Unit 3: Geodetic survey of a fault scarp
In this unit, students will design a survey (TLS and/or SfM) of a fault scarp. After conducting the survey in the field, students will analyze the data to identify the number and magnitude of possible fault displacement(s) by measuring offsets in the point cloud as well as calculate the recurrence interval of the fault based on either a known age or scarp morphometric age (or both). The goal is to create a brief report summarizing the methods used and Quaternary history of displacements on the fault. An optional extension exercise (Unit 3.5) has the students conduct a hillslope diffusion analysis is using MATLAB. Fault scarps are the topographic evidence of earthquakes large and shallow enough to break the ground surface, and are evidence of Quaternary fault activity. A primary goal of studying exposed scarps is to gain insight into the magnitude and frequency of fault slip. Scarps typically begin as step-shaped landforms and deteriorate with age through erosion. In some cases, the form of the scarp may record evidence of more than one earthquake, distinguished by a change in scarp slope. Assuming the same surface processes, the relative age of fault scarps can be determined by their morphology (shape).
Unit 3 Learning Outcomes
Students are able to:
- Design and conduct a geodetic survey of a fault scarp and understand the basic principles of earthquake faulting and erosion modeling of topographic profiles to estimate fault deformation history
Use existing geodetic data set of a fault scarp and understand the basic principles of earthquake faulting and erosion modeling of topographic profiles to estimate fault deformation history
- Articulate the societally important impetus for fault scarp analyses
Unit 3 Teaching Objectives
- Cognitive: Facilitate design a more complex geodetic survey and apply geodetic survey methods to a fault scarp analysis.
- Behavioral: Promote student ability to set up and run a more complex geodetic survey with necessary supporting equipment and observations.
Context for Use
The content in Unit 3 was designed for upper-level geoscience majors in a course with field components. It can work in a field camp over the course of approximately one long day of field work and data analysis. It can also work in an academic year course such as structural geology, tectonics, geomorphology, geophysics, remote sensing/GIS, or field methods. It can be conducted over approximately one lecture (introduction to geodesy to study fault scarp analysis) and two labs (conducting a small survey of a feature, exploring and interpreting the collected data). Another option is to collect data during a one- or two-day field trip, followed by data processing and analysis during subsequent class periods. If a LiDAR scanner or SfM collection platform is not available, this may be used in a classroom setting with a prepared data set. The material works well for a group of approximately twenty (or fewer) students with an instructor and teaching assistant/s. The ideal number of students for this exercise using TLS is twelve, as they can break into four teams of three, which means each feature will have four scan positions (doable with a group of students in eight to ten hours) and gives each student time for hands-on scanner time. The number of students is more flexible using SfM, as data may be collected on multiple platforms at the same time so many students can have hands-on data collection time. If both survey techniques are being used, larger classes can be accommodated but an additional teaching assistant might be helpful. Student experience with field observations, field maps, and trigonometry, along with other basic calculation skills is expected. In a field course, this unit is ideally situated mid-way through the course, as students will already have some field experience. The details of designing and conducting a survey are covered in Unit 1-TLS and Unit 1-SfM, so Unit 1 (for the technique/s of interest) is a necessary precursor to Unit 3. It is not necessary to complete Unit 2 in order to do Unit 3. Unit 3 and Unit 3.5 ("Optional exercise") cover the same topic, but Unit 3.5 focuses on using MATLAB for hillslope diffusion analysis. Unit 3.5 can replace Part D of the regular Unit 3 student exercise, if the instructor wishes for a more sophisticated analysis. Unit 5 is the summative assessment for the module.
Description and Teaching Materials
1) Geodetic survey
Prior to departing for the field site, check equipment against the equipment list to ensure all pieces are present. Students will need all their typical field supplies, including a calculator. Upon arrival to the site, introduce the area of interest and give a brief background. Facilitate a group discussion about the scarp: Introduce the process of scarp formation, including some discussion of transport-limited versus weathering-limited scarps. What kind of faulting created this scarp? Can vertical fault scarps form from strike slip faulting? Why use geodetic survey techniques to characterize a fault scarp? This introduction to the site should conclude with a group discussion of the project goals. With these goals in mind, students should determine the ideal camera locations / collection path or scanner positions as well as target placements. Students should then break into small teams of three to five people to set up the scanner (if doing a TLS survey), targets, and GPS, like they did in Unit 1 and/or Unit 2. The team that set up the scanner for Unit 1 and/or Unit 2 should set up targets so more students get hands-on experience with the scanner. If doing a TLS survey, teams will rotate and conduct a minimum of one scan each. Completing the scan resolution parameters worksheet should be optional for this unit. If doing an SfM survey, students should rotate (ideally in pairs) to operate the collection platform. While not collecting survey data, a team should be working on material for their write-up, a site map of the survey including target/GPS locations, scanner positions or camera locations / collection path, recording metadata, calculating scan resolution parameters on their provided worksheet, and taking field notes, like those they would take at any other geologic outcrop. If more time-filling activities are needed, provide a previously collected profile of the scarp to students so they can measure the scarp elevation at 28 points to input into the provided hillslope diffusion modeling Excel sheet. After conducting the survey, take down equipment and inventory prior to leaving the field site. This portion of the unit should take approximately eight to ten hours for a TLS survey and four to six hours for an SfM survey, although these estimates depend on the scale of the scarp.
