Lecture 4 - Earthquakes, the basics

Question 1

What is the difference between a right stepping and a left stepping right lateral fault?

Answer 1

Right stepping - The land is pulled apart at the fault bend whenever the fault moves, It creates a hole that could become a basin. Left stepping - The land is pushed together when the fault moves. Eventually could grow into a mountain

Question 2

Where is the epicenter on the Earth’s surface?

Answer 2

Directly above the hypocenter.

Question 3

What is the hypocenter?

Answer 3

Where the rupture began

Question 4

Why does the epicenter not fall on the trace of the fault at the surface?

Answer 4

The dip of the fault.

Question 5

What are the basic principles of how a seismometer works?

Answer 5

• The earth moves moving the framework of the seismometer. - The suspended heavy weight and then pen beneath remain relatively stationary. - As the concrete base moves the stationary pen draws an ink line on paper on the rotating drum.

Question 6

Define amplitude, wavelength and the period of waves

Answer 6

• Amplitude: height of the waves above starting point - Wavelength: distance between successive waves - Period: time between successive waves = 1/frequency

Question 7

Give standard strengths of body and surface waves

Answer 7

Body waves - 0.5 to 20 Hz Surface waves - 0.005 to 0.1 Hz

Question 8

Name and describe the two types of body waves

Answer 8

• P-waves: Travel fastest, like the push-pull motion of slinky toy (4.8km/sec in granite) - S-waves: Move in an up and down motion perpindicular to direction of advance, like the waves in a shaken rope. (3km/sec in granite)

Question 9

What are surface waves?

Answer 9

• Created when body waves hit surface. - They are similar to the waves created by throwing a rock into a lake

Question 10

What kinds of waves compose the surface waves?

Answer 10

• Love waves (horizontally polarized) and Raleigh waves (cause horizontal and vertical movement) - Travel slower than body waves but less attenuation because of long wavelength and low frequency

Question 11

LEARN

Question 12

How are earthquakes located using triangulation?

Answer 12

• Need three seismic stations
• Measure the P-S arrival times and convert these to distance
• Triangulate using radius given by P-S travel times

Question 13

Name and briefly describe the use of the three magnitude scales

Answer 13

• Mercalli: measures damage done by an earthquake
• Richter scale: measures the energy released during an earthquake (M_L=log₁₀A-0.15+1.6log₁₀D) where A is the amplitude of the P-wave in mm
• Moment magnitude (M): The seismic moment of the earthquake; the rigidity of the Earth x the amount of slip on the fault plane during the earthquake

Question 14

Potential energy is stored in the crust in the form of built-up stress. During an earthquake, this stored energy is transformed. What does it result in?

Answer 14

• Cracks and deformation in rocks
• heat
• radiated seismic energy (Es)

Question 15

What is the seismic moment a measure of?

Answer 15

The total amount of energy that is transformed during an earthquake. Only a small fraction of the seismic moment is converted into radiated seismic energy which is what seismographs register.

This means that 99% of the energy produced by earthquakes we cannot measure!

Question 16

Why is the moment magnitude a better measurement than the seismic moment for larger earthquakes?

Answer 16

It measures directly the amount of strain energy released by movement along rupture surface

Question 17

Why are buildings with ‘soft’ first stories especially susceptible to building damage during earthquakes?

Answer 17

No bracing on the first floor and no shear walls. Stresses are concentrated at the joints between the first and second floors. They give way causing the floor to flatten.