Recently for me, earthquakes went from the theoretical to the real. On February 28 at 10:54, I and the rest of the Northwest were treated to a live demo of what a quake really feels like. I found information on the Olympia, WA. event at ANZA Olympia Earthquake. Scroll down the page to see the recorded waves from that event. The large graph excursions were almost strong enough to knock me off my feet.
Now I know the feeling, but descriptions don't work because it is unlike any other feeling. My speculation is that the solidity of the earth is ingrained at the midbrain level, below the conscious, and when that is disturbed, the reaction comes from there. I can only describe it as a profound uneasiness. Having been a sailor for decades, I can tell you that rough water is nothing like an earthquake.
So much for the feelings. What actually does happen? More importantly, how is it caused and is there any way to predict better than the current plus or minus hundred year accuracy?
To understand what causes earthquakes, you need to picture the internal structure of the earth. On the surface where we live, there is ocean and continents. The floor of the ocean and the continents are the earth's crust, the outermost layer which is 3 to 25 miles thick. Under that layer is the mantle which extends down 1789 miles. The mantle changes in density as it gets further down due to pressure and heat. At the center of the earth is an inner and outer metallic core totalling 2166 miles in radius. A very rough approximation would be a golf ball inside a softball which has a thin film of water on it. We live on that thin film.
Most earthquakes are breaks in the earth's crust, caused by the slow movement of mantle rock. The mantle is driven by heat generated within the earth's core and mantle, and the very slow flow of mantle rock pushes and pulls the continental plates. These forces cause the plates to crack, which shakes the ground we live on. Thus the causes of earthquakes lay ultimately at the core of the earth. While it may boggle the mind, our solid ground floats on a dense layer of magma (hot rock) which is not perfectly rigid. The continents are like rock icebergs afloat on a sea of slow moving magma.
Geophysics has advanced substantially in the last twenty years. The advent of computers being applied to the problem of finding oil has led to a better set of tools for studying the layers of the earth. There are a number of universities who have major programs in geophysics. Here is a seismic resource list showing universities, data centers and research groups. Sesmic Resources
Echo sounding for oil has only probed the upper part of the mantle, less than ten miles down and more than forty miles above the diffuse mantle/magma boundary. Even a large non-nuclear explosion doesn't generate a strong enough wave to hear the echo from the crust/mantle boundary.
However, there are three sources of energy that will reach much further - nuclear explosions, earthquakes and large volcanic eruptions. During the cold war, monitoring for underground nuclear tests generated a very large data set that represents all the seismic disturbances detected during that time. Since then, monitoring for earthquakes has continued to add to that data. See the IDA Global Seismic Network for much more.
Collecting all of the data about the earth is a challenging job. The US Department of Commerce NOAA (National Ocean and Atmospheric Administration) has the largest collection. It is in the NGDC (National Geophysical Data Center). Solid earth geophysics is just one part of their collection of data, models, reports and studies.
Thanks to modern computer capabilities, scientists have learned a lot about the inner earth. A good overview how much can be found at the IGPP: Whole Earth Geophysics. This is the Institute of Geophysics and Planetary Physics. The site has links from Array Seismology to Seismic Surface Waves. In addition, there are available computer models and data for download and experimentation at: Mantle Models. These are the Scripps 3D Mantle Models used by scientists.
Information about the earth from a wide variety of chemistry and physics sources are now being organized by a new web site: Earth Reference. The geophysics of earth are at the REM site. Under that site are several links to models and information. Online and downloadable models can be found at Models.
One of the visually interesting and instructive sites is Earth Density . This shows the earth with distortions exaggerated, and the density differences that help drive mantle flow. The next visual site is on the topography of the earth, at Topography. These pictures give a hint of the complexity of the internal map of the earth.
Finally, some insight into how the rock compresses as it moves deeper into the earth is shown at Compression. While none of this qualifies as light reading, it is very helpful to see graphic models that visualize some of the results of the scientific study of the earth.
Because the source of quakes is beyond our reach, even if we knew how to control the magma flow, we would have no ability to apply that nonexistent knowledge. Since there are no cures for earthquakes possible or even speculative at our current level of science, we will continue to experience them. While control is out of the question, prediction is not. But is this our best option to deal with the destructive power of quakes? Prediction is based on knowledge of the facts, and an understanding of the process. Those we have in general terms, but it is far short of the detailed knowledge and understanding required to simulate the mechanics in detail.
