EARTHQUAKES Flashcards
- Earthquake:
the ground shaking cases by stresses released in Earth
o Factors that determine destructiveness
Location, magnitude, depth, distance from epicenter, secondary effects, architecture
EQ stresses
seismic waves (kinetic energy), displacements along the fault (kinetic energy), heat and other energy
megathrust EQs
plate boundary EQ
up to M9 occurs every 200-800 years
causes large tsunamis
- Understand why most earthquakes IN Canada occur on the west coast
o Subduction zone at a convergent Boudry
North America plate w/ Juan de Fuca plate and pacific plate
North American plate EQs
* Large damage potential closer to surface =more shaking
Juan de Fuca plate EQs
* Deeper and very frequent
o Quantitative 1-10
measures magnitude
richter scale
moment magnitude scale
Richter’s scale
based on aptitude of seismic waves, which is related to energy released
* Not really used by geologists, despite being well known
Moment magnitude scale:
is based on the amount of fault slippage produces, the size of surface area that ruptured, and the nature of the materials that faulted, which is more accurate than Richter’s scale
qualitative
1-12 intensity
Modifies Mercalli scale
Modifies Mercalli scale
what we feel
* 1: felt by very few
* ~5: general panic
* 12: total destruction
o Magnitude
Characterizes an EQs relative size
Each whole number represents a tenfold increase in the measured amplitude and 32 times more energy release
- Use plate tectonics to predict where earthquakes occur, and how earthquake depth and magnitude is ties to different types of plate tectonic boundaries.
o Materials respond differently (strain) to stress. Brittle deformation leads to an earthquakes
o Deep focus at convergent boundaries
o Intermediate focus at transform boundaries?
o Shallow focus at divergent boundaries
o Subduction zone earthquakes (convergent boundaries)
Largest possible earthquakes/ deep focus (meteorite impacts can be larger)
Convergent boundaries
Type of stress here: compression
Type of faults: reverse (or thrust)
Three locations of earthquakes
* #1: subduction interface earthquakes.
o Can be huge @ M9.4) megathrust EQs
* #2 EQs in the subducting or overriding plate
o Moderate size, largest = ~M7
* #3 Benioff earthquakes
o Very deeps: 100s of km)
o Little/ no damages at surface
o Deeper focus as moved down the Benioff zone
Megathrust rupture zone located on the coast of Vancouver island
* 12-30 minutes from EQ till initial tsunami waves arrive
* Last one: jan 26 1700 @ M9 (records in Japan and first nations(oral) )
o Continent- continent convergent Eq zones
Very large (not as large as subduction) up to ~M8
Convergent boundaries
Type of stress: compression
Type of fault: reverse (or thrust)
India= thrust fault
o Divergent EQ zones
Small to moderate EQs usually less than M5
Most are underwater
Type of stress: tension/extension
Type of fault: normal
Ex: east African rift, mid Atlantic ridge
* Rift valley forms
o Transform boundaries
Oceanic (majority) very small Eqs
Continental- shallow mostly small earthquakes, some larger usually up to M6 or M7
Type of stress: shear
Type of fault: strike-slip
Ex: San Andreas fault, Queen Charlotte fault
o Body waves
P- wave (primary/ pressure)
* Particles move parallel to the direction wave propagates
* Can travel through all matter
* Is the fastest
S- waves ( shear/ secondary)
* Particles move perpendicular to direction wave propagates
surface waves
Rayleigh wave: vertical and horizontal motion parallel to wave travel direction (like ocean wave)
Can travel through all matter
o Seismograph: the basic idea
Mass suspended by a spring
Seismograph moves, the mass does not
Motion causes pen to move
EQ detectors
P waves arrive first
* Twice as fast as the others
* Over 300km (our distance from the subduction zone)
* P waves will take 60 sec to arrive
* Surface waves will take 100 sec to arrive
* If we can detect p waves, we have enough time to “take shelter”
o Seismograph readings
Three seismographs gives us triangulation
The earthquake epicenter is where the three circles meet
o EQ forecasting
Scientists cannot yet predict EQs due to the complicated nature of subsurface stress and strain
* They use probability
Long-term forecasts
(10- few thousand years)
Medium range predictions
(few years to a few months)
Short range predictions
(few days or hours)
paleoseismology
uses past earthquake data to predict future occurrences
o Areas which are overdue for an earthquake are in a seismic gap and have built up strain
- Paleo earthquakes:
rock records also show past EQs through disrupted sediment layering
o The faults
o Measurements
(tiltmeters and radon detectors)
Measure dilatancy
increase in volume due to built up stress in a rock
Tiltmeters:
measures tilting and swelling (dilatancy) of rocks
Seismographs: BC has 30, 200 all over Canada
japan more than 4000
How we perceive earthquakes depends
- Magnitude
- Foundation (ground) material
- Duration
- Building construction
o In general
Brick and concrete- bad
Wood, steal, and reinforces concrete – good
5 EQ hazard types
ground shaking
liquefaction
fire
tsunami
landslides
landslides
very common after EQs
Usually, people blame these deaths on the EQs not the landslide but
Very common in BC and mountains around the world
After EQs landslides can increase for up to 10 years
tsunami
Can be caused by vertical motion of tectonic plates (or landslide, or meteorites, or volcanoes)
fire
Shaking breaks gas and electrical lines (often at the same place)
Van- secondary fire hydrant system- connected to ocean incase mountain connection fails
liquefaction
Shaking causes wet loose (unconsolidated) soils to liquify (act like a liquid)
Ground loses cohesion and flows
Bad for buildings
Richmond and delta are at major risk to liquification since the ground is mostly loose sediments
o Ground shaking
Buildings may fall down
Most deaths and injuries in EQs are from building collapse
Bedrock is much more stable than sediments
Building materials
* Wood is better than brick since it is flexible
* Building codes need to meet seismic standards in EQ prone areas
o Interplate
EQs between two plates, initialed near plate boundaries
o Intraplate
EQs that are large and damaging, located within a single plate, and can occur far from plate boundaries
short term predictions
o Chinese scientists made the first successful prediction of major EQ in 1975 based on foreshocks: M7.5 haicheng earthquake
o Other precursory phenomena include pre-seismic deformation of the ground surface, emission of radon gas seismic gaps along faults, and anomalous animal behaviour
- EQ planning
o Development o hazard-reduction programs
o Engineering and land-use adjustments to earthquake activity
o Development of a warning system
