Earthquakes 3 Flashcards
Elastic Rebound Theory
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- pate tectonic forces stress rocks -> deform
- Elastic deformation of lithosphere
- Brittle failure / deformation so litho eventually breaks
- stored elastic energy is released and deformed rocks go back to original state
(energy stored on either side of failure)
What theory describes how earthquakes occur?
elastic rebound theory
-> we feel the waves/vibration produced by elastic rebound
Describe the processes of plates at the boundary
- Plates lock = stress & deformation
- Plates release = earthquake & tsunami
does a rupture propagate far?
only along a portion of the fault
-> some areas are still locked
Describe how earthquakes occur in terms of the brittle and elastically deformed crust
- brittle crust ruptures at hypocentre
- elastically deformed crust rebounds and vibrates
- waves + energy propagate away in all directions
where is the epicentre in relation to the hypocentre?
directly above it
Which type of seismic wave hurt us?
surface waves
what are the two types of body waves?
- Pressure or primary
- Shear or secondary
P - waves
- fastest
- particle motion parallel to propagation (like slingy)
- can move through solids or fluids
S - waves
- slower than p waves
- particle motion is perpendicular to propagation (like waves)
- can ONLY move through solids
Can S waves move through fluids?
no
Can P waves move through fluids?
yes
Surface waves
- the damaging waves
- require a surface, cannot travel thru water (?)
- slower than body waves
What are the types of surface waves?
- Rayleigh wave
- Love wave
Rayleigh wave
- vertical and horizontal motion parallel to wave direction
- like an ocean wave
Love wave
- horizontal movement perpendicular to wave travel direction and parallel to ground surface
- like when u wave a rope on the ground
rank the seismic waves based on speed
- P (6 km/s)
- S (3.5 km/s)
- Surface (2-3 km/s)
From a seismograph, how can we tell if we are far or close to the epicentre?
far: bigger gap between the P, S, and Surface waves
close: smaller gap ^^
On a seismograph, which type of wave appears the largest + longest
surface
How do we determine where the hypocentre is?
We use time delay between the P and S wave arrivals
-> need at least 3 seismometers
How many seismometers do we need to pinpoint the hypocenter?
at least 3
1 = know how far
2 = gives two points
3 = tada! TRIANGULATION
we use scales that measure earthquakes based on _____ & _____
Magnitude and Intensity
What scale do we use to calculate earthquake magnitude?
Moment Magnitude (M_w)
- logarithmic
- quantitative measure of size
- proportional to energy release
EQ Magnitude: earthquake hazard is proportional to
energy release
As EQ Magnitude increases, what grows?
both rupture area and duration of shaking grow
-> and risk!
EQ Magnitude is based on what wave property?
wave amplitudes recorded by seismometer (slip area)
For each increase on the Magnitude scale, what factor do ground motion and energy released increase by?
ground motion: by a factor of 10
energy released: by a factor of 32!
How much bigger is the energy released in a M8 EQ compared to M6?
32 x 32
~ 1000
What is the ground motion difference between a M6 and M8 EQ?
10 x 10
~ 100
What is the Moment Magnitude (M_w) based on?
- based on the area of rupture and slip on fault
What scale do we use to calculate EQ intensity?
Modified Mercali Scale
- ranges from 1 to 12
What does the Modified Mercali Scale measure?
Qualitative estimate of how ground motion affects population and structures
- perception of ground shaking intensity
- observed damage
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The factors that influence our perception of EQ intensity
- Magnitude
- Distance from hypocentre
- Structural resistance
- duration
- ground material
Seismic waves ____ as they travel
weaken
Shaking from a Mw 9 EQ will last about
4-5 minutes
what ground materials are worse in an EQ?
wet sediments = more intense
rock and compact sands = less intense
Building design in an EQ
Good: flexible, wood, concrete WITH steel
Bad: brick, stone, concrete without steel
