An earthquake is the motion produced when stress within the earth builds up over a long period of time until it exceeds the strength of the rock, which then fails by breaking along a fault
Earthquake motion can be considered in two parts:
- The movement during fault rupture produces a range of vibrations, or seismic waves, that are radiated outwards.
- The vibrations of engineering significance occur at frequencies from less than 0.2 Hz to 20 Hz (periods from about 5 seconds down to about 0.05 seconds). This is just below the range of ordinary sound vibrations.
- This motion can be measured as displacement, velocity or acceleration.
- The waves travel at varying speeds depending on the type of rock, but usually in the range 3 to 8 km per second for rocks within 30 km of the earth’s surface. This compares with about 0.3 km per second for the speed of sound waves through air.
- The motion is complex (non-stationary) – there are sharp arrival pulses followed by slow decay. The total duration depends on the size or magnitude of the earthquake.
- The vibrations last much longer than the fault rupture duration because different wave types travel at different speeds, and also the waves are reflected back and forward by interfaces within the earth.
- An earthquake produces a permanent displacement across the fault. Fault displacements vary from a few millimetres in very small earthquakes to a few metres in very large earthquakes.
- The rupture area on the fault plane varies significantly with the size of the earthquake. The length of a rupture may vary from a few metres for small earthquakes, to a few kilometres for moderate earthquakes, to over 100 kilometres for very large earthquakes.
- Once a fault has been produced, it is a weakness within the rock, and is the likely location for future earthquakes.
- After many earthquakes, the total displacement on a large fault may build up to many kilometres, and the length of the fault may propagate for hundreds of kilometres.
- A small earthquake will rupture a small fraction of an existing fault area with a small displacement, while a large earthquake may rupture most or all of the existing fault area. Thus the maximum earthquake size depends on the size of the existing fault area.
- Most faults are not simple flat dipping planes. They are affected by varying geological structures, and may vary in direction and dip angle, or have complex fractures, with parallel and en echelon segments. They may occur over zones hundreds of metres or kilometres wide.
Stress and Strain
Stress Within the Earth
- It is possible, but not strictly accurate, to think of stress within the earth as being compression, tension or shear, giving reverse faults, normal faults and strike-slip faults respectively.
- It is better to think of tectonic stress in terms of three orthogonal principal stress directions, ranked as maximum, middle and minimum.
- It is the difference between maximum and minimum principal stress that causes an earthquake. The earthquake is a shear movement along a fault plane. The fault plane is usually at right angles to the plane containing the maximum and minimum principal stresses, at an angle less than 45° (often 30° to 35°) to the maximum principal stress, depending on the friction properties of the fault material.
- The principal stresses may be oriented in any direction, but geological processes and the free surface of the earth often constrain one or other to be near vertical.
- If the minimum principal stress is vertical, then horizontal compression gives reverse faulting.
- If the middle principal stress is vertical then, with both maximum and minimum stresses horizontal, strike-slip faulting results.
- If the maximum principal stress is vertical, the resulting normal fault motion is equivalent to that from horizontal tension.
Strain Within the Earth
- As stress builds up within the earth, the rocks will gradually be deformed, or strained.
- This strain stores considerable elastic strain energy with the rock.
- When the earthquake occurs, part of the elastic energy is released as seismic waves which radiate from the source, and part as heat.
The Time Interval Between Earthquakes
In an active area like Japan or Papua New Guinea, it may take tens or hundreds of years for the elastic strain energy to accumulate in the rocks.
In areas of low activity, like Australia, it may take hundreds of years to build up energy for even a moderate earthquake, and tens or hundreds of thousands of years to build up for a large earthquake.
During an earthquake, this energy is released in seconds.
The Earthquake Cycle
Quiescence, building up of energy.
Precursory activity may occur in the high stress situation for months or years.
Foreshocks minutes to days before main shock.
Main shock, largest event in the cycle.
Aftershocks occur in the following days to weeks.
Adjustment activity may last years to centuries, and cover the area surrounding the main shock.
Quiescence again, lasting much longer than the other phases (hundreds of years in active area to millions of years).
The cycle is not periodic (earthquakes do not occur after equal time periods) because stress build-up at any given site is affected by the earthquake activity in the surrounding area.