Lecture 7: earthquakes

Question 1

Earthquakes:

Answer 1

They result from the rupture of rocks along a fault.

Energy from an earthquake is released in the form of seismic waves.

They are mapped according to the epicentre; the focus is located directly below the epicentre.

They are measured by seismographs and compared by magnitude

Question 2

Surface rupture:

Answer 2

forms fault scarp, amount of slip on fault

Question 3

Epicenter:

Answer 3

point of surface above focus

Question 4

Focus:

Answer 4

where rupture on fault plane started

Question 5

Earthquake magnitude:

Answer 5

The magnitude of an earthquake is expressed as a number to one decimal place.
This type of measurement was first developed by Richter in 1935
The Richter scale was a measure of the strength of a wave 100km from the epicentre.
Since then, more accurate methods have been developed and the Richter scale is no longer in use.

Question 6

The moment magnitude scale:

Answer 6

Today, earthquakes are measured using the moment magnitude scale (M)

The scale is determined by:
- The area ruptured along the fault
- The amount of movement along the fault
- The elasticity of the crust at the focus

Like the Richter Scale, it is a logarithmic scale.

Example: an M7 earthquake represents 10 times the amount of ground motion as an M6 earthquake.

Question 7

Magnitude and frequency of earthquakes:

Answer 7

Except for very large earthquakes, the magnitude on the moment magnitude scale is like the Richter scale.

The strongest earthquake to occur is M9.5 in Chile In 1960.

In Canada, it is M 8.1 in B.C. in 1949.

There are only a few M9+ earthquakes each century

Question 8

Earthquake intensity:

Answer 8

The modified Mercalli intensity scale is a qualitative scale based on damage to structures and the effect on people.

It is based on 12 categories.

Question 9

Earthquake processes:

Answer 9

Earthquakes are most common at or near plate boundaries.

Motion at plate boundaries is not usually smooth or constant.

Friction along plate boundaries exerts force (stress) on the rocks, exerting strain or deformation

When the stress exceeds the strength of the rocks, there is a sudden movement along a fault.

Thus, faults are considered seismic sources.
Identifying faults is necessary to evaluate risk of an earthquake in an area

Not all faults reach the earths surface

Question 10

Blind faults:

Answer 10

faults that are located below the surface

Question 11

The movement (or rupture) starts at the focus and propagates in all directions, called

Answer 11

seismic waves.

Question 12

Fault types:

Answer 12

There are two basic types of geologic faults distinguished by the direction of the displacement of rock or sediment.

Strike-slip faults:
Displacements are horizontal

Dip-slip faults:
Displacements are vertical

Question 13

Strike-slip faults:

Answer 13

The San Andreas fault is the best example of this type.
Less damaging

Question 14

Drip-slip faults:

Answer 14

There are three types:
- Reverse fault
- Thrust fault
- Normal fault

They are comprised of two walls on an incline defined by miners:

1. Footwall: where miners placed their feet
2. Hanging wall: where miners placed their lanterns

Question 15

reverse fault

Answer 15

The hanging wall pushing up relative to the footwall inclined at a steep angle (angle over 45 degrees)

Question 16

thrust fault

Answer 16

These are like reverse faults except the angle is 45 degrees or less

Question 17

normal faults

Answer 17

The hanging wall has moved down relative to the footwall

Question 18

Fault activity:

Answer 18

In terms of activity, faults can fall into one of three categories:
Active
Movement during the past 11,600 years
Potentially active
Movement during the past 2.6 million years
Inactive
No movement during the past 2.6 million years

Question 19

Tectonic creep:

Answer 19

Definition: the slow movement of rock or sediment along a fracture caused by stress
It is also referred to as a fault creep.

This can damage roads and building foundation (movement of a few cm per decade).
Along these faults, periodic sudden displacements producing earthquakes can also occur.

Question 20

Seismic waves:

Answer 20

generated by fault rupture travel within the body of the earth and others travel along the surface.

Question 21

Body waves:

Answer 21

These include P and S waves
Travel through the body of the earth

Question 22

P wave

Answer 22

Primary waves (compressional) push out
They move fast with a push-pull motion and can travel through solids or liquids

Question 23

S wave

Answer 23

They are also called secondary or shear waves - S like pattern
They move more slowly in an up-and-down motion and can only travel through solids

Question 24

surface waves

Answer 24

Definition: seismic waves that form when P and S waves reach Earths surface and then move along it
These waves move more slowly than body waves
Surface waves are responsible for damage near the epicenter.

Question 25

Factors that determine the shaking people experience during an earthquake:

Answer 25

Magnitude
Distance to epicenter
Focal depth
Direction or rupture
Local soil and rock types
Local engineering and construction practices

Question 25

seismographs

Answer 25

record the arrival of waves to a recording station.
Because P waves travel faster than S waves, they appear first on a seismogram
Earthquake shaking decreases with distance from the epicentre.

