Earthquakes Flashcards
Where are the effects of earthquakes greatest?
In built up areas
What are the two main causes of earthquake fatalities?
Soilifluction
Landslides
What type of earthquake is more frequent?
Small earthquakes are more frequent than big ones
What affects the severity of an earthquake?
The depth of the focus
What was the magnitude of the Tainan earthquake in Taiwan on 5th February 2016?
6.4
Where do the largest events occur?
Where there is subduction
What is the depth of the earthquake like if it is not caused by subduction?
If it isn’t a subduction earthquake then it is a shallow earthquake
Why are some poorer areas devastated by earthquakes?
Due to finance-
Concrete is expensive so cans are used as foundations
Money is made by selling the top (building) but foundations have to be financed themselves which leaves them poorly constructed
What is an issue with compression?
Compression causes more stress which leads to higher pressure, this means the effects are more destructive
What is the first thing heard when an earthquake occurs?
A noisy p wave
When do people start to notice earthquakes?
When they are magnitude 4 or above
When was the first study of earthquakes?
1906 after the San Francisco earthquake
Where do most earthquakes occur?
Near plate boundaries
Define hazard
The potential to cause harm
Define risk
The likelihood of harm in defined circumstances
Give two examples that prove that not all events are equal
Kokoxili earthquake (Central Tibet) in 2001 had no fatalities M 7.8- high seismic hazard, low seismic risk Weichuan earthquake in China May 2008 had nearly 90,000 fatalities - high seismic hazard, high seismic risk
What are risks?
This is the probability that an event can occur in a given time by the damage that the occurrence would cause
What is the risk equation?
Hazard x (people + property0
Define earthquake
An abrupt movement in the earth caused by the sudden release of slowly accumulated strain
What is an earthquake?
A sudden slip on a fault resulting in ground shaking and radiated seismic energy caused by the slip from the hypocenter or focus
Define epicentre
The place on the surface of the earth under which an earthquake rupture originates
Define fault
A fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture
What are the 4 types of fault?
Normal fault
Right-lateral fault
Reverse fault
Left-lateral fault
Where do normal faults normally occur?
Where rifting occurs (at divergent plate boundaries) where two plates or blocks divert away from each other
What is the motion of normal faults?
Vertical motion
Give an example of an earthquake on a normal fault
Dixie Valley, Nevada 1954 M 7.3
What is the motion of a reverse fault?
Vertical motion
Give an example of an earthquake caused by a reverse fault
Chi-Chi Taiwan 1999 magnitude 7.6
Where do most reverse faults occur?
In a subduction setting (convergent plate boundary) where two plates or blocks converge towards each other
What do reverse faults lead to?
Changes in the topography as one part moves up and one moves down
Give an example of a strike slip fault earthquake
Izmit, Turkey 1999 M 7.6
Where do most strike slip faults occur?
In a transform fault setting (conservative plate boundary) where two plates or blocks slide away from each other
What are the two types of strike slip faults?
Right lateral and left lateral
What is the motion of strike slip faults?
Lateral motion
What seismic waves are released by earthquakes?
P and S body waves and surface waves
What are the 5 factors that control the level of shaking?
Magnitude Distance Geology Building style- construction not height Duration of shaking
How can geology affect the intensity of the shaking?
Local soils amplify the shaking
What is earthquake magnitude a function of?
Energy release
What is the main scale for measuring earthquake magnitude?
Richter Magnitude Scale
What sort of scale is the Richter scale?
Logarithmic
What does it mean that the Richter scale is logarithmic?
The amplitude of the seismic waves recorded by a seismograph goes up 10 times for each point on the scale
What happens to the energy of the earthquake if the amplitude on the seismograph increases by a factor of 10?
There is an increase in energy by a factor of 33
What is more common, foreshocks or aftershocks?
Aftershocks are more common
Where do aftershocks occur?
Along the surface rupture of the main event
What are 4 indirect effects of earthquakes?
Collapse of structures Fire Liquifaction Landslides Tsunamis
How many people died as a result of the 2003 Bam earthquake?
30,000
How many people were injured as a result of the 2003 Bam earthquake?
30,000
What percentage of buildings were damaged or destroyed as a result of the 2003 Bam earthquake?
85%
What was the magnitude of the 2003 Bam earthquake?
M6.5
What was the magnitude of the Boxing Day 2004 earthquake?
9.2
what are the 3 forms of earthquake records?
Instrumental
Historical
Geological
What is an issue with instrumental records?
They are not available everywhere and records are only since 1950
What is an issue with historical records?
They are fragmentary anecdotal and not everywhere
What do geological records show?
Recent fault activity, tectonic geomorphology
What don’t undeformed beds experience?
Stress
What does extension do?
It makes faults and regional thinning
What does compression make?
Faults and folds
What does shearing do?
It displaces layers horizontally and can result in strike-slip faulting
What is elastic?
All solids including the Earth’s crust
What are the 3 Fs of earthquakes?
Forces
Faults
Friction
What do jerky motions on faults cause?
Earthquakes
What is the periodic earthquake model?
Earthquakes occur when stress reaches a critical value
Stress is released back down to an ambient value
Therefore size and time are predictable
What is the size predictable earthquake model?
Earthquakes occur at some point when stress is above a critical value
Stress is released back down to the ambient value
therefore size is predictable but time is not
Explain the time predictable model
Earthquakes occur when stress reaches the critical value
Variable stress is released by the earthquake
Therefore time is predictable but size is not
What are 6 ways to cope with seismic risks?
Hazard maps Frequency/magnitude of earthquakes in specific regions Building planning Building regulations Emergency planning and insurance Tsunami monitoring systems
What does a hazard map do?
It takes into account active faults, landslide hazards and geology
What is an issue with hazard maps?
They don’t take into account building density so the full risk isn’t shown
What is one important method of coping with seismic risks?
Making sure buildings are resistant to shear
How can resistance to shear be improved?
Supporting corners in buildings
What sort of bedrock is safer to live on?
Solid bedrock
What is the Richter scale based on?
Resistance
