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Imaging Intraplate Faults in Puerto Rico With New Aeromagnetic Data: The Roles of Fault Reactivation and Tectonic Inheritance

Description: 

Puerto Rico experiences damaging shallow (< 10 km) intraplate earthquakes, such as the 2019-ongoing earthquake sequence (up to Mw6.4) in the SW part of the island, as part of a complex deformation zone associated with oblique subduction ~200 km to the north. We use new high-resolution airborne magnetic and radiometric data over Puerto Rico and the surrounding shelf, mapped Quaternary-active faults, and a new geologic map compilation to better understand relations between intraplate seismicity and geologic features.

Aeromagnetic data show a dominant NW-SE fabric, with most lineaments trending N55W-N80W, but with exceptions. The Great Northern and Great Southern Puerto Rico fault zones (GNPRFZ and GSPRFZ, resp.) form the island’s structural framework; they are expressed as complex, 4-8-km wide zones of parallel and sub-parallel lineaments that terminate to the west in curved patterns resembling horsetail structures. The GNPRFZ extends farther east than previously mapped curving to the north to trend ENE near and past Culebra Island. The GSPRFZ appears to extend offshore to the SE near Salinas, possibly beyond the survey area.

Contrasting that dominant NW-SE structural grain, Quaternary-active fault mapping (based on lidar, aerial photography, and various field observations) shows the importance of ~E-W-striking faults, such as the previously mapped Salinas and South Lajas faults and additional faults mapped in the Lajas Valley and near the southern part of the GSPRFZ. These faults are rarely aligned directly with magnetic anomalies along their full length; they more commonly overlap small sections of larger NW-SE-trending anomalies, especially where mapped at the surface. A key exception is the N70W-striking Punta Montalva (PM) fault, which is well-aligned with a >20 km-long magnetic lineament extending from offshore to La Parguera; we note that the PM fault was seismically active along 5-6 km of its length during the 2019-ongoing sequence. Comparisons between these different datasets suggest the importance of tectonic inheritance, where pre-existing zones of weakness are reactivated in an evolving stress regime.

Session: Earthquakes, Lithospheric Structure, and Dynamics in Stable Continental Region - I

Type: Oral

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Anjana

Student Presenter: No

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