Lecture Twenty Nine - Earthquakes Flashcards
What is an earthquake?
A sudden movement on an existing fault or the formation of a new fault.
What is a fault?
A fault is a break or fracture in the Earth’s crust.
Stored elastic energy is suddenly released causing ground movement and seismic waves.
Types of faults:
Normal fault - extension.
Reverse fault - high angle compression.
Thrust fault - low angle compression.
Slip-strike fault - Compression.
What is the elastic rebound theory?
Occurs when elastic strain begins to build up between to faces in the Earths crust (on a fault line), eventually this strain is released by the elastic energy being relesed very quickly (in a matter or seconds or minutes).
What are the different types of seismic waves?
P-waves: primary waves = compression.
S-waves: secondary waves = sinusoidal waves.
Love waves: L-waves = lateral undulations.
Rayleigh waves: R-waves = rolling motion.
Where do Earthquakes occur?
Mostly along tectonic boundaries.
Mostly at less than 100km into the Earth.
Most deformation in the Earth upper crust takes place on faults at plate boundaries.
Relatively cold rocks (T<400 degrees C) fail in a brittle manner and produce Earthquakes.
Hot rocks deform in a plastic (ductile) manner, but can still prodcue earthquakes, just less likely.
What is the focus and epicentre of an earthquake?
Focus: the location of an Earthquake in 3D, the loaction of the rupture of the crust.
Epicentre: the point at the Earth’s surface above the focus.
The ‘rupture zone’ is the part of the fault that really mvoed during an earthquake.
Deep earthquakes do not repture the surface.
Explain how the magnitude of earthquakes in measured.
The magnitude is a measurement of the size of an earthquake.
Magnitude effectively measures then amount of energy released.
Was primarily measured by the Richter scale - M = log10(A) + B.
M = Magnitude.
A = Amplitude on seismograph trace.
B = Distance correction factor - depends logaruthmically on the distance between the seismometer and epicentre and is 0 at epicentre.
Richter magnitude scale is Logarithmic, that is, a magnitude 5 earthquake is 10 times stronger than a magnitude 4 earthquake.
Magnitude is now measured using Moment Magnitude (Mw) Scale.
Looks at the amount of slip and sie of reupture where siesmic waves were generated.
This scale is better at telling earthquakes apart.
Describe the effects of different magnitude earthquakes on the richter scale.
Less than 3.5 = generally not felt.
3.5-5.5 = often felt, but rarely causes damage.
Under 6 = Slight damage in well designed buildings. Major damage to poorly constructed buildings over a small region.
6.1-6.9 = Can be destructive in areas up to 100km across where people live.
7-7.9 = Major earthquake. Can cause damage over large areas.
8+ = Great earthquake. Can cause damage over hundereds of km.
What effects can an earthquake have on the earth?
Sudden ground motion:
Can be affected by the type of ground wave travel through.
Rocks shake less than sediment.
Can cause tsunamis.
Landslides:
Liquifaction:
Seismic waves cause water saturated semi-consolidated or unconsolidated sediments to become a fluid like slurry and lose strength.
This can force water to the surface, and cause mud to behave like quicksand.
It can also cause sandy sediments to rise to the surface, making ‘sand volcanoes.’
Uplift and subsidence:
How can earthquakes be predicted?
Earthquake prediction in time is very difficult.
Hard to predict when ‘snap’ will occur.
Earthquakes appear to be cyclic, although not a regular cycle.
We can preduct regions of high and low earthquake probability according to:
- Average time between earthquakes.
- Seismic gap = The seismic gaps is a part of a fault that has not has an earthquake for a while (part of a fault that is due to slip).
- Stress mapping = Look at the stress build-up in the rocks along a fault line, and measures and predict areas of high and low stress.
