Lecture 2 Flashcards
1
Q
What do earthquakes result from? How is energy from an earthquake released? How are they mapped and measured?
A
- result from the rupture of rocks along a fault
- energy from an earthquake is released in the form of seismic waves
- mapped according to the epicentre, the focus is located directly below the epicentre
- they are measured by seismographs and compared by magnitude
2
Q
How can earthquake magnitude be measured quantitatively?
A
- expressed as a number to one decimal place; first developed by Richter but not used anymore
- Richter scale was a measure of strength of a wave at a distance of 100 km the epicentre
-Moment Magnitude Scale (M)
- scale is determined by area ruptured along a fault, amount of movement along the fault, elasticity of the crust at the focus
- similar to Richter scale, it is a logarithmic scale
- M7 earthquake represents 10x the amount of ground motion as an M6 earthquake
3
Q
What is the strongest earthquake to ever occur?
A
- M 9.5 in Chile in 1960
- In canada, it is M8.1 in BC in 1949
- except for very large earthquakes, Richter and M scale are similar
- there are only a few M9+ earthquakes each century
4
Q
How can earthquake intensity be measured qualitatively?
A
- Modified Mercalli Intensity Scale
- based on damage to structures and the effect on people
- based on 12 categories (1: felt by few people, 2: felt by a few people at rest especially on upper floors…12: damage is total and waves are seen on ground surface)
5
Q
Where are earthquakes most common?
A
- at or near plate boundaries
- 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
6
Q
Describe the movement that occurs at a fault during an earthquake
A
- starts at the focus and propogates in all directions, called seismic waves
- faults are considered seismic sources
- identifying faults is necessary to evaluate risk of an earthquake in a given area
- not all faults reach the Earth’s surface
- blind faults are located below the surface
7
Q
How are earthquakes distributed geographically?
A
- they occur in the same places over and over again
- in Vancouver island, there is satellite monitoring for land that might be uplifting (when oceanic plate sinks it is crushing continental plate and may push it up a few mm)
- many in Oklahoma from fracking
8
Q
What are the two basic types of geologic faults?
A
- strike-slip faults: displacements are horizontal, San Andres (slabs move horizontally)
- dip-slip faults: displacements are vertical
9
Q
What are the 3 main types of dip-slip faults?
A
- reverse faults: hanging wall has moved up relative to footwall inclined at an angle steeper than 45 degrees
- thrust faults: these are similar to reverse faults except the angle is 45 degrees or less
- normal faults: the hanging wall has moved down relative to the footwall
- they are comprised of footwall and hanging wall
10
Q
How is fault activity determined?
A
- active: movement during the past 11600 years
- potentially active: movement during the past 2.6 million years
- inactive: no movement during the past 2.6 million years
11
Q
What is tectonic creep?
A
- the slow movement of rock or sediment along a fracture caused by stress
- also referred to as fault creep
- can damage roads and building foundations (movement of a few cm per decade)
- look for cracks in road, bricks of home, foundation chipping, driveway cracks
- along these faults periodic sudden displacements producing minor earthquakes can also occur
12
Q
What are different kinds of seismic waves?
A
- some travel within the body of the Earth and others travel along the surface
- body waves: P waves and S waves
- P waves: primary/compressional waves, move fast with a push pull motion and can travel through solids or liquids
- S waves: secondary/shear waves, move more slowly in an up and down motion and can only travel through solids
- surface waves: seismic waves that form when P and S waves reach Earth’s surface then move along it
- surface waves move more slowly than body waves
- responsible for damage near the epicentre
13
Q
What factors determine the shaking people experience during an earthquake?
A
- magnitude
- distance to the epicentre
- focal depth
- direction of rupture
- local soil and rock types
- local engineering and construction practices
14
Q
How does earthquake shaking appear on a seismograph?
A
- seismographs 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
15
Q
How is distance to the epicentre determined?
A
- the difference between 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
- epicetnre is located where the circles intersect; this is triangulation
16
Q
How does focal depth affect shaking at the surface?
A
- seismic waves become less intense as they spread outward toward the surface
- the greater the focal depth, the less intense the shaking at the surface
- this reduction of energy is referred to as attenuation
- waves spread the shaking out
17
Q
How does direction of rupture affect shaking?
A
- earthquake energy is focused in the direction of the rupture
- this is known as directivity and contributes to increased shaking
- radiated waves are sometimes stronger in one direction along the fault
18
Q
How does local soil and rock types affect shaking?
A
- dense homogenous crust can transmit earthquake energy quickly
- seismic energy slows down in areas with heterogeneous, folded, faulted crust
- implication: earthquakes in eastern NA are felt over larger areas than those in western NA
19
Q
What is amplification?
