A new study on the 2024 magnitude 4.8 earthquake near Tewksbury says the event likely ruptured a previously unmapped fault in the New Jersey Highlands rather than the better-known Ramapo Fault nearby.
The paper, first published Aug. 11, examined the source of the earthquake by combining aftershock data, field mapping, friction experiments and stress analysis.
On April 5, 2024, a "notable" earthquake and several aftershocks rocked the east coast Friday, federal officials said.
The initial magnitude 4.8 earthquake was detected about 10:23 a.m. with an epicenter near Lebanon, according to the United States Geological Survey, which monitors earthquakes. A magnitude 2.0 earthquake was detected about seven miles west of Bedminster at noon.
According to the study, the 2024 earthquake occurred near the roughly 300-kilometer-long Ramapo Fault, but that fault is "misoriented for slip in the current tectonic stress regime."
Researchers reported that the upward projection of relocated aftershocks aligned with a zone of NNE-striking topographic traces of fault scarps.
The paper says those features mark "a previously unmapped fault zone in the New Jersey Highlands," which the authors designated the Mountainville Fault.
The authors wrote, "We henceforth designate this fault as the 'Mountainville Fault.'"
The same summary states, "The upward projection of aftershocks aligns with the topographic (LiDAR) trace of a previously unmapped fault zone 3.5 km west of the epicenter."
The study also says that bedrock exposures along that lineament showed clusters of subparallel paleo-slip surfaces and fracture sets that the authors identified as signs of an immature fault.
The fault is described as having characteristics of a deeper blind fault core, with mapped surface structures representing Riedel shears rather than a fully developed surface-breaking fault.
It says the Mountainville Fault "lacks a breakthrough principal slip surface (core)," a feature the authors said is characteristic of an "immature" fault zone. The paper adds that laboratory friction testing and slip tendency analysis predicted "frictional instability" of the fault.
The authors also reported that the experiments predict earthquake occurrence mostly at aftershock depths and suggest possible concealed fluid-rock alteration at the shallower mainshock hypocenter.
The broader finding reaches beyond one earthquake.
The study says the Mountainville Fault's traits match other structurally investigated earthquake sources in the region, "demonstrating the seismic hazard of elusive, critically oriented immature intraplate faults."
The authors similarly wrote that "immature basement faults may constitute important seismic hazards in intraplate settings than previously thought."
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