From Hypocentral Seismicity Patterns to 3D Fault Models

Travis V. Alongi, Austin J. Elliott, Rob Skoumal, Alex Hatem, & David R. Shelly

Published September 8, 2024, SCEC Contribution #13674, 2024 SCEC Annual Meeting Poster #120

Three-dimensional (3D) fault geometry plays a crucial role for understanding earthquake behavior and refining seismic hazard models. However, detailed subsurface fault structures are often poorly constrained due to low resolution imaging methods or the absence of geophysical imaging observations. Along some fault systems, abundant microseismicity appears to delineate fault structures at seismogenic depths, providing information about their position and shape. Here, we present an approach to construct 3D fault geometries from hypocentral seismicity patterns. Our method integrates established machine learning techniques to cluster events based on their spatial density. Subsequently, to model long and continuous fault surfaces, clusters are linked according to their 3D spatial orientation and relative positions. The linked clusters are then fit with a functional form that best represents the observed seismicity patterns. Finally, we evaluate the resulting fault surfaces by assessing the goodness of fit of a modified power law to the spatial distribution of seismicity relative to the modeled surface. The developed workflow provides a low-user-input solution for reconstructing 3D fault geometries at depth from readily available earthquake catalogs and provides the opportunity to explore geometric controls on earthquake behavior.

As a proof of concept, we determine the fault geometries at the San Andreas-Calaveras Fault junction in Central California. Our results agree within +/- 400 meters with previously published 3D fault models constrained by methods other than aligning along seismicity (Brocher, 2005; Field et al., 2014). We further tested the applicability and limitations of the algorithm in areas with varying earthquake densities, as well as in regions of differing apparent fault complexity at depth.

Citation
Alongi, T. V., Elliott, A. J., Skoumal, R., Hatem, A., & Shelly, D. R. (2024, 09). From Hypocentral Seismicity Patterns to 3D Fault Models. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)