Earthquakes

Question 1

What is a fault?

Answer 1

Rupture in Earth’s crust along which rocks on one side move past those on the other.

Question 2

What are the three primary types of fault?

Answer 2

Strike slip: plates move up/down (horizontally) from one another (transform).

Normal: plates pull apart (divergent).

Reverse: hanging wall moves over footwall (convergent).

Question 3

What are two, more minor types of faulting?

Answer 3

Thrust: lower angle reverse fault (<45 degrees).

Blind thrust: occurs underground, ductile rock layers fold, go largely unnoticed.

Question 4

The horizontal line on the dipping surface is what?

Answer 4

Strike.

Question 5

What is stress and strain, pertaining to rocks?

Answer 5

Stress: forces applied on rock.

Strain: change in shape of rock in response to stress.

Question 6

Typical rocks deform _____.

Answer 6

Elastically.

Question 7

Opposite sides of faults are typically deformed instead of moving. Movement occurs along the fault when rocks undergo what?

Answer 7

Brittle failure.

Question 8

Rock mass fails suddenly by faulting when what force occurs?

Answer 8

Shear stress > shear strength.

Question 9

What are three elements of earthquakes in divergent margins (normal faults)?

Answer 9

Common, low magnitude, shallow focus.

Question 10

What is perhaps the most notable transform boundary?

Answer 10

San Andreas fault system.

Question 11

Earthquakes in transform boundaries tend to be what two things?

Answer 11

High magnitude, shallow focus.

Question 12

Convergent boundaries are caused by what kind of motion and consist of what type of faulting?

Answer 12

Horizontal motion.

Reverse faults.

Question 13

Earthquakes in convergent boundaries tend to be what two things?

Answer 13

Shallow to deep focus, large magnitude.

Question 14

What are seismic waves?

Answer 14

Energy released from the fault slip traveling outward.

Question 15

Define the focus (hypocentre) and the epicentre.

Answer 15

Focus (hypocentre): location of movement along the fault.

Epicentre: point on the map directly above the focus.

Question 16

Describe primary waves (P-waves) in terms of wave style, speed, and where they can travel.

Answer 16

Compressional (succession of elastic compressions/expansions).

Fastest (4-7 km/sec in bedrock).

Travel through all materials.

Question 17

Describe secondary waves (S-waves) in terms of wave style, speed, and where they can travel.

Answer 17

Shear waves (succession of transverse vibration).

Arrive after P-waves (2-5 km/sec in bedrock).

Only travels through solids.

Question 18

Rayleigh and Love waves are what? What are they triggered by, and what is their speed?

Answer 18

Series of violent vibrations on surface.

Triggered by P-waves reaching surface.

Velocity ~90% of S-waves.

Question 19

Seismic waves _____ like light.

Answer 19

Refract.

Question 20

Is the Richter Scale commonly used? How is it determined and how is each level stronger than the other?

Answer 20

Not commonly used except in local small earthquakes.

Determined from logarithm of the amplitude of waves recorded by seismograph.

Each level 10 times stronger than previous level.

Question 21

The Moment Magnitude Scale estimates what? How is it calculated into a magnitude?

Answer 21

Estimates the total energy released by an earthquake.

Calculated from area of fault and distance the ground moved along fault, derived from analysis of all waveforms recorded from shaking. Seismic moment computed, converted to a magnitude.

Question 22

The seismic moment is a measure of the size of an earthquake based on what three things?

Answer 22

Area of fault rupture.

Average amount of slip.

Force required to overcome the friction sticking the rocks together that were offset by faulting.

Question 23

For dip slip faults, what will seismographs feel?

Answer 23

Depending on location from focus sphere, some will feel “push” from focus, others initial “pull” towards.

Question 24

The Mercalli Intensity Scale is determined by damage done to structures. What is it dependent on, and what is the damage in question related to?

Answer 24

Depends on distance from fault rupture.

Related to crustal rocks.

Question 25

While each whole number increase in magnitude represents a tenfold increase in the measured amplitude, it represents a _____ times more energy release.

Answer 25

32.

Question 26

What do seismometers do?

Answer 26

Record the vibrations from earthquakes that travel through the Earth.

Question 27

The effect of earthquakes is up to what vertically and horizontally?

Answer 27

Up to 1-2 m/s horizontal, 1 g vertical.

Question 28

One effect of earthquakes is soil liquefaction. What is this?

Answer 28

Soil particles lose contact as result of shaking, leads to soil losing strength and acting as fluid.

Question 29

List four effects of earthquakes that could lead to displacement.

Answer 29

Mass movement.

Fire.

Disease.

Famine.

Question 30

What magnitude was the Sumatra-Andaman (2004) earthquake, how long did it last, how tall was the tsunami, and how much water was displaced?

Answer 30

9.1

10-12 minutes.

20-30 m.

30 km3.

Question 31

Damage to built structures is dependent on what five things?

Answer 31

Earthquake magnitude.

Distance from epicentre.

Duration of earthquake.

Aftershock.

Location (rural/urban, developed/undeveloped, populated/unpopulated).

Question 32

Damage to built structures is also dependent on _____ properties and _____ codes.

Answer 32

Substrate; building.

Question 33

Small buildings are more affected by what waves?

Answer 33

Short and frequent (high frequency).

Question 34

Tall buildings are more affected by what waves?

Answer 34

Long waves farther apart (low frequency).

Question 35

The 1985 Mexico City earthquake was an 8.0 magnitude earthquake that killed 9,500 and displaced over 100,000. What are the statistics for the buildings that collapsed or were seriously damaged?

Answer 35

60% in the 6-15 story range.

Question 36

Earthquake-prone countries have building codes to mitigate impacts. What is key?

Answer 36

Enforcement.

Question 37

Which relatively recent earthquake led to casualties and damage due to poor building code enforcement?

Answer 37

L’Aquila, Italy (2009).

Question 38

Damage in Haiti during 2010 was due mainly to poor or non-existent building codes. What could they not withstand?

Answer 38

Aftershocks, soil liquefaction.

Question 39

The world’s largest earthquake ever recorded was what? What was the magnitude and depth?

Answer 39

“The Great Chilean Earthquake” (1960).

9.5 magnitude, 25 km depth.

Question 40

Long term prediction for earthquakes is based on what?

Answer 40

Historical records.

Question 41

List five short-term precursors for earthquakes?

Answer 41

Changes in foreshock patterns.

Changes in resistivity of bedrock close to fault zone.

Changes in water table.

Tilting or uplift around fault zone.

Abnormal animal behaviour.

Question 42

Is earthquake prediction currently possible?

Answer 42

No, but probabilities are forecast with seismic hazard maps.