Seismic Waves Questions 1, 4, 5

By Deirdre Peck & Steve Gabriel

1. What are the types of seismic waves? How are they different from one another?

http://walrus.wr.usgs.gov/infobank/programs/html/school/moviepage/03.01.19.html

Geologists divide the seismic waves that travel through the Earth's interior into two basic types,

   "primary" or "P-waves" and

   "secondary" or "S-waves."

A P-wave is a compressional wave that makes the rock vibrate parallel to the direction of its movement.

Since it is a very fast wave traveling through rock at between four and seven kilometers per second, the P-wave is the first wave to arrive at a recording station following an earthquake.

This is also known as a longitudinal wave

An S-wave, on the other hand, has a shearing motion that makes the rock vibrate perpendicular to its path.

This movement slows the S-wave, so that it travels at two to five kilometers per second or about half the speed of the P-wave.

This is why S-waves arrive as secondary waves at the Earth's surface.

There is another important difference between P-waves and S-waves.

Although both can pass through solid rock, only P-waves can also pass through gases and liquids.

This is also known as a transverse wave

Wave animations found at:

http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

2. What are sources of seismic waves?  From Wikipedia

A seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun. Seismic sources can provide single pulses or continuous sweeps of energy, generating seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract and are recorded by receivers, such as geophones or hydrophones.[1]

Seismic sources may be used to investigate shallow subsoil structure, for engineering site characterization, or to study deeper structures, either in the search for petroleum and mineral deposits, or to map subsurface faults or for other scientific investigations. The returning signals from the sources are detected by seismic sensors (geophones or hydrophones) in known locations relative to the position of the source. The recorded signals are then subjected to specialist processing and interpretation to yield comprehensible information about the subsurface.[2]

3. How are seismic waves measured?

Seismic waves are basically measured by keeping an instrument still and having the inertia of the earth movement record the on the instrument.

http://earthquake.usgs.gov/learn/topics/seismology/keeping_track.php

S waves are emanating vertically at the surface of the earth.

P waves would generate the horizontal waves on the surface