Earthquakes in NJ And Their Hazards

For the spring semester of 2009, I had a major project/term paper due for two separate classes- Soil Science and GIS (geographic information systems aka, digital mapping). I decided to pick a topic that could work for both classes to cut down on my workload- earthquakes in NJ and the related soil hazards. I was inspired to do earthquakes in NJ because of a conversation I had had with my dad about an earthquake he had felt in NJ many years ago. I was curious to find out more about the topic, and my research project was born. A week after picking this topic, a small earthquake hit northern NJ as I was chatting with a friend online. He described it as shaking the whole house as if a bomb went off nearby. Instead it was a magnitude 3.0 earthquake epicentered near Dover, NJ. 


The following is the presentation I gave on my project. All maps are made by me unless otherwise noted.

I love this picture for the title slide- it's so true! People don't normally think that earthquakes happen in NJ, or the East Coast for that matter, yet there are so many geologic features that indicate the area was seismically active.

Earthquakes are caused by the accumulation of stress in the Earth's crust. The  stress is caused by the movement of the Earth's tectonic plates. The tectonic plates do not all move together at the same rate, so strain accumulates in more areas than others. When the strain is more than the bedrock can handle, the bedrock breaks to relieve that stress and releases energy in the form of rapid vibrations- earthquakes.

Earthquakes are measured in two ways- magnitude and intensity. The most commonly reported is the magnitude. For example the Haiti earthquake in 2010 was a magnitude 7. The magnitude is a logarithmic scale, so a magnitude 2 is ten times stronger than a magnitude 1, and a magnitude 3 is ten times stronger than a magnitude 2, and so on. Intensity is the amount of shaking felt at the surface, and is affected by the hardness of the bedrock (harder bedrock transmits vibrations better than soft bedrock) and the distance from the epicenter.

The Modifired Mercalli Scale measured the intensity of an earthquake. The earthquake that struck Virginia in August this past year was probably an intensity of about VI to VII in the areas closer to the epicenter. That earthquake was felt ever so slightly here in NJ, so I would guess the intensity this area was II to III.

This is a seismicity hazard map from the USGS. The red and brown colors indicate areas with a high risk for earthquakes. The lighter colors show areas with low risk for earthquakes.  The dark red and brown in California makes sense- there's a tectonic boundary running through there; it's very seismically active. But what's with the dark red and brown in South Carolina and the Midwest? There are no plate boundaries there! You'll also notice a slight yellow area near the epicenter of the August earthquake in Virginia.

How are intracontinental (within a continent) earthquakes caused? The most accepted theory is that the strain accumulated millions of years ago, and that the strain is still being released. The dark red and brown area in the Midwest from the previous picture is know as the New Madrid Seismic Zone, and has historically produced earthquakes to rival those that occur in recent times in California. During the winter of 1811-1812, four earthquakes with a magnitude around 8 occurred there. It was so strong that the course of the Mississippi River changed (see insert, I did not make the map.) and the river temporarily flowed backwards. Large cracks opened up in the ground, and the landscape was devastated in areas. The earthquake was even felt as far away as Boston, MA, where church bells started ringing because of the ground shaking. There weren't a lot of people living in the area of the earthquake epicenter at the time, so the damage wasn't too bad.

NJ has a number of faults and folds in the northern section of the state (and none down south of Trenton). The rippling of the bedrock from stress accumulation is what causes these geologic features. In NJ, these faults are not currently active, but they still hold some stress that could potentially be released.

I looked at where earthquakes have been epicentered, at least since record keeping began. You can see that most of the earthquakes are occurring in the northern part of the state. The two bar graph show the frequency of occurrence of magnitudes and depths of epicenters. The tall bars at zero are the data points for which there is no magnitude or depth data. The magnitudes show a relatively normal distribution, with the peak around 2, which is nothing to sneeze at. It does trail off to the stronger magnitude side, with a few outliers at magnitude 5.  The same pattern shows for the depth occurrences. Most earthquakes are shallow, but that means they are stronger (felt more at the surface) than ones epicentered deeper.

There have been 3 instances of magnitude 5 earthquakes happening in NJ- in 1737, 1783,  and 1884. Damage was rather minor back then. But the number of people living here and the amount of infrastructure was no where near the levels it is today. This map shows a comparison of population densities around the times that these strong earthquakes occurred in NJ, and a map showing the population density as of the most recent census (which at the time of making this presentation was 2000). The population densities of NJ in 1790 and 1880 were rather low, except for a concentration near NYC. If you look at the population density recently.... its obvious that a LOT more people live here. Now add the fact that geologists estimate this area has a magnitude 5 earthquake about once every 100 year, and the last one was in 1884.... and that is very frightening.

One of the soil related earthquake hazards is liquefaction, which is kind of like quick sand, in which structures can sink into the soil. It happens when saturated, loose, sandy soils with uniform particle sizes are shaken. The water fills the pore space, and suddenly, the soil loses it's strength, and things can sink. If the particle sizes of the soil are distributed evenly, then when the shaking occurs, smaller particles will fill in the pore space, and strength is maintained.  For this map I identified the soils classified as sandy and loose. Most of those soils occur in southern NJ.

Landslides are another soil related earthquake hazard. Soil particles lose their strength between each other and gravity takes over, and large chunks of hillsides or cliffs come loose. For this map, I identified the types of soils that have already had a lanslide occur, and then found  the location of that soil in other parts of the state. The data points show where landslides have historically occured. One of them was actually caused by something similar to an earthquake. A landslide in Paterson, on Garret Mountain, occured due to the vibrations of the nearby railroad. Other notable landslides are the one that fell on top of an illegally parked car in  2002, and the landslide that destroyed a rock that looked like Hitler in 1947.

In conclusion, most earthquakes in NJ are not strong, though there is the potential for a large one to strike. If this were to happen, I cringe to imagine the effects. The infrastructure damage would be horrible and even if it were minor, would seriously snarl traffic and our day to day lives. What if a crack form in the Lincoln Tunnel or George Washington Bridge? I expect pandemonium. Since most epicenters are in northern NJ, I put the risk of liquefaction at medium low for the southern part of NJ. I think northern NJ is in the clear for that one. Not so much for landslides, however. I put the risk of landslides from a strong enough earthquake at pretty high, due to the soil types and topography. And since most of the landslides occurred without the aggression of the soil caused from an earthquake I think its very possible for an earthquake to cause a landslide.

And of course, my references. What kind of scientist would I be with out them?