What are Earthquakes?

Are caused by the shaking or trembling caused by the sudden release of energy
Usually associated with faulting or breaking of rocks – cracking of the plates also called faults.
Continuing adjustment of position results in aftershocks

The Earth’s crust is subjected to huge forces
Very large forces can fracture the rock
This can be seen as fault lines in the rock layers
The Focus and Epicenter of an Earthquake
The point within Earth where faulting or cracking of the rock begins is the focus
The point directly above the focus on the surface is the epicenter

The cause of most major earthquakes is the strain that builds up along faults at or near boundaries between lithospheric plates.
A fault is a break in the lithosphere along which movement has occurred.
The depth at which an earthquake originates depends upon the type of plate boundary involved
Causes of Earthquakes
At divergent boundaries such as the M.O.R, earthquakes tend to occur within the first 30 km of the surface
This is the case for transform boundaries as well
At subduction zones, however, as one plate plunges into the mantle, the focus of the earthquakes can be as far down as 700km below the surface

The energy released in an earthquake travels in waves.
Waves that travel from the focus of an earthquake through Earth are called body waves
Every earthquake produces two different types of body waves – P waves, S waves
P waves – compressional waves also known as the Primary wave
P waves squeeze and stretch rock material as they pass through Earth – can travel through everything – solid rock, magma, ocean, and even air
The second group of waves – the S waves or secondary waves – cause the particles of rock material to move at right angles to the direction in which the waves are travelling.
S waves can only travel through solid material but not through liquids or gases
The velocity of P and S waves depends on the type and density of the material which they travel through
Velocity is greater through materials that are rigid and dense
Through all types of solid material, S waves travel at a little more than half of the speed of P waves

These are the waves that travel along the Earth’s surface
Surface waves are produced when P and S waves reach the surface
The two types of surface waves are Love waves and Rayleigh waves
Love waves cause particles to move from side to side, Rayleigh waves travel more slowly and cause particles to move in elliptical patterns.
Surface waves cause considerable damage to structures

We measure earthquakes using a seismograph – it measures the seismic waves that are sent out by the earthquake
Earthquakes are measured on the Richter Scale
6.1 earthquake on richter scale is 100 times more powerful than a 5.1 event

Seismographs – b/c earthquakes produce different types of wave motions, there are different types of seismographs to measure them
In a seismograph, a heavy weight is attached to the base anchored in bedrock – this allows the weight to stay still even in an earthquake
A record sheet called a seismogram is placed on the drum which records the wavy lines of an earthquake

P waves are the first waves to be recorded by a Seismograph, followed by the S waves and then surface waves.
The further the Seismograph is from the source of the earthquakes, the longer the lag time between the P waves and S waves
A travel – time graph shows the relationship b/w P and S wave arrival times and the distance from an earthquakes epicenter

Since a Seismograph tells scientist the distance to an earthquake, it does not tell them the exact location of the earthquake
To determine the exact location of an earthquake, we need to know the distances from at least three different stations in order to plot an epicenters location
After getting data from three stations, draw a circle from the station, the point at which all three circles meet is the epicenter

The magnitude of an earthquake can also be measured using the seismogram.
The scale of earthquake magnitude was introduced by Charles F Richter in 1935
So each increase in a whole number in the Richter magnitude is 31 times more powerful than the last number
ie – magnitude 7 is 31 more powerful than a magnitude 6