#s-wave

S Wave Shadow Zone

From and early age, we are all taught that the interior of the Earth can be divided into four distinct layers; a solid crust, a plastic mantle, liquid outer core, and solid inner core. But have you ever wondered how we know these layers exist? Obviously, nobody has every managed to drill down to the center of the Earth to check. The deepest hole drilled so far, the Kola Superdeep Borehole in Russia, is just over 12 km deep, far shy of the 6, 371 km that would be required to reach the center of the Earth. Most of what we know about the interior structure of the Earth comes from studying how seismic waves generated by large earthquakes move through the planet. By using networks of sophisticated seismometers installed in bedrock around the world, geophysicists can record even the smallest vibrations. Seismic waves that travel through the Earth are generally divided into two types; compressional (P waves) and shear (S waves). P waves compress and expand material in the direction the wave is propagating. S waves shear the material perpendicular to the direction the wave is propagating. P waves travel much faster than S waves, and can be differentiated on a seismograph on the basis of their arrival times. A great way to visualize how these waves move is with a slinky, here’s an example: http://bit.ly/2xXRXgq

Liquids cannot sustain shear forces, meaning S waves can only propagate through solid material. When an earthquake occurs, seismic waves are generated at the hypocenter and propagate in all directions through the Earth. S waves are able to pass through the solid crust and mantle and can detected by seismometers on the other side. When an S wave crosses the boundary between the mantle and the outer core, some of the energy contained in the wave gets reflected back toward the surface, and some is transmitted through the core as a P wave. In the 1930’s, Beno Gutenberg found a lack of certain previously predicted S wave recordings at seismometers located more than 103 degrees of latitude from where the earthquake took place. This is known as the ‘S wave shadow zone’ for a given earthquake, and is directly caused by the inability of S waves to pass through the outer core.

CD

http://bit.ly/2xySNPt http://bit.ly/1m0UbMt http://bit.ly/2wMJT2c Fowler, C.M.R. 2005. The Solid Earth: An Introduction to Global Geophysics (2nd Edition). Cambridge University Press, Print.

Locate a quake

This pretty awesome app is about to be available on the iphone and is currently available on an ipad. It lets you take seismic data and in real time locate the epicenter of an earthquake using data from seismic stations.

The epicenter of an earthquake is the point directly on the Earth’s Surface above where the fault ruptured. When a fault breaks and waves travel outwards, they do so at speeds that are pretty well known depending on the type of rock. There can be a lot of variability near a quake, but on average the Earth is pretty homogeneous – seismic waves at a distance of a few hundred kilometers from an earthquake will only have their speeds change by a few percent depending on the rocks they go through. In other words, remember your physics – distance = rate x time, so if you know the rate, you have 1 of the 3 variables.

Earthquakes produce several types of waves – waves that travel along the surface called Rayleigh and Love Waves, and waves that travel through the planet called P and S waves. It is the P and S waves that are used in this app as those travel through the planet itself. Each of those waves has a speed that, in the earth’s lower crust and upper mantle, is well characterized.

The P-wave is faster than the S-Wave. The P-Wave will always arrive before the S-wave, and the difference in time between them depends on how far you are from the quake. In other words, from the same equation above – the separation between the P and S waves, which you see a person highlight on this app, tells you the distance from the earthquake.

A single seismic station can therefore tell you how far away an earthquake was. It creates a circle of distance around you (with small error bars around it showing how the small variation in wave speeds in the crust propagates). Once you’ve identified P and S waves from an earthquake, you know the epicenter must be somewhere in that ring.

Finally, the app lets you select other stations and locate the first arrival of a p-wave in those stations. Once you add in data from a second station, you have 2 different circles of distance and you narrow the location of the epicenter down to 2 points. Once you have three stations, you have uniquely identified the epicenter in 3-dimensional space, as only 1 point on the map will produce the observed set of waves at each position.

They recommend using this app as a tool for education, outreach, emergency management, and even scientists who want to quickly confirm data taken elsewhere.

-JBB

Video credit: https://www.youtube.com/watch?v=alX8yhMCngs

Read more: http://www.src.com.au/software/quick-quake/

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