On Shaky Ground 2: How Earthquakes Teach Us About Geology

My first earthquake was the Loma Prieta earthquake on October 17, 1989. The 6.9 magnitude earthquake is also known as the World Series Quake because  millions saw the earthquake live on TV as they watched Game 3 at Candlestick Park.

The epicenter of the earthquake was located about 10 miles northeast of Santa Cruz on the Loma Prieta segment of the San Andreas fault system. I was on a commuter bus in Berkeley at the time; I didn’t feel a thing. But, many other people did.

More than 200 buildings were damaged in San Francisco’s Marina District. Forty-two people died when the upper level Cypress Street off-ramp of the Nimitz Freeway collapsed into the lower dock. Hundreds of Oakland residents were displaced when the buildings they lived in or worked in were closed because of structural damage.

Earthquakes are destructive. They cause property damage, injuries and loss of life. But, we can also learn a lot from earthquakes.

Earthquakes are caused by the interactions of tectonic plates. They generally occur at the boundaries where two or more plates meet. We can identify plate boundaries by mapping large amounts of earthquakes.

In this map, strong earthquakes from 2012-2014 are shown in red. Earthquakes from the last week are also shown.plateboundariesYou can see that most earthquakes occur in distinct bands. These bands outline the boundaries of Earth’s tectonic plates. The discovery that earthquakes occur in bands actually contributed to the idea of plate tectonics.

My students learn that there are three types of plate boundaries: divergent boundaries where plates move apart and new crust is formed; convergent boundaries where plates move together and oceanic crust is subducted or pushed down into the mantle; and, transform boundaries where plates move past each other. We can use earthquakes and volcanoes to determine the type of boundary at a map location.

When I added the Smithsonian Institutions Holocene volcanoes layer to the map, it looks like this. volcanoesThe yellow volcanoes are volcanoes that have been active over the last 10,000 years. As you can see, most of these volcanoes occur in the same area as earthquakes. There are some exceptions.

Volcanoes that occur far from plate boundaries are “hot spot” volcanoes. These form when crust travels over a mantle plume, an area in the mantle that is extra hot. Hawaii is a chain of hot spot volcanoes.Hawaii

There are also places where there are earthquakes an no volcanoes. The coast of California is one of those places.


In California, the San Andreas fault marks a transform boundary where the Pacific Plate is moving past the North American plate. Volcanoes only occur at divergent and convergent boundaries. The volcanoes to the east are extinct leftovers from Basin-Range rifting (divergent boundary).

We can use earthquake depth to determine if a map boundary is convergent or divergent. In this map, depth is shown by the size of the circle. Deeper earthquakes appear larger.


Shallow earthquakes (less than 75 km deep) often occur at mid-ocean ridges. These are long chains of underwater volcanoes where tectonic plates move apart and new crust is formed. Deep earthquakes occur at subduction zones where oceanic crust is being pushed under continental crust.


When this occurs, the subducting plate bumps and scrapes against the overriding plate. This causes deep earthquakes. When the subducting material reaches a depth of about 660 km, the rock becomes soft enough to flow and earthquakes stop.

Non-map images are from Creative Commons (Wikipedia). My map can be found here.


On Shaky Ground Part 1: Earthquakes in Virginia

It’s been just over four years since a 5.8 magnitude earthquake in the Central Virginia Seismic Zone damaged the Washington Monument. That was a significant earthquake for our area.

Last Sunday (9/27/15) the Central Virginia Seismic Zone experienced another earthquake.  The magnitude 2.0 quake was one of hundreds of small earthquakes that have occurred in the area over the past four years. And, InsideNova feels the need to report every single one.

So, is Virginia really a seismically active area? I placed esri’s USA Earthquake Risk map on top of a topographic base map. The map shows potential ground shaking intensity from earthquakes, an estimate of the amount of damage an earthquake is likely to cause in an area.  I set the scale so the highest risk in shown in dark red while the lowest risk appears as dark blue. I added the past week’s worth of earthquakes (from esri Disater Response) to the map. You can see the our 2.0 quake in Central Virginia.

Seismic zones

Moost of Virginia falls in the medium blue range indicating a very low risk of damage from earthquakes. Our recent earthquake is small compared to the many of the other earthquakes that occurred in the U.S. last week.

