Sunday, April 7, 2024

New Jersey Earthquake (Part II)

In Part I of this post, I introduced the recent (April 5) New Jersey earthquake felt here in the Hudson Valley. Let's now talk a little about the geology.

The east coast of the U.S. has a long history stretching back almost 1.5 billion years (compared to the 4.5 billion year history of the Earth as a whole). During that time, our area experienced four major mountain-building events (which geologists call orogenies from the Greek work for mountain - oros). These mountain building events were due to collisions of crust from plate tectonic movement. Most of those mountains have since weathered away leaving remnants of rock that formed miles below the surface. There were also two major rifting events in that time period as blocks of crust pulled away again after those collisions (sometimes they also stuck around!).

All of these collisions and rifting events left folded rocks and faults. Many people are under the mistaken impression that faults only occur out west but there are faults you can see in rock outcrops on the side of the road anywhere in the Hudson Valley. But, unlike those currently active faults out west (like the infamous San Andreas), most of the faults here in our area formed hundreds of millions of years ago and have been inactive for a long time. But, as we'll see, sometimes they can be reactivated.

Most active faults are found near tectonic plate boundaries. The San Andreas, for example, is basically the plate boundary between the North American Plate and the Pacific Plate.

Here on the east coast, however, we're a couple thousand miles from the nearest plate boundary which is in the middle of the Atlantic Ocean at the Mid-Atlantic Ridge which separates the North American Plate from the Eurasian and African Plates. However, as tectonics plates jostle each other at the plate boundaries, stresses are transmitted into the interior of the rigid plates. These stresses can sometimes trigger preexisting fault movement to form what are called intraplate earthquakes (earthquakes in the middle of a tectonic plate).

Typically, these intraplate earthquakes aren't very large, but occasionally moderate ones can occur and very occasionally large ones (look up the 1811/1812 New Madrid, MO or the 1886 Charleston, SC quakes).

OK, so now let's look at New Jersey. New Jersey is made up of several physiographic provinces.

As you can see, the boundary between the Highlands and the Piedmont regions is a fault called the Ramapo Fault.

The NJ Highlands region extends northeastward in the Hudson Highlands of New York and represents rocks that formed miles below an ancient Himalayan-scale mountain belt formed during an event called the Grenville Orogeny over a billion years ago.

The Piedmont region is underlain by sedimentary rock intruded by igneous lava flows forming linear ridges. These formed when the supercontinent of Pangaea began rifting apart around 200 million years ago. This formed what's called the Newark Basin or Newark Rift (which extends upwards into Rockland County, NY).

The boundary between the Highlands and the Newark Basin (Piedmont) is a major fault which formed at that time - the Ramapo Fault. In the diagram below, the grays, yellows, and reds represent sedimentary rocks which filled the Newark Basin as it was pulling apart and rifting while the black colors represent the igneous intrusions of magma and lava flows (the most famous being the Palisades Sill along the Hudson River). The NJ Highlands are off to the left (west) of this diagram in orange (the same rocks also underlie the basin).

I will pick this up in Part III in a day or two.

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