This document discusses using topographic position index (TPI) analysis and landform identification to locate geologic faults. TPI compares elevation of each land area to the mean elevation of surrounding areas, and identifies landforms like canyon bottoms, gentle slopes, steep slopes, and ridge tops. Comparing TPI results at different scales can filter out smaller non-fault features. The method successfully identified some fault-related landforms but had issues with scale. Future work involves refining the method and combining TPI with other data to more accurately locate faults.

1. Geologic Process Identification through Landform Analysis
Ian Thomsen

5. Topographic Position Index
From Jenness 2006

6. Inherently Scalable
Adjusts to capture features of different scales
•Quickly
•Easily
•Repeatable
Jenness, 2006

8. •Comparing elevation of each cell to the mean of all cells in an annulus of radius 100 cells (3000 meters) around it.
•Only interested in large, regional-scale features
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9. Slope Tool

10. TPI < -6 = Canyon bottom
-4 < TPI < 3 , Slope < 12° = Gentle Slope
-4 < TPI < 3 ; Slope > 12° = Steep Slope
3 < TPI < 6 = Steep Slope
6 < TPI = Ridge top
Raster Calculator

11. Focal Statistics Operations
Areas of statistical operation can be customized to the scale of interest
For example:
•4500m square window discards signal from smaller features
•19500m window only captures largest features

12. Identify all points which are classed as Steep Slope or Ridge Top at 4500x4500m scale but only class as Shallow Slope at 19500x19500m scale
•Filters out all features too large to be fault generated
•Removes smaller topographic relief
Raster Calculator

13. Pamir

21. Results
•Method successfully marks landforms which match characteristics of fault scarp structures
•Successfully removes other topography from consideration
But…
•Issues of scale

28. Results
•Defines SOME topographic relief as fault related, but not all
•MORE problems of scale

35. Results
Obvious problems:
•Where are the faults?! They SHOULD be around here!
Two possibilities:
•There is no fault here
Or
•The scale of features of interest is lost in more recent features

36. Problems identifying fault features with TPI
Extremely scale dependent
Scale must be uniform
Difficult to detect topography of interest within other geologic/erosional features

37. Implications of work
Check the accuracy of map data
Identify sites where fault-generated features are more and less likely to be found
Can be applied to any geological features whose scale can be isolated from “background” topography

38. Future work
Develop statistical methods to quickly combine
TPI
Lithologic changes
Seismic activity
Test method accuracy in the field
Refine algorithm
Apply TPI analysis to other geologic mapping projects