Open-source tools for making geometrically-complex fault surfaces

Three--dimensional fault geometries are critical for realistic analyses of crustal deformation, seismic hazard, and rupture dynamics, yet 3D fault datasets are sparse outside a few well--studied regions. We present a set of free and open-source QGIS plugins that enable users to construct accurate 3D fault surfaces from common types of geological and geophysical data. The workflow combines (1) CrossSectionDigitizer, for georeferencing and extracting fault geometries from map-referenced cross--sections; (2) Contours2Mesh, for generating triangular meshes from manually drafted contours; and (3) a modified Qgis2threejs plugin for interactive 3D visualization. Together, these tools allow geoscientists to build complex, non-planar fault surfaces in a familiar GIS environment without specialized modeling software. The methods are illustrated with examples from the Yakima Folds Province.