ShockwavesAt any plate boundary stress and strain can build up along the joint, due to friction created by masses of rock trying to move past each other. This is greatest along the Wadati-Benioff zone within destructive plate boundaries, with the 9.5 Mw earthquake in Chile (1960) being the largest. The Benioff Zone is an area of seismicity corresponding with the slab being thrust downwards in a subduction zone. The different speeds and movement of the rock at this point produce numerous earthquakes. Stress is also great along transform (conservative) boundaries, with the 7.6 Mw earthquake of 1992 on the San Andreas Fault being the largest. There is less friction at constructive plate boundaries and so magnitudes are lower, for example, the largest recent earthquake in Iceland was 6.5 Mw in 2000, but they are usually smaller.

The focus of an earthquake is the point at which the strain is released. This sends earthquake (seismic) waves in all directions, being strongest at the point on the Earth’s surface directly above the focus, known as the epicentre. There are several types of earthquake (seismic) wave, and all move the ground in a different way. The P waves (primary) arrive first and have a short wavelength and travel quickly through the crust. S waves (secondary) arrive a few seconds later and have a longer wavelength and a velocity of 4 km/s, causing more destruction. In the November 2011 Tohuku earthquake in Japan the S waves reached Tokyo 90 seconds after the release of stress at the focal point.

There are additional earthquake (seismic) waves, such as the Rayleigh wave and the Love wave. When all shockwaves are combined over a short period of time, maybe 30 seconds, it is no surprise that buildings without an aseismic design get severely damaged and collapse. This is because the ground moves in many different ways.

Secondary impacts of Earthquakes

Secondary hazards resulting from an earthquake may include a tsunami, which can have far reaching impacts spatially (for example Indian Ocean 2004), landslides (for example Kashmir 2005) and liquefaction (for example Christchurch 2011), which are localised impacts. Landslides occur when earthquake (seismic) waves loosen rock or unconsolidated material on steep slopes; material loses cohesive strength and moves downwards under the influence of gravity. Landslides are more common on hilly or mountainous areas, and may hit small settlements within its topography, and block or destroy roads and railways, making it difficult to rescue people or supply emergency aid and rebuilding materials. Liquefaction occurs in lowland areas where shaking sorts the ground material to the point where it acts as a fluid.

TsunamiTsunamis are a major secondary hazard because of their potential destructive ability and spatial area that they can cover. A tsunami usually

consists of a sequence of waves with deep troughs in between, because the water is drawn upwards into each wave. In deep water tsunamis may hardly be noticeable due to long wavelengths, but in shallow water the friction at the base of the wave slows them down, shortening the wavelength and allowing them to build in height (maybe 20 times higher than in open water). Tsunamis are created by water column displacement, mostly by undersea plate movements where part of the seabed is thrust upwards (or downwards) very quickly, or by explosive volcanic eruptions on a volcanic island (which may involve landslides or cone collapse into the sea), or underwater landslides such as at continental shelf edges. The Tohoku tsunami took 22 hours to cover the Pacific Ocean in a complex pattern due to reflection and refraction by islands and landmasses, and the bathymetry of the ocean.

LiquefactionLiquefaction is the process by which water-saturated material can temporarily lose normal strength and behave like a liquid under the pressure of strong shaking. Liquefaction occurs in saturated soils (ones in which the pore space between individual particle is completely filled with water). An earthquake can

cause the water pressure to increase to the point where the spoil particles can move easily, especially in poorly compacted sand and silt. On Tuesday 22 February 2011 at 12.51 p.m. Christchurch was badly damaged by a magnitude 6.3 earthquake, which killed 185 people and injured several thousand. The earthquake epicentre was near Lyttelton, just 10 kilometres south-east of Christchurch’s central business district. The earthquake occurred more than five months after the 4 September 2010 earthquake, but is considered to be an aftershock of the earlier quake.Liquefaction was much more extensive than in the September 2010 earthquake. Eastern sections of the city were built on a former swamp. Shaking turned water-saturated layers of sand and silt beneath the surface into sludge that squirted upwards through cracks. Properties and streets were buried in thick layers of silt, and water and sewage from broken pipes flooded streets. House foundations cracked and buckled, wrecking many homes. Despite the damage to homes, there were few serious injuries in residential houses in liquefaction areas. However, several thousand homes will have to be demolished, and some sections of suburbs will probably never be re-occupied.