Lecture 3 Earthquakes Part 1

Earthquakes Part 1

What is a fault?

Fractures in rocks along which movement occurs that develop when crust experiences deformation

What happens on either side of a fault?

Movement, called slip

What is the hanging wall and footwall?

Hanging wall - rock above the fault plane

Footwall - rock below the fault line

Normal Fault

hanging wall moves down fault plane relative to footwall.

T/F Normal Faults occur due to tension stress along divergent plate boundaries

True.

Reverse Fault

Hanging wall moves up a fault plane, relative to footwall due to compression stress along convergent plate boundaries.

Which type of fault results in crustal extension and thinning?

Normal Faults

Which type of faults result in crustal shortening and thickening?

Reverse Faults

T/F Reverse Faults occur due to tensions stress along convergent plate boundaries.

FALSE.

• due to compression stress along convergent plate boundaries.

What is a thrust fault?

Type of reverse fault, gentle slop <30 degree angle

• canadian rockies

The fault plane is vertical in which type of fault?

a. Normal Fault

b. Thrust Fault

c. Reverse Fault

d. There are no vertical fault planes

e. Strike-Slip Faults

e. Strike-Slip Faults

Movement occurs _______ to the strike line and the strike line is the ________ line of the fault plane.

Parallel, Horizontal

What are the two types of strike-slip displacement?

Left-lateral (sinistral): the opposite block appears to move left.

Right-lateral (dextral): the opposite block appears to move right.

San Andreas Fault characterized by what kind of displacement

Right lateral (dextral) displacement.

Elastic deformation

• rocks can bend / deform under stress, return to original shape when stress is removed

What is elastic rebound?

• when bent rock on both sides of a fault suddenly straighten out.

Elastic rebound theory

Seismic waves generated due to elastic rebound, explains earthquakes.

Most earthquakes occur on

reactivated fault lines

What is stick-slip behavior?

Cycle of stress build-up and eventual slippage along a fault

What is the focus (hypocenter)?

Location inside the earth where seismic waves began

→ Shallow Focus <70km deep

→ Intermediate Focus 70-300km deep

→ Deep Focus Earthquakes >300km

What is the epicenter?

Point on earth’s surface directly above the focus

What are foreshocks, mainshocks, and aftershocks?

Foreshock - cluster of small earthquakes preceding the mainshock

Mainshock - largest earthquake of the sequence

Aftershock - cluster of small earthquakes following the main shock. Each one smaller than the last, frequency diminishing over time. Hours days years.

What are the two types of body waves travellign through earth’s interior?

P-Waves and S-Waves.

Primary Waves - fast, 6km/s

Secondary Waves - come later, slower 3.6km/s

Surface waves travel on earth’s surface, slower than body waves. What are the two types?

L-Waves (Love Waves) - horizontal shearing motion, like snake

R-Waves (Rayleigh Waves) - vertical motion of rolling undulations

P waves: ______ motion _____ to the wave

Compression, Parallel

S waves: ______ motion _______ to the wave

Shearing, Perpendicular

What is the key difference in P- and S-wave motion?

P waves move in compressional motion parallel to wave and can move through solid, liquid and gas

S waves move in shearing motion perpendicular to the wave

• travel only through solids, cannot go through liquid, stop at core mantle boundary, which is how we know liquid is in earth.

Most shaking comes from

Surface waves

• L waves side to side

• R waves up and down

What is attenuation?

• waves weaken with increasing distnace from the focus

• as energy spreads out it covers more area.

What is amplification?

• waves slow down as they enter weaker rocks or sediements

• slowing velocity causes wavelength to shrink but amplitude to grow.

Which causes more shaking? Soft ground or hard rock?

Soft Ground due to amplification

What happens to seismic waves as they travel further from the focus?

a. their energy becomes more concentrated

b. their amplitude always increases

c. they weaken due to energy spreading out

d. frequency increases

c. they weaken due to energy spreading out

Why does attenuation occur?

a. waves enter softer rock

b. energy spreads over larger area

c. wavelengths become shorter

d. rocks become stronger.

b. energy spreads over larger area.

what happens when seismic waves enter soft sediments?

A. Velocity increases and amplitude decreases
B. Velocity decreases and amplitude increases
C. Velocity and amplitude both decrease
D. Velocity and amplitude both increase

B. Velocity decreases and amplitude increases

What is the difference between reflection and refraction?

Reflection - wave energy bounces away from boudnary

Refraction - wave enregy crosses boundary and changes angle.

How does rock density affect refraction?

→ crossing into less dense material, wave bends away from boundary

→ crossing into denser material, wave bends toward boundary

What is a seismograph, and what is a seismogram?

Seismograph: instrument that measures and records ground motion.

Seismogram: the recorded trace produced by the seismograph.

In what order do seismic waves arrive on a seismogram?

→ P-waves → S-waves → surface waves.

What is S-P time?

The time difference between the arrival of the S-wave and P-wave. A larger S-P time means the earthquake is farther from the recording station.

• allows calculation of distance from epicentre with 3 seismograms

How does a seismogram work?

a. The moves as ground shakes.

b. The frame moves as ground shakes, the weight and pen do not.

c. The weight moves as the ground shakes

b. The frame moves as ground shakes, the weight and pen do not.

How is an earthquake's epicenter located?

Using triangulation with three seismograms/stations.

What is earthquake magnitude

A measure related to the size/energy released by an earthquake.

What is the moment magnitude scale (Mw)?

The modern magnitude scale based on factors such as rock rigidity, fault area that slipped, and average displacement.

What happens to energy when magnitude increases by 1?

→ About 32× more energy is released.

What happens to ground-motion amplitude when magnitude increases by 1?

→ It increases by 10×.

Great earthquake has magnitude of

• >8.0

• total damage

Major Earthquake

• 7.0-7.9

• extreme damage

strong earthquake

• 6.0-6.9

• moderate to serious damage

Light earthquake

• 4.0-4.9

• felt by most, slight damage

Minor Earthquake

• <3.9

• felt by some, hardly any damage

How does earthquake depth affect shaking?

→ Shallower earthquakes generally produce stronger shaking at the surface than deeper earthquakes, all else being equal.

T/F: Normal faults occur due to tension stress, whereas reverse faults occur due to compression stress.

True

T/F: Normal faults are typically associated with convergent plate boundaries, whereas reverse faults are typically associated with divergent plate boundaries

FALSE.

Normal faults → divergent boundaries → tension stress.

Reverse Faults → convergent boundaries → compression stress

In ______ the hanging wall moves down the fault plane relative to the footwall.

a. Reverse fault

b. Left lateral strike slip

c. Right lateral strike slip

d. Normal faults.

d.       Normal faults.

T/F Reverse Faults occur due to compression stress along divergent plate boundaries.

FALSE.

→ Occur due to compression stress along convergent plate boundaries.

T/F S Waves represent shearing motion perpendicular to wave propagation.

True

Which of the following is incorrect?

a. Seismic waves are vibrations generated by earthquakes

b. In elastic deformation rocks can recover their shape.

c. San Andreas fault is characterized by left-lateral displacement

c. San Andreas fault is characterized by left-lateral displacement

T/F: P-Waves Represent compression motion perpendicular to wave propagation direction.

False

• compression motion parallel to wave propagation.