Week 7: Earthquakes

Earthquakes

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

Earthquake magitude

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 100 km from the epicentre.

\
Since then, Richter is not used anymore

The moment magnitude scale

Today, earthquakes are measured using the Moment Magnitude scale (M).

\
The scale is determined by:

* The area ruptured along a 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

\
\

Example question:

Compare an M5 to an M2 -how much more shaking would there be

* 1000 times
* 10 x 10 x 10 (3 times more powerful)

Magnitude and frequency

Except for very large earthquakes, the magnitude on the Moment Magnitude Scale is like the Richter Scale.

\
There are only a few M9+ earthquakes in each country

The strongest earthquake to ever occur

The strongest earthquake to ever occur is M9.5 in Chile 

in 1960. 

\
In Canada, it is M8.1 in B.C. in 1949.

Which scale is quantitative and which is qualitative?

__**Qualitative:**__

* Modified Mercalli intensity scale

\
__**Quantitative:**__

* Moment Magnitude intensity scale

Modified Mercalli intensity scale

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.

Earthquakes are most common _______________

near or on plate boundaries

Earthquake processes

motion at plate boundaries is not usually smooth or constant

\
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.

Seismic sources

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

\
Thus, faults are considered seismic sources.

\
Identifying faults is necessary to evaluate the risk of an earthquake in an area

\
Not all faults reach the earths surface. Blind faults are located below the surface

Earthquake distribution

fault types

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^^

\
example: San Andreas Fault

Dip-slip faults

Displacements are $vertical$

\
three types:

* Reverse fault
* Thrust fault
* Normal fault

\
Comprised of two walls on an incline defined by minors:

\

1. Footwall (where minors placed their feet)
2. Hanging wall (where minors placed their lanterns)

Reverse fault

The hanging wall has moved up relative to the footwall at an angle steeper than 45 degrees

Thrust fault

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

Normal fault

The hanging wall has moved down relative to the footwall

Fault activity

In terms of activity, faults can fall into one of three categories:

\
__**Active**__

* movement during the past ==11, 600 years== (based on geologic time scale)

\
__**Potentially Active**__

* movement during the past $2.6 million years$

\
__**Inactive**__

* $no movement$ during the past $2.6 million years$

\

Tectonic creep

Definition:

* The low movement of rock or sediment along a fracture caused by stress

\
It is also referred to as *fault creep*.

\
This can damage roads and building foundations (movement of a few cm per decade).

\
Along these faults, periodic sudden displacements producing earthquakes can also occur

Seismic waves

Some seismic waves generated by fault rupture travel within the body of the Earth and others travel along the surface.

\
__**Body waves:**__

* These include P waves and S waves

\
__**P waves:**__

* they also called primary or compressional waves

\
They move fast with a push-pull motion and can travel through solids or liquids

S wave

also called secondary or shear waves

* move slowly, in an up-and-down motion and can $only travel through solids$

P waves vs S waves

Surface waves

Definition:

* seismic waves that form when P and S waves reach the surface and then move along it

\
These waves move more slowly than body waves.

Earthquake shaking

Factors that determine the shaking people experience during an earthquake:

* magnitude
* distance to the epicentre
* focal depth
* direction of rupture
* local soil and rock types
* local engineering and construction practices

Seismographs record the arrival of waves to a recording station.

Because P waves travel faster than S waves , they appear first on a seismograph

\
Earthquake shaking decreases with distance from the epicentre

Distance to the epicentre

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

Locating an earthquake

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

\
\*must use 3!!!

Focal depth

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.

\
This reduction of energy is referred to as %%**attenuation**%%

Direction of rupture

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

Local soil and rock types

The local geology influences the amount of ground motion.

\
* Dense homogenous crust can transmit earthquake energy quickly

\
Seismic energy slows down in areas with heterogeneous, folded, faulted crust.

\
**Implication:**

* Earthquakes in eastern North America are felt over larger areas than those in western North America

Amplification

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

Alluvial

soil that was deposited by water

amplification has historically enhanced damage in *__________________* area earthquakes

San Fransisco

Shake maps

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

\
(why we have 2 scales)

The earthquake cycle

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 earthquake.

