Taking a Picture of the Earth Deep Beneath Our Feet

Taking a Picture of the Earth Deep Beneath Our Feet

Figure 1 | Poor image quality -Little information is given to aid
in the understanding of this image.

    The figure to the right  (Figure1) may look a bit incomprehensible. Even through you have more information in some locations than others, regarding the shapes and colors, it may still be puzzling to figure out what is going on in this image. The poor image quality leaves much room for interpretation, if we want a better idea of what Figure 1 is we will need more information for higher resolution. 

    In the past, lack of information to create higher resolution images has been a problem in creating seismologists’ picture of the continents.  Seismologists seek to reveal the geologic structure (the type and thickness of differing rock layers) many kilometers beneath our feet. Seismologists look into the earth with seismograms collected from seismic stations. Seismograms contain information about four main things: an earthquake, the station, background movement, and the geologic 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” a little bit of the earth’s structure at each station. The effect is similar to a taking a telescope and pointing it into the ground at each station. Until recently, the images of the geologic structure from seismic stations have been fairly unclear (like Figure 1). The National Science Foundation funded project to improve the image (or resolution) of the interior of the earth called Earthscope USArray.

    Earthscope USArray densely packed seismic stations across the continental US and Alaska to collect information about the earth beneath our feet. The increased station coverage (Figure 2) allows us to gather more information and create a higher resolution image of the geologic structure. The idea is similar to increasing the resolution of an image. If we look at Figure 3, we see the higher resolution image of Figure 1. Now that we have more information we can better understand what’s going on in the picture, just like understanding the continent with Earthscope USArray.

Figure 2 | Left: Earthscope USArray station coverage Right: Prior Station coverage 

Figure 3 | Left: improved image of Figure 1with more information (Now
 we can see the image was grumpy cat) Right: Poor image quality with little
information. 

    The dense seismic stations provide the highest resolution image of any continent, allowing scientists to look at the earth in a whole new way. The vast majority of the human population lives on a continent. In fact, you are likely on one right now. It may be surprising to find then, that we know very little about the structure within these continents. In fact, scientists have found a layer present in almost all continents 100 kilometers (km) beneath our feet that has not been able to be explained –we don’t know why it is there or what the consequences of this layer have been.  Some have suggested that this layer may be related to the locations of large earthquakes that occur in the middle of the continents. In the United States most earthquakes are located on the west coast, however if we look at locations of some of the largest earthquakes and earthquake hazard areas in Figure 4, you will find bright red areas associated with high hazard in the St. Louis, MO area. If there is a correlation between some characteristic of the 100km layer within the continents and the high hazard areas within continents of the US, we may be able to where other large earthquakes in continents may occur. Even if no correlation exists, this layer might preserve billion year old secrets to continental formation, alteration, and breakup and provide insight to the past, present, and future of our continent.

Figure 4 | Hazard map from United States Geological Survey. Areas with high seismic hazard are in
warmer colors with red being the most hazardous, while cooler colors indicate low seismic
hazard  with dark blue being the least hazardous. 

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

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

To make your own earthquake map (like  Figure 4)  visit:

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


References: 

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

What is Going on in the Earth's Belly?

When Geoscientists first imaged the earth’s interior, they probably thought what any parent first thinks when looking at a sonogram: “what the hell is that?”. Seismic Tomography is perhaps the oldest method of trying to image the interior of a large object we could not easily get a camera inside.

Just like looking inside a pregnant woman with acoustical waves (sonography), seismologist use earth vibrations to image what is going on deep down in the planet. They have used earthquakes, nuclear explosions, and even vibrations related to the ocean, to look at the rattles and shakes of the earth and produce 3D images of what they think the structure is way down below. In a nutshell, that is what Seismic Tomography does.

Unfortunately though, the seismologists who make these images cannot refine their craft as the medical industry has done: they have never put the earth on an operating table and actually looked inside to see if they are using their tool correctly. They have been able to do it in shallow depths, the upper crust, the earth’s most outer layer of “skin”, but getting any deeper is still a very difficult technological feet. So, they have to make assumptions, argue how the computer should display the signals coming from their instruments, and add some “artistry” to how the image is displayed.  With all these variables floating around that can drastically affect the picture they produce, it is a really good thing they at least do not have to worry about malpractice insurance.

Sonogram of Child in Utero

Tomographic Cross Section of Velocity "Anomalies" of the earth