[No more witty title/meme tie-ins]
A Discussion of Seismic Activity and the Types of Seismic Waves
A recapitulation by Aaron Kaufman
P –waves , also called longitudinal are the primary type of wave, involving areas of compression and rarefaction. The particles travel parallel to the direction of the wave, and are quick but gentle. S-waves , also called transverse waves, are the secondary type of wave, involving peaks and troughs in the manner of a sine wave. The particles travel perpendicular to the direction of the wave. These are slower but deadlier than P-waves. Pwaves travel at 5-7 km/s, while S-waves travel at 3-4 km/s . S-waves can’t travel through water, and this fact serves as the greatest evidence for a liquid outer core. Surface waves only propagate along the surface, and generally cause the most damage in a quake.
Distance = (velocity primary
* velocity
secondary
* Δ time) / (velocity primary
* velocity
secondary
)
To locate the epicenter of a quake, it is necessary to take 3 points and analyze the time gap between the p- and s-waves. 3 circles are drawn from the location where the distance was taken, and the radius of the circle is the distance. The three circles should intersect at only 1 point, which is the epicenter. It is important to note that the velocity of a wave goes down with increasing temperature, and up with increasing pressure.
Love waves are surface waves that are really hard to describe. The particles move in circles or ellipses in the direction of the wave, if that helps. They are the most devastating in the immediate vicinity of the epicenter. Rayleigh waves are where the particles are moving circularly, doing a sort of barrel roll. Ocean waves are like this.
A seismometer is a device used to measure the intensity of a quake. It consists of a pen held above the ground by springs, with the tip of the pen touching a moving sheet of paper. When a quake hits, the springs absorb the shock, and the pen doesn’t move, while the paper does, creating a zigzag line. The paper can be oriented up-down or left-right to measure different waves. Seismographs are what seismometers produce:

Earthquake waves have similar properties to light. They can reflect, magnify, and take the path of least time. Note how the waves bend to go through the liquid outer .

When waves have to go through the liquid outer core, the s-waves cannot pass. There is a smaller interval within the s-wave shadow where p-waves cannot pass, called the p-wave shadow. Waves can also reflect off of the boundaries between the core and the mantle, and reflecting waves can hit areas already hit. These may go on indefinitely, as there are reflections from the Northridge quake still going on.
The Mercali scale was the first scale used to record earthquake size. It went from 1-7, based on entirely subjective measures. According to wikipedia, a modified version is still used today in some places, but mostly it isn’t used. More recent is the Richter scale , which measures quakes based on the logarithm of their amplitude on a seismogram, plus a constant to make it roughly equate to the Mercali scale. Today we mostly use the moment magnitude scale , which measures the energy released by a quake.
Moment magnitude = Log
10^2/3
E + K = (2/3) Log E + K, where K is a constant to make it equate to the Richter scale.
It is noticeably difficult to distinguish very small quakes and fat people falling over, for example. We can distinguish quakes from bombs because bombs have more P and less S.
But at any rate, damage depends on, among other things, soil type and construction standards. Sandy soil with high water content will liquefy , or act as a liquid, and the building on it will begin to sink. The San Fernando has lots of this problem. Building materials are also important for many reasons. Wood and steel are flexible and bend while shaken, whereas stone or brick or clay buildings will collapse and kill the inhabitants. As such brick buildings are illegal in the Valley. In the east, where tornadoes are the problem, all buildings are made of brick. So they’d be fucked in an earthquake and we’d be fucked in a tornado. The maximum size for a quake is 9, because there are practical limits on the size of faults, and how much friction can be generated.
There are several types of faults. The most common is the strike-slip , where all of the rock displacement is horizontal. The San Andreas is such a fault. We’re lucky in that
Wikipedia has lots of nice pictures for us. Normal faults have a hanging wall and a footwall , the former of which moves up while the latter of which moves down. The displacement is all vertical, and the movement is along a diagonal 30 degrees from the plane of the surface. Reverse thrusts are the opposite of normal faults, but with a complementary angle:

Strike-slip faults are due to lateral forces parallel to the fault plane. Normal faults are due to stretching, and reverse faults are due to compression. When there is stretching, the result is a series of peaks ( horsts) and troughs ( grabens) :
The entire Southwestern US is a giant series of these. The angular edges are usually smoothed out significantly, but sometimes they aren’t, and the cliffs are called scarps .
Thrust faults are low-angled reverse faults, and blind thrust faults are when thrust faults are covered over by layers of sediment. There are possibly hundreds hidden beneath LA right now. Northridge was caused by one:

Alongside the San Andreas fault are thousands of little, parallel faults, because when stretched, rock will not break in only one place. Along fault lines there are layers of lithified fault gauge , which is powdered rock created when faults grind themselves up.
The blocks are very nonporous, and as a result, do interesting things to the water table.
The table usually flows under the surface, but when it encounters a layer of this fault gauge, it gets blocked and has to push upward to the surface, forming a lake or oasis.
Many civilizations sprouted along these. These are very useful to paleoseismologists, because the plant material sinks and doesn’t rot, and then can be carbon dated, and this can be used to find earthquake dates.
The New Madrid Fault is located in New Madrid, Missouri. It is right below St. Louis, and is the remains of an old plate boundary.
The American Ice sheet was an ice sheet that extended down through North America and covered the entire northern half of the continent in kilometers of glacial ice. Long
Island is a result, as are the millions of lakes in Minnesota. Since the mantle acts like a viscous fluid, under all that extra weight it slowly bent inward. Now that the ice is melted, the mantle is slowly bobbing back up like an ice cube dropped in water, except much slower. It’s called elastic rebound .