chapter 6 - earthquakes

What is the focus of an earthquake?

The location where the earthquake begins. directly under the epicentre

what do earthquakes result from

they result from the rupture of rocks along a fault

what are earthquakes compared by

magnitude

what are the 3 types of dip-slip faults, sketch it

Reverse Fault - The hanging-wall has moved up relative to the footwall inclined at an angle steeper than 45 degrees. - Evidence of reverse fault is underground (layers don't line up) Thrust Fault - These are similar to reverse faults except the angle is 45 degrees or less. Normal Fault: - The hanging-wall has moved down relative to the footwall.

What are seismic waves?

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

Natural Service Functions of Earthquakes - what are 4 examples

- 1 Faults provide pathways for the downward flow of surface water (into soil) - 2 They can channel groundwater to surface discharge points (springs) o Very important in dry areas - 3 New mineral resources can be found - some minerals are preferentially deposited in faults. - 4 Scenic landscapes (hills, valleys) can develop in fault zones over millions of years. i.e. mountains in B.C. -> because two plates collided

- what 6 factors impact how much earthquake shaking people feel

- 1. Magnitude (M8 vs. M5) - 2. Distance to the epicenter - 3. Focal depth o Earthquakes from deeper down tend to lose energy as they travel to the surface, so closer to surface = more damage - 4. Direction of rupture o is the fault N- S, E-W, because shaking occurs along the fault - 5. Local soil and rock types - 6. Local engineering and construction practices o Haiti has weak building codes, japan has strict ones -> socioeconomics

what are the 4/5 stages of the earthquake cycle

- 1. an inactive period - 2. A period where accumulated strain produces (deformation) minor earthquakes (minor = M3 or less) - 3. A period of foreshocks prior to a major release of stress (this stage does not always occur) o M4/M5 -> clue before an EQ - 4. a period where the mainshock occurs allowing the fault to release built-up stress - mainshock: the strongest earthquake in a series of associated earthquakes o associated -> maybe 2-week period - 5. a period of aftershocks with epicenters in the same general area as the main shock - The time between each stage varies.

3 areas at risk for earthquakes in Canada

- 3 areas: 1. Subduction zone on west coast 2. Nunavut, Arctic § Up there, the land was covered by a huge ice sheet, when it melts, the crust rises § with no weight of the ice, prone to EQs 3. Major faults near Southern Quebec and Eastern Ontario

- what is the formula for aftershocks

- As a general rule, the amount of aftershocks that occur on a given day after a mainshock can be forecasted by a formula: aftershocks on a given day = aftershocks on first day / given day - 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 / 7 = 29 **QUESTION ON FINAL EXAM

- what are body waves - what are the two types (2 names for each, and movement type)

- Body waves: These include P waves and S waves. move within the earth. P waves o They are also called primary or compressional waves o They move fast with a push-pull motion and can travel through solids or liquids (through liquid mantle, all layers) S waves o They are also called secondary or shear waves. o They move more slowly, in an up-and-down motion and can only travel through solids

- what is the earthquake cycle (definition) - what idea is this based on -

- 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. - As stress continues to increase, the deformed material will eventually rupture (for next EQ)

Intraplate Earthquakes - what is the definition of this - how strong are they compared to plate boundary EQs - why can they cause a lot of damage? - how large of areas can feel these - what are two active intraplate zones in North America - example of one EQ felt over the whole continent - two cities in U.S. at risk for intraplate EQs

- Definition: An earthquake on a fault in the interior for a continent, far from a plate boundary o i.e. any in Ontario - These earthquakes are typically smaller than plate boundary earthquakes. - However; damage could be considerable due to lack of preparedness - Because of dense (homogenous) continental bedrock, these earthquakes are felt over large areas. - There are two relatively active intraplate zones in North America: Central Mississippi River Valley and St. Lawrence River Valley o Mississippi River is in middle of North America, St. Lawrence River passes through Quebec - The New Madrid earthquakes in Missouri (1811-12) were over M7.5 and felt over the entire continent. o The recurrence interval in this area is likely several hundred years, in like 2100 - St. Louis and Memphis City are at-risk for intraplate earthquakes

definition of amplification - when does this occur and how does it happen - definition of alluvial soil - example of an area impacted by this

