Lecture 7: Earthquakes

What creates an earthquake?

A rupture of rocks along a fault

Energy from an earthquake is released in what form?

Seismic waves

Earthquakes are mapped according to the ____

Epicentre

Where is the focus located

Directly below the epicentre

They are measured by ____ and compared by _____

• seismographs

• magnitude

What did the Richter Scale measure?

the strength of a wave 100 km from the epicentre

What scale do we use to measure earthquakes today?

The Moment Magnitude Scale

The Moment Magnitude Scale is determined by: (3 things)

1. The area of rock ruptured along a fault

2. The distance of movement along the fault

3. The eslasticity of the rock at the focus

Like the Richter Scale, it is a ______ scale

Logarithmic

An M7 earthquake represents ___x the strength of an M6 earthquake

10x

What is the strongest earthquake ever recorded o the Moment Magnitiude Scale?

M9.5 in Chile in 1960

What scale do we use to measure intensity?

The Modified Mercalli Intensity Scale

What is the The Modified Mercalli Intensity Scale?

This is a qualitative scale based on damage to property and the effect of an earthquake on people.

Where are earthquakes most common? Why?

At or near plate boundaries

• Friction along plate boundaries exerts a 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

The earthquake process:

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

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

3. The rupture starts at the focus and propagates in all directions in the form of seismic waves

What is a fault located below earth’s surface called?

Blind faults

What are the 2 basic type of geologic faults distinguished by?

the direction of the displacement of rock

Two types of faults and direction of displacement:

Strike-slip faults: displacements are horizontal

• transform

Dip-slip faults: displacements are vertical

• normal, thrust, reverse

Example of a strike-slip fault

San Andreas

What are the 3 categories of Dip-Slip faults?

1. Reverse faults

2. Thrust faults

3. Normal faults

What are the two walls in dip-slip faults?

1. Footwall —> where miners placed their feet

2. Hanging-wall —> where miners placed their lanterns

Reverse fault:

The hanging-wall has moved up relative to the footwall and is inclined at an angle above 45 degrees    

Thrust Fault:

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

Normal Fault:

The hanging-wall has moved down relative to the footwall

What are the 3 categories of fault activity?

1. Active

2. Potentially active

3. Inactive

Active faults:

movement during the past 11,600 years

Potentially active faults

movement during the past 2.6 million years

Inactive faults:

no movement during the past 2.6 million years

What is tectonic creep? (or fault creep)

The extremely slow movement of rock along a fracture caused by stress

What is bad about tectonic creep?

• can damage roads and building foundations

• can also produce earthquakes if sudden displacement across these faults occurs

What are body waves?

seismic waves generated by fault rupture that travel within the body of the earth

2 types of body waves?

1. P waves

2. S waves

p-waves:

• also called primary or compressional waves

• move relatively fast with a push-pull motion

• can travel through solids or liquids

• fast

• push-pull (back and forth)

• solids AND liquids

s-waves:

• also called secondary or shear waves

• they move relatively slow in an up-and-down motion

• can only travel through solids

• slow

• updown

• ONLY solids

What are surface waves?

Seismic waves that form when P waves and S waves reach Earth’s surface and then move along it

• move more slowly than body waves

• surface waves are responsible for damage near the epicentre

• slower

6 factors that determine earthquake shaking:

1. magnitude

2. distance to epicentre

3. focus depth

4. direction of rupture

5. local soil and rock type

6. local engineering and construction practices

What waves appear first on a seismogram?

P waves

• because they travel faster

How do we determine the distance to the epicentre?

Measuring the difference between the arrival times of the P waves and S waves at different locations

• gets calculated at 3 different seismic stations

• a circle with a radius equal to that distance is drawn around the station

How do we locate an earthquake (epicentre)?

Triangulation —> where the 3 circles intersect

What is focus depth?

Seismic waves become less intense as they spread outward toward the surface

What is the relationship between focus depth and shaking?

Inverse

• the greater the focus depth, the less intense the shaking at the surface

greater focus depth = less shaking

What is directivity?

Earthquake energy is more focused along the geographic direction of rupture

• contributes to increased shaking

• waves are stronger along the direction of the fault

What kind of crust transmits energy quickly vs slowly?

Dense, homogenous crust —> transmits seismic energy quickly

Heterogeneous, folded, faulted crust —> slows down seismic energy

Why do we feel more shaking in eastern North America than western North America?

The underlying bedrock in eastern North America is more homogeneous

What is amplification?

An increase in ground motion during an earthquake

P waves and S waves slow down as they travel through _____ sand, gravel, clay, soil, etc

Alluvial

What does alluvial mean?

any material deposited by water

What is the definition of The earthquake Cycle?

A hypothesis that explains successive earthquakes on a fault overtime

• based on the idea that strain drops abruptly after an earthquake and then slowly accumulates until the next earthquake

• As stress continues to increase, the deformed rocks with eventually rupture

The Earthquake Cycle (diagram)

The Earthquake Cycle stages:

1. Inactive period

2. Period of strain

3. Period of foreshocks prior to major stress release (doesn’t always happen)

4. Period where the mainshock occurs, allowing the fault to release the stress

5. Period of aftershocks

* time between each stage varies

What is a foreshock?

a small to moderate earthquake that occurs shortly before and in the same general area as the mainshock

What is a mainshock?

the largest earthquake in a series of associated earthquakes

What is an aftershock?

a small to moderate earthquake that occurs shortly after and in the same general area as the mainshock

How can we forecast the number of aftershocks that occur on a given day after a mainshock?

