Earthquakes Flashcards
What are tectonic plates?
Can either be continental or oceanic. The Earth is broken up into several ‘plates’ of (average 100m thick), part crust and part mantle (different minerals in each).
How are earthquakes, volcanoes and mountains formed?
By the plates interacting at their boundaries.
What is a mainshock?
Is the biggest earthquake in the sequence.
What is a foreshock?
Are any SMALLER earthquake BEFORE the mainshock (in the same region).
What is an aftershock?
Are any SMALLER earthquake AFTER the mainshock (in the same general region). They generally diminish in frequency and intensity over a period of several months.
Although aftershocks are not intense as the mainshock, they can sometimes cause more damage to structures as they have already been weakened.
Note: It is impossible to say whether a particular earthquake after a previous one would have happened anyway (still actively working on this).
What is an earthquake?
Earthquakes are natural geological phenomena caused by the sudden and rapid movement of a large volume of rock.
The violent shaking and destruction caused by an earthquake are the result of rupture and slippage along fractures in the Earth’s crust called faults.
What is the focus?
The focus is the point where the earthquake occurred (underground). Often between 5km-700km.
What is the epicentre?
The epicentre is the point directly above the focus on the surface of the Earth.
what is released from earthquakes?
A massive amount of energy is released as SEISMIC WAVES.
What are seismic waves?
Seismic waves area form of elastic energy causing vibrations in the material that transmits them.
Earthquakes generate waves that radiate outwards in all directions from the focus.
How many strong earthquakes on average are there each year.
On average 75. But there are thousands of earthquakes happening each year but many of them are small and can only be detected by sensitive instruments.
What damage do earthquakes often create when near a large population.
Shaking of the ground and liquefaction of soils brings, wreaks havoc on, buildings, roads and other structures.
As well as power and gas lines rupturing, causing numerous fires that become uncontrollable especially when water mains have been broken by the earthquake.
What causes earthquakes?
- Energy released by atomic explosions or the movement of magma in Earth’s crust can generate earthquake-like waves but are generally WEAK.
- Tectonic stresses acting over tens of thousands of years slowly deform the crustal rock on both sides of a fault. When deformed by differential stresses, rocks bend and store elastic energy. Eventually, the fictional resistance holding the rocks together is overcome. Slippage allows the deformed (strained) rock to ‘snap back’ to its original stress-free shape.
In summary:
Earthquakes are produced by the rapid release of static energy stored in rock that has been deformed by differential stresses. Once the strength of the rock is exceeded, it suddenly ruptures, causing the vibrations of an earthquake.
How do we know that Earth is not a static planet?
One way is that fossils of marine organisms have been found thousands above sea level.
And when earthquakes occur you can see the displacement of fences, roads etc.
What does the rigid lithosphere move over?
The rigid lithosphere moves above the weaker but still solid asthenosphere.
