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Flashcards in Earthquakes Deck (34)
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1
Q

Faults

A

fractures in along which displacement occurs

2
Q

Focus (hypocentre)

A

source location of the earthquake on the failt

3
Q

epicenter

A

point on Earth’s surface directly above the focus

4
Q

seismic

A

of, subject to, or caused by an earthquake or Earth vibration

5
Q

What are body waves and which types of body waves are there?

A
  • Body waves travel through earth
  • P-waves (Primary) travel faster
  • S-waves (secondary) travel slower
6
Q

What are surface waves and which types of body waves are there?

A

Surface waves are slower and cause greatest destruction

  • L-waves have largest amplitude, shearing motion
  • Rayleigh waves have rolling motion
7
Q

How to seismometers work?

A
  • mass attatched to moveable frame
  • motion of mass damped by its interia
  • relative motion recorded on rotating drum, magnetic tape, or digitally
8
Q

How to locate an earthquake

A
  1. Mearuee amplitude of largest wave on record
  2. Use time between P and S waves to find distance to epicenter
  3. Line between distance and ampllitude gives Richter magnitude
9
Q

How many seismic records are needed to locate the epicenter of an earthquake?

A

3

10
Q

Focal dapth classifications:

A
  • Shallow: 0-70km
  • intermediate: 70-300km
  • deep: > 300km
11
Q

Intraplate earthquakes

A
  • destructive

- strain accumulation not directly related to plate movements, but due to localised strain accumulation

12
Q

Natural vs anthropogenic earthquake examples

A

Natural:

  • volcanic
  • tectonic
  • landslide-generated

Anthropogenic:

  • rock burst
  • fluid injection or extraction
  • reservoir-triggered
  • explosion
13
Q

How are the Mercalli Scale and Richter Scale different

A
  • Mercalli Scale is based on damage and effects as a measure of earthquake intensity
  • RIchter scale is based on seismographic record
14
Q

Mercalli Scale advantages

A
  • no strong ground motion seismometers in mainly seismically active regions
  • help construct a record of past seirmic activity, important for risk assessment
15
Q

Mercalli Scale drawbacks

A
  • subjective, inconsistent
  • affected by factors such as geology
  • no use in uninhabited areas
  • cannot distinguish between proximal and distal quakes
  • local assessment often reflects maximum rather than average
16
Q

P-wave magnitude

A
  • From the P-wave amplitude

- not affected by focal depth to source

17
Q

Surface wave magnitude

A
  • amplitude of largest wave in surface within a period

- better approximation to size of a quake

18
Q

Earthquake intensity relates to _______

A

ground motion during an earthquake

19
Q

Perception of earthquake intensity depends on:

A
  • magnitude
  • distance
  • foundation
  • structural resistence
  • duration of motion/shaking
20
Q

Old crust is ____ at transmitting ______ than young crust

A
  • more effective

- seismic energy

21
Q

Material amplification

A

seismic waves slow as they move from bedrock to loose sediment

22
Q

Structural collapse depends on:

A
  • magnitude and duration of strong ground motion
  • distance from epicentre
  • ground response
  • building construction and maintenance
23
Q

Resonance

A

taller buildings are more sensitive to lower frequencies, shorter buildings are more sensitive to high frequencies

24
Q

Liquefaction

A
  • loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking
  • causes major subsidence, fracturing, sliding of surface, buildings can silk and tilt
25
Q

Subsidence

A

earthquakes can raise or lower land 10m or more during one event

26
Q

How to earthquakes cause tsunamis?

A
  • vertical displacement of fault

- large undersea landslide

27
Q

How do earthquakes cause fires?

A
  • ground shaking ruptures power and gas lines

- damage ot water mains prevents fire fighting efforts

28
Q

Factors contributing to fires

A
  • wooden buildings
  • narrow streets
  • inadequate water
  • fractured gas mains
  • overturned stoves
29
Q

Difficulties with forecasting

A
  • only as good as the available catalogues

- poor for low frequancy/high magnitude seismicity

30
Q

Earthquake precursors

A
  • groundwater changes
  • atmospheric effects
  • gas release
  • anamolous animal behaviour
  • changes in seismic velocities
  • crustal deformation
  • changes in magnetic/electrical properties of rocks
  • foreshocks
31
Q

Cascadia subduction zone

A
  • shallow, deep, or intraplate

- subduction of Juan de Fuca plate beneath NA plate

32
Q

Evidence of past megathrust earthquakes in Cascadia

A
  • oral history of Cowichan people
  • tree rings
  • buried lowlands
  • tsunami deposits
  • written tsunami records from Japan
33
Q

How many megathrust earthquakes have happened in Cascadia in the lest 6000 years?

A

13

34
Q

average time between megathrust earthquakes in Cascadia

A

every 500-600 years