New Jersey Earthquake (Part III)

In Part I of this post, I introduced the recent (April 5) New Jersey earthquake felt here in the Hudson Valley. In Part II, I talked a little about the geologic setting. Let's now finish up by discussing the Ramapo Fault.

As I mentioned in the last post, the Ramapo Fault or, more accurately, the Ramapo Fault System since it's not just a single fault trace is the western boundary of the Newark Basin - a rift valley that opened up when the supercontinent of Pangaea broke apart starting some 200 million years ago. On the east side of the fault are the Hudson Highlands of New York and the New Jersey Highlands to the south - the rocks that formed deep in the Grenville mountain belt over a billion years ago.

This fault has been around a long time with its major activity occurring during the time of the dinosaurs as Pangaea rifted apart. Afterwards, it settled into a long period of quiescence only occasionally readjusting.

These types of earthquakes are called intraplate earthquakes because they occur in the interior of continental plates far from plate boundaries (in this case, the center of the Atlantic Ocean at the Mid-Atlantic Ridge). Why do they occur? Because movements of the tectonic plates at their boundaries cause stresses that can be transmitted long distances into the middle of the rigid tectonic plates. The old faults, formed hundreds of millions of years ago, may be oriented such that those stresses just happen to be enough to trigger their movement.

Intraplate earthquakes do occur in places like New Jersey. Below is a map of earthquakes from the late 1700s until recent. While most intraplate earthquakes are barely felt, they may also reach moderate levels and, in rare cases, become strong. This typically causes more damage in places like the Northeast where we don't routinely build to earthquake standards as the do in California.

If you want to worry, Google the 1886 Charleston, SC earthquake or the 1811-1812 New Madrid, MO earthquakes. Areas which show up very nicely on this seismic hazard map of the U.S.

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.

New Jersey Earthquake (Part I)

So I was in my office yesterday morning (Friday, April 5), engrossed in proofreading an important document going out to our college's accrediting agency, when I vaguely felt something. My colleague, in the adjacent office said "Was that an earthquake?" and I first thought maybe it was someone working on the roof (which they've been doing a lot lately) or it was a strong wind gust (but looking out the window showed everything calm). I pulled up the National Earthquake Information Center and, low and behold, a magnitude 4.8 earthquake in northwestern NJ popped up. So I guess I did feel it, but didn't recognize it as an earthquake at the moment. Same is true for a lot of other people I talked to at the college.

As a geologist, a moderate earthquake in New Jersey doesn't overly surprise me. While not common, they do occur. A few aftershocks have also occurred, the strongest a 4.0.

Here's a picture you can find in a lot of geology textbooks. It illustrates, using the Virginia 5.8 earthquake in 2011 (also felt in the Hudson Valley) how east coast earthquakes are felt over a much larger area than west coast earthquakes of the same magnitude.

This difference is due to the fact that rocks in the east are older, colder, and more intact than rocks in the west. Out west, the rocks are much more fragmented by faults and these faults attenuate (weaken) the energy from earthquakes. Here's a map of felt shaking from the New Jersey quake.

So how often do earthquakes occur in New Jersey? Here's a nice map.

I will continue this discussion in a second post in a day or two...