Week 7

Question 1

what is an earthquakes?

Answer 1

ruptures of rocks along a fault

Question 2

how is energy from an earthquake released?

Answer 2

in the form of seismic waves

Question 3

how are earthquakes mapped?

Answer 3

according to the epicentre

the focus is located directly below the epicentre

Question 4

how are earthquakes measured, specifically what tool is used, and compared?

Answer 4

measured by seismographs and compared by magnitude

Question 5

how is the magnitude of an earthquake measured? (what type of scale is used)

Answer 5

note, that it’s expressed as a number to one decimal place

the Richter scale was the first developed measurement, but it is no longer used

today, the Moment Magnitude scale is used

both scales are logarithmic scales

e.g. M7 = 10 x M6

Question 6

who first developed magnitude as a measurement of earthquakes? what was the scale called?

Answer 6

Charles Rich, a seismologist

the Richter scale was a measure of the strength of a wave 100km from the epicentre

the Richter scale is no longer used

Question 7

describe the Moment Magnitude scale (M)

Answer 7

the current scale used to measure the magnitude of earthquakes

it is determined by…
— the area of rock ruptured along a fault
— the distance of movement along the fault
— the elasticity of the rock as the focus

measurements are written as M(number)

there is on official upper limit on the Moment Magnitude scale (no cap??)

Question 8

what is the strongest earthquake recorded on the Moment Magnitude scale globally and in Canada?

Answer 8

M9.5 Chile

and M8.1 in BC

— note, there are only a few M9 earthquakes each century

Question 9

what is the Modified Mercalli Intensity scale?

Answer 9

it’s a qualitative scale based on the damage to property and the effect of an earthquake

Question 10

what is the qualitative scale used to measure Earthquakes called?

Answer 10

the Modified Mercalli Intensity scale

it’s a qualitative scale based on the damage to property and the effect of an earthquake

Question 11

describe the earthquake process

Answer 11

earthquakes are most common as or near plate boundaries

motion at plate boundaries is not smooth or constant

friction along plate boundaries exerts a force/stress on the rocks, causing strain or deformation

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

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

thus, faults are considered seismic sources

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

Question 12

what are blind faults?

Answer 12

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

not all faults are found at the Earth’s surface… Blind faults are located below the surface

Question 13

what are seismic sources?

Answer 13

faults

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

Question 14

what are the 2 basic types of geologic faults? how to distinguish between the 2?

Answer 14

can be distinguished by the direction of the displacement of rock

strike-slip faults
—- displacements are horizontal

dip-slip faults
—- displacements are vertical
—- there are 3 types of dip-slip faults

Question 15

give an example of the strike-slip fault

Answer 15

the San Andreas fault

Question 16

what are the 3 categories of the dip-slip fault? differentiate between them

Answer 16

reverse faults
—- the hanging wall has moved up relative to the footwall and inclined at an angle above 45 degrees

thrust faults
—- there are like reverse faults except the angle is 45 degrees or less

normal faults
—- the hanging wall has moved down relative to the footwall

Question 17

what is a dip-slip fault? describe it

Answer 17

a fault where the displacements are vertical

they are comprised of 2 inclined walls, as defined by miners:
—- footwall: where miners placed their feet
—- hanging-wall: where miners placed their lanters

Question 18

what are the types of faults, according to activity? differentiate between them

Answer 18

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

what is a tectonic creep?

Answer 19

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

also referred to as “fault creep”

along these faults, periodic sudden displacements can produce earthquakes

this can damage roads and building foundations (movement of a few cm per decade)

Question 20

how seismic waves generated?

Answer 20

some seismic waves are generated by fault rupture travel within the body of the Earth and other travel along the surface

Question 21

what are the types of seismic waves?

Answer 21

body waves
—- includes P waves and S waves

surface wavesw

Question 22

what is a P wave?

Answer 22

a type of seismic wave

also called primary or compressional waves

they move relatively fast with a push-pull motion and can travel thru solids or liquids

Question 23

what is a S wave?

Answer 23

a type of seismic wave

also called secondary or shear waves

they move relatively slow in an up and down motion (s for snake!) and can only travel thru solids

Question 24

what is a surface wave?

