Earthquakes

Although associated with plate boundaries, earthquakes can occur anywhere.  On 27th February 2008 there was an earthquake of 5.2 on the Richter scale centred on the Lincolnshire town of Market Rasen.  It damaged buildings and caused a great deal of alarm, but nobody was killed

At 14.28 on Monday 12th May 2008 there was a catastrophic earthquake in China of magnitude 7.9 (on the Richter Scale) that cost tens of thousands of lives. 

Earthquakes happen when there is movement along fault lines.  The movement is not smooth; the rocks have a lot of friction against each other, and elastic forces build up as the fault line moves.  Then the rocks break releasing the elastic energy in the form of shock waves (and not a little heat).

Where the earthquake occurs is called the focus.  The point on the ground immediately above the focus is called the epicentre.  The seismic (earthquake) waves propagate spherically from the focus.  There are four main types of earthquake wave:

The speeds are those in surface rocks.  In more dense rocks, the waves travel faster.  The speed a wave travels at is governed by:

• The Young's Modulus of the rock;
• The density of the rock.

The equation is:

The Physics codes are:

• E - Young's Modulus (= stress ÷ strain) - Units Pa;
• r - density (= mass ÷ volume) - Units kg m^-3.

At boundaries, refraction occurs, and Snell's Law applies:

While earthquakes are destructive, they are remarkably helpful to scientists in helping to determine the structure of the Earth. 

Nobody has ever been to the centre of the Earth (and, if they did, they would be fried).  The deepest hole ever bored is about 30 km, compared with the radius of the Earth of 6000 km.  So all what we know about the structure of the Earth comes entirely from what we have gleaned from seismic data, collected from a large network of seismometers.

Seismometers (or seismographs) produce a trace called a seismogram.

Note how the P-wave arrives first, then the S-Wave.  Remember that these are body waves; the other two only travel along the surface.

The intensity of the earthquake is usually measured on the Richter Scale, which represents the energy released.  It is a logarithmic scale, so a magnitude 4 earthquake has 100 times the energy of a magnitude 2.

The Mercalli Scale represents the intensity, or the observed effects.  The two scales are shown in the table below: