Crust? Mantle? Core?

Move over, adult world, here come the 8^th graders. They are a LOT smarter than WE were in the 8^th grade.

Q: Hello Mr./Mrs. Geologist, I am sending you this email because I am wondering about something. How can you be sure about the dimensions of say: the mantle, or the inner core? I am an 8th grader and I would be very grateful if I was sent a response to this question?

- Oliver McE

A: The short answer is seismology.

The long answer is that when medium to large earthquakes occur around the Earth, they send a substantial amount of wave energy through the body of the Earth. Seismologists have learned how to use a large and sophisticated global network of seismometers to time the arrivals of many different kinds of waves – and analyze their characteristic signatures. Some of these waves (specifically S-waves or shear waves) cannot propagate through liquids. At the same time, P-waves (pressure waves, similar but lower frequency than the sounds you hear with your ears) will refract, or bend, as they cross boundaries with different physical properties. Refraction is what light does as it crosses the air-water boundary in a pool - so fish in a stream look like they are shallower than they actually are.

By carefully timing the arrivals of the different types of waves, seismologists can discern different physical properties at different depths. Among these, the Crust-Mantle, Mantle-Core, and Outer Core-Inner Core boundaries stand out very clearly.

The analysis is much more complicated than this, or course, but it's pretty obvious when S-waves can be seen up to a certain distance away around the curve of the Earth from the earthquake source - but not beyond. This means that a solid-liquid boundary has been crossed.

Here are some helpful links that will let you visualize what I'm talking about:

http://en.wikipedia.org/wiki/Structure_of_the_Earth

This is a good place to start.

http://www.bing.com/images/search?q=earth%27s+structure+layers&qpvt=earth%27s+structure+layers&qpvt=earth%27s+structure+layers&FORM=IGRE

I got this rather complicated link by doing a search for "earth's structure layers".

http://www.bing.com/images/search?q=seismic%20waves%20diagram&qs=IM&form=QBIR&pq=seismic%20waves&sc=8-13&sp=2&sk=IM1

I got this by doing a search for "seismic waves diagram".

There are other ways that we geophysicists can "see" different structures inside the Earth, including

2. torsional oscillations of the Earth in response to very large earthquakes,

3. study of the Earth's complex dipolar and changing secular magnetic field,

4. ground-based and satellite-based gravity and gravity gradient measurements,

5. mathematical modeling, and

6. high-pressure, high-temperature laboratory experiments.

The combination of  all these approaches taken together has allowed geophysicists to actually do something called "tomography" - sort of like a CT-scan or an MRI scan of the human body that doctors use - to actually see complex details beyond just simple layering in the deep Earth. Taken together, we are becoming increasingly confident of things like where the down-going oceanic slabs beneath the continents are, and what the minerals – and metals – at different depth might be. 

How deep can earthquakes occur?

Earthquakes are caused by the sudden release of accumulating strain in the Earth's upper crust or buried fragments thereof. This means that rocks must be breaking for an event to be detected by human beings. However, deep in the Earth's interior, rocks are no longer brittle. With overlying rock pressure and increased heat, they become plastic, so they cannot break. 

Q: Is it possible that earthquake can happen at the inside of the earth? Like Mantle or outer core, inner core?

- Sarah G

A: Many things may be theoretically possible, but once you get below the point of plasticity in the Earth's crust there will be no rock-breaking earthquakes. This aseismic depth - where rocks deform plastically - varies as a function of temperature, pressure, and rock constituents. I believe the deepest earthquakes ever recorded are at around 800 km. These happen along subducted, down-going oceanic crust that is still dragging some of its cooler thermal regime with it. 

When an earthquake occurs, is it possible to tell what the aftermath is going to be like?

There are earthquakes – and then there are their aftermaths. After my house stops shaking and I get up off the floor – assuming I still can – what else will happen?

Q: Hello! When an earthquake occurs, is it possible to tell what the aftermath is going to be like? If so, is there any way to prevent it?