Answer 24

seismic waves that form when P waves and S waves (body waves) reach the Earth’s surface and then move along it

these waves move more slowly than body waves

surface waves are responsible for damage near the epicentre

Question 25

what factors determine the shaking that people experience during an earthquake?

Answer 25

magnitude distance to the epicentre focus depth (how deep is the earthquake) direction of the rupture local soil and rock type local engineering and construction practices ------- the combination of all these effects results in widespread variation in the shaking felt in the vicinity of an earthquake... therefore, 2 earthquakes that have the same magnitude can have very different impacts

Question 26

describe how a seismograph records the waves

Answer 26

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

Question 27

how is the distance to the epicentre determined via seismographs?

Answer 27

the difference between the arrival times of the P waves and S waves at different locations determine the distance to the epicentre the distance to epicentre is calculated at 3 different seismic stations a circle with a radius equal to that distance is drawn around the station the epicentre is located where the circles intersect, this process is called triangulation

Question 28

what is triangulation?

Answer 28

used to locate the epicentre by intersecting the 3 circles drawn.... the distance to epicentre is calculated at 3 different seismic stations a circle with a radius equal to that distance is drawn around the station

Question 29

how does the focus depth affect the intensity of shaking felt at the surface?

Answer 29

seismic waves become less intense as they spread outwards toward the surface therefore, the greater the focus depth, the less intense the shaking at the surface

Question 30

what is directivity?

Answer 30

earthquake energy is more focused along the geographic direction of rupture it contributes to increased shaking ------------ radiated waves are some times stronger along the direction the fault

Question 31

how does the local soil and rock type affect an earthquake? give an example

Answer 31

the local geology influences the amount of ground motion dense homogenous crust can transmit earthquake energy quickly but, seismic energy slows down in areas with heterogenous, folded, faulted crust therefore, earthquakes in eastern North America are felt over larger areas than those in western North America ----- the underlying bedrock in eastern North America is more homogenous than in the west ----- the mountain range near central California prevented the earthquake's shaking from reaching further distances than the one near central Virginia

Question 32

how are surface waves amplified? give an example

Answer 32

P waves and S waves slow down as they travel thru alluvial sand, gravel, clay, soil, etc. as the waves slow, some of their energy is transferred to surface waves amplification has historically enhanced damage in San Francisco area earthquakes

Question 33

what is the earthquake cycle?

Answer 33

a hypothesis that explains successive earthquakes on a fault over time 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 rocks will eventually rupture

Question 34

what are the steps of the typical earthquake cycle?

Answer 34

inactive period strain produces minor earthquakes foreshocks prior to a major release of stress (this stage doesn't always occur) mainshock occurs allowing the fault to release built-up stress aftershocks ---------- the time between each stage varies

Question 35

what is a foreshock?

Answer 35

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

Question 36

what is a mainshock?

Answer 36

the largest earthquake in a series of associated earthquakes

Question 37

what is an aftershock?

Answer 37

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

Question 38

what equation is used to calculate the amount of aftershocks that occur on a given day after a mainshock?

Answer 38

aftershocks on first day following an earthquake divided by given day after an earthquake

Question 39

where do most earthquakes occur?

Answer 39

along plate boundaries e.g. pacific ring of fire, middle east, Himalayan mountains

Question 40

which North American cities are at high risk of earthquakes?

Answer 40

Anchorage, Vancouver, Victoria, Seattle, Portland, San Francisco, Los Angeles, Mexico City (ALL of which are on the west coast!)

Question 41

what are plate boundary earthquakes?

Answer 41

earthquakes that occur on faults which specifically separate plates

Question 42

what are the 3 types of plate boundary earthquakes?

Answer 42

strike-slip earthquakes thrust earthquakes normal fault earthquakes

Question 43

what is a strike-slip earthquake? commonly found where? give an example

Answer 43

earthquakes that occur along transform faults where plates slide horizontally past one another they are common in California along the San Andreas fault (a strike-slip fault) the best known strike-slip earthquake is the Loma Prieta earthquake that disrupted the 1989 World Series in Oakland, California

Question 44

what is a thrust earthquake? commonly found where? how strong are they?

