Aftershock imaging with a rapid response array for the April 14, 2025 magnitude 5.2 Julian, California earthquake

Binayak Parida, Abhijit Ghosh, Shankho Niyogi, Heather A. Ford, Guadalupe Bravo, Ashley Stroup, Adam Margolis, Axel J. Periollat, Shiori Nakaya, & Rebecca Leung

Submitted September 7, 2025, SCEC Contribution #14923, 2025 SCEC Annual Meeting Poster #TBD

On April 14, 2025, an Mw 5.2 earthquake occurred about 5 km south of Julian, California (U.S. Geological Survey). The hypocenter was about 14 km deep. Julian lies in the Peninsular Ranges between the Elsinore and San Jacinto fault systems. Around Julian, strands of the Elsinore and San Jacinto systems are close together and trend northwest to southeast. The traces step past one another, creating an interconnected fault corridor. The Elsinore fault has a slip rate near 5 mm per year, below the 20 to 25 mm per year of the southern San Andreas and generally lower than the San Jacinto. This setting benefits from dense short-term deployments that improve imaging of the nearby complex system of faults.

We installed 36 nodal seismometers to augment the existing permanent network. UC Riverside was the only entity that deployed the temporary nodal seismometers for this event. A node was installed within meters of the epicenter. Deployment began on April 16 and was completed within four days. The array then recorded continuously through May 19, providing about one month of data. Stations surrounded the mainshock to provide nearly complete 360-degree azimuthal coverage, which improves detection and location accuracy.

After quality control and preprocessing, we will create an AI-based earthquake catalog using the AI-PAL algorithm (Zhou et al., 2025), combining our nodal seismometers with the existing SCEDC stations in this region. Phase picks will be used to associate and locate earthquakes. Absolute locations will be refined with double difference relocations to reveal fine scale fault geometry. We will compare the AI-based earthquake catalog with the standard SCEDC catalog to measure gains in detection and any reduction in magnitude of completeness. The catalog will capture more small aftershocks, lower the magnitude of completeness, and improve locations.

With the resulting high-resolution catalog, we will carry out spatiotemporal analyses that include aftershock rate decay, clustering, and migrations. We will determine the geometry of the fault in the Julian region in detail and study the triggering of the Elsinore fault, which is about 7 km northeast of the epicenter. This catalog with improved locations and more detections will give us a clearer picture of the near fault structure in the Julian region and provide a framework for future studies.

Key Words
Julian earthquake, nodal seismometers, rapid response array

Citation
Parida, B., Ghosh, A., Niyogi, S., Ford, H. A., Bravo, G., Stroup, A., Margolis, A., Periollat, A. J., Nakaya, S., & Leung, R. (2025, 09). Aftershock imaging with a rapid response array for the April 14, 2025 magnitude 5.2 Julian, California earthquake. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology