12.2 Final Installment: Earthquakes

Tectonic plates have a lot of friction Tectonic plates have a lot of friction. There is constant pressure on them, but it takes a whole lot of energy to overcome the inertia of something with the mass of a tectonic plate.

When enough pressure builds up, the plates push free and slide past one another. This results in an earthquake.   The point where this slippage takes place is called the focus (plural foci). It is the location INSIDE the earth where the quake starts.

Directly above this point on the surface of the earth is the epicenter.   Different plate interactions can cause quakes at different depths (ex: subduction vs. transform)

All of this shaking causes a variety of seismic waves All of this shaking causes a variety of seismic waves. Studying earthquakes by examining seismic waves is called seismology. There are several types of waves that can be formed: P waves, S waves, and L waves. (see chart on p. 529)

Seismic Wave Symbol Description   Primary Wave P -Type of body wave -Fastest speed (6 km/s) -Ground squeezes and stretches in direction of wave travel -Travels though solids, liquids, and gases Secondary Wave S -Slower than P waves (3.5 km/s) -Ground motion perpendicular to direction of wave travel -Cannot travel through liquids. Surface Wave L -Travels along earth’s surface -Slowest -Makes ground ripple, like the surface of a pond when you throw in a rock.

It is actually by studying these seismic waves that we figured out the composition of the layers of earth. Changes in speed and direction of the waves allow us to predict what type of material they move though. Much like light waves, the seismic waves bend (refract) when changing mediums. Also, S waves are not detected on the opposite side of the earth from an earthquake. This means the outer core must be a liquid.

We like to measure earthquakes based on how much “shaking” takes place We like to measure earthquakes based on how much “shaking” takes place. Machines that measure this shaking are called seismometers (or seismographs). One of the earliest known seismographs was from China and dates back over 3000 years.

Modern seismographs produce a physical record of ground motion Modern seismographs produce a physical record of ground motion. This is called a seismogram.   One of the first accepted ways of measuring an earthquake is called the Mercalli scale. It was based on the effects of the earthquake, not how much shaking was involved. The problem with this scale is that it makes it very hard to compare earthquakes in different places.

You may have heard of something called the Richter scale You may have heard of something called the Richter scale. It also measures earthquakes, but it measures how much shaking takes place. The shaking (or strength) of an earthquake is called its magnitude.

The Richter scale uses magnitude. It is a logarithmic scale The Richter scale uses magnitude. It is a logarithmic scale. That means the difference between a “1” and a “2” on the Richter scale is actually 10x. So the difference between and “3” and a “5” would be 10 x 10 = 100.

There is no maximum limit to the Richter scale, however, a quake that would measure over 10 on the scale would require so much energy that it is practically impossible. The biggest quake on record was in Chilie and measured 9.5

7.9 on the Richter scale is about equal to 10 of the nuclear bombs dropped on Japan during WWII.