By Afroz Ahmad Shah
During any earthquake a large amount of strain energy is released, which travels as waves in all directions through the layers of the Earth, reflecting and refracting at each interface. The waves are called seismic waves or earthquake waves. These are similar to sounds waves, which are created through a disturbance in materials (media), for example when we talk to anyone, our voice disturbs the air and the energy is carried away from us towards the listener. This carries energy away from its point of origin. Similarly, when we throw a pebble in a pond, it generates waves, which are carried further away through a process which transfers the energy but NOT the matter. If you drop a big boulder the waves move further away. However; unlike waves in water which are confined to a region very near to the water’s surface, earthquake waves can also propagate through the interior of the Earth, because some of these waves can pass through solid, liquid or a gaseous medium. These waves are used by scientists to know the internal configuration of the earth and its layered structure, which otherwise was quite hard to infer. The velocity of an earthquake wave depends on a number of factors, for example, the nature and composition of a rock, temperature etc. Generally, in a harder and compact rock, these waves move faster while they get slowed down in unconsolidated sediments. This can be dangerous, because once an earthquake wave enters unconsolidated medium, it looses velocity and its wavelength (the spacing between the adjacent crests) decreases but not its energy. This results in severe shaking; for example in 1989 the Loma Prieta quake caused a greater damage to a region, which was previously a part of the bay area and has been filled in. However; nothing significant occurred in most of the Oakland.
During an earthquake, the energy is carried away from the point of origin of an earthquake and is dissipated to far-off places depending upon the energy released during a quake, for instance during a recent magnitude 9 earthquake in Japan, which was caused because of the tectonic movement of plates. As we know, there are four tectonic plates in and near Japan, the Eurasian plate, the North American plate, the Pacific plate and the Phillippines sea micro-plate. The continuous push on the Pacific plate drags it down under the North American plate at about 8cm/year, along the megathrust fault boundary. The resistance to its downward pull via friction builds up the strain energy along this boundary, which ultimately fails via fracturing, because the rocks no longer withstand the continuous push.
The big Japanese quake was initiated on one of the portions of this fault, which slipped along an area of the fault roughly 500km long and up to 200km wide. The rupture happened about 24 km (15 miles) below the surface and the elastic energy released was enough to set the ground rippling at frequencies and amplitudes people can perceive at a distance of even greater than 2,500 kilometers. This slippage of the rocks vertically uplifted a huge water column to form tsunami waves, which traveled to far off places, even as far as Chile. It is reported that this magnitude 9.0 earthquake was more than 500 times stronger than the recent January 12 Haiti earthquake, which suggests the powerful impact of the earthquake and the waves of tsunami that it generated.
There are two basic types of seismic waves, those which can travel within the earth’s interior (called body waves), these are Compressional (P) and Shear (S) waves and those which can travel on or near the earth’s surface (called surface waves), these are Rayleigh and Love waves (Figure).
Compressional, primary or P waves:
This kind of wave is called a primary (P) wave, because they are the first to arrive during an earthquake. It is like an ordinary sound wave (longitudinal wave) and has alternate bands of compression (pushes) and dilation (pulls), which cause back and forth shaking in the same direction as the direction of propagation (Figure). So, if you observe anything on ground is shaking in say an east-west direction, then, it means that the P wave is either coming from east or west. These waves typically travel at a velocity of ~ 5 – 7 km/s in Earth’s crust; >~ 8 km/s in Earth’s mantle and core; 1.5 km/s in water; 0.3 km/s in air.
Secondary, S, Shear, Transverse
S wave is also known as secondary wave, because it arrives in seconds. These are transverse waves, which mean that the shaking is perpendicular to the direction of propagation (Figure). If during an earthquake, S waves are coming from east or west, then the shaking will be either north-south/up down or to some angle in between. These waves travel at a velocity of ~ 3 – 4 km/s in typical Earth’s crust; >~ 4.5 km/s in Earth’s mantle; ~ 2.5-3.0 km/s in (solid) inner core. However; they can only travel through a medium which does not allow east shear motion and therefore cannot pass through fluids.
L, Love, Surface or long waves
The second type of surface wave was discovered in 1911 by an Englishman, Augustus Edward Hough Love. These waves travel only on the surface of the Earth and are a combination of compression and shear (Figure). Their velocity is about ~ 2.0 – 4.5 km/s in the Earth, which depends on frequency of the propagating wave. They are called long waves, because of their longest wavelengths and are the most damaging waves during an earthquake. This is primarily because these waves travel on the surface and are not spreading out into three dimensional planes (within the earth), and often retain their biggest amplitude.
R, Rayleigh, Surface waves, Long waves, Ground roll
These waves were first mathematically described by English physicist John William Strutt, 3rd Baron Rayleigh, (1842 –1919). He along with William Ramsay, discovered the element Argon for which he earned the Nobel Prize for Physics in 1904. In the Rayleigh waves, particles make an elliptical movement against the propagation direction, which is in contact with water waves, where the particles make circular paths against the direction of propagation. Hence, their motion is retrograde (Figure). Their velocