Seismicity Flashcards
Causes of Earthquakes
Earth’s rocks are subjected to forces that tend to bent, twist or fracture them they deform. These stresses cause earthquakes
Seismic waves
Waves emanating from the focus
- body waves
- surface waves
Body waves
Travel in all directions through the body of the earth
- P waves
- S waves
Surface waves
- Do not travel through the earth, travel along paths parallel to the earths surface.
- They cause up and down and side to side movements.
- Slower than S waves
Seismometers
An instrument used to record seismic vibrations to create a seismograph.
-P waves arrive before S waves
Richter scale
- Scale of earthquake size
- Measures the amplitude of a wave
- Tenfold increase for each number
- Uses computers and advanced technology
Moment magnitude
- No maximum or minimum
- Measures the total energy released by an earthquake
- Includes area of the faults rupture and the slippage along the fault
Mercalli scale
- Asses the intensity of ground shaking and building damage over large areas
- Applied after the earthquake
- Surveys
- Subjective
What happens during an earthquake
- Shaking
- Ground rupture (along the fault line)
- Fire (secondary effect as power lines e.t.c rupture)
- Landslide and debris/rock fall
- Liquification
- Aftershocks
- Tsunami
Liquefaction
A process that occurs in water saturated unconsolidated sediment, sand and soil flows rather than remains solid.
Aftershocks
- Further earthquakes
- May make it dangerous for a rescue effort to be made
Tsunamis
- Giant ocean waves
- Generated when a body of water is disturbed
- Waves amplify as they reach the coast
Short term effects of earthquakes
- Ground shaking causes damage
- Schools destroyed
- Immediate death and injury
- shocked, hungry and homeless people
- Landslides
- Liquefaction
- damage to power stations
- Panic, fear and hunger
Medium term effects of earthquakes
- Fire
- Education suspended
- Bodies not buried, and long term disabilities from injuries
- NGO’s provide supplies
- -Flooding
- Power cuts
- Civil disorder
Long Term effects of earthquakes
- High unemployment
- repair of infrastructure
- Long term lost generation
- Trauma and grief
- Pre fab homes may become permanent
- Loss of farmland
Long term forecasting
- Paleo seismology, studies pre historic earthquakes, and they have recurrence intervals, Long term forecast can be made.
- Seismic gaps, an area along a fault where no earthquakes have occurred recently
Short term predictions
- Precursor event may signal an earthquake is coming
- Ground uplift and tilting
- Foreshocks
- Water levels in wells drop or raise
- Emission of radon gas
- Strange animal behaviour
Equipment used to measure earthquakes
- Levelling
- Laser reflector
- Seismometer
- Gravity meter
- Strain meter
- Water table level
- Radon gas sensor
- Satellite surveying
Human induced earthquakes
–Hoover Dam construction (600 earthquakes)
-Toxic waste injected into hazardous waste disposal wells in 1960’s
-Nuclear testing in Nevada
(Due to increasing fluid pressure in the rocks which reactive older faults)
Mitigating earthquake hazards
- Strict building codes (California)
- Stepped profile of buildings
- Bracing
- deep foundations
- steel framed buildings e.t.c
1986 San Fran Earthquakes
- Richter scale 7.1
- Killed 40 people
- Strict building codes
1986 Armenia Earthquakes
- Magnitude 6.9
- No earthquake proof buildings
- 25,000 killed
P waves
- P waves, travel with a velocity dependent of the elasticity of the rock, moves via compression, very like a sound wave
- Less destructive
S waves
- S waves, shear waves or secondary waves, travel with a velocity dependent on the rigidity and density of the rock, shear a material or change its shape (Love waves shake side to side and Rayleigh waves shakes up and down)
- More damaging
- Don’t travel through liquids
