In known history, Singapore has not experienced an earthquake.

Singapore is located in an area sandwiched by the Java trench in the west and south, and the Philippine plate and trench in the east.

Thus Singapore is located in a seismically stable zone, free from earthquakes.
Most of the shallow and bigger earthquakes are in western Sumatra and Java because of the deep subduction zones to the west-east and south-north of the Java trench. Earthquakes that occur near Singapore are usually deep earthquakes and weak.

However this does not exclude us from being affected by weak tremors once or twice a year due to a strong earthquake located in western Sumatra.
The tremors are weak and are felt by a few people living on high floors over certain specific areas of the Singapore.

The Meteorological Services Division has set up a compact network of seismic sensors around the island to monitor the effects of these earthquakes from western Sumatra. The data are made available to engineers to help them ascertain the tremor effects on structures. These data are also made available to researchers in the universities. The information is also made available to the general public whenever a tremor incident occurs in Singapore.

The earth is divided into many layers. The outermost layer is called the upper crust or lithosphere, and is about 100 km thick. The lithosphere is generally in a solid state and rigid. We build our buildings and live on the surface of the lithosphere. Below the lithosphere is a layer of softer mantle rock that is less brittle and deforms easily (compared to the lithosphere). This layer extends to a depth of about 700 km.

The lithosphere is the part of the earth where most earthquakes occur. The lithosphere is not a continuous shell but is divided into different sections called plates. These plates are not stationary, but 'move' or 'glide' on top of the softer rock below, with speeds of around 2-10 cm per year. When two plates meet or collide, we have a boundary. What happens at this boundary is important. Usually one plate will bend beneath another plate at the boundary. As we are dealing with a generally solid rigid plate, we can expect tremendous stress, strain and tension as the plates bend, especially in the areas around the boundary of the plates. These stresses, strains and tensions produce stress and strain cracks on the plates, which are called faults. e) When stress and strain on certain parts of the plates exceed the threshold that can be sustained by the elasticity of the rocks in that area, the plate ruptures, releasing strain energy which is called an earthquake. This energy is transmitted through the plates in the form of seismic waves, heat and sound. The amount of energy released will determine the magnitude or strength of the earthquake.

The public can obtain information on earthquakes/tremors from the Police Hotline and the Meteorological Services Division hotline at 6542-7788 during a tremor incident. Similar information can also be obtained from the NEA website: www.nea.gov.sg.

An epicentre is the point on the Earth's surface directly above the hypocentre of an earthquake. The hypocentre is the calculated location of the point within the Earth where the earthquake originates.

Simply stated, the magnitude of an earthquake can be determined by the distance of the observer from the earthquake and the strength of the seismic waves measured using specialized instruments called seismometers.

There are different types of magnitude scales used to measure the magnitude of an earthquake. Each scale measures different characteristics of the seismic waves. In most earthquake media reports, the value of the different magnitude scales is confused with the Richter magnitude scale.

Seismic waves can be classified into 3 types according to their wave period.

Body waves are the P and S waves and these have periods of around 0.01 seconds to 50 seconds. Surface waves such as the L and R waves have periods of around 10 to 350 seconds. Other free oscillations have longer periods from 350s to 3600 seconds.

The Richter scale is obtained from the measurement of the peak amplitude of ground motion from a Wood-Anderson seismometer and the difference of arrival times between the P and S waves. Note that Richter scale (Ml) has a period of measurement of 0.1s and 1.0s. What this means is that it can only be used to measure earthquakes less than 500km away. That is why the Richter scale is sometimes called the Local Scale.

Another scale, the Body-wave magnitude scale (Mb) has a period of measurement of 1.0s and 5.0s. This frequency range is the frequency range of the P and S waves (body waves) of the seismic waves. Another scale, the Surface-Wave magnitude scale (Ms) has a period of measurement of 20s. This frequency range is within the surface wave range of the seismic waves. Finally the Moment Magnitude Scale (Mw) has a period of measurement of greater than 200s. Of these scales, Mw is used in the scientific seismology community to determine the magnitude of an earthquake, although the Richter scale is most commonly used.