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. 

Wild images of Martian meteorites Lead to Information About Mars’s Ancient Surface and Atmosphere

By James W. Dottin III

                    

               

“Are those fossils of a piranha and a griffon”? This may be the first question to pop into your mind! Your second question may be, “wait was there life on Mars”? Unfortunately, the answer to both questions is no. These are two images of a Martian volcanic rock, found in Antarctica, that formed skeletal-type minerals within itself.  Although, the cool photos fail at answering the question of life on Mars, they do provide us with valuable information about how the surface of Mars interacted with its atmosphere. Previous studies on these Martian rocks have told us that when you use chemistry to separate sulfur from the Martian rocks and analyze it closely, you receive data that tell you that early on in Mars’s lifetime, the sun was ripping apart molecules that are bound by sulfur in the atmosphere in cool and unusual ways. The images tell us how the sulfur got into the rocks!!

I was able to use a microscope with a camera to take really zoomed in pictures of the Martian meteorite and load the images into a computer program that is able to detect small differences in color within the image. The colors of interest in these images are the light/tan/gold color of the “fossil looking minerals” and the really tiny bright gold/yellow specs which are the sulfur minerals. I hypothesized that the we found sulfur in these rocks due to a process called “assimilation”, a lava flow picking up material as it flows across a surface. A proposed chemical reaction suggested that we should expect a certain amount of “fossil looking minerals” to sulfur minerals. The computer software calculated the amount of “fossil looking minerals” and sulfur minerals (by distinguishing their color differences) and confirmed that the hypothesis was indeed correct!

So what’s the overall big picture? Billions of years ago, the sun ripped a bunch of sulfur molecules apart, allowing the sulfur to fall onto the surface of mars. A volcano erupted, spewing lava over the surface, allowing the lava to capture and incorporate the sulfur into the flow. The lava cooled and became rock. A large asteroid hit mars and ejected large amounts of surface material into space. Some of the surface material traveled through space and made its way to earth and fell in Antarctica. Brave meteorite hunters found the Martian rock, it was delivered to me, and I delivered this information to you. Thank you for reading my blog and stay tuned!

Tectonics on Earth's "Twin" Planet

Venus had often been called Earth’s twin planet.  While it has many similarities to Earth- size, density, bulk composition, radiogenic heat production- it turns out to be a very different world.  There are surface temperatures of 740 K (hot enough to melt lead) and an atmosphere of almost entirely carbon dioxide (a runaway greenhouse effect).  Another important difference is its absence of plate tectonics.  Earth appears to be the only planet with plate tectonics, which is the theory that Earth’s lithosphere (the crust and upper mantle) is made of individual mobile plates that can collide, spread apart, or slide past one another.  Although Venus currently lacks plate tectonics, it is impossible to know if it existed before 500 Mya.  It was around this time that Venus underwent a global resurfacing event; this age has been constrained with crater count data.  The physics behind this catastrophic process is still mysterious, and some suggest it could be cyclic.  Instead of plate tectonics, Venus is currently in stagnant lid convection regime, which means that tectonics is driven by basal shear from mantle convection (Hot mantle rises and cools, then descends. The same principal as boiling water on the stove- see Fig. 1).

Figure 1: A simulation showing convection in a one-plate planet, such as Venus. Red indicates warm, rising mantle, and blue is cold, descending mantle. (credit: Walter Kiefer and Louise Kellogg, lpi.usra.edu)     

On Earth, plate tectonics require a global network of shear zones (areas of low strength and concentrated deformation) throughout the entire lithosphere.   The localization processes are dominated by water content and minerals with layered sheet-like fabrics, such as micas.  Venus’ lithosphere is dry and lacks layered fabrics, and yet the rift zones of Venus are remarkably Earth-like.  The area of Beta Regio (Fig. 2) has often been compared to the East Africa Rift System due to their similar geologic morphology.  Beta Regio has a rift called Devana Chasma, which is part of a triple junction system similar to the one seen in East Africa.  Beta Regio is a volcanic highland region; the elevation and other geophysical data suggest that it is the result of a young underlying mantle plume (a thermal perturbation which rises through the mantle and impinges on the base of the lithosphere…think of a lava lamp!).  For Devana Chasma to be as narrow as it is, we expect some form of localization to be active to concentrate the deformation into a localized region.  It is likely that the presence of melt will have a significant impact on localization.  The basal shear force induces a small stress on the lithosphere, and may not be enough to cause rifting.  The addition of gravitational potential energy due to the area’s high elevation might be enough to induce rifting.  

