Some quakes already felt in Rhode Island
By Kevin Krajick, Columbia Climate School
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| Peak ground velocity from a magnitude 4.8 earthquake under Tewksbury township, N.J., (gold star) on April 5, 2024. Shaking was relatively weak at the epicenter, but spread unexpectedly far, predominantly to the northeast. Credit: Won-Young Kim/Lamont-Doherty Earth Observatory |
On April 5th, 2024, a magnitude 4.8 earthquake struck
northern New Jersey’s Tewksbury township, triggering widespread alarm. Although
the region occasionally experiences small tremors, this was the largest since
1884, when an earthquake of approximately magnitude 5 struck under the seabed
off Brooklyn.
According to existing models, the earthquake should have
caused substantial damage at its epicenter, yet it remained largely unscathed.
In contrast, relatively distant New York City shook much harder than expected,
resulting in minor damages. Outsize shaking extended all the way to Virginia
and Maine. A recent study suggests why this happened, challenging existing
assumptions about regional earthquake hazards.
“There was some peculiar behavior,” said study coauthor Won-Young Kim of the Columbia Climate School’s Lamont-Doherty Earth Observatory
While 4.8 is not a major quake in global terms, people in
the highly populous U.S. Northeast are not used to anything that big. The U.S.
Geological Survey (USGS) estimates it was felt by some 42 million people; a
USGS online portal that
crowd-sources first-person reports of shaking received nearly 184,000
entries―the most ever from any U.S. quake, according to a
companion paper about the event. Both papers were recently published
in the journal The Seismic Record.
Surprising Findings at the Epicenter
Hours after the quake, Kim and colleagues headed to the
epicenter to survey the situation. “We expected some property damage―chimneys
knocked down, walls cracked or plaster fallen, but there were no obvious
signs,” said Kim. “We talked to police officers, but they were not very excited
about it. Like nothing happened. It was a surprising response for a magnitude
4.8 earthquake.”
The surface motion generated by earthquakes is measured on
the Modified
Mercalli Intensity Scale. Based on the magnitude, the quake’s depth (a
fairly shallow 5 kilometers, or 2.9 miles), and area geology, existing models
posit that a 10-kilometer area around the epicenter should have seen intensity
VII shaking on this scale, described as “very strong.” Most well-designed and
built structures would probably get off without much damage, but others of
lesser design or materials could collapse, especially unreinforced masonry
walls and chimneys.
However, no one at or around the epicenter reported
intensity VII shaking or anything close to it. Damage was limited to minor
cracking in some drywall and a few items knocked off shelves. The only
exception: an already crumbling grist mill built in the 1760s of unreinforced
stone, and already largely a wreck. About 3.5 miles from the epicenter, part of
the mill’s facade toppled.
Assessing Regional Earthquake Patterns
Usually, earthquake shaking fades out in a more or less
symmetrical bull’s eye pattern from the source. But that did not happen either;
stronger-than-expected shaking extended far out, mainly to the northeast, and
to a lesser extent other directions.
In Newark, N.J., some 20 miles from the epicenter, three row
houses were partly toppled, and dozens of people had to be evacuated. Residents
of New York City, 40 or 50 miles away, reported intensity IV motion, with
sustained vibrations of windows, doors, and walls. More than 150
buildings reported
minor damage, mainly superficial cracks in masonry.
However, inspectors ordered two Bronx buildings to erect
protective sidewalk sheds when cracks appeared in their facades, and a Brooklyn
public school had to close
its gym for repairs because of vertical step-shaped cracks along an
interior wall. Gas and
water lines developed leaks as far off as the lower Hudson Valley, and
on Long Island, the front of someone’s Jeep slumped into a suddenly opened
sinkhole. Even people in parts of New Hampshire, some 280 miles away, reported
intensity III shaking, similar to a big truck passing by.
Geological Insights and Fault Analysis
To understand what happened, Kim and colleagues at South
Korea’s Seoul National University analyzed so-called Lg waves. These are a type
of low-frequency wave of energy that bounces back and forth between the Earth’s
surface and the Moho―the
boundary between the Earth’s crust and the mantle, which in this area lies
about 35 kilometers down. The analysis suggested that the quake occurred on a
previously unmapped fault that runs south to north. The fault is not vertical
but rather dips eastward into the Earth at about a 45-degree angle.