- Definition: An increase in ground motion during an earthquake. - P and S waves slow as they travel through alluvial sand, gravel, clay, soil, etc. o Alluvial -> soil deposited by water (ex. during a storm or when sea levels rose) o as they move through looser soil, wave intensifies → actually spreads over bigger areas (see in images) - As the waves slow, some of their energy is transferred to surface waves - Amplification has historically enhanced damage in San Francisco area earthquakes. - Alluvial soil in Oakland, just across the water from San Francisco

Plate Boundary Earthquakes definition: - what are the 3 types

- Definition: Earthquakes that occur on faults separating lithospheric plates o lithosphere = land, or chunks of land, or chunks of crust, oceanic crust even - There are three types: - Strike-slip earthquakes - Thrust earthquakes - Normal fault earthquakes

Surface Waves - definition - how fast do they move - where do they cause damage

- Definition: Seismic waves that form when P and S waves reach the Earth's surface and then move along it - These waves move more slowly than body waves. (- Things like trees, mountains etc. can slow these waves down) -> still cause damage - Surface waves are responsible for damage near the epicenter (Epicenter is on the surface NOT UNDERGROUND)

Tectonic Creep definition another name what can it damage how can an earthquake occur along the fault what are clues of this

- Definition: The slow movement of rock or sediment along a fracture caused by stress o aka fault creep. - This can damage roads and building foundations (movement of a few cm per decade). - Along these faults, periodic sudden displacement producing earthquakes can also occur - What are clues that this is happening? cracks in brick of foundation, cracks in wall, cracks in driveway o Since earthquakes can't be forecasted important to look for clues

Liquefaction - what is the definition of this - when can this occur - what kind of damage can occur

- Definition: The transformation of water-saturated sediment from solid to liquid o Soil has to be saturated with water - 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 silt may flow upward along fractures in the overlying solid material - This effect can cause extensive damage

Ground Rupture - what is this - what is the name of something this forms - what structures can these impact

- Displacement along faults causes cracks in the surface. - During strong earthquakes, fault scraps can be produced that extend for hundreds of kilometers (can be as long as the fault) o The displacement of land is the fault scrap o Dip-slip - Ground rupture can uproot trees, collapse buildings, and destroy bridges, tunnels, and pipelines.

- explain how direction of rupture affects the earthquake shaking felt - what is a name for this - which direction of waves are more strong

- Earthquake energy is focused in 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

- where are earthquakes most common near

- Earthquakes are most common at or near plate boundaries.

- why do earthquakes cause death and destruction - what's the best we can estimate about future earthquakes - what group is developing programs to reduce EQ hazard - what area in Canada will be hit by a large EQ soon? in how many years?

- 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 o Trying to forecast o Know Vancouver will be hit with a large earthquake in next 50 years

- when is the Moment Magnitude Scale different than the Richter Scale - what's the strongest EQ to occur (country, magnitude, year) - what's the strongest EQ to occur in Canada (country, magnitude, year)

- Except for very large earthquakes, the magnitude on the Moment Magnitude Scale is similar to the Richter Scale. - The strongest earthquake to ever occur is M9.5 in Chile in 1960. - In Canada, it is M8.1 in B.C. in 1949.

- how can landslides occur from earthquakes - what type of area is at risk for many landslides after an EQ

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

- how can fires occur after an earthquake - which earthquake had most damage occur due to fires

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

- what are hazard maps - what information do they have

- Hazard maps identify areas of risk associated with earthquake effects. - They include areas prone to liquefaction, zones of possible ground rupture and historic epicentres. o This map is for Seattle, red -> liquefaction zones o Map powerlines, pipelines, major roads etc..

- why is it necessary to identify faults - what type of faults cannot be seen - how can fault locations be used for earthquake mapping

- Identifying faults is necessary to evaluate the risk of an earthquake in a given area. - Not all faults reach the Earth's surface -> Blind faults are located below the surface. o San Andreas can be seen from an airplane, but not all faults are like that (blind faults) o If we can map them out, can see where earthquakes might happen

- what will accurate forecasts of earthquakes be likely based on ? - 4 precursors for earthquakes

- If accurate forecasts are possible, they will most likely be based on precursors: - 1 The pattern and frequency of earthquakes o Based on foreshocks and microearthquakes (M1, M2) - 2 Land-level change o Uplift or subsidence (sinking) may precede earthquakes o GPS stations can recognize small changes in elevation - 3 Seismic gaps along faults o Areas along a fault that have not seen recent earthquakes may be more likely to experience one - 4 Physical and chemical changes o Changes in groundwater levels and the chemistry may occur if rocks expand prior to an earthquake