- Tyler H

A: Urban engineers and earthquake specialists have intensively studied the consequences of earthquakes for at least a century and a half now. As a result, they have a pretty good understanding of what the consequences of a large earthquake will be like: how much damage will be caused by the initial event, and how much damage (and subsequent disruption) by aftershocks. Perhaps surprisingly, most of the truly catastrophic damage of the 1906 San Francisco earthquake came from the fires that raged for days following the initial massive shaking event. 

Engineers and urban planners have a good idea of where the water mains and power lines will be cut off, and where the economic corridors (food and fuel distribution via highways and railways, for instance) will be disrupted. With the advent of the internet and computer systems, warehousing of food in southern California has been consolidated in far fewer locations than during the 1960's. This means that a serious potential side effect of a large San Andreas earthquake will be... starvation. 

Earthquakes cannot yet be predicted, though that hope drives some of the most brilliant research minds I have personally ever met. Consequently, efforts have been under way for decades now, instead, on *mitigating* a large earthquake's effects. One aspect of this is "retro-fitting" older buildings to make them more earthquake resistant (not earthquake proof). Hand in hand with this are efforts to establish more stringent building codes - so newer buildings are already pretty earthquake resistant when they are first built. 

By hosting events like the Great Southern California Shakeout, the US Geological Survey, along with local and regional emergency personnel and individuals like you and I, all become more aware of what to expect from one of these terrible events. If we have an idea of what will happen, we can build realistic contingencies into our planning. As an individual or a family you can take part in this process by having a "72-hour kit" of food, water, and other necessities on hand in your own home. If your cultural or religious tradition encourages you to help others, you may also want to "amp up" your emergency supplies to 6 months or a year's worth in order to help your neighbors. 

We will never find enough money to make ALL buildings and ALL infrastructure 100% earthquake-proof. We CAN make buildings more earthquake RESISTANT, however. I bought a house in Washington State 12 years ago that met the more stringent modern building codes, so the insurance company gave me a 15% discount on my home owners insurance. But my house is only somewhat more resistant to earthquakes than one built in the 1950's - again, it's not earthquake-proof. I asked how much earthquake insurance might cost me to add to my home owners policy? I was told this would add 15% more onto my premium. 

I told my insurance agent to go ahead and add it on - it balances out in several senses of the word.

What is the most shock absorbing material for a house

Here's another person concerned about their house. I think  it's a GREAT idea for people to think like this, as so much  of our family resources are locked up in where and how we  live. The geology under our feet really DOES affect the  quality of our lives - often in ways we may not even realize  ...yet. Q: What is the most shock absorbing material for a house  to withstand a Category 3-4 Earthquake? Also, if you had  to make a model of a house with no connecting edges, what   types of modifications would you suggest that would with- stand that  earthquake? Thank you and have a great day! - Betsy D

A: Most people would prefer to live in a house that did not let wind, rain, mice, and insects in, so I assume most people live in houses "with connecting edges."

THAT said, if your house is made of reinforced concrete, it will likely survive a M=3-4 shock, but there would be cracks everywhere. Larger events than M=3-4 have in the past led to "pancaking" - whole floors collapsed onto floors below them, with the terrible consequences you can imagine. If a house is constructed of cinder blocks or brick, it is common for facades - whole walls - to collapse. A wood-framed house, however, will flex to a certain degree. Cracks in drywall can easily be taped and sealed and painted over. 

My house is wood-framed, and the foundations are interconnected reinforced concrete, excavated down into bedrock. Before I bought the house, I checked the surrounding greenway for any evidence of landslides (trees bent near the base, etc.) and found none. Perhaps my education may then prove to be useful here... eventually. However, in the event of a M = 8+ subduction earthquake, all bets are off. Destruction will be widespread, not only in the houses we live in, but in the infrastructure that provides us with food, water, heat, and electricity. 

Perhaps, then, a limited lifetime may also prove useful - I hope not to be around to see this.