Lecture 9

Question 1

Earthquake

Answer 1

Deformation of rocks stores elastic energy in the rock. Suddenly released if that rocks breaks=earthquake

Question 2

What is an Earthquake

Answer 2

The brittle fracture (failure) surface represents a fault. Most earthquakes occur on existing faults that are locked.

These processes occur in “cold” rocks, typically less than 70 km deep

Question 3

Deep Earthquakes

Answer 3

• Go 70 km into mantle
• Wadati Benioff zone marks deep earthquakes in colder and brittle subducted crust
• helps scientists understand what happens to subduction get crust

Question 4

Focus

Answer 4

Aka hypocenter

The point where energy is first released and seismic waves originate

Question 5

Epicentre

Answer 5

The point of the Earth’s surface directly above the focus

Question 6

Measuring ground shaking

Answer 6

Seismographs are used to measure and record the seismic waves caused by earthquakes

This info is then used to determine the magnitude and focus (epicentre) of the earthquake

Question 7

Magnitude: Richter Scale

Answer 7

• based on maximum recorded amplitude of seismic waves on a seismogram
• corrected for the seismographs distance from epicentre
• log scale

Each step on scale multiplies amplitude by 10

For energy released, each step roughly corresponds to to 32X

Question 8

Magnitude: seismic moment magnitude

Answer 8

Used more commonly than Ritcher scale, which assumes the focus of the earthquake is a point (not always the case)

Takes into account that many earthquakes release energy over a large area

Question 9

Magnitude: Mercalli intensity scale

Answer 9

A qualitative scale based on the shaking people feel and damage caused

NOT corrected for distance=higher intensities closer to epicentre (unlike Ritcher and MM)

Question 10

Seismic waves

Answer 10

1. Body waves

2. Surface waves

Question 11

Body waves: P waves

Answer 11

First to arrive (primary)
Similar to sound waves (compressional)
Propagates by volume change in rock
Vibration direction is parallel to wave propagation 
Pass through solid liquid or gas 
1-14 km/s (water to base of mantle)

Question 12

Body waves: S waves

Answer 12

Secondary
Shear waves-no volume change but waves change the shape of the rock
Vibration direction perpendicular to wave propagation
Only move through solids (can cause damage)
1-8 km/s (loose sediment versus base of mantle)

Question 13

Surface waves

Answer 13

Form when body waves reach the surface

Slower but larger than body waves

Cause most damage in earthquakes

Tend to be smaller from very deep earthquakes

Question 14

Locating Earthquakes

Answer 14

Distance from the focus is determined from interval between S and Parrival at seismograph

However can’t determine the origin from one measurement

Question 15

To locate and earthquake

Answer 15

Use triangulation

• P and S waves may be moving through different areas
• where they all intersect gives us an idea of where the focus/epicentre of the earthquake occurs
• if our epicentre is deep then it may be off

Question 16

Hazards of earthquakes

Answer 16

1. Liquefaction
2. Shaking destroying infrastructure and buildings (Primary Effects)
3. Landslides
4. Land subsidence/Uplift
5. Fire