A
- an increase in ground motion during an earthquake
- P and S waves slow as they travel through alluvial sand, gravel, clay, soil
- alluvial: deposited by water, loose unconsolidated soil
- as the waves slow, some of their energy is transferred to surface waves
- has historically enhanced damage in San Fran area earthquakes
20
Q
What is the earthquake cycle?
A
- a hypothesis that explains successive earthquakes on a fault
- 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
21
Q
What are the stages of the earthquake cycle?
A
Inactive period
- strain produces minor earthquakes
- period of foreshocks prior to a major release of stress (does not always occur)
- foreshock: small to moderate earthquake that occurs shortly before and in the same general area as the mainshock (M4-5)
Mainshock
- mainshock occurs allowing the fault to release built up stress
- mainshock: largest earthquake in a series of associated earthquakes
- period of aftershocks with epicentres in the same general area as the mainshock
- rock is settling after big event where there is more shaking
- always aftershocks after major earthquake
- aftershock: small to moderate earthquake that occurs shortly after and in the same general area as the mainshock
- time between stages varies
22
Q
How can the amount of aftershocks be forecasted?
A
-aftershocks on given day=aftershocks on first day after/given day
23
Q
What geographic regions are at risk from earthquakes?
A
- 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, los angeles, mexico city
- however, not all areas at risk of earthquakes are near plate boundaries
24
Q
Where are earthquakes located in Canada?
A
- africa was pushing into NA which created mountains
- in the north crust is rising which may account for earthquakes
25
What are plate boundary earthquakes?
- earthquakes that occur on faults separating lithospheric plates
- strike-slip, thrust, and normal fault earthquakes
26
What is a strike-slip earthquake?
- occur along transform faults where plates slide horizontally past one another
- they are 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
27
What is a thrust earthquake?
- these occur on faults that separate converging plates
- they are also called subduction earthquakes
- common off of coast of BC, Washington, and Oregon
- strongest on Earth and can produce tsunamis
- subduction zone is the only place M9 can be produced
28
What is a normal fault earthquake?
- 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
29
What are intraplate earthquakes?
- 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
- because of dense continental bedrock these are felt over large areas
- two relatively active intraplate zones in NA: Central Mississippi River Valley and St Lawrence River Valley
- The New Madrid earthquakes in Missouri 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 events
- St Louis and Memphis are at risk
30
What are the primary and secondary effects of earthquakes?
- primary: ground shaking, surface rupture
- secondary: liquefaction, land-level change, landslides, fire, tsunamis
31
What is ground rupture?
- displacement along faults causes cracks in the surface
- during strong earthquakes, fault scarps can be produced that extend for hundreds of kms
- fault scarp: a linear escarpment at Earth's surface formed by movement along a fault during an earthquake
- can uproot trees, collapse buildings, and destroy bridges, tunnels and pipelines
32
What is liquefaction?
- the transformation of water-saturated sediment from solid to liquid
- 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 sands and slit may flow upward along fractures in the overlying solid material
- this effect can cause extensive damage
33
What are landslides?
- 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
34
How can ground shaking cause fires?
35
Describe the characteristics of the Haiti earthquake?
- poorest country in Western hemisphere for many years
- M7 earthquake in 2010
- one of the worst natural disasters in history; death toll over 160 000
- epicentre was 25km from port au prince, most buildings in city were destroyed
- occured along a transform fault
- destruction enhanced by poor construction materials and a lack of building codes
- landslides affected slums in the hillsides surrounding the city
36
37
What are 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
38
What human activities cause earthquakes?
- several human activities are known to trigger small to moderate earthquakes
- weight from water reservoirs produced by dams can create new faults
- injecting liquid waste deep in earth can increase pressure and cause slippage along fractures
- fracking
- testing nuclear weapons leads to explosions that may increase strain in an area
39
How can earthquake hazard be minimized?
- 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 US are developing programs to reduce hazard from earthquakes
40
What are the goals of earthquake hazard reduction programs?
- improve national seismograph networks
- develop awareness of earthquake sources
- determine earthquake potential
- predict effects of earthquakes on buildings
- communicate research to educate the public
41
How did Alaska plan for earthquakes?
-trans-alaska oil pipeline construction was altered where it crossed the denali fault
42
How is seismic risk estimated?
- hazard maps identify areas of risk associated with earthquake effects
- they include areas prone to liquefaction, zones of possible ground rupture, historic epicentres
43
What are precursors to earthquakes?
1. 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 fault that have not seen recent earthquakes may be 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
44
How can earthquakes be forecasted?
- 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 15-30 seconds of warning and only warn of an earthquake that has already occurred
45
What is the perception of the earthquake hazard?
- 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
46
What are community adjustments that can be made to earthquake hazard?
- not possible to prevent people from living in earthquake prone areas
- 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 available
47
What personal adjustments should be made for earthquakes?
- most 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
