Earthquakes Flashcards
Give a description of earthquakes
Earthquakes have a spatial distribution in linear belts/clusters. They occur anywhere along plate boundaries and dd boundaries, 90% occur along plate boundaries
What are earthquakes?
A sudden movement of the earths crust
What is the depth of shallow, intermediate earthquakes and deep earthquakes?
Shallow (<70km depth)
Intermediate (70-300km)
Deep (>300)
Why do some earthquakes occur away from boundaries?
Reactivation of old fault lines
Longer pressure builds up…
…stronger the earthquake
Where do rocks obtain the strength?
Interlocking and cementation of grains or crystals of which they are built
What are earthquakes meander using?
A seismograph
What do high peaks and troughs show on a seismograph?
Worse or more violent earthquake
How many violent earthquakes does the world experience on average each year?
20-50
What is the magnitude of a earthquake measured on?
A Richter scale
What is the scale of the Richter scale?
0-9
On a Richter scale, and earthquake measured at 6 is how many times greater than a quake measured at 5?
Ten times more powerful
What instrument is used to measure the effects of the quake on people?
A Mercalli scale
How many degrees of severity are present on a Mercalli scale?
12 degrees of severity
What scale, abbreviated as Mw, is preferred for measuring earthquakes?
The moment magnitude scale
What does the Mw use to base the severity of the earthquake?
Total moment release of the earthquake
What is moment?
Moment is the product of the distance a fault moved and the force required to move it
What is a logarithmic scale?
It means for every whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by the seismograph goes up in tens
What are the magnitudes 1 and 8 power equivalent to?
1- 6 ounces of TNT exploding
8- 6 million tonnes of TNT exploding
What are shock waves?
Energy released by and earthquake
What is the focus?
Where shockwaves are released from
What is the epicentre?
The point on the surface directly above the focus where the waves are felt the strongest
What are P waves?
P waves or primary waves are compression waves
They move the fastest
Compress the rock in front of them and elongate behind it (to and fro)
That can move through solids liquids and gas
What are S waves?
Secondary S waves or shear waves, undulate causing an up and down and side to side motion as they roll through the earth
They can only move through a solid
Do more damage than P waves
P and S waves travel through the interior of the earth so therefore they are…
Body waves
What are L waves?
The surface L waves are the surface waves whose motion is restricted to near ground surface
Travel more slowly than S waves
There are two types of L waves, what are they called and there properties?
Love waves- vibrate at Rift angles to the direction of transmission but have no vertical displacement
Rayleigh waves- have both vertical and horizontal movement in a vertical plane pointing in the direction of the transmitted wave
Where is the point of maximum displacement of shock waves?
The focus
In what year were 4 kinds of seismic waves detected?
1909
Seismographs close to the epicentre picked up waves doing what?
P and S waves close to the epicentre moving slowly and further away P waves and S waves were moving faster
When these shock waves meet different rock densities, what happens to the waves?
They are reflected and refracted
If shock waves pass through denser rocks they?
Speed up
When shock waves pass through less dense rocks they?
Slow down
From the MOHO, was was detected?
Slower waves had travelled from the focus of the earthquake through the upper layer of the crust but, by contrast foster waves have passed through denser material in the earths core, this denser material speeded up the waves and deflected them
So what was suggested about the earths crust due to the MOHO?
The change in density from 2.9g/cm3 to 3.3g/cm3 marked the boundary between the earths crust and the mantle below
What does MOHO stand for?
The Mohorovicic discontinuity
Later a shadow was found in a area between 105o and 142o, why could they not detect shock waves from within?
Shock waves had passed from a liquid to a solid, this S waves would stop and P waves would be refracted, this was effectively the boundary between the mantle and the core
What are the primary earthquake effects?
- collapsing buildings
- falling debris
- roads, bridges collapsing
- shock and panic
- broken gas pipes
- loss of water supply
- loss of medical supplies
- damaged communications
- Broken water mains
- power lines destroyed
- ports destroyed
- poor quality housing flattered
- shock waves and ground shaking
What are the secondary effects of a earthquake?
- fires
- landslides and avalanches
- tsunamis
- diseases
- economic losses
- soil liquefaction
- fear of future tectonic events
- homelessness
The scale of a earthquake disaster depends on…
- strength of the earthquake
- depth of focus
- nature of bedrock
- building design
- population distribution
- wealth of country
How do humans cause earthquakes?
- Rocky Mountain in Denver Colorado, injected into underlying rocks during 60’s. Minor earthquakes occurred in a previously earthquake free zone, 1962-1965 700 minor quakes
- underground nuclear testing, 1968, 120,000 tone bombs in Nevada triggered 30 minor earthquakes
- Hoover dam, as lake filled underlying rocks were adjusted and increased over 40 km3 of water and 6000 minor earthquakes in 10 years