- example of how geology impacts earthquake shaking in two sides of north America

- Implication: Earthquakes in eastern North America are felt over larger areas than those in Western North America - Two earthquakes with similar magnitude (6.0 and 5.8) - Red -> only California felt it - The one in central Virginia was felt in southern Ontario and Chicago as well -> because this was a relatively homogenous crust (could feel it over a larger area)

- what are the 3 categories of fault activity

- In terms of activity, faults can fall into one of three categories: - Active o Movement during the past 11,600 years - Potentially Active o Movement during the past 2.6 million years - how do we know this? carbon dating, other methods of dating rock - Inactive o No movement during the past 2.6 million years - *Faults that were labelled inactive can still have earthquakes, we've been surprised in the past

- is it possible to stop people from living in EQ prone areas? - what type of risk must be attempted to minimize? - 4 ways to reduce that risk

- It is not possible to prevent people from living in earthquake prone areas. - Therefore, we must take a number of steps to minimize seismic risk: o 1 Critical facilities should be located as safely as possible o 2 buildings must be designed to withstand vibrations (in many cases, retrofitting is required for old buildings) o 3 education is a component of preparedness (workshops, training sessions, earthquake drills) o 4 Earthquake insurance should be made available - Available in California

- what scale is used today to measure earthquakes - what 3 qualities are used in this scale

- Moment Magnitude scale (M) - The scale is determined by: o Area ruptured along a fault o The amount of movement along the fault (distance) o The elasticity of the crust at the focus (Elasticity -> how stretchy, flexible is the crust)

- what do most earthquake casualties result from - where should one hide during an earthquake

- 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

- where do most earthquakes occur along - 3 examples of areas - what are some north american cities most at risk

- Most earthquakes occur along plate boundaries: - Pacific Ring of Fire (all around Pacific Ocean), Himalayas Mountains (India pushing into continental Asia), Middle East - North American cities at high risk of earthquakes: o Anchorage, Vancouver, Victoria, Seattle, Portland, San Francisco, Los Angeles, Mexico City -> all along plate boundaries - However, not all areas at risk of earthquakes are near plate boundaries.

- explain how focal depth affects the earthquake shaking felt - what is the name for the reduction of energy

- 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

- two types of effects from earthquakes and examples of each

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

- what magnitude earthquakes can human activity trigger - 4 examples of human activities

- Several human activities are known to trigger small to moderate earthquakes (M4 or less) - 1 The weight from water reservoirs produced by dams can create new faults o Reservoir -> man-made lake behind a dam - 2 Injecting liquid waste deep into the Earth can increase pressure and cause slippage along fractures o i.e. injecting nuclear waste in unpopulated areas, deserts, like Nevada - 3 Fracking -> inject water to release natural gas o Lots of small EQs in Oklahoma and Texas - 4 Testing nuclear weapons leads to explosions that may increase strain in an area o In unpopulated areas like Nevada

- what changes with every number increase in an earthquake scale (i.e. M5 vs. M8)

- Similar to the Richter Scale, it is a logarithmic scale. - Each number increases by 10 times the intensity - M8 vs. M5 is 1000 times more intense (10x10x10)

- what do survivors of strong earthquakes experience - how does one community's experience with an EQ affect other places - what have EQs exposed about buildings

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

- example of planning for earthquakes in Alaska - which earthquake happened there

- 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.

- what is name for the qualitative scale used for earthquakes, what does it measure - how many categories does it have

- 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

Strike-Slip Faults - which direction is the displacement - what is the best example of this - sketch a picture of this

- The San Andreas Fault is the best example of this type - splits the river - Notice no up or down with this - Displacements are horizontal

- what are Shake Maps used for?

- The combination of all of 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 - Notice: that these are roman numerals (quantitative scale) - Two maps -> diff locations (LA, Seattle) - More shaking in Seattle

- what determines a station's distance to the epicentre - how many seismic stations measure the distance to the epicenter - what's the name of the process used to determine the epicentre

- The difference between the arrival times of the first P and S waves at different locations determine the distance to the epicentre. (P waves arrive first) - The distance to the epicenter is calculated at 3 different seismic stations - A circle with radius equal to that distance is drawn around the station - The epicenter is located where the circles intersect; this process is called triangulation.

