# GEOL 1700 Midterm: Earthquakes - Study Guide Key - Part II - Spring 2017

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STUDY GUIDE FOR EARTHQUAKES

AND EARTHQUAKE HAZARDS – PART II

General Study Questions - Short and Not-So-Short:

1. What are seismic waves? Describe the major types and sub-types of Seismic Waves (i.e., where

they travel, how they travel, types of matter through which they travel, their relative velocities, etc.).

When an earthquake occurs, it releases energy in the form of waves that radiate from the earthquake

source in all directions. The different types of energy waves shake the ground in different ways and also

travel through the earth at different velocities. The fastest wave, and therefore the first to arrive at a

given location, is called the P-wave. The P wave, or compressional wave, alternately compresses and

expands material in the same direction it is traveling. The S-wave is slower than the P wave and arrives

next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling.

Surface waves follow the P- and S-waves.

Primary or P-waves propagate through materials in a push (compressional waves) and pull

(Dilational) type motion in the direction of travel. Since P-waves cause a change in volume, and

since all 3 states of matter resist a change in volume, P-waves can travel through all 3 states of

matter. Average velocity in the upper crust is ~6 km/s. Because they are the first to arrive at a

seismograph station after an earthquake, they are called P-waves for Primary Waves.

Shear or S-waves travel along the same paths as P-waves but only at about 60% of their velocity.

S-waves have a horizontal component of motion that vibrates perpendicular to the vertical

component of motion in the direction of propagation. S-waves only change the shape of materials

in which they travel so they can only travel through solids and cannot travel through liquids and

gases since these 2 states of matter do not resist a change in shape. S-waves cause some of the

damage during EQs. Their average velocity in the upper crust is ~3.5 km/s. Because Shear Waves

travel more slowly than P-waves and always reach any given seismograph recording station at

some time after the P-waves arrive, they are called S-waves for Secondary Waves.

Surface Waves travel only along the Earth’s surface and are called Long Waves or L-waves

because they have a long wave period, which is the time required for successive wave crests

to pass a fixed reference point. Thus, surface waves collectively are called L-waves for Long

Period Waves. Surface waves are important to planners and engineers because they cause

most of the vibrations that cause damage. There are 2 types of L-waves.

A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical

motion, with no transverse, or perpendicular, motion.

Love Waves travel in a similar fashion to S-waves, that is, with particle motion at

right angles to the direction propagation. However, Love waves have no vertical

component of displacement. The horizontal displacement/motion produced by Love

waves is what causes most the damage during EQs.

2. How can P-wave and S-wave from one seismograph station be used to determine the distance to an

earthquake epicenter from this seismic station? Describe how seismologist geographically locate

the epicenter of an earthquake. (Review your Virtual Earthquake Exercise)

Because the P- and S-waves always travel along the same paths, but at different speeds, the lag in the

arrival of the S-wave behind the P-wave is proportional to the distance from the EQ epicenter. So the

greater the lag time or difference in arrival time between the P- and S-wave, the greater the distance to

the EQ epicenter. Thus, the distance from any seismograph station to an EQ epicenter can be

determined using Time-Distance Graphs or by solving simultaneous equations for 2 unknown variables

3. What is Earthquake Magnitude (what does it measure?), and what scale is used to measure it.

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Describe how this scale works. Why are there so many Earthquake Magnitude Scales?

“Earthquake Magnitude” <http://earthquake.usgs.gov/learn/glossary/?term=magnitude>

The magnitude of an earthquake is determined from the logarithm of the amplitude of

waves recorded on a seismogram at a certain period. The Richter Scale plus the

Moment Magnitude Scale + others

Earthquake size, as measured by the Richter Scale is a well known, but not well

understood, concept. The idea of a logarithmic earthquake magnitude scale was first

developed by Charles Richter in the 1930's for measuring the size of earthquakes

occurring in southern California using relatively high-frequency data from nearby

seismograph stations. This magnitude scale was referred to as ML, with the L standing

for local. This is what was to eventually become known as the Richter magnitude.

As more seismograph stations were installed around the world, it became apparent

that the method developed by Richter was strictly valid only for certain frequency and

distance ranges. Because of the limitations of the original Richter magnitude scale,

ML, a new, more uniformly applicable extension of the magnitude scale, known as

moment magnitude, or Mw, was developed. In particular, for very large earthquakes

moment magnitude gives the most reliable estimate of earthquake size. New

techniques that take advantage of modern telecommunications have recently been

implemented, allowing reporting agencies to obtain rapid estimates of moment

magnitude for significant earthquakes. The USGS has used the Mw Scale since 2002.

4. What is Earthquake Intensity (what does it measure?), and what scale is used to measure it.

Describe how this scale works and how it can be used to reconstruct the Magnitude of

Earthquakes that occurred before 1935 (i.e., before development of the Richter Scale)

“Mercalli Scale” <http://earthquake.usgs.gov/learn/glossary/?term=intensity>

The Mercalli Scale is based on observable EQ damage. From a scientific standpoint,

the Richter scale is based on seismic records while the Mercalli is based on observable

data which can be subjective.

Thus, the Richter scale is considered scientifically more objective and therefore more

accurate. For example a level I-V on the Mercalli scale would represent a small amount

of observable damage. At this level doors would rattle, dishes break and weak or poor

plaster would crack. As the level rises toward the larger numbers, the amount of

damage increases considerably. The top number, 12, represents total damage.

The following table gives intensities that are typically observed at locations

near the epicenter of earthquakes of different magnitudes.

Magnitude / Intensity Comparison

Magnitude

Typical Maximum

Modified Mercalli Intensity

1.0 - 3.0

I

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3.0 - 3.9

II - III

4.0 - 4.9

IV - V

5.0 - 5.9

VI - VII

6.0 - 6.9

VII - IX

7.0 and

higher

VIII or

higher

5. What is the difference between the magnitude and intensity scales?

Magnitude and Intensity measure different characteristics of earthquakes.

Magnitude measures the energy released at the source of the earthquake.

Magnitude is determined from measurements on seismographs. Intensity

measures the strength of shaking produced by the earthquake at a certain

location. Intensity is determined from effects on people, human structures,

and the natural environment.

6. How are Great Earthquakes and Major Earthquakes defined, and what is the annual, global

Frequency of each? Know “Major” and “Great”.

Descriptor

Magnitude

Average Annually

Great

8 and higher

1

Major

7 - 7.9

18

Strong

6 - 6.9

120

Moderate

5 - 5.9

800

Light

4 - 4.9

6,200 (estimated)

Minor

3 - 3.9

49,000 (estimated)

Very Minor

< 3.0

Magnitude 2 - 3: about 1,000 per

day

Magnitude 1 - 2: about 8,000 per

day

7. Compare and contrast Earthquake Hazards to Earthquake Risks.

<http://earthquake.usgs.gov/learn/glossary/?term=earthquake risk>

<http://earthquake.usgs.gov/learn/glossary/?term=earthquake hazard>

Earthquake hazard

Earthquake hazard is anything associated with an earthquake that may affect

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