Giant layers of rocks below the ground in western New Jersey have been shifting slightly — sometimes once a day, sometimes a few times a day — all because of a sizable earthquake on April 5 that rumbled in Hunterdon County and shook houses, stores and office buildings hundreds of miles away.
The occasional shaking during the past two weeks has been caused by smaller earthquakes, known as aftershocks, and the number of those has now grown close to 100, according to data from the U.S. Geological Survey, the government agency that monitors earthquake activity around the world.
While many of the aftershocks in New Jersey are so small that few people have even noticed them, each quake is being closely monitored and providing vital clues to the USGS and a team of researchers from Rutgers University and several other universities.
The main mystery they hope to solve is this: Where exactly in the ground did the April 5 quake occur?
Based on past history and the initial data that emerged from seismometers, researchers are fairly certain that the initial magnitude 4.8 quake originated near the Ramapo fault, which runs diagonally from southeastern New York state across northern New Jersey and into eastern Pennsylvania.
However, no one knows for sure whether the quake occurred directly along the main part of the Ramapo fault or on one of the many fissures connected to the fault, according to the USGS and Rutgers researchers.
“The Ramapo fault actually splits into different segments that form the border between New Jersey’s Highlands and the Newark Basin,” said Ken Miller, a distinguished professor of geology in the Department of Earth and Planetary Sciences at Rutgers. “Sometimes it’s just lumped together and called the Ramapo fault system or the border fault.”
The main fault basically runs northeast to southwest, but some preliminary data indicated the initial quake in early April had a north-south movement, suggesting the trembler may have occurred on a “splay,” one of the branches that split off from the main fault, Miller said.
Dara Goldberg, a research geophysicist for the U.S. Geological Survey, said it’s not a lock that the Ramapo fault was the culprit on April 5. She noted the Ramapo is just one of several known faults — a fracture or zone of fractures between two blocks of rock — in the Hunterdon County area. Two others are the Tewksbury fault and the Flemington fault.
Dozens of earthquake sensors
To help pinpoint the source, strength and duration of the aftershocks, the USGS recently installed five new seismometers within a mile or two of Readington Township, the epicenter of the April 5 trembler. In addition, researchers from Rutgers, Columbia University, Yale University and the University of Texas are in the process of installing up to 100 smaller seismic sensors in the same general region, Miller said.
Those narrow cylinder-shaped devices are placed about 9 inches into the ground, and can detect the location and magnitude of even the smallest earthquakes.
Miller said all of those sensors should be in place within a few days, and most will remain in New Jersey for a few weeks. The five USGS sensors are expected to operate for several months.
The USGS monitoring devices, known as “aftershock kits,” were deployed a week and a half ago and “record strong motion and high frequency,” the agency noted.
“The sensors will send data in real time back to the USGS’s National Earthquake Information Center in Golden, Colorado for analysis,” the agency said. “Some of the ‘kits’ will stand alone and others will be co-located with already-existing stream gauges monitored by USGS’s New Jersey Water Science Center.”
Miller and Goldberg said each aftershock will provide clues into which specific fault or section of a fault caused the initial earthquake.
“Knowing details about the earthquake source addresses many research questions,” Goldberg noted. “For one, it helps us to understand the state of stress within the Earth’s crust in this location. Identifying more aftershocks helps to gain insight to the causative fault, as well as the structure of that fault with increasing depth.”
“Understanding where earthquakes occur in the past can help improve seismic hazard models and indicate where earthquakes could happen again in the future,” she added, “both by using seismicity and delineating fault extents.”
More aftershocks to shake N.J.?
The USGS’s current forecast estimates a 1% chance of a magnitude 4 or larger aftershock and a 9% chance of a magnitude 3 or higher aftershock in the next month. Over the next year, there’s a 3% chance of a magnitude 4 or higher and a 21% chance of a magnitude 3 or higher.
Goldberg said the strongest aftershock in New Jersey so far — a magnitude 3.7 that occurred in the early evening on the same day as the initial quake — generated more than 12,000 reports from people who felt it. (That trembler was originally reported as a magnitude 4.0 but was later downgraded to 3.7 after new data was analyzed.)
But some of the smaller aftershocks were also felt. “We have received tens to hundreds of DYFI (Did You Feel It) responses for many of the aftershocks in the magnitude 2 range as well,” Goldberg noted.
The DYFI system allows citizens to report whether they felt any shaking, how strong it was and whether any damage occurred after any earthquake.
“There are many factors that go into whether a person will feel an earthquake,” Goldberg said. “One factor is magnitude, of course. Another is how deep the earthquake is (shallow earthquakes are felt more strongly). It also depends on the geology underfoot and what you might be doing when the earthquake strikes.”
The magnitude 4.8 quake on April 5 was the strongest earthquake centered in New Jersey since 1938, when an early morning quake of the same magnitude shook the Trenton area. The 1938 quake was felt as far north as Jersey City and as far south as Delaware.
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Len Melisurgo may be reached at LMelisurgo@njadvancemedia.com or on X at @LensReality.
