Saturday, September 3, 2011

Earthquakes - Virginia's 2011 quake


Here's the government's official public report of the quake in case anyone wants the specifics http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/se082311a.php#summary.  My presentation was more for earthquakes in general.

We all know that the tectonic plates are constantly shifting. This is essentially the reason earthquakes occur. As the plates shift, they bend, stretch and compress against one another, causing a buildup of potential energy. When this potential energy becomes great enough, it overcomes the force of static friction between plates, and they slip against one another, creating millions of tons of force as the plates grind against each other; essentially converting potential energy to kinetic energy. This kinetic energy is released as radiated energy (seismic waves), fracture energy (ruptures), and thermal energy (heat).
Physics helps us to understand the destructive forces behind the seismic waves. These waves come in two forms; Body Waves and Surface Waves, each with two subtypes.
Body waves resonate through the interior of the earth and are quicker and shorter-lived than surface waves. The two subtypes are P-waves (primary) and S-waves (secondary). P-waves are compression waves (like sound waves) that compress and extend the rock mass in the direction it travels (through both solids and liquids). S-waves are shear waves. Their motion is perpendicular to the direction of travel, and thus are the ground-shaking waves (cannot travel through water however).
Surface waves are slower moving, but longer lasting waves, and more damaging due to their location and unique properties of the waves. The two subtypes are Love waves and Rayleigh waves. Love waves are like S-waves put on their sides, moving the ground from side to side in a horizontal plane (solids only), while Rayleigh waves move vertically and horizontally in a vertical plane (solids and liquids).
It's important to note that these waves are unpredictable because they travel at different rates through different substances, and are reflected/refracted at the interface between rock types. Thus they readily change direction and properties (P-waves can convert to S-waves for instance). - post written by John Kahler

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