- Earthquakes and Volcanoes
Earthquakes and Volcanoes
The earth’s crust is in constant motion, leading to seismic and volcanic activity that is both threatening and awe-inspiring
The outer layer of the solid earth, the crust, is divided into tectonic plates which are in constant and gradual motion. In Singapore’s case, we are located on stable continental crust of the Eurasian Plate. The boundaries between tectonic plates are where the crust often experiences the greatest stress. When one plate is forced beneath the other, a process known as subduction, both deformation and melting of the crust can occur. This is the underlying reason for earthquakes and volcanoes at plate boundaries. The “Pacific Ring of Fire” is an informal name for the plate boundaries that encircle the Pacific Ocean. This seismically and volcanically active region stretches from New Zealand in the Western Pacific, through Indonesia, the Philippines, Japan, Russia, to Canada, USA, Mexico, Peru and Chile on the Eastern Pacific.
Key fault zones in our region are shown in the figure below.
Zone of convergence between the Australian Plate, Eurasian Plate and Philippine Plate. Significant faults are marked. Thrusts are where compression leads to shortening of the crust across the fault zone. Subduction of one plate beneath the other is occurring along the Sunda Trench, Manila Trench and Philippine Trench. After Metcalfe (2011, Geological Society of London Special Publications.)
The motion of the tectonic plates creates stresses both within them and at their boundaries. The stress induces weaknesses and eventually breaks to develop in the crust, known as faults. Faults can experience no movement and be undetectable for many years because friction ‘locks’ their surfaces. However when the fault zone undergoes sufficient stress to overcome this friction, and there is sudden movement, this is known as an earthquake.
Two examples of subduction involving ocean and land (left), and ocean with ocean (right). The lithosphere and asthenosphere describe regions of the crust with differing physical properties (Image credit: US Geological Survey). Subduction earthquakes can be highly devasting.
Earthquakes can vary greatly in size and impact, from barely detectable micro-tremors to massive destructive events such as the magnitude 9.2 Sumatra-Andaman earthquake on 26 December 2004. The largest earthquakes tend to occur on subduction zones. However earthquake-generating faults can also occur within a tectonic plate. In particular, the Sunda Trench stretching around Sumatra and Java, and the Manila Trench off the coast of the Philippines, are the sites of many large earthquakes in our region.
The point within the earth where the fault rupture initiates is called the hypocentre (sometimes also known as the focus). This is usually located below the ground surface, sometimes at a depth of tens or even hundreds of kilometres. The ‘depth of the earthquake’, generally calculated from the depth to the hypocentre, is an important parameter as shallow earthquakes tend to be more damaging than deep ones. The term epicentre is used to describe the point at the ground surface directly above the hypocentre. The magnitude of the earthquake describes its size. While most people have heard of the “Richter scale”, this scale has its limitations and is now considered outdated. Most scientists use the improved “moment magnitude” scale instead, which takes into account the area of the fault rupture. For every 1 unit increase in moment magnitude 1, the energy released increases by about 32 times.
Earthquakes cannot be predicted in the same way that weather is forecast, due to complexities in the earth’s crustal composition and how they respond to stress. Geoscientists are therefore unable to pinpoint the day and hour in which an earthquake will strike. Instead, they identify particular zones where the risk is high. Where high quality data are available, it is possible to estimate the likelihood of earthquake occurrence in a particular area over a period of years or decades. There is also no reliable precursor to earthquakes that can provide an early warning. It has been suggested that certain chemicals released during faulting, patterns of seismic activity, electromagnetic signals or even animal behaviour can signal the onset of an earthquake. However these have not proven to be robust. So far no reliable precursor to an earthquake has been found. Earthquake forecasting and prediction remains an active area of research.
A tsunami is a series of large waves formed in the sea or near the coast, usually generated by submarine earthquakes, landslides or collapse of a volcano during an eruption. The most powerful tsunami can travel over thousands of kilometres in hours and can reach enormous heights at the coast.
(Image credit: International Tsunami Information Centre)
Where earthquakes are the underlying cause, the most devastating tsunami occur when there is a large-magnitude ‘thrust’ event occurring at shallow depth (see figures above). This combination tends to generate very large displacements of water, as in the cases of the Indian Ocean tsunami / Sumatra-Andaman earthquake (26 December 2004) and the Tohoku earthquake and tsunami (which led to the Fukushima nuclear accident, 11 March 2011). A well-known tsunami induced by a volcano collapse occurred during the eruption of Krakatoa (Krakatau) in August 1883. Local towns and villages were destroyed by waves nearly 50 metres in height.
Singapore, protected by surrounding land masses and shallow seas, is relatively well-sheltered from tsunami. Nevertheless, MSS maintains an early warning system and issues advisories to the public if necessary.
The boundaries between tectonic plates are where the crust often experiences the greatest stress. When one plate is forced beneath the other, a process known as subduction, both deformation and melting of the crust can occur. This is the underlying reason for earthquakes and volcanoes at plate boundaries.
The behaviour of volcanoes is closely related to the chemical composition of their magma/lava. This is in turn a function of their location and geological history. This is why there are such pronounced differences between the gentle eruptions and dark lava of volcanoes on Hawaii versus the more explosive eruption seen at Mount Pinatubo in the Philippines, in June 1991.
Southeast Asia is one of the most active volcanic regions in the world. However, the effects are seldom felt by Singapore as we are hundreds of kilometres from the nearest volcano, and because the area of ash fall is controlled by the wind direction.
Singapore is not affected by the kinds of hazards experienced in the immediate vicinity of a volcano. However on the occasions when a volcanic eruption is sufficiently large and the winds are blowing towards Singapore, ash emissions can affect the air quality in Singapore. The severity and duration of such an event can vary. Fortunately, such occurrences are relatively rare: The last documented case was the eruption of Mount Pinatubo in Luzon Island, Philippines in June 1991. The air quality in Singapore went into the moderate or unhealthy range for 3 days (Based on the new PSI scale with 24-hour PM2.5 included, the readings were 91, 112 and 91 for 17, 18 and 19 June respectively.) following the volcanic eruption on 14 Jun 1991. Air travel in Singapore was not affected despite major disruptions close to Pinatubo itself.
All the different components of the earth (atmosphere, oceans, ice, land, the subsurface and living organisms) interact with one another. However earthquakes neither affect everyday weather nor are affected by it.
Volcanic eruptions, in contrast, can affect the weather and this is well documented. Volcanoes tend to release large amounts of sulphur dioxide. Volcanoes also release ash particles at a range of sizes, which can attract water droplets and could possibly have an effect on the electrical charge around them. In the vicinity of a volcano, it is common to observe rain, lightning, and thunder. Large eruptions can have a global scale, multi-year effect. The ash and gases from large eruptions can be transported globally by high-level winds. Sulphur dioxide and other aerosols are known to have a ‘dimming’ effect. This was why the June 1991 eruption of Mount Pinatubo in the Philippines led to slightly cooler than usual temperatures worldwide, in some regions by as much as 0.5 degrees C. A similar phenomenon occurred in April 1815 when Tambora erupted in Indonesia. Tambora is thought to have lowered global temperatures by as much as 3 degrees Celsius, and in parts of Europe and North America, 1816 was known as “the year without a summer.”