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Hello Meteor - Surface Waves


Hello Meteor - Surface Waves

A tsunami is a natural disaster which is a series of fast-moving waves in the ocean caused by powerful earthquakes, volcanic eruptions, landslides, or simply an asteroid or a meteor crash inside the ocean. A tsunami starts as a small wave and builds up into a larger wave., a tsunami starts suddenly. The waves travel at a great speed across an ocean with little energy loss. They can remove sand from beaches, destroy trees, toss and drag vehicles, houses and even destroy whole towns and cities. Tsunamis can even be caused when a meteorite strikes the earth's surface, though it is very rare. A tsunami normally occurs in the Pacific Ocean, especially in what is called the ring of fire, but can occur in any large body of water like lakes and seas. If the slope of the coast is not steep, the water may pull back for hundreds of meters. People who do not know of the danger often remains at the shore.

Tsunamis are often called seismic sea wave or tidal waves because they usually rise and fall more slowly than ordinary ocean surface waves. This name is misleading, because tsunamis are not related to tides; they merely rise slowly as a series of fast-moving waves in the ocean caused by powerful earthquakes or volcanic eruptions.

It's common sense to expect the energy from the explosion to arrive at a later time at stations further away, which it does. Notice how the sharp peak corresponds to distance? We can actually make a plot of this and learn some more from the data. To do this, pick a feature that is easily identified in each waveform (we used the first trough) and record how many seconds after the blast it arrives at the instrument. We then plot that on the x-axis of a graph and the distance of the station from the blast on the y-axis. The result should be something like that shown in figure 2. Now we can use some basic math to figure out how fast this energy was traveling. The red line on the figure is the "best fit line" to the data. We use some basic statistics (a linear regression) to make this line, but any plotting program will do it for you. A line has a slope (how steep it is) and a y-intercept (where it touches the y-axis when x is zero). The slope of a line is how much the y values change per a certain change on the x axis, often taught as "rise over run" in the classroom. The slope of this line turns out to be about 3km/s. That's a pretty reasonable speed for surface waves (which these are) through the ground!

The p-wave could be hard to see because 1) it's going to be relatively small, and 2) there are waves from an earthquake in Tonga arriving about the same time as the meteorite explosion. We know the waves we picked aren't from the tonga event, those would have arrived at all the stations at almost the same time because they were reflecting off the Earth's core. It would be an interesting project to play with trying to pick p-waves and/or estimate their arrival window by guessing the height of detonation.

We don't have to stop here though. This morning I saw this youtube video, a compilation of people recording the shockwave. The meteorite had streaked past, exploded, and they were recording this when the shock wave hit. Shockwaves behave in a funny way, but luckily it's been studied a lot by the government. Why? Nuclear weapons! Seismologists are commonly employed to determine if a nuclear test has taken place, and estimate it's size, location, etc. A lot of very interesting information on air-blast and it's interaction with buildings can be found in the book "The Effects of Nuclear Weapons". The book has lots of formulas and relations that could make many interesting lab exercises, but we'll just discuss reflection in this post.

Just as Earth was about to have a close encounter with asteroid 2012 DA14, the people of Chelyabinsk, Russia had a personal experience. Before we talk about both 2012 DA14 and the Chelyabinsk event some terminology needs to be set out. A meteoroid is a s


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