\
\

The cycle stages

A typical cycle has several stages:

\
* an inactive period
* a period where the *mainshock* occurs allowing the fault to release built-up stress
* a period where strain produces minor earthquakes
* a period of foreshocks prior to a major release of stress

\
\
The time between each stage varies.

foreshock

a small to moderate earthquake that occurs before a large earthquake in the general vicinity of the large earthquake

mainshock

The largest earthquake in a series of associated earthquakes

aftershock

a small to moderate earthquake that occurs after a large earthquake in the general vicinity of the large earrthquake

Aftershocks

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

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

\
200 divided 7 = 29

Geographic regions at risk from earthquakes

Earthquakes are not randomly distributed.

\
Most earthquakes occur along plate boundaries:

* Pacific ring of fire
* Himilaya mountains
* Middle east

\
North American cities at high risk of earthquakes:

* Anchorage, Vancouver, Victoria, Seattle, Portland, San Francisco, Los Angeles, Mexico City

\
However, not all areas at risk of earthquakes are near plate boundaries.

Plate boundary earthquakes

* what are the 3 types?

Definition:

* earthquakes that occur on lithospheric plates

\
\
There are three types:

* Strike-slip earthquakes
* Thrust earthquakes
* normal fault earthquakes

Strike-slip earthquakes

These earthquakes occur along %%transform faults%% where plates slide horizontally past one another.

\
* Common in California along the San Andreas fault

\
The best-known strike-slip earthquake is the Loma Prieta earthquake that disrupted the 1989 World Series in Oakland, California.

Thrust earthquakes

\
These earthquakes occur on ^^faults that separate converging plates^^.

\
They are also called subduction earthquakes.

\
They are common off the coast of B.C. Washington and Oregon

\
These earthquakes are __**strongest**__ on earth (some are larger than M9) and can produce tsunamis

Normal fault earthquakes

These 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

Intraplate earthquakes

Definition:

* an earthquake on a fault in the interior of a continent, far from a plate boundary

\
These earthquakes are typically smaller than plate boundary earthquakes.

\
However, damage could be considerable due to lack of preparedness

Intraplate earthquake examples

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.

\
Recurrence interval in this area is likely several hundred years

recurrence interval

The time between successive (or consecutive) events

effects of earthquakes

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
* tsunamis

surface rupture

Displacement along faults causes cracks in the surface.

* during strong earthquakes, fault scarps can be produced that extend for hundreds of kilometers

fault scarp

A linear escarpment along a fault where a vertical displacement has occurred

Surface rupture

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

Liquefaction

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 pressure decreases, the sediment compacts and regains its strength

Liquefaction

Watery sand and silt may flow upward along fractures in the overlying solid material.

\
This effect can cause extensive damage.

Landslides

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

Fires

Ground shaking and rupture can sever power and gas lines, starting fires.

\
Appliances may topple over causing gas leaks that ignite

\
Approximately __**80%**__ of the damage during 1906 San Francisco earthquake was caused by fire 🔥

Natural service functions of earthquakes

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

Earthquakes caused by human activity

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 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.

Minimizing the earthquake hazard

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 U.S are developing programs to reduce the hazard from earthquakes

Earthquake hazard reduction programs

The programs have 5 goals:

* improve national seismograph networks
* develop awareness of earthquake sources
* determine earthquake potential
* predict effects of earthquakes on buildings
* communicate research to educate the public

Planning for earthquakes

The denali earthquake in Alaska (2002) demonstrated the value of planning for earthquakes

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

Estimating seismic risk

Hazard maps identify areas of risk associated with earthquake effects.

\
They include areas prone to liquefaction, zones of possible ground rupture and historic epicentres

Precursers to earthquakes

If accurate forecasts are possible, they will most likely be based on precursors:


1. **The pattern and frequency of earthquakes**
* based on foreshocks and microearthquakes

\
2. **Land-level change**
* uplift or subsidence may precede earthquakes
* GPS stations can recognize small changes in elevation

\
3. **Seismic gaps along faults**
* areas along a fault that have not seen recent earthquakes may be a more likely to experience one

\

4. **Physical and chemical changes**
* Changes in groundwater levels and chemistry may occur if rocks expand prior to an earthquake

Earthquake forecasting

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.

Perception of the earthquake hazard

Survivors of strong earthquakes often report traumatic stress.

\
Typically, one communities experienced with an earthquake has not stimulated other communities to enhance their preparedness

\
\
Earthquakes have exposed shoddy construction practices.

Community adjustments to the earthquake hazard

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)



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

\
* earthquake insurance should be made available

Personal adjustments

Most earthquake casualties result from building collapse and falling objects.

\
During an earthquake, it is best to stay away from windows and tall furniture.

\
The safest locations are under desks or tables