Question 26

Distance to the epicentre:

Answer 26

The difference between the arrival times of the first P and S waves at different locations determine the distance to the epicentre.
The distance to the epicentre is calculated at 3 different seismic stations
A circle with radius equal to that distance is drawn around the station

Question 27

Locating an earthquake:

Answer 27

The epicentre is located where the circles intersect; this process is called triangulation.

Question 28

Focal depth:

Answer 28

Seismic waves become less intense as they spread outward toward the surface.
Therefore, the greater the focal depth, the less intense the shaking at the surface.

Question 29

Attenuation:

Answer 29

this reduction of energy based on focal depth

Question 30

Direction of rupture:

Answer 30

Earthquake energy is focused along the direction of rupture.
This is known as directivity and contributes to increased shaking.
Radiated waves are sometimes stronger in one direction along the fault

Question 31

Local soil and rock types:

Answer 31

The local geology influences the amount of ground motion.
Sense homogeneous crust can transmit earthquake energy quickly
Seismic energy slows down in areas with heterogenous, folded, faulted crust.
Implication: earthquakes in Eastern North America are felt over a larger area than those are in Western North America

Question 32

Amplification:

Answer 32

Definition: an increase in ground motion during an earthquake

P and S waves slow as they travel through alluvial sand, gravel, clay, soil, etc.

As the waves slow, some of their energy is transferred to surface waves.

Question 33

Alluvial:

Answer 33

soil that was deposited by water - very soft, lots in the great lakes region

Question 34

shake maps

Answer 34

The combination of all these effects results in widespread variation of the shaking felt in the vicinity of an earthquake.
Therefore, two earthquakes that have the same magnitude can have very different impacts.

Question 35

The earthquake cycle:

Answer 35

Definition: a hypothesis that explains successive earthquakes on a fault
It is based on the idea that strain drops abruptly after an earthquake and then slowly accumulates until the next earthquakes.
As stress continues to increase, the deformed material will eventually rupture.

Question 36

A typical cycle has several stages:

Answer 36

1. Inactive period
2. A period where strain produces minor earthquakes
   Could be a few or hundreds of years
3. A period of foreshocks prior to a major release of stress
   Doesn’t always occur
   Foreshocks: a small to moderate earthquake that occurs shortly before and in the same general area as the main shock
4. A period where the mainshock occurs allowing the fault to release built-up stress
   Mainshock: the largest earthquakes in a series of associated earthquakes
5. A period of aftershocks with epicentres in the same general area as the main shock
   A small to moderate earthquake that occur shortly after and in the same general area as the mainshock.
   The time between each stage varies.

Question 37

Aftershocks:

Answer 37

Generally, the number of aftershocks that occur on a given day after a mainshock can be forecasted by a formula:
Formula: Aftershocks on given day = aftershocks on first day after / given day

Question 38

Geographic regions at risk from earthquakes:

Answer 38

Earthquakes are not randomly distributed.
Most earthquakes occur along plate boundaries:
Pacific ring of fire, Himalaya mountains, middle east
North American cities at high risk of earthquakes:
Anchorage, Vancouver, Victoria, Seattle, Portland, San Francisco, LA and Mexico city
However, not all areas at risk of earthquakes are near plate boundaries.

Question 39

Plate boundary earthquakes:
Definition

Answer 39

earthquakes that occur along faults separating lithospheric plates

Question 40

Plate boundary earthquakes: There are 3 types:

Answer 40

Strike-slip earthquakes
Thrust earthquakes
Normal fault earthquakes

Question 41

Strike-slip earthquakes

Answer 41

These earthquakes occur along transform faults where plates slide horizontally past one another.
They are common in California along the San Andrea Fault
The best-known strike-slip earthquake is the Loma Prieta earthquake that disrupted the 1989 world series in Oakland, California.

Question 42

Thrust earthquakes:

Answer 42

These earthquakes occur on faults that separate converging plates.
They are also called subduction earthquakes.
They are common off the coast of BC, Washington, and Oregon
These earthquakes are the strongest on Earth (some larger than M9) and can produce tsunamis

Question 43

Normal fault earthquakes:

Answer 43

These earthquakes occur on faults associated with divergent plate boundaries.
They are common along the mid-Atlantic ridge
Most are located under oceans and are generally smaller than M6

Question 44

Intraplate earthquakes:

Answer 44

Definition: an earthquake on a fault in the interior of a continent far from plate boundaries
These earthquakes are typically smaller than plate boundary earthquakes.
However, damage could be considerable due to lack of preparedness
Because of dense continental bedrock, these earthquakes are felt over large areas.
There are two relatively active intraplate zones in north America.
Central Mississippi river valley
St. Lawrence River Valley
The new Madrid earthquakes in Missouri (1811-12) were over M7.5 and felt over the entire continent.
The recurrence interval in this area is likely several (few) hundred years
Can be expecting another M7 earthquake soon

