Unraveling Secrets Deep Beneath Our Feet:

Exploring the detailed structure within the continents using Earthscope USArray

    The vast majority of the human population lives on a continent. In fact, you are likely on one right now. The part of the continent on which you stand could have been stable since before even the first bacteria existed, hundreds of million to several billion years ago. It may be surprising to find then, that we know very little about how these continents formed and how they have remained stable over most of earth’s history. (Find out how old the part of the continent you are on right now, check out Figure 1 created by Rob Butler at the University of Leeds.)

Figure 1: The age of the continents
Ma stands for million of years ago. Dark blue regions within the continents
are the oldest ( formed over 2500 million years ago) while yellow regions
within the continents are the youngest (Formed less than 200 million years ago)
Rob Butler- University of Leeds 
http://www.see.leeds.ac.uk/structure/assyntgeology/extra_info/ehistory.htm
 

   The continents have remained, while so much else on earth has changed – they preserve billion year old secrets of their formation, alteration, and breakup and provide insight to the past, present and future of continental stability. Understanding more about what is beneath our feet does more than just help us understand the long lasting stability of the continents. We may all be more familiar with one hazard associated with the geologic structure (the type and thickness of differing rock layers)  – earthquakes . Understanding what makes up the continents beneath our feet – specifically the geologic structure - can help us address more pressing questions such as: What determines the locations of large earthquakes that happen within the interiors of continents? (See Figure 2 for the locations of the some of the largest earthquakes since 1980, are any near you?) These types of questions led to a National Science Foundation funded project to improve the image of the interior of the earth called Earthscope USArray.

  

Figure 2: Earthquake and Hazard map of the Continental United States
This map shows earthquakes with magnitude 4.8 and larger that have been recorded since 1980 as white circles. Magnitude 4.8 earthquakes have been chosen as the smallest  shown on the map as they can be easily felt and may cause small structural damage. This map also shows areas prone to earthquake hazard in the United States. Regions that have high earthquake hazard are in warmer colors while low earthquake hazard areas are in cooler colors. Notice some areas within the continents have high earthquake hazard. Figure made using USGS Hazard Program

    Earthscope USArray uses seismic stations spaced 70 kilometers apart across the continental US and Alaska to collect information about the earth beneath our feet (Figure 3).  These seismic stations collect seismograms, which contain information about four main things: an earthquake, the station, background movement, and the structure of the earth. If we remove the earthquake, station, and background information, we are left with information from the earth’s structure. This is one method seismologists use to “see” into the earth’s structure at each station. The effect is similar to a taking a telescope and pointing it into the earth at each station. Using the many stations increases the resolution of the images we have of the continent’s structure, and just like increasing the number of pixels in a picture, gives us a more clear idea of what is beneath our feet (Figure 4). Earthscope USArray provides the highest “pixel” or resolution image of any continent. Allowing scientists to look at the earth in a whole new way. 

Figure 3: Earthscope USArray Stations
Earthscope has added seismic stations across the US. This figure shows the improvement of station coverage from prior to the Earthscope USArray project (right) to after (left). Stations are represented by any color triangle. 

Figure 4: An example of resolution improvement
Increasing the number of stations is similar to increasing the number pixels in an image like these smilies. The left could be likened to poor station coverage (the right image in Figure 3) going to a higher and higher resolution with better station coverage ( the left image in Figure 3). The edges of the smilies become more clear with higher pixel coverage, just like the geologic structure beneath the earth becomes more clear with higher station coverage. 

For more information about Earthscope and projects being done, visit:

Earthscope - http://www.earthscope.org/ 

To make your own earthquake map (like  Figure 2)  visit:

USGS Earthquake Hazard Program - http://earthquake.usgs.gov/earthquakes/search/

References:

Butler, Rob, Claire Gordon, and Martin Krabbendam."Making New Continents." The Moine Thrust.  University of Leeds. 

"Earthquake Hazards Program." Earthquake Hazards Program. US Department of the Interior -  United States Geological Society, 26.Aug, 2014.