EqSimage: A Python Package for Fault Imaging from Earthquake Similarity

Imaging of underground structures is a primary objective of all geophysical methods. In areas exhibiting natural or anthropogenic seismic activity, recorded and accurately located earthquakes can be used to map the faults that host them. Initial images can be further improved by the use of relative relocation techniques and seismic events with highly similar waveforms (multiplets).

In this work, we present EqSimage, a Python package designed to identify multiplets, perform their relative relocation using double-difference technique, and delineate potential fault planes. The package supports both continuous and triggered seismic data, which can be read directly from disk or downloaded from data centers using FDSN web services or ArcLink. Signal similarity is evaluated through cross-correlation of three-component seismic data. To distinguish groups of similar events, several clustering algorithms are available, including SciPy hierarchical clustering with cross-correlation coefficient as a distance metric and clustering based on pick times only. Subsequently, the double-difference relocation of all identified multiplets is carried out using the original hypoDD software by Waldhauser (2001), version 2.1b. Finally, the relocated events are divided into groups and best-fitting plane is determined for each group using the FaultNVC software (Sawaki et al., 2025).

EqSimage performs all processing steps automatically based on a single configuration file. The output includes a relocated earthquake catalog in QuakeML format and estimated fault-plane orientations. Additionally, several visualization tools are provided at individual stages of the workflow in order to assess the performance of configuration parameters. These tools include visualization of identified multiplets (waveforms and relocated hypocenters), cross-correlation matrices, relocated events, and inferred fault planes.

We demonstrate the capabilities of EqSimage using several datasets representing different types of anthropogenic seismicity, including injection induced seismicity, reservoir triggered seismicity and mining induced seismicity. Case studies are presented from The Geysers geothermal field (California, USA), the Song Tranh 2 water reservoir (Vietnam), and a seismically active underground mine in Poland.

 

References:

Waldhauser, Felix, hypoDD -- A program to compute double-difference hypocenter locations, U.S. Geological Survey Open-File Report 01-113, 2001.

Sawaki, Y., Shiina, T., Sagae, K., Sato, Y., Horikawa, H., Miyakawa, A., Imanishi, K., & Uchide, T. (2025). Fault Geometries of the 2024 Mw 7.5 Noto Peninsula Earthquake From Hypocenter-Based Hierarchical Clustering of Point-Cloud Normal Vectors. J. Geophys. Res.: Solid Earth, 130(4), e2024JB030233.

This research was supported by research project no. 2022/45/N/ST10/02172, funded by the National Science Centre, Poland, under agreement no. UMO-2022/45/N/ST10/02172. This work was also partially supported by a subsidy from the Polish Ministry of Education and Science for the Institute of Geophysics, Polish Academy of Sciences.