Question 45

Recurrence interval:

Answer 45

the time between successive events

Question 46

Effects of earthquakes:

Answer 46

Several different effects related to earthquakes contribute to deaths and property destruction.
Primary effects:
Ground shaking, surface rupture
Secondary effects:
Liquefaction, land-level change, landslides, fire, tsunami

Question 47

Surface rupture:

Answer 47

Displacement along faults causes crakes in the surface.
During strong earthquakes, fault scarps can be produced that extend for hundreds of kilometers

Question 48

Fault scarp:

Answer 48

a linear escarpment at earths surface formed by movement along a fault during an earthquake
Surface rupture can uproot trees, collapse buildings, and destroy bridges, tunnels, and pipelines.

Question 49

Liquefaction:

Answer 49

Definition: the transformation of water saturated sediment from solid to liquid
This may occur during strong earthquakes when water pressure becomes high enough to suspend particles of sediment within the soil.
Once the pressure decreases, the sediment compacts and regains its strength
Watery sand and slit may flow upward along fractures in the overlying soil material.
This effect can cause extensive damage.

Question 50

Landslides:

Answer 50

Ground shaking produced by an earthquake can cause rock and sediment to move downslope.
A single earthquake in a mountainous area can cause thousands of landslides

Question 51

Fires:

Answer 51

Ground shaking and rupture can serve power and gas lines, starting fires.
Appliances may topple over causing gas leaks that ignite
Approximately 80% of the damage during the 1906 San Francisco earthquake was caused by fire.

Question 52

Natural service functions of earthquakes:

Answer 52

Faults provide pathways for the downward flow of surface water.
They can channel groundwater to surface discharge points (springs)
New mineral resources can be found; some minerals are preferentially deposited in faults
Scenic landslides (hills, valleys) can develop in fault zones over millions of years.

Question 53

Earthquakes caused by human activity:

Answer 53

Several human activities are known to trigger small to moderate earthquakes.
The weight from water reservoirs produced by dams can create new faults
Injecting liquid waste in the earth can increase pressure and cause slippage along fractures
Fracking or mining
Testing nuclear weapons leads to explosions that may increase strain in an area.

Question 54

Minimizing the earthquake hazard:

Answer 54

Earthquakes cause death and destruction because they often occur with little warning.
At present, we can forecast the likelihood that an earthquake will occur in an area, but not exactly when it will occur.
The geological surveys of Canada and the USA are developing programs to reduce the hazard from earthquakes

Question 55

Planning for earthquakes:

Answer 55

The Denali earthquake in Alaska (2002) demonstrated the value of planning for earthquakes
Where the Tran- Alaska oil pipeline crossed the Denali fault, its construction was altered to withstand a large earthquake.

Question 55

Earthquake hazard reduction programs:
The programs have 5 goals:

Answer 55

1. Improve national seismograph networks
2. Develop awareness of earthquake sources (faults)
3. Determine earthquakes potential
4. Predict effects of earthquakes on buildings
5. Communicate research to educate the public

Question 56

Estimating seismic risk:

Answer 56

Hazard maps identify areas of risk associated with earthquake effects.
They include areas prone to liquefaction, zones of possible ground rupture, and historic epicentres

Question 57

Precursors to earthquakes:

Answer 57

If accurate forecasts are possible, they will most likely be based on precursors:
The pattern and frequency of earthquakes
Based on foreshocks and microearthquakes
Land-level change
Uplift or subsidence may precede earthquakes
GPS stations can recognize small changes in elevation
Seismic gaps along fault
Areas along a fault that have not seen recent earthquakes may be more likely to experience one
Physical and chemical changes
Changes in groundwater levels and chemistry may occur if rocks expand prior to an earthquake.

Question 58

Earthquake forecasting:

Answer 58

There have been modest incidences of successfully forecasting earthquakes.
All forecasts must be scientifically reviewed before they are released
Research projects along the San Andreas fault are aiding in understanding the conditions that occur before an earthquake
Current earthquake warning systems provide 15 to 30 seconds of warning and only warn of an earthquake that has already occurred.

Question 59

Perception of the earthquake hazard:

Answer 59

Survivors of strong earthquakes often report traumatic stress.
Typically one community experience with an earthquake has not be stimulated other communities to enhance their preparedness
Earthquakes have exposed shoddy construction practices.

Question 60

Community adjustment to the earthquake hazard:

Answer 60

It is not possible to prevent people from living in earthquake prone areas.
Therefore, we must take several steps to minimize seismic risk:
Critical facilities should be located as safely as possible
Buildings must be designed to withstand vibrations (in many cases, retrofitting is required)
Educations is a component of preparedness (workshops, training sessions, earthquake drills)
Earthquake insurance should be made available