2) Data exploration and interpretation
After collecting survey data, students will then visualize and analyze the data they collected in the field. If doing a TLS survey, this can be done immediately after returning from the field. If doing an SfM survey, an eight-to-twelve-hour gap is needed to do the processing to generate the model based on the field photos. Students or the instructor should begin running the model upon returning from the field. Give a brief presentation (~10 minutes) covering the scientific background for the fault scarp analysis, including the measurements students should make of the profiles. After this, attach a projector to a laptop to open the program and walk students through the data visualization process, specifically how to crop the data to examine cross-sectional profiles of the scarp. It is best to look through the included Data Processing and ExplorationManual prior to this part of the unit. If students did Unit 2, a comprehensive review of the software is unnecessary because they will use the same data exploration skills used in that unit. To familiarize students with the hillslope diffusion, Part B of the student exercise is an introduction to hillslope diffusion worksheet. If under time constraints or students are familiar with hillslope diffusion and associated calculations, Part B can be skipped.
If doing a TLS survey, students should first colorize the point cloud data using the RGB imagery collected during the survey. They can then choose how they would like to project the data. It is most helpful to color points by true color for this unit. For both survey types, data should be rotated to an ideal orientation to represent true fault geometry as determined by each individual student (or pairs/teams of students, if computers are limited) based on their field data. After this, students should measure the displacement on the fault: If weathering-limited (i.e., a bedrock scarp), this is a simple measurement of the amount of exposed bedrock. If transport-limited, students must extrapolate displacement from the length of the scarp slope and the angle of the slope. If the scarp shows characteristics of both, have the students use a combination of these methods. Students will do these measurements on three profiles. They will also need to collect profile data to input to Excel (28 measurements) if the scarp is transport-limited.
These measurements are used to either model the recurrence interval (weathering limited) or the hillslope diffusion (transport limited). Based on these calculations, students can write a brief summary of the history of the fault, including the displacement, earthquake magnitude, and the estimated recurrence interval.
Unit 3.5 - Alternative hillslope diffusion analysis using Matlab rather than Excel
Unit 3.5 works best as a substitute for Part C of the regular Unit 3 student exercise. It covers the same material by with a more sophisticated analysis. Student experience in the programs ArcGIS and MATLAB would be helpful, but not required, as students will learn all the skills necessary to complete this unit in the unit. More instructor notes for Unit 3.5 can be found in the file below.
- Introduction to Fault Scarps (PowerPoint 2007 (.pptx) 34.9MB Oct6 23)
- Overview of methods of analyzing fault scarps
- Primary student assignment files:
Unit 3 Student Exercise - Fault Scarp Survey (Microsoft Word 2007 (.docx) 1.5MB Oct6 23)
Unit 3 Student Exercise Part B (Microsoft Word 2007 (.docx) 146kB Jul17 16)
Excel sheet for hillslope diffusion (Excel 2007 (.xlsx) 151kB Mar28 16)
- Includes assignment sheet, sketch page, notes page, and scan resolution parameters worksheet (included for TLS surveys only)
- Model of hillslope diffusion in Excel; students may input their profiles from the field or pre-measured profiles (see below prepared data sets).
- Optional student assignment files:
Unit 3.5 Student Exercise - Optional hillslope diffusion with Matlab (Microsoft Word 2007 (.docx) 1.3MB Oct6 23)
Unit 3.5 - Optional exercise instructor overview (Microsoft Word 2007 (.docx) 154kB Oct6 23)
Unit 3.5 optional exercise MATLAB code (Zip Archive 79.5MB Mar28 16)
- MATLAB version of hillslope diffusion; use with more computationally advanced students. This is best used as a substitute for Part C of the regular Unit 3 student exercise.
- SfM Field Methods Manual for Students (Microsoft Word 2007 (.docx) 6.8MB Jun28 23)
TLS Field Methods Manual (Microsoft Word 2007 (.docx) 32MB Oct6 23)
- Includes considerations for survey design, an equipment list for students to fill in, and specifics of TLS or SfM data collection. Applies to all units.
- SfM Data Processing and Exploration Manual (Agisoft Metashape software) (Microsoft Word 2007 (.docx) 14.5MB Jun29 23)
TLS Data Processing and Exploration Manual (RiScan Pro software) (Microsoft Word 2007 (.docx) 5.2MB Oct6 23)
- General guides to using the software for data exploration and some processing. Applies to all units.
