What is an earthquake?
Earthquakes are the result of slow-building energy within the Earth’s crust, as tectonic plates move against each other, which is relieved by a sudden movement of those plates. This outburst of energy produces seismic waves that ripple across land and water from the initial point of rupture.
The Earth’s crust is made up of a jigsaw puzzle of roughly 15-20 rigid, floating continental and oceanic plates that undergo continual change, generating energy as they shift, slide and collide with each other. The margins and nature of interaction between these plates determine the major geological features and activities of the Earth’s surface and create systems of earthquake faults.
A relatively recent theory, plate tectonics only achieved widespread acceptance in the 1960s, after being pioneered by German geophysicist, Alfred Wegener in 1912. Alfred developed the theory of ‘continental drift’, which contended that the position of the Earth’s continents were not fixed, but were instead, mobile and drifting about the Earth’s surface.
It is also the basis of the theory for how the world transitioned from the supercontinent Pangaea, through to Gondwanaland and then individual continents that we see today.
What are Faults?
Fault lines mark the boundaries between two tectonic plates. The movement of these plates creates visible fractures and discontinuity, better known as faults. However, faults rarely consist of single, clean fractures, and thus are referred to as ‘fault zones’ to denote their complex deformation.
From a landscape point of view, they often manifest as mountain or volcanic chains or areas of raised formations. Some of the more well-known faults include the Himalayan fault that gave rise to the world’s highest mountain range; the San Andreas fault line in the US, the Southern Alps in New Zealand, and the Ring of Fire in the Pacific.
While individual earthquakes may uplift a few metres of land, “over a long time, with many earthquakes, displacements can be large, for example, over 400km along the alpine fault in New Zealand,” says Professor Brian Kennett from the Research School of Earth Sciences at the Australian National University.
Faults can be categorised into three distinct types
Strike-Slip fault: A strike-slip fault occurs where portions of the Earth’s crust move sideways, and the result is a horizontal motion along the fault. California’s San Andreas Fault, which has caused some of the most devastating earthquakes in the region, is a strike-slip fault.
Normal fault: A normal fault occurs where the deeper part of the crust is drifting away from an overlying part. Normal faults are generally associated with the divergence of plates. The East African Rift Zone and the Basin and Range Province in North America are two regions where normal faults are causing a divergence of the Earth’s crust.
Reverse or thrust fault: A reverse fault is when the rock above the fault plane is displaced upwards (opposed to gravity). A reverse fault with a small dip angle (the angle between the horizontal surface and the fault) is called a thrust fault. These faults can produce larger earthquakes than strike-slip faults. Both the 2004 Sumatra earthquake and tsunami, and the 2015 Nepal quake were a consequence of their proximity to a thrust fault.
Measuring earthquake magnitude
Earthquakes are measured by the seismic waves that emanate from the epicentre (originally, the Richter scale, but in more recent times the Moment Magnitude scale).
The Richter scale determines the earthquake’s magnitude by measuring the amplitude of seismic waves generated by the quake. The logarithmic scale ranges from 1-10, with each increase in number denoting a tenfold increase in the amount of energy released. No earthquake has been recorded at 10. The biggest earthquake on record is the 1960 Chile quake, which measured 9.5.
The length of the fault affected, the amount of earth displaced and the depth of the earthquake all contribute its severity. Typically, about 100 earthquakes greater than M1.5 occur every day, but most of these are less than M4. The bigger the earthquake, the less frequently they occur.
Measuring earthquake intensity
The Mercalli scale describes the intensity and features of an earthquake, based on its observed effects. The scale gives an indication of the effects of an earthquake and ranges from I (not felt), up to XII, which is the total destruction of an area.
Shallow earthquakes (<15km deep), tend to cause the most damage because there is not as much earth for the energy to travel through, and so not much energy is lost.
|I||Not felt||Not felt, except by few perople.|
|II||Weak||Felt only by a few people, especially on upper floors of buildings.|
|III||Weak||Noticable by people indoors, especially on upper floors of buildings. Standing cars may rock slightly. Vibrations similar to the passing of a truck.|
|IV||Light||Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors rattle; walls make cracking sound. Sensation like heavy truck striking building. Standing cars rocked noticeably.|
|V||Moderate||Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.|
|VI||Strong||Felt by all, many frightened. Some heavy furniture moved. Damage slight.|
|VII||Very strong||Damage negligible in buildings of good design and construction; slight-to-moderate damage in well-built ordinary structures; considerable damage in poorly built or badly designed structures.|
|VIII||Severe||Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.|
|IX||Violent||Damage considerable in specially designed structures. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Landslides occur.|
|X||Extreme||Some well-built wooden structures destroyed; most masonry and frame structures, and foundations destroyed. Rails bent.|
|XI||Extreme||Most well-built wooden structures destroyed; most masonry, frame and foundation of structures destroyed. Rails bent. Specially designed earthquake-resistent buildings may have minor damage. Landslides.|
|XII||Extreme||Almost total devasation of all poor-to-moderately built structures. Specially designed structures may have moderate damage. Landslides.|
Can earthquakes be predicted?
Scientists are able to predict the general area – through location of faults – in which most major earthquakes are most likely to occur. However, there is no method to accurately predict the exact time, place and magnitude of an earthquake. “The circumstances that lead to failure depend on a cascade of interactions, that are chaotic and intrinsically unpredictable in detail,” says Brian.
Concerted efforts are being geared towards minimising risk associated with earthquakes, particularly in vulnerable regions, such as Kathmandu Valley. This is done by assessing both the seismic hazard and vulnerability of a region.
Many seismically hazardous countries, such as Japan, have installed significant resources into identifying possible precursors to major earthquakes, as well taking vast precautionary measures to forewarn their citizens, relaying safety measures in times of crisis.
Brian says that with sufficient information on prior activity it is possible to make broad forecasts of likely locations of earthquakes, and to assign plausible probabilities of occurrence. “However recent great events like the 2004 Sumatra-Andaman and the 2011 Tohoku-Oki, Japan make it clear that that our available records are not long enough for this to work well at the global scale,” he says.
“When we compare weather to earth processes, with timescales comparable to the growth of finger nails at the fastest, the equivalent of tomorrow in weather is a hundred years or more for earthquakes,” he says. “Very large events may take a thousand years or more to build up to failure, so indications will be very subtle and hard to pick up. The final trigger may be another smaller earthquake in the same area.”
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