism Solutions for Geologists by Vince Cronin
The following citations provide source parameter information for a selection of the earthquakes available on the Visible Earthquakes InSAR Tool.
- Aiquile Earthquake, Bolivia 1994
- Dinar, Turkey 1995
- Eyidoǧan, H. and A. Barka, The 1 October 1995 Dinar earthquake, SW Turkey, Terra Motae, 8, 5, 479–485.
- T.J. Wright, B.E. Parsons, J.A. Jackson, M. Haynes, E.J. Fielding, P.C. England, and P.J. Clarke, 1999, Source parameters of the 1 October 1995 Dinar (Turkey) earthquake from SAR interferometry and seismic bodywave modelling. Earth and Planetary Science Letters, 172, 1-2, 23–37.
- Damxung, China 2008
- Bie, L., Ryder, I., Nippress, S.E.J.k and R. Bürgmann, 2014, Coseismic and post-seismic activity associated with the 2008 Mw 6.3 Damxung earthquake, Tibet, constrained by InSAR. Geophysics Journal International, 196, 2, 788-803.
- Qiao, X., Yang, S., Du, R., Ge, L., and Q. Wang, 2011, Coseismic Slip from the 6 October 2008, M w6.3 Damxung Earthquake, Tibetan Plateau, Constrained by InSAR Observations. Pure and Applied Geophysics, 168, 10, 1749-1758.
- El Mayor Cucapah, Mexico 2010
- Oskin, M.E., Arrowsmith, J.R., Corona, A.H., Elliott, A.J., Fletcher, J.M., Fielding, E.J., Gold, P.O., Garcia, J.J.G., Hudnet, K.W., Liu-Zeng, J., and Teran, O.J., 2012, Near-Field Deformation from the El Mayor–Cucapah Earthquake Revealed by Differential LIDAR. Science, 335, 6069, 702-705.
- Wei, S., Fielding, E., Leprince, S., Sladen, A., Avouac, J.P., Helmberger, D., Hauksson, E., Chu, R., Simons, M., Hudnut, K., Herring, T., and Briggs, R., Superficial simplicity of the 2010 El Mayor–Cucapah earthquake of Baja California in Mexico. Nature Geoscience, 4, 9, 615-619.
- Southern California Earthquake Data Center - El Mayor Cucapah Earthquake
- Haida Gwaii, Canada 2012
- Landers, CA 1992
- Northridge, CA 1994
- Zhang, J., Kuge, K., Lay, T., and Tsuboi, S., 1997, Determination of earthquake source mechanisms using teleseismic 30–140 s waves: The January 17, 1994, Northridge earthquake. Journal of Geophysical Research Solid Earth, 102, B4, 8159-8169.
- Southern California Earthquake Data Center - Northridge Earthquake
- South Napa, CA 2014
- F. Guangcaia, L. Zhiweia, S. Xinjianb, X., Binga, and D. Yanan, 2015, Source parameters of the 2014 Mw 6.1 South Napa earthquake estimated from the Sentinel 1A, COSMO-SkyMed and GPS data. Tectonophysics, 655, 139–146.
- D.S. Dreger, M.H. Huang, A. Rodgers, T. Taira, and K. Wooddell, 2015, Kinematic Finite‐Source Model for the 24 August 2014 South Napa, California, Earthquake from Joint Inversion of Seismic, GPS, and InSAR Data. Seismological research Letters, 86, 2A, 327-334.