Aftershocks on given day

= aftershocks on 1st day after / given day

Example: If 200 aftershocks occurred on the first day after the mainshock, how many aftershocks are likely to occur on the 7^th day after the mainshock?

200 / 7 = 29

Are earthquakes randomly distrubuted?

No

Where do most earthquakes occur?

Along plate boundaries

• Ex. Pacific Ring of Fire, Himalaya Mountains, Middle East

Some occur intraplate

What North American cities are at high risk of earthquakes:

• Anchorage

• Vancouver

• Victoria

• Seattle

• Portland

• San Francisco

• LA

• Mexico City

What areas are at high risk of intraplate earthquakes?

Memphis & St. Louis

What are plate boundary earthquakes?

Earthquakes that occur on faults which specifically separate plates

What are 3 types of plate boundary earthquakes?

1. Strike-slip

2. Thrust

3. Normal

Strike-Slip Eathquakes

• occur along transform faults where plates slide horizontally past one another

• Example: San Andreas fault

Thrust Earthquakes

• occur on faults that separate converging plates

• also called subduction earthquakes

• Common off the coast of B.C., Washington, and Oregon

• These earthquakes are the strongest on Earth and can produce tsunamis

Convergent plate boundary / subduction zones

Normal Fault Earthquakes

• occur on faults associated with divergent plate boundaries

• common along the Mid-Atlantic Ridge

• Most are located under oceans and are generally smaller than M6

Convergent plate boundaries / mid-ocean ridges

Intraplate Earthquakes

• An earthquake on a fault in the interior of a continent, far from a plate boundary

• typically not as strong as plate boundary earthquakes.

• damage could be considerable due to lack of preparedness

Why are intraplate earthquakes felt over large areas?

Because of dense continental bedrock

Where are the 2 active intraplate zones in North America?

1. Central Mississippi River Valley

2. St. Lawrence River Valley

What is recurrence interval?

the time between successive events

example: the recurrence interval in areas on active intraplate zones is likely several hundred years

Earthquake hazard map

Primary effects of earthquakes

• Ground shaking

• surface rupture

Secondary effects of earthquakes

• Liquefaction

• land-level change

• landslides

• fire

• tsunamis

What is a fault scarp?

a linear escarpment at Earth’s surface formed by movement along a fault during an earthquake

What is surface rupture and what is its effects?

• Displacement along faults causes cracks in the surface.

• Surface rupture can uproot trees, collapse buildings, and destroy bridges, tunnels, and pipelines

What is liquefaction?

The transformation of water-saturated sediment from solid to liquid during an earthquake

• It occurs 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

How do earthquakes cause landslides?

Ground shaking produced by an earthquake can cause rock and sediment to move downslope

How do earthquakes cause fires?

Ground shaking and rupture can start fires by severing power and gas lines.

• Appliances may topple over causing gas leaks that may ignite

• ~ 80% of the damage during the 1906 San Francisco earthquake was caused by fire

What are the natural service functions of earthquakes?

• Faults provide pathways for the downward flow of water

• They can channel groundwater to surface discharge points (springs)

• New mineral resources can be exposed —> Some minerals are preferentially deposited in faults

• Scenic landscapes (hills, valleys) develop in fault zones over millions of years.

How can earthquakes be caused by human activity?

• Fracking (breaking of rock to get oil)

• The weight from water reservoirs produced by dams can create new faults

• Injecting liquid waste deep in the Earth can increase pressure and cause slippage along fractures

• Testing nuclear weapons leads to explosions that may increase strain in an area

Earthquake Hazard Reduction Programs (5 goals)

1. Improve national seismograph networks

2. Develop awareness of earthquake sources

3. Determine earthquake potential

4. Predict effects of earthquakes on buildings

5. Communicate research to educate the public

What is a great example of the value of planning for earthquakes?

The Denali earthquake in Alaska in 20

• Where the Trans-Alaska oil pipeline crossed the Denali fault, its construction was altered to withstand a large earthquake

How do we identify areas of seismic risk?

Hazard maps

• They include areas prone to liquefaction, zones of potential ground rupture, and historic epicentre

What are the 4 precursors to earthquakes to help with forecasting?

1. The pattern and frequency of smaller ruptures

   • Based on foreshocks and microearthquakes 

2. Land-level change

   • Uplift or subsidence may precede earthquakes  

   • GPS can recognize small changes in elevation 

3. Seismic gaps along faults

   • Areas along a fault that have not seen recent earthquakes may be more likely to experience one 

4. Physical and chemical changes

   • Changes in groundwater level and chemistry may occur if rocks expand prior to an earthquake  

Current warning systems provide how much of a warning?

15-30 seconds

• only warn of a major earthquake that has already occurred

How can we minimize seismic risk?

• critical facilites should be located as safely as possible  

• Buildings must be designed to withstand vibrations (retrofitting may be required) 

  • Diagonal beams prevent building from collapsing  

• education is a component of preparedness (workshops, training sessions, earthquake drills)

• -Earthquake insurance should be made available in high-risk areas 

What causes most casualties from earthquakes?

building collapse and falling objects