General earthquake background
What are they?
- Shaking caused by a rapid release of energy
- Common on our planet-occur every day-more than a million detectable each year.
- Result of plate movements
How large are they?
- Most=small- <3Mw (moments magnitude)
- However large earthquakes can destroy infrastructure and cause mortality
- Highest death toll - 1556 Shen-shu earthquake, China- 820,000 deaths
- Overall 3.5 million predicted deaths over the last 2,000 years
Moments magnitude scale:
- abbreviated to MMS; denoted Mw
- measures the size of earthquakes in terms of the energy released
- also considers the rigidity of the Earth moved, the slip on the fault and the size that slipped.
Causes of Earthquakes
- Sudden motion along a newly formed crustal fault
- Sudden slip along an existing fault
- Movement of magma in a volcano
- Volcanic eruption
- Giant landslide
- Meteorite impact
- Nuclear detonation
A fault slip=most common
Features of earthquakes
Hypocentre Where fault slip occurs. Seismic waves expand outwards from this point.
Epicentre= Land surface right above the hypocentre. Used in relation to settlements nearby.
Types of fault in the crust
Faults are produced by rock breaking apart. Once created they are always a zone of weakness: pre-existing faults can always be reactivated.
- Resulting from expansion/stretching: Hanging wall moves down compared to the footwall.
- Resulting from compression. Hanging wall moves up compared to the footwall.
- Where two blocks slide laterally past each other. No vertical motion.
- Common fault on compressional mountain belts. The slope of the fault is not very steep-e.g. 30o.
Where are faults?
Found everywhere in the crust
Can be active or inactive
Not all faults reach the surface- blind faults= invisible
A fault scarp is produced when there is displacement at the land surface
2 types of wave: Body + surface waves.
Body waves arrive first by travelling through the Earth's interior- they are of higher frequency:
P waves:primary/compressional- travel by expanding + compressing material- which moves back and forwards parallel to wave direction. Fastest type of wave- can also travel through solids, liquids and gases.
S waves: secondary/shear- travel by moving material back +forth perpendicular to wave direction. Slower than p-waves, can only travel through solid material.
Surface waves travel only through the crust; slower+lower frequency- cause damage
L-waves: love waves- move the ground back and forth like a writhing snake. Slowest + most destructive
R-waves: rayleigh- the ground ripples up and down like an ocean wave. Die out with depth
Insturments that record ground motion- both vertical and horizontal.
- \P waves (fastest- arrive first)
- S waves
- Surface (slower- arrive last)
Arrival times can be used to work out the time delay between the waves and how far away the hypocentre of the earthquake must have been to produce the gaps between the waves. This is because the seperation between each type of wave increases as the distance from the epicentre increases.
To do this: compare with a Time travel curve graph
- This plots the increasing dely in arrivals and can be used to find the distance to the epicentre
- If different seismometer stations do this- they can triangulate and find the epicentre. Yay! :XD
Measuring an earthquake
Magnitude-amount of ground motion
Mw- uniform measurement- Normalized to reduce impact of distance
Logarithmic each new value = 10x bigger than the last.
Intensity- severity of damage
Mercalli intensity scale- degree of shaking damage- Uses roman numerals
Damage decreases as distance from epicentre increases
Earthquakes and plate boundary types
Divergent: Mid-ocean ridge
- Normal faults @ spreading ridge axis'
- Strike-slip @transforms
- Tend to be shallow- <10km deep
Divergent- Continental rifts e.g. East African rift/ Rio Grande rift, mexico
- Normal faults due to stretching.
Convergent e.g. Andes
- Large thrust faults @ contact between plates: Subducting plate bends the overriding plate- this can snap back. Both shallow+deep. But below 660km= rare as mantle is more ductile here.
Transform - e.g. San Andreas, California
- mainly stike-slip
Only around 5% earthquakes do not occur on plate boundaries
Theories as to how?
- Remnant crustal weakness in failed rifts or shear zones?
- Stresses in the aesthenosphere transmitted up to the lithosphere?
1811-1812: New Madrid Earthquake
- 3 Magnitude 7 earthquakes reversed the flow of the Mississippi river
- Due to an ancient rift zone?
Factors in earthquake damage
- Magnitude/energy of earthquake
- Depth of hypocentre
- Distance from epicentre
- Nature of sub-surface materials
- Duration of the shaking
Human factors e.g. level of development, building regulations, precautionary measures in place etc.
As well as the earthquake...
Landslides + avalanches
- Material on steep slopes could fail either during the shaking or afterwards due to structures being weakened.
- Where water is pushed up from the ground in certain locations- draining other areas- this can cause land to subside in sections and buildings to slump downwards or collapse
- Shaking may topple stoves, candles etc
- Broken gas lines may ignite
- Water and sewage pipes may be broken and leak
Emergency services may struggle to respond due to disruption of roads etc.
Results from displacement of the ocean floor
This could be due to:
- Submarine landmines
- Volcanic eruptions
Faulting displaces a large volume of overlying water
The thrust up colum of water then spreads out in a rippling effect in all directions
Imperceptible in deep water, long wavelength + travels extremely quickly - up to 100's kmph
In shallow waters- friction causes the bottom of the wave to slow, forcing the water to build up in heigh to sometimes over 10m.
This then surges on to land + can cause huge destruction
Indian Ocean Tsunami, 2004
Location: Sunda oceanic trench, North-west Sumatra
Spped: 800km per hour
Height on land: 15m in some areas
Mortality: Close to 1/4 million people
Tohoku earthquake, Japan, 2011
Pacific plate lurched upwards and moved the seafloor upwards by 30cm
Magnitude 9 earthquake
Scientists had predicted an earthquake would occur soon- but not one of that magnitude.
Japan experiences many earthquakes- stringent building codes- helped prevent widespread collapse. Also- public had a minute warning due to early warning system.
Still: dust cloud- fog-like mist; buildings swayed; fires
Tsunami did the main damage- submerged low-lying land up to 10km fom the coast
$300 bn damages. Around 16,000 deaths -mostly due to tsnumai. But over 2,000 missing.
also caused a level 7 nuclear meltdown + has affected the country's policy on energy ever since