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The Seismic Sources of Northeastern US: New Insights Into Their Detailed Geological Structure and Reactivation Mechanics

Description: 

Major seismic events along the northeastern US commonly rupture previously unmapped basement-hosted faults, among which only a few have been subsequently studied in detail. The high population density of this region elevates its seismic risk. Availability of bedrock outcrops in the vicinity of seismicity provides a natural laboratory to explore how the nature of seismic sources and the mechanics of strain release implicate the seismic hazards of stable continental regions. We focus on the Ramapo Seismic Zone in New York-New Jersey, which hosted the2003 Mw3.4 Milford, NJ earthquake, and the recent 2024 Mw4.8 Tewksbury, NJ earthquake, and evaluate the relationships between seismicity clusters, hypocentral slip planes, and the geological structure of the [projected] surface exposures of the ruptured faults. Further, we analyze over three decades of local seismicity predating 2024 with wave paths entirely in the upper crust (<10 km) to explore the relationships between the ENE-WSW regional maximum horizontal compressive stress, basement discontinuities, and fault reactivation. In this talk, we will show results demonstrating that: 1) the causative faults of these earthquakes are oriented NNW-to-NNE or NW-SE, striking at high angles to the presumably inactive large Ramapo Fault, 2) the surface exposures of the causative faults are characterized by gougeless, poorly coalesced paleo-slip surfaces, indicating rough, immature faults, and 3) NNW-to-NNE fast shear wave polarization directions dominate the upper crust of the seismic zone, along with WNW-ESE and NE-SW secondary trends. Most intriguing, our results show that events with propagation paths that sample the source region of the 2024 Mw4.8 Tewksbury earthquake have ~NNE-striking dominant fast-splitting direction parallel to the ruptured fault, indicating aseismic shear dilatancy of the fault zone prior to the Mw4.8 event. The seismic anisotropy-based illumination of critically stressed, potentially seismogenic, elusive immature faults presents a promising direction for seismic hazard mitigation in stable continental regions.

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

Type: Oral

Date: 4/17/2025

Presentation Time: 10:45 AM (local time)

Presenting Author: Folarin

Student Presenter: No

Invited Presentation: Yes

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