- Riegl TLS Field Operation Manual and Workflow (Acrobat (PDF) 7MB Oct19 16)
- This workflow document was developed by EarthScope for TLS field engineers using Riegl VZ400 scanners. It is largely applicable to several other Riegl scanners. While additional training and experience is needed to independently run a Riegl scanner, it can be a useful reference for instructors to understand the basics of Riegl scanner operation. If updates are published to this document, they will be available on the GAGE Knowledge Base(search "TLS"). Applies to all units.
- Prepared data sets These include a data set and contextualization information to provide to students if field work is not possible.
- Hebgen fault scarp data
Profile of Hebgen Lake fault scarp 1957 and 2008 (Excel 2007 (.xlsx) 10kB Mar30 16)
- Includes profiles of a scarp at Hebgen Lake from 1959 and 2008.
- Landers fault scarp data (SfM) (Zip Archive 1238.5MB Jul3 23), Landers, California
- The Landers data is a set of photographs of the Landers fault scarp.
- El Mayor Cucapah fault scarp - simple photo set (SfM) (Zip Archive 66.9MB Jun30 23), El-Mayor Cucapah Earthquake Fault Scarp, Baja, California
- Includes a set of photographs of the El-Mayor Cucapah Fault Scarp
- Hebgen fault scarp data
Teaching Notes and Tips
General advice on making the module work in field courses can be found on the module Overview page.
Fault scarps are frequently covered in vegetation. If this is the case for the scarp you survey with the students, use the section included in the Data Processing and Exploration Manual to guide students on how to remove vegetation from the scarp to make it clear for analysis.
SfM does not "see through" vegetation in the same way that TLS does, so it may be difficult or impossible to clean this data in the same way. As such, scarps with less vegetation are ideal for SfM surveys. If a clean scarp is not available, have students take inaccuracies due to vegetation into account in their work.
If you wish to have students compare point clouds or triangular meshes generated from the two different techniques (TLS and SfM), an open source software that may be of assistance is CloudCompare. A short overview of CloudCompare can give you sense for its general capability, while the main CloudCompare website includes several tutorial videos. An OpenTopography workshop "Applications of High Resolution Topographic data to the Earth Sciences" offers several hours of video instruction on the use of the software on second day of the workshop program and a general tutorial is also available: CloudCompare General Tutorial (Microsoft Word 2007 (.docx) 14.5MB Jun29 23).
If Internet access is available, you may have students view the area of interest the evening before. Groups can design the survey (placing targets and scan positions) in Google Earth using imagery and then print their design or load onto a tablet. After viewing the field site in person, students can discuss the validity of their original design and update as needed.
Much of the formative assessment can be done through observations of and discussions with students individually, in pairs, or periodically in the whole group. Students can also hand in their work from the field, including field notes (atmospheric conditions, metadata) and sketch of survey setup. The work for formative assessment could be graded based on completion.
Summative assessment for Unit 3 is based on the completed student exercise. The exercise includes a section on the motivation for doing the survey and design and a section on the recent history of the fault. An assessment rubric is included in the student exercise. Summative assessment for the module as a whole will be evaluated at the end of the module in Unit 5. If students complete Unit 3 in addition to Unit 1, the Unit 5 topics we recommend that you choose from are "sequence stratigraphy," "channel sands," and "dinosaur footprints."
References and Resources
- Stewart, I.S. and Hancock, P.L., (1990) What Is a Fault scarp?: Episodes, 13 (4), 256–263.
- Wallace, R.E., (1977) Profiles and Ages of Young Fault Scarps, North-central Nevada: GSA Bulletin, 88, 1267–1281.
- Wallace, R.E., (1980) Degradation of the Hebgen Lake Fault Scarps of 1959: Geology, 8 (5), 225-229.
- Wells, D.L. and Coppersmith, K.J., (1994) New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement: Bulletin of the Seismological Society of America, 84 (4), 974–1002.
- Introduction to fault scarps
- Scarp definition: Stewart, I. and Hancock, P.L., (1990), What Is a Fault Scarp?: Episodes, 13 (4), 256-263. This paper explains what a fault scarp is, what fault scarps look like as a result of different types of faulting, and other morphological characteristics of fault scarps.
- Fault scarp analysis for research applications
- Avouac, J.-P., (1993) Analysis of Scarp Profiles: Evaluation of Errors in Morphological Dating: J. Geophys. Res., 98, 6745–6754.
- Haddad, D.E., Akciz, S.O., Arrowsmith, J.R., Rhodes, D.D., Oldow, J.S., Zielke, O., Toke, N.A., Haddad, A.G., Mauer, J., and Shilpakar, P., (2012) Applications of Airborne and Terrestrial Laser Scanning to Paleoseismology: Geosphere, 8, 771-786.