...when a build up of pressure in the crust is suddenly released and the ground shakes violently..
The point within the crust where the pressure is released in known as the focus, which is found at a range of depths:
- Shallow focus - 0-70km
- Intermediate focus - 70-300km
- Deep focus - 300km - 700km
The place on the earth's surface above the focus is called the epicentre.
Seismic shock waves have their highest level of energy at the focus - it decreases as the waves spread out. It recieves the highest amount of energy so is the most potentially hazardous location
Types of Seismic Waves
- P-waves (Primary) - fastest waves shaking the earth back and forth. They move through solids and liquids
- S-waves (Secondary) - slower waves moving sideways, shaking the earth perpendicularly to the direction of travel. They do more damage than P waves but do not travel through liquids
- Surface waves - travel nearer to the surface and are the slowest but most destructive type of wave. They include L-waves (Long) which move the ground sideways and Raleigh waves which move the ground up and down
P and S waves travel through the interior of the Earth so are recorded on a seismograph
The majority of earthquakes occur along plate boundaries - the most powerful are associated with destructive margins. At conservative margins, the boundary is marked by a fault, movement along which produces the earthquakes.
On the San Andreas Fault, California (which represents the boundary between the North American and Pacific Plate) the system consists of a broad complex zone in which there are a number of fractures of the crust
Some occur away from plate boudaries and are associated with the reactivation of old faultlines. It has been suggested, human activites could cause some minor earthquakes - large reserviors put pressure on the surface rocks or subsidence of deep mine working
Magnitude and Frequency
The magnitude of an earthquake is the amount of energy released by the event. it is measured on the Richter Scale, a logarithmic scale where each unit represents a 10fold increase in strength and 30 fold increase in energy released. The intensity of an earthquake is measured on the 12point mercalli scale, which reflects the effects of the event.
Frequency varies greatly between - seismically active regions and seismic zones within the shiled areas.Seisometers measure and record shock waves caused by earthquakes and are used in establishing patterns of seismic activity that help predict earthquakes.
Afterchocks are earthquakes that follows on from the main event and may last for months after. They are generated by earth settling back after disruption of their first displacement. Some are significant in their own right and hamoer rescue efforts - ie the 9.3 Indonesian earthquake of 2004 was followed by a 6.1 aftershock
The severity of the effects depends on factors such as:- magnitude, distance from epicentre and the underlying geology
- Tsunamis - usually triggered by earthquakes and submarine landslides
- Landslides and Avalanches - slopes fail due to ground shaking
- Liquefaction - violent disruption of the ground causes it to become liquid like, shaking causes increased pore water pressure which reduces the effective stress. This reduces the shear strength of the soil so it fails more easily. It causes the movement of groundater - the surface may appear dry but sometimes excess water comes to the surface through cracks, bringing liquefied soil and creating soil volcanoes. This damages strucures and threatens victims.
- Human Impacts:- Depends on population density and distance from epicentre. Primary - structures collapse; gas, water and electricity disrupted. Secondary - fires from gas leaks, disease, scarce water and loss of trade