Extreme Earth - Earthquakes

Nature can be incredibly powerful and destructive; the recent earthquakes in Haiti and Chile have reminded us of that, so here we take a look at what causes the earth to act in such a way.

A dilapidated marketplace in Port-au-Prince after the quake in January

Earthquakes range in size from a small rumbling below the earth’s surface to buildings collapsing and the earth’s crust tearing apart. This can often be followed by smaller quakes or aftershocks which compound the damage caused. But what causes earthquakes and can we do anything to stop them?

An earthquake is essentially a sudden release of energy in the Earth’s crust. They occur anywhere within the earth where elastic strain energy has been stored up. Frictional resistance caused by irregularities in fault structures cause tectonic plates to become locked in position, but these plates continue to move relative to each other. Over time, stress is placed on the plates, and elastic strain energy builds up along the fault surface. Eventually, this energy needs to be released and the plates suddenly move relative to one another.

January’s quake in Haiti was caused by a slip on the east-west fault system – the

Damage to Port-au-Prince

Enriquillo-Plaintian Garden fault – where the North American and Caribbean plates slide past each other. This is a strike-slip fault where the two sides of the fault glide past each other horizontally.

“The Caribbean plate moves eastwards with respect to the North American plate. In this region, motion is on two major E-W trending fault systems, one in the north of Haiti and one in the south," said Dr Neil Thomas, principal lecturer from the school Geography, Geology and the Environment at Kingston University.

“A build up of stress was released suddenly as the fault system in the south unlocked on January 12th, resulting in the main earthquake and the subsequent aftershock sequence. On these types of fault systems, earthquakes occur at shallow depths and the Haiti main earthquake occurred at a depth of only 13km below the surface,” he added.

The magnitude 7.0 quake was the worst Haiti had experienced in two centuries. It was followed by several unusually strong aftershocks – quakes of smaller magnitude which occur as the crust around the displaced fault plane adjusts to the effects of the main shock – which caused more damage to the capital and many nearby towns.

“Aftershocks occur partly because the fault system does not release all of the

Damage to a road caused by the quake in Chile Credit Cristian Felipe Muñoz Cabezas

stress in one go.  Over a period of time, which varies depending on the fault system concerned, energy is released in the lead up to a major event (so-called foreshocks), during the main event and in an aftershock sequence,” Dr Thomas said, “Though not unequivocally confirmed, the last major event on the EP Garden fault system, before the January 12th event, was in 1860 and before that in 1770 and 1751.  So, stress has been building for a long time and that, coupled with readjustment, may be why the aftershock sequence contained some very large earthquakes.”

In February, Jean-Max Bellerive, the Haitian Prime Minister stated that 200,000 people had been killed and a further 300,000 people had been injured. Figures estimate that two million people were left homeless as a result, and the capital – and much of the surrounding area – was left in ruins. Most of the damage was located at the epicentre – 15km southwest of Port-au-Prince, 13km below the Earth’s surface. Bellerive also estimated that 250,000 residencies and 30,000 commercial buildings had collapsed or were severely damaged – the Presidential Palace, National Assembly Building and the main jail included.

Quakes can also be caused by normal and reverse faulting – dip-slip faults, where displacement occurs along the fault in the direction on the dip and movement is on a vertical plane. Normal faults – where one plate slips downwards relative to the other – usually occur along divergent boundaries and are accompanied by an extension of the Earth’s crust.

In reverse or thrust faults, one plate is pushed upwards – usually at a convergent

fault types

boundary – and is coupled with a shortening of the Earth’s crust. Around 90% of the world’s earthquakes – and 81% of the largest – occur in the Pacific Ring of Fire on the margins of the Pacific Ocean. Such earthquakes are often accompanied by volcanic activity, which is rife in these areas.

“The situation here is even more complex because when plates move towards each other they can either collide but one does not 'dive' under the other (the Himalayas is an example area for this), or they can collide and the denser plate is pulled down underneath the lighter one (a process we call subduction)” Dr Thomas said.

The Chilean quake is an example of a reverse fault and measured 8.8 on the moment magnitude scale – just under 100 times more forceful than the quake in Haiti. It occurred along the boundary where the Nazca and South American tectonic plates converge. The quake’s epicentre was offshore, about 8km west of Curanipe, and the quake itself caused at least 720 deaths according to President Michelle Bachelet. NASA also report that the quake has shifted the Earth’s axis changing its rotation, causing shorter days of about 1.26 microseconds (a microsecond is one millionth of a second).

While most earthquakes are caused by movements of the Earth’s tectonic plates, they can also be induced by human activity. Drilling and injecting liquids into wells, coal mining and oil drilling can all disrupt the Earth’s crust. Constructing large dams and buildings can also be contributing factors. An earthquake in China’s Sichuan Province in May 2008 is believed to have been caused by the Zipingpu Dam, which caused the pressure of the fault to fluctuate over 500 miles away and accelerated the movement of the fault. The 8.0 magnitude quake lasted two minutes and killed at least 68,000 people.

Earthquakes are evident from the shaking and movement of the ground caused by the rupturing of geological faults. They create seismic waves which travel through the Earth’s interior and can be recorded by seismometers all over the earth. Primary or P-waves arrive first, followed by secondary or S-waves.

“For P waves, the disturbance is in a series of compressions and dilations (squeeze and stretch) parallel to the direction of wave motion through rocks. For S waves, disturbance is perpendicular to the direction overall wave motion, in a shearing action. This shear motion takes place in two orthogonal directions (essentially horizontal and vertical),” Dr Thomas said.

Surface waves tend to cause more damage than P and S waves – they are guided by the surface of the earth – and can be larger in amplitude and can be the largest signals on a seismogram.

“These waves move at, or just below, the surface of the earth and in major earthquakes they are the reason why observers sometimes see the ground visibly roll. There are two types of surface wave – Rayleigh (R) waves and Love (L) waves. The R waves progress in a rolling motion just below the surface and are a major cause of vertical ground shaking. L waves progress in a 'snake-like' motion across the surface and are a cause of horizontal shaking. Ground motion in earthquakes results from a complex combination of these different mechanisms.” Dr Thomas said.

While it’s impossible to prevent an earthquake, geologists are devising ways to predict when a tremor may take place. They examine the faults in an area – measuring the amount of pressure on the fault line each year since the last major tremor, and the energy and power of the last quake – to determine how much pressure it takes for a fault to generate an earthquake.

“Unlike other tectonic hazards such as volcanoes, earthquakes are very difficult to predict. Indeed, earthquake science now talks of statistical (probabilistic) forecasting, rather than prediction,” said Dr Thomas, “There has been a significant amount of progress in the field of earthquake forecasting in the past five years and we hope that this will gather more pace in the decade to come.  However, no seismologist will ever be able to say with certainty that an earthquake will occur on a given date at a given time, with a given magnitude, at a given depth in a given epicentral location.”

Dr Thomas says it is impossible to accurately predict the effects that any earthquake will have because this depends not only upon the characteristics of the earthquake but also many other factors so the priority for the future has to be ensuring that quake-prone countries have well-planned, co-ordinated disaster risk reduction measures. This should include engineering solutions to make structures more earthquake-resistant, scientific monitoring of seismic activity, education for people in quake-prone areas and well-organised and rehearsed strategic responses. The movement of tectonic plates causes destruction and devastation on major scales, whether it be through earthquakes or secondary hazards associated with them. They cannot be prevented and it is impossible to predict when an earthquake is going to occur, how big it’s going to be and what damage it will cause. But what is clear is that quake-prone areas need to be better prepared in order to avoid the major death toll and chaos caused by the recent quakes in Chile and Haiti.