Earthquakes Flashcards
Faults
fractures in along which displacement occurs
Focus (hypocentre)
source location of the earthquake on the failt
epicenter
point on Earth’s surface directly above the focus
seismic
of, subject to, or caused by an earthquake or Earth vibration
What are body waves and which types of body waves are there?
- Body waves travel through earth
- P-waves (Primary) travel faster
- S-waves (secondary) travel slower
What are surface waves and which types of body waves are there?
Surface waves are slower and cause greatest destruction
- L-waves have largest amplitude, shearing motion
- Rayleigh waves have rolling motion
How to seismometers work?
- mass attatched to moveable frame
- motion of mass damped by its interia
- relative motion recorded on rotating drum, magnetic tape, or digitally
How to locate an earthquake
- Mearuee amplitude of largest wave on record
- Use time between P and S waves to find distance to epicenter
- Line between distance and ampllitude gives Richter magnitude
How many seismic records are needed to locate the epicenter of an earthquake?
3
Focal dapth classifications:
- Shallow: 0-70km
- intermediate: 70-300km
- deep: > 300km
Intraplate earthquakes
- destructive
- strain accumulation not directly related to plate movements, but due to localised strain accumulation
Natural vs anthropogenic earthquake examples
Natural:
- volcanic
- tectonic
- landslide-generated
Anthropogenic:
- rock burst
- fluid injection or extraction
- reservoir-triggered
- explosion
How are the Mercalli Scale and Richter Scale different
- Mercalli Scale is based on damage and effects as a measure of earthquake intensity
- RIchter scale is based on seismographic record
Mercalli Scale advantages
- no strong ground motion seismometers in mainly seismically active regions
- help construct a record of past seirmic activity, important for risk assessment
Mercalli Scale drawbacks
- 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
P-wave magnitude
- From the P-wave amplitude
- not affected by focal depth to source
Surface wave magnitude
- amplitude of largest wave in surface within a period
- better approximation to size of a quake
Earthquake intensity relates to _______
ground motion during an earthquake
Perception of earthquake intensity depends on:
- magnitude
- distance
- foundation
- structural resistence
- duration of motion/shaking
Old crust is ____ at transmitting ______ than young crust
- more effective
- seismic energy
Material amplification
seismic waves slow as they move from bedrock to loose sediment
Structural collapse depends on:
- magnitude and duration of strong ground motion
- distance from epicentre
- ground response
- building construction and maintenance
Resonance
taller buildings are more sensitive to lower frequencies, shorter buildings are more sensitive to high frequencies
Liquefaction
- 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
Subsidence
earthquakes can raise or lower land 10m or more during one event
How to earthquakes cause tsunamis?
- vertical displacement of fault
- large undersea landslide
How do earthquakes cause fires?
- ground shaking ruptures power and gas lines
- damage ot water mains prevents fire fighting efforts
Factors contributing to fires
- wooden buildings
- narrow streets
- inadequate water
- fractured gas mains
- overturned stoves
Difficulties with forecasting
- only as good as the available catalogues
- poor for low frequancy/high magnitude seismicity
Earthquake precursors
- groundwater changes
- atmospheric effects
- gas release
- anamolous animal behaviour
- changes in seismic velocities
- crustal deformation
- changes in magnetic/electrical properties of rocks
- foreshocks
Cascadia subduction zone
- shallow, deep, or intraplate
- subduction of Juan de Fuca plate beneath NA plate
Evidence of past megathrust earthquakes in Cascadia
- oral history of Cowichan people
- tree rings
- buried lowlands
- tsunami deposits
- written tsunami records from Japan
How many megathrust earthquakes have happened in Cascadia in the lest 6000 years?
13
average time between megathrust earthquakes in Cascadia
every 500-600 years