It’s not surprising that the West Coast of the United States is covered in red and orange. The California, Oregon, and Washington coasts are active margins. This means that the edge of the continent coincides with the edges of one or more tectonic plates (in this case: North American, Juan de Fuca, Pacific). Geologic activity such as earthquakes and volcanoes generally occurs at the edges of tectonic plates.

The East Coast is a passive margin. The eastern edge of the North American tectonic plate is far out in the Atlantic ocean. So, geologic activity on the East Coast and most of the U.S. is relatively rare as indicated by the blue coloring.

Behind California is an area known as the Basin Range province. This is the remnant of an ancient rift zone. In the early Miocene (about 17 ma), the North American continent began to stretch and thin. But, before the continent could rift into two tectonic plates, seismic activity stopped. This left a network of faults which still responds to the stresses from activity on the West Coast. The New Madrid fault zone which underlies Alabama, Arkansas, Illinois, Indiana, Kansas, Kentucky, Mississippi, Missouri, Oklahoma, Tennessee and Texas is another example of an ancient rift zone.

But, not all rift zones are ancient. The red area in Northern New York and New Hampshire indicates activity from the St. Lawrence rift system, an active rift zone that runs along the St. Lawrence River. Perhaps one day, North America will split and a new Canadian plate will form.

If you are looking carefully you might notice that there are medium blue areas in the middle of continents far from any plate boundaries that have recently experienced larger earthquakes. Is that rifting? Does that mean we should worry?

No, it’s not rifting, but we may have reasons to worry. The cluster of earthquakes in Oklahoma includes several with a magnitude between 2.0 and 3.0. These small earthquakes have been linked to fracking rather than seismic activity. The 3.2 earthquake near Stamford, NY may also be caused by human activity. It is likely the result of water pressure from the Gilboa Dam.

Like Oklahoma and New York, Virginia has networks of ancient faults that can be reactivated by human activity. However, seismologists believe that most of our earthquakes are simply the result of old faults moving because of sea floor spreading in the Atlantic Ocean.

You can view my map here.

Where Will I Put the Bottled Water?

When we were in class on Wednesday night, Hurricane Joaquin seemed to be headed straight for Virginia. I’m pretty well prepared, but I donated last year’s bottled water stash in the spring. So, on Thursday morning, I stopped at Target to stock up on bottled water which is now sitting in the hallway because I don’t have space for it.

The latest forecasts show that I didn’t really need to rush and buy water; Joaquin isn’t likely to reach the East Coast. But, thoughts of hurricanes inspired a map.


I mapped esri’s active hurricanes and recent hurricanes as well as hurricane tracks from historic hurricanes (in blue, from Maps.com) on top of a base map. Hurricane Joaquin’s path is shown in black, while the yellow dots represent the tracks of recent hurricanes. As you can see, we might be safe from Hurricane Joaquin, but (historically speaking) it’s only a matter of time until a hurricane does hit Virginia.

You can see the ArcGIS Online map here.

I think I’ll keep the water.


Hi, I’m Sara Lubkin. I am a GIS certificate student at Northern Virginia Community College.

I am also a geologist who studies fossil insects, paleoclimate and the current effects of climate change. I earned my PhD from Cornell University in 2008 and worked in web and social media marketing for a while.

I now teach introductory geology classes at the University of Mary Washington. When I’m not teaching or learning, I do research. I am currently collaborating with the NASA DEVELOP Program and the Virginia Institute of Marine Science on a project tracking harmful algal blooms (HAB) in the Chesapeake Bay. I am also a mom.

For me, GIS is connecting data to geography. When non-scientists ask me what GIS is, I tell them that I put information on maps so I can better analyze and interpret.  I was a scientist before I studied GIS, but GIS has allowed me to ask and answer different types of questions.

Before studying GIS, my fossil insect research focused on describing new species and determining how species were related to other species and modern species. I was getting frustrated with this work and wanted to do something more meaningful. Now, I am able to connect Pleistocene fossil insect species to specific climate zones and create maps of Pleistocene glaciation. I am hoping to use this research and my work with NASA DEVELOP to transition into the study of climate change and its effects on the Earth.