- how does local geology (soil and rock types) impact earthquake shaking - how does this impact eastern vs. western North America

- The local geology (soil and rock types) influences the amount of ground motion. - Dense homogenous crust can transmit earthquake energy quickly. o surface waves can quickly travel through homogeneous crust o Homogenous -> crust is even, same rock type o Heterogenous -> many different rock types (ex. places with mountains) - Seismic energy slows down in areas with heterogeneous, folded, faulted crust.

- how is the value of EQ magnitude of an earthquake expressed - who developed the first EQ measurement scale, what year and what's the name of the scale - what did this scale measure - is this still in use?

- 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 at a distance of 100 km from the epicentre. - Since then, more accurate methods have been developed and the Richter Scale is no longer in use.

what are the 5 goals of earthquake hazard reduction programs

- The programs have 5 goals: - 1 improve national seismograph networks (seismic stations) - 2 develop awareness of earthquake sources o find faults, more blind faults - 3 Determine earthquake potential o If fault is inactive, potentially active or active - 4 Predict effects of earthquakes on buildings - 5 Communicate research to educate the public o Educate without instilling fear

- how many M9+ earthquakes occur each century - what M is very common - what range M can we not usually feel

- There are only a few M9+ earthquakes each century (100 years) - M2 are very frequent, but we cannot feel any under M3

what are the 2 Fault Types - what is the difference between them

- There are two basic types of geologic faults distinguished by the direction of the displacement of rock or sediment. - Strike-slip faults o Displacements are horizontal - Dip-slip faults o Displacements are vertical

- there have been ______ incidents of successfully forecasting earthquakes - who reviews EQ forecasts and why - where are research projects occurring to study EQs, what is being studied - how long of a warning do current EQ warning systems provide

- There have been modest incidences of successfully forecasting earthquakes - All forecasts must be scientifically reviewed before they are released o Don't want to send a false alarm - Research projects along the San Andreas Fault are aiding in understanding the conditions that occur before an earthquake. o Universities in California - Current earthquake warning systems provide 15 to 30 seconds of warning and only warn of an earthquake that has already occurred.

Strike-Slip Earthquakes - where do these earthquakes occur - where are they common - what's the best known strike-slip earthquake

- These earthquakes occur along transform faults where plates slide horizontally past one another. - They are common in California along the San Andreas Fault o San-Andreas fault is near San Francisco - The best-known strike-slip earthquake is the Loma Prieta earthquake that disrupted the 1989 World Series in Oakland, California.

Normal Fault Earthquakes - where do these occur - example of area that these are common - how strong are these

- These earthquakes occur on faults associated with divergent plate boundaries. - They are common along the Mid-Atlantic Ridge o Atlantic is getting wider, but doesn't have many tsunamis b/c the EQs are weak - Most are located under oceans and are generally smaller than M6

Thrust Earthquakes - where do these earthquakes occur - another name for these - 3 states/provinces that these are common - how strong are these earthquakes, what can be a secondary effect of them

- These earthquakes occur on faults that separate converging plates. - They are also called subduction/dip-slip (thrust) earthquakes. o dip slip - one plate is sinking beneath the other - They are common off the coast of B.C., Washington and Oregon (these states are south of B.C.) - These Earthquakes are the strongest on Earth (some are larger than M9) and can produce tsunamis.

what are the two parts of a dip-slip fault (sketch this)

- They are comprised of two walls on an incline defined by miners: o Footwall (where miners place their feet) o Hanging-wall (where miners placed their lanterns) Both are rocks

- where do earthquakes tend to happen in relation to the past - 2 examples of areas where they tend to occur - where in western Canada are there large faults - what is changing about vancouver island? - what device is used to detect uplift

- all the earthquakes that have happened since 1988, tend to happen in the same areas over and over o Subduction zone, mid-Atlantic ridge - big faults in Victoria BC - Vancouver Island is rising slowly (uplifted) because faults coming together (subduction zone) - GPS can detect uplift (which is a zone of deformation)

how is the energy from an earthquake released

in the form of seismic waves

- describe the motion/forces at plate boundaries - how does this start an earthquake - where does the movement start (name of spot), and how does it continue? - what is another name for a fault

o Motion at plate boundaries is not usually smooth or constant - Not constant = ex. won't move for 100 years, then randomly move o even though not moving, they're pushing, force, friction → causes stress o 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. o Each rock has shear strength - The movement (or rupture) starts at the focus and propagates in all directions, called seismic waves. - Thus, faults are considered seismic sources.

what are earthquakes measured by

seismographs