Answer 44

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

Question 45

what is the strongest type earthquake on Earth?

Answer 45

the thrust earthquake some are larger than M9 and they can produce tsunamis

Question 46

what is a normal fault earthquake? commonly found where? how strong are they?

Answer 46

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 47

what is an intraplate earthquake?

Answer 47

an earthquake on a fault in the interior of a continent -- far from a plate boundary these earthquakes are typically not as strong as plate boundary earthquakes however, damage could be considerable due to lack of preparedness and because of the dense continental bedrock which allow the quakes to be felt over large areas

Question 48

what are the 2 relatively active intraplate zones in North America?

Answer 48

Central Mississippi River Valley and St. Lawrence River Valley but Memphis and St. Louis are cities that also have risk of faults

Question 49

what is a recurrence interval?

Answer 49

time between successive/consecutive events

Question 50

what are the primary effects of an earthquake?

Answer 50

ground shaking surface rupture

Question 51

what are the secondary effects of an earthquake?

Answer 51

liquefaction land-level change landslides fire tsunamis

Question 52

what are surface ruptures?

Answer 52

displacement along faults cause cracks in the surface during strong earthquakes, fault scarps can be produced that extend for hundreds of kilometers surface rupture can uproot trees, collapse buildings, and destroy bridges, tunnels, and pipelines

Question 53

what is a fault scarp?

Answer 53

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

Question 54

what is liquefaction? how can earthquakes cause liquefaction?

Answer 54

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 deceases, the sediment compacts and regains its strength watery sand and silt may flow upward along fractures in the overlying rock creating small mounds this effect can cause extensive damage

Question 55

how can earthquakes cause fires? give an example

Answer 55

ground shaking and rupture can start fires by severing power and gas lines appliances may topple over causing gas leaks that may ignite approximately 80% of the damage during the 1906 San Francisco earthquake was caused by fire

Question 56

what are the natural service functions of earthquakes?

Answer 56

faults provide pathways for the pathways for the downward flow of surface 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 year

Question 57

describe how human activity can cause earthquakes

Answer 57

several human activities are known to trigger small to moderate earthquakes the weight from water reservoirs produced by dams can create new fault injecting liquid waste deep in the Earth can increase pressure and cause slippage along fractures ----- and fracking too testing nuclear weapons leads to explosions that may increase strain in an area

Question 58

how can the hazard of earthquakes be reduced?

Answer 58

currently, we can forecast the likelihood that an earthquake will occur in an area but not precisely when it will occur

Question 59

what are the 5 goals of the hazard reduction program the Geological Survey of Canada and US has developed for earthquakes?

Answer 59

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

Question 60

what earthquake demonstrated the value of planning for it?

Answer 60

the Denali earthquake in Alaska in 2002 where the Trans-Alaska oil pipeline crossed the Denali fault, its construction was altered to withstand a large earthquake

Question 61

what are hazard maps?

Answer 61

they identify areas of risk associated with earthquake effects they include areas prone to liquefaction, zones of potential ground ruptures, and historic epicentres

Question 62

what are precursors to earthquakes that can potentially be used to forecast earthquakes?

Answer 62

the pattern and frequency of smaller ruptures ---- based on foreshocks and microearthquakes land-level change ---- uplift or subsidence may precede earthquakes ---- GPS can recognize small changes in elevation seismic gaps along faults ---- areas along a fault that have not seen recent earthquakes may be more likely to experience one physical and chemical changes ---- changes in groundwater level and chemistry may occur if rocks expand prior to an earthquake

Question 63

describe the warning that earthquake forecasting can currently provide us with

Answer 63

current warning systems provide 15 to 30 seconds of warning only warn of a major earthquake that has already occurred

Question 64

how can communities adjust to earthquake hazards (i.e. minimize seismic risk)?

Answer 64

critical facilities should be located as safely as possible buildings must be designed to withstand vibrations (retrofitting may be required) education is a component of preparedness (workshops, training sessions, earthquake drills) earthquake insurance should be made available in high-risk areas

Question 65

in an earthquake, what can individual do to avoid harm?

Answer 65

stay away from windows and tall furniture the safest locations are under desks or tables