Sandstone, Northern Colorado

The review questions are now posted.

Also, I posted a condensed set of slides for Geologic Time. You will need to know all of this information. It is a lot to learn, so please start studying now!

When a fault slips, the elastic energy that was stored in the rock is released. Some of that energy is converted into frictional heat, but MOST of that energy turns into seismic waves (vibrations).

Imagine plucking a guitar string. Elastic energy from the string is converted into waves, making music.

There are 3 types of seismic waves:

P waves (primary)

S waves (secondary)

surface waves

P and S waves are called body waves because they travel through the Earth’s interior. Surface waves travel only along the surface.

Body Waves

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All 3 waves travel at different speeds

P waves are the fastestS waves are second fastestSurface waves are the slowest

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P waves are compressional waves (the same as sound waves -when you speak, you create P waves in the air)

P waves push and pull material in the direction that they travel.

P waves travel through solids, liquids, and gases.

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S waves are shear waves, and they move material at right angles (90o) to the direction that they travel.

S waves travel ONLY through solids. They cannot travel through liquids or gases.

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Surface waves occur only on the surface, and are similar to waves that we see on water.

There are 2 types of surface waves. One type moves the ground in a rolling motion, and the other type moves the ground in a sideways motion.

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Seismograph (also called a seismometer): Instrument used to record seismic waves

Seismic waves are recorded by seismographs at many seismic stations around the world.

Old-fashioned seismograph Modern seismometer

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Seismology is the branch of geology that studies seismic waves

Geologists who study seismology are called seismologists

Seismogram: The visual record of the vibrations, recorded by a seismograph.

seismogram

Remember, P waves are the fastest, S waves are the second fastest, and surface waves are the slowest.

So, P-wave arrives first, followed by an S wave, and then finally surface waves.

seismogram

How do seismologists determine the focus of an earthquake?

In the mid 1900’s seismologists started keeping records of earthquake vibrations at many different seismic stations.

If they knew the location and time of original location (i.e., the focus) of the earthquake, they could plot how long it took for the different vibrations (such as P waves and S waves) to reach the different seismic stations.

How do seismologists determine the focus of an earthquake?

Here is an example of what that looked like.

P-wave

S-wave

Other P and S waves that bounced around the Earth’s interior

How do seismologists determine the focus of an earthquake?

As more data was collected, seismologists noticed that this chart was very consistent.

The main result was this:

The farther away the focus of the earthquake was from the seismic station, the longer the time gap between the P and S wave arrivals at that station.

THEREFORE, by knowing the time gap between the P and S wave vibrations, you can determine how far away your station is from the focus of the earthquake.

How do seismologists determine the focus of an earthquake?

A seismologist at a seismic station may observe the following seismogram that shows a P wave, followed by an S wave arriving at the seismic station.

How do seismologists determine the focus of an earthquake?

The seismologist would then measure the time difference between the P wave and the S wave.

Almost 8 minutes

How do seismologists determine the focus of an earthquake?

The seismologist would then find where that time difference between the P and S wave fits on the plot.

P-wave

S-wave

How do seismologists determine the focus of an earthquake?

Here’s a nicer looking view of the same thing

How do seismologists determine the focus of an earthquake?

Once seismologists determine the distance to the focus for several different seismic stations, they can triangulate to find focus of the earthquake.

How do seismologists determine the focus of an earthquake?

In summary:

Right after an earthquake:

1) At each seismic station, seismologists measure the time difference between the P wave arrival and the S wave arrival.

2) They use that time lag to determine how far away the focus of the earthquake was from that station.

3) By comparing how far away the earthquake focus was from several different seismic stations, they can triangulate the location of the earthquake focus.

Earthquake Magnitude

There are several ways in which the intensity of an earthquake is measured.

Here we will cover:

Richter Magnitude Scale – which is the most popular with the media

Moment Magnitude Scale – which is most popular with seismologists

Earthquake Magnitude

Both scales are logarithmic – this means that each number on the scale represents an earthquake that is 10x stronger than the previous number.

Examples:

A magnitude 6 is 10x stronger than a magnitude 5

A magnitude 7 is 10x stronger than a magnitude 6

A magnitude 8 is 10x stronger than a magnitude 7

A magnitude 8 is 100x stronger than a magnitude 6

A magnitude 8 is 1000x stronger than a magnitude 5

Earthquake Magnitude

Both scales are corrected for distance.

The farther the P, S, and surface waves travel away from the focus, the less intense their vibrations become. Therefore, stations near the focus record stronger vibrations than stations further away from the focus.

If this was not corrected, each station would record a different magnitude. To correct for this, each station applies a correction for its distance away from the focus. Therefore, each station will record the earthquake as having the same magnitude.

Richter Magnitude Scale

The magnitude of the earthquake is determined by the amplitude of the vibrations.

Earthquake Magnitude

The problem with the Richter scale is that it doesn’t adequately measure the energy of the earthquake.

Earthquakes can come in different varieties. Some are quick with high amplitude vibrations, while others are long-lasting with lower amplitude vibrations.

The Richter magnitude would be bigger for a quick earthquake with high amplitude vibrations; however, a longer-lasting earthquake with lower amplitude vibrations may actually be more powerful.

To fix this problem with the Richter scale, seismologists developed the Moment Magnitude Scale.

Earthquake Magnitude

Hiroo Kanamori (CalTech) invented the moment magnitude scale which is now used by all seismologist.

Moment Magnitude Scale

The preferred scale used by seismologists because it is a measure of the physical processes of the earthquake itself.

To calculate the moment magnitude, seismologists integrate (a calculus term that means “sum up”) the entire seismogram.

In contrast, the Richter Magnitude is based only on the part of the seismogram that includes the largest vibration.

The moment magnitude of an earthquake tells you something about its:

1) total amount of energy released by the earthquake

2) the size of the fault

Earthquake Magnitude

Earthquake Magnitude

Modified Mercalli Intensity Scale

It is not scientific. It relies upon human observations, not actual measurements.

You don’t need to know the items on this scale

By comparing seismograms at many different seismic stations, seismologists can determine what type of fault caused the earthquake.

A seismic station over here would first see a “push”

A seismic station over here would first see a “push”

A seismic station over here would first see a “pull”

A seismic station over here would first see a “pull”

Seismologists create maps of which seismic stations have a “push” and which seismic stations have a “pull”. These maps are called focal mechanisms.

A 3D representation of how a fault emits P and S waves that either “push” or “pull”. Arrows pointed out indicate “push” and arrows pointed in indicate “pull”.

You don’t need to know the details of this slide

For example, many earthquakes in this region have “focal mechanisms” that indicate that rocks are moving along normal faults.

Large earthquakes are less common than small earthquakes

For example, there are thousands of magnitude 4 earthquakes every year, but only about 10 (on average) magnitude 7 earthquakes every year.

Some Notable Large Earthquakes that have occurred in the past

San Francisco 1906 Estimated Magnitude 8.0