According to the analysis, the movement was rapid and
complex―a circular combination of the two sides of the fault sliding
horizontally against each other (known as strike-slip motion) and one side also
shoving itself up and over the other (known as a thrust). Once the rupture
started, it spread horizontally to the north. Usually much of the energy from
such a quake takes the path of least resistance―that is, straight up, to the
surface, where pressure on the rock is the least. That is what makes the epicenter
the most dangerous place to be.
That was not the case here, the researchers say. Instead, much of the energy headed downward, along the fault’s dip, and continued until it hit the Moho. Then it bounced back up, emerging among other places under New York City, which was right in the way. Then the wave bounced back down and re-emerged further away in New England, somewhat weaker, and so on, until it petered out. The long-distance echoes were likely strengthened by the fact that most rocks underlying this region are hard and dense, and conduct energy efficiently, like the ringing of a bell.
Historical Perspective on Seismic Activity
The area from Philadelphia to southwestern Connecticut has
seen some 500 known quakes from the 1600s to the present, but many others have
almost certainly gone unnoted before modern seismic instruments came along.
Most are so faint that few, if any, people feel them, and the vast majority of
other quakes have been harmless. However, the threat could be greater than
previously thought, according to an
earlier paper led by Lamont-Doherty seismologist Lynn Sykes.
Long-Term Seismic Risks Reassessed
These quakes are not caused by ongoing movements of giant
tectonic plates like those in much more hazardous places like California.
Rather, they emanate from ancient fault zones dating as far back as 200 million
years, when what is now Europe tore away from what is now North America,
cracking up the subsurface with massive earthquakes. Some of these crumbly
zones are still settling and readjusting themselves, and occasionally parts of
them move with a jolt.
Based on the short historical record, earthquakes the size
of April’s or slightly larger occur roughly every 100 years. But based on the
sizes of known faults and other calculations, Sykes et al. have suggested that
the area could see a magnitude 6 every 700 years, and a magnitude 7 every 3,400
years. The magnitude scale is exponential, so a magnitude 6 is 10 times more
powerful than a 5, while a magnitude 7 is 100 times more powerful than a 5. No
one knows if such quakes have occurred in human time or could, but if one did,
it would be catastrophic.
Ongoing Research and Future Implications
The April 5 quake has brought about a spurt of new research.
In cooperation with the USGS and other researchers, Kim helped place a
temporary network of dozens of seismometers near the epicenter to monitor
aftershocks, which continued for weeks. These signals are being used to better
map various details of the quake and the area’s faults.
Lamont-Doherty structural geologist Folarin Kolawole and
colleagues have been mapping
numerous bedrock fractures near the epicenter caused by past
earthquakes of indeterminate ages. These could well be millions of years old,
says Kolawole, but they could also point to current, unmapped zones of weakness
lurking below.
Meanwhile, Lamont-Doherty geologist William Menke is working
to document
possible prehistoric quakes in the more recent past. New York’s
Harriman State Park, just over the border from New Jersey, is littered with
giant boulders dropped onto the surface when glaciers from the last ice age
melted, some 15,000 to 20,000 years ago. Many are precariously balanced in
their original positions. Menke’s hypothesis: if he can calculate the
earthquake force that would be required to tip the boulders over, he can rule
out an earthquake of that size, at least for that time period.
Kim said the new study suggests the need to re-evaluate how shaking from any future sizable quake may be distributed across the region. “Some that are not even that big could maybe focus energy toward population centers. If [the April] earthquake was just a little stronger, or a little closer to New York City, the effect would be much greater,” he said. “We need to understand this phenomenon and its implications for ground motion prediction.”
Reference: “Rupture Model of the 5 April 2024 Tewksbury, New
Jersey, Earthquake Based on Regional Lg‐Wave Data” by
Sangwoo Han, Won‐Young Kim, Jun Yong Park, Min‐Seong Seo and
YoungHee Kim, 18 September 2024, The
Seismic Record.
DOI:
10.1785/0320240020
