Seismology

Seismology - Earthquakes II: "X-Raying the Earth"...

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Earthquakes II: "X-Raying the Earth" 1. Waves are detected by the motion of the ground, relative to a stationary mass hung on a pendulum or spring, suitably amplified. 2. The arrival times of the P & S waves are used to compute the distance to the EQ focus and locate it. 3. Most (85%) of EQ's are less than 64 km deep, in great belts along plate boundaries. Rest of EQ's extend down to 700 km in dipping planar zones marking the cores of subducted oceanic lithosphere. 4. Studies of the arrival times of seismic waves yield evidence about the internal structure of the Earth: Reflection methods compute the depth of velocity changes ( e.g., layering in sedimentary rocks, core/mantle boundary) from the 2-way vertical travel time. Refraction methods determine seismic velocity (and rock type) from the speed of horizontal waves ( e.g., distinguishing crust from mantle). Tomography measures velocities in 3-D within the Earth by measuring travel-times with many look angles, and solving a large inverse problem.
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1 c:\work\classes\ess1\seismolo.doc EARTHQUAKES II: X-RAYING THE EARTH 1. Waves are detected by the motion of the ground, relative to a stationary mass hung on a pendulum or spring, suitably amplified. When the Earth, foundation, building, and seismograph are shaking, you need a stable, unmoving mass to use as a reference. As Newton said, “A body at rest tends to remain at rest unless acted upon by an outside force.” Every seismometer (“shaking-meter”) includes a mass. We cannot isolate it from outside force, or gravity would make it fall! However, we can hang a mass on a string (pendulum) to greatly reduce the transmission of lateral forces when the seismometer moves sideways. We can hang another mass on a long, soft spring to try to keep the vertical force nearly constant, even when the seismometer moves vertically. Thus, a particular seismometer can only measure one of the 3 components of ground motion (up-down, N-S, or E-W). It will take 3 seismometers to completely record the motion. Engineering note: A pendulum or mass-on-spring has its own natural oscillation time, or period. It is only a good recorder for waves which pass in less time than this (waves of shorter period). In order to make the best possible seismometer, a lot of cleverness goes into increasing the natural period of the pendulum or mass-on-spring, without making
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Seismology - Earthquakes II: "X-Raying the Earth"...

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