Until prediction of earthquakes can be made better than weather prediction, we might well suffer more from missed predictions than surviving the quakes. The problems with prediction are large and fall into two categories. First is our lack of detailed knowledge, which we can solve over time.
The second category is one of our ability to accurately calculate consequences, which we already see limits to in weather prediction. The same cause, chaos or chaotic behavior, is the cause of the problem for weather and probably quakes.
The concept of chaos is simple. It essentially says that in a complex system, extremely small causes can have unpredictable results. The weather saying is that a butterfly's wings in Arizona could cause a tornado in Kansas. So even if we could know everything about the causes of quakes, and computation was not a limit, we might still fail to accurately predict quakes because of causes that are below the noise level.
For the foreseeable future, we will have to rely on better engineering and construction to reduce damage and injuries, and better response when a big quake does hit. Earthquake engineering has been tested up to magnitude 7 force, which has been the economic limit for most buildings. Events such as the Alaska quake of magnitude 8.6 would damage even buildings built to current earthquake standards.
One unusual experiment was done just eleven months before the February 2001 quake hit. When the old Kingdome (football stadium in Seattle) was scheduled for demolition, the University of Washington Geophysics program coordinated placement of dozens of portable recording devices at one kilometer intervals around Seattle. The triple impact of the Kingdome implosion was recorded out to 7 miles away, yielding detailed information about local earth structures. See Kingdome Demolition.
Based on the results of Washington state's ten year effort to improve earthquake safety, the approach of strengthening buildings and highways does work. After a 6.8 magnitude quake, there were only two deaths from that cause. However, had that been a shallow quake such as happen in California, there would have been more damage and injuries.
Note: Special thanks to Dr. Svetlana V. Panasyuk of Harvard University. Her clarification of the details of the earth's core and earthquakes and her contributions on the web contributed significantly to my understanding of the process. Any mistakes in this column are mine.
Earthquakes can be separated into three classes: Shallow, less than 20 miles down; Deep, down to the mantle boundary at 50-60 miles; and Very Deep, in the upper mantle from 60 to about 400 miles. Shallow and deep refer to which part of the continental plate is being affected. The whole earthquake subject is referred to as Plate Tectonics.
Shallow quakes are the most destructive because they deliver their energy to a smaller surface area than deep quakes. Energy is measured by the Richter or Magnitude scales and is calculated after the quake from several recordings of the event.
Deep quakes are caused by a continental plate that is being pushed or pulled into the mantle. These forces are caused by the very slow flow of the hot rock under great pressure. Hotter rock rises and pushes on the plate while cooler rock descends and pulls on the plate.
The primary cause of the heat inside the earth is radioactive decay. About half of the heat is developed in the smaller core, making it hotter than the mantle. The other half is generated in the large mantle. Mantle near the core gets hotter and expands, making that part of the mantle less dense. This creates a force lifting the hotter rock up while cooler rock flows in to replace it. The result is a slow current of rock from the hot core through the cooler mantle, pushing the continents around. It takes millions of years for hot rock to reach the crust since it moves a few inches per year.
Geophysicists have no analogy to the famous "Voyage to the Center of the Earth" by Jules Verne. Except for deep mines (one mile), deep wells (several miles) and a few deep holes in the ocean floor, all our information on the inside of the earth is calculated from the reception of powerful sound waves. These waves are from atomic explosions, earthquakes and large volcano explosions.
The technique is called Seismic Tomography. It is similar to that of ultrasound tomography, such as is used to examine an unborn child. The scale is immensely larger, the computation much more demanding, but the results are a general picture of density boundaries inside the earth. Actual pressures and temperatures are calculated from estimating the composition of the mantle and the density of the rock above a specific depth.
Rock is not completely incompressible. It does take a lot of heat and pressure, and only the inside of the earth can generate. A few specialized laboratories can create pressures similar to the lower crust, but not any deeper. As the rock compresses and heats going down, it reaches a place where it can exist in a slightly different atomic arrangement that resists the pressure better. Over a distance of dozens of miles, the rock gradually changes into the denser form. This denser form will add to the downward motion until all of the surrounding rock is in the same form. More information is available at the REM site mentioned above.
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