Figure 2: Image of Beta Regio. The black lines are regions that were not imaged. (credit: Rathbun et al. Formation of Beta Regio, Venus

It is thought that the addition of the appropriate localization mechanism(s) and including the fact that tectonics are not driven by the same forces on Venus as they are on Earth, that an agreeable model for rifts on Venus can be developed.  By understanding the tectonic regime of Venus we are able to better constrain its evolutionary history and interior processes.  Venus can also represent an appropriate analogue for early Earth, prior to the initiation of plate tectonics, which will allow us to better understand how it developed here.

See the Earth's interior through noise

What does a local earthquake sound like

For many of us, the first impression of earthquakes is hazard, disaster or catastrophe. However, on the other side, seismologists can make use of the record from seismic wave because of the information it carries from the deep interior of the Earth. This even leads to the fact that for tens of years, seismologists struggles in studying most part of the Earth’s interior due to the lack to seismicity in those areas. Now there might be a new way of exploring the Earth that meets the interests of both the public and scientists -- seismic noise tomography.

Detecting seismic signals from noise(Weaver, 2005). Among the diffusive wave field of noise, the detectors record random signals. Infrequently a wave may travel through both detectors. As a result, the signals are weakly correlated.
Like the definition of most noises, seismic noise is a non-interpretable or unwanted component of the signal recorded by seismometers. The recent study is challenging this assumption. The main idea of seismic noise tomography is to detect and amplify the common feature between two noisy waveforms from different detectors. Specifically, cross-correlation function is used here as a measure of similarity between two seismic records. The physical meaning of this coincidence is the propagation of the weak but repeated signals through the Earth between these seismometers. By looking through a monthly long noise record and adding up the detected signal, seismologists are able to provide a strong, clear cross-correlation function between a pair of seismic stations. This information is then translated into the time needed to travel through this path. Combined with the distance; the velocity of the Earth’s interior is obtained. While the analysis of one pair of stations only provides the velocity structure in a limited area, same work can be done between various pairs of seismometers within a dense seismic array. Then seismologists can use tomographic techniques to generate a map of seismic wave velocity.

Group-speed maps constructed by cross-correlating 30 days of ambient noise between USArray stations. (A) 7.5-s-period Rayleigh waves. (B) 15-s-period Rayleigh waves. Black solid lines show known active faults. White triangles show locations of USArray stations used in this map. (Shapiro et al., 2005)

The noise tomography result from Shaprio et al. has been suggested to be consistent with geological structures in the studied area. As more and more dense seismometer arrays become available, such work can be carried out throughout the Earth with numerous possible applications.

Reference

Shapiro, N. M., Campillo, M., Stehly, L., & Ritzwoller, M. H. (2005). High-resolution surface-wave tomography from ambient seismic noise. Science,307(5715), 1615-1618.

Weaver, R. L. (2005). Information from seismic noise. Science, 307(5715), 1568-1569.

How do Geoscientists “Hear” the Melt Underground?

Earthquakes generate “earthquake sounds” (terminologically called seismic waves) that travel though rocks and fluids. Seismic waves have different velocities in different materials. By listening to the sound from the inside Earth, seismologists tell where the “sound” is from and what kind of materials it might have traveled through. And that gives us the information about the Earth interior. Figure 1 shows us how fast the seismic wave travels at different depth of the Earth.

                                    Figure 1: Seismic velocities in the Earth

If we zoom in the velocity profile, we can see a low velocity zone (LVZ) around depth 100 km. Seismic-wave velocities rely on the property of the material in which the wave propagates. Generally, the harder and denser the material is, the faster the wave travels in it. As depth increases, the rock density gets higher. So the question now is why there is a velocity decrease at the LVZ. Many studies were done on this topic, and currently the most convincing interpretation is partial melt. As depth increases, the temperature also increases. At LVZ, part of the rocks starts melting, which generates fluids that cause the seismic velocity anomaly.

                                                Figure 2: At about depth 100 km underground, there is a low velocity zone.

However, the details of how the melt shape and amount affect the velocity are not totally clear. Geoscientists want to find a quantitative relationship between the melt and the seismic velocity change. First, they measured the shape of the melt using X-ray scanning in the lab. Figure 3 shows the melt shapes in partially molten rock cubes at different degrees of molten. The next step, which hasn’t been done yet, is to simulate the seismic wave propagation on the computer and then compare the calculated velocities with our observations. Since we are not able to dig into any depth of our Earth, a combination of the seismic methods and rock physics analysis would be particularly helpful with exploring the Earth interior.

                                                Figure 3: Melt distribution in partially molten rock samples

Figure1 and figure 2 are from wikipedia, figure 3 is from Miller, Kevin J., et al. "Experimental quantification of permeability of partially molten mantle rock." Earth and Planetary Science Letters 388 (2014): 273-282.