The 2024 Mw4.8 New Jersey Intraplate Earthquake: Preferential Rupture of an Immature Fault in Frictionally Unstable Basement Rocks

Abstract

Intraplate regions commonly host energetic earthquakes on less‐prominent faults, raising questions on how fault structure may influence intraplate seismogenesis. We investigate the causative fault of the 2024 Mw4.8 New Jersey earthquake which occurred near the misoriented 300‐km long Ramapo Fault, by integrating seismicity‐guided field geological mapping with friction experiments and slip tendency analysis. We find that the up‐dip projection of relocated aftershocks aligns with a zone of NNE‐striking topographic (LiDAR) traces of fault scarps, marking a previously unmapped fault zone in the New Jersey Highlands, henceforth designated as “Mountainville Fault (MF).” In basement outcrops along the topographic lineaments, we observe a predominance of NNE‐striking/E‐dipping, slickensided, gougeless fracture clusters with horsetail tip‐splays, indicating a brittle shear zone origin. The fractures are poorly coalesced, and comprise steeply‐dipping sets that are crosscut by gently‐dipping sets. At the surface, MF lacks a break‐through principal slip surface (core), and the geomorphic scarps and fractures strike at ∼15–25° obliquity to the mainshock's nodal plane, suggesting that the field‐mapped structures represent the Riedel‐shears of a deeper blind fault core, characteristic of an “immature” fault zone. Laboratory‐constrained frictional behavior of sampled representative fault rocks, and slip tendency computation from focal mechanism‐resolved stress field predict frictional instability of the MF. Additionally, the friction experiment results suggest a possible contribution of concealed fluid‐rock alterations at the depth of the mainshock. Altogether, these MF characteristics fit those of other structurally investigated earthquake sources in the region, demonstrating the seismic hazard of elusive, critically‐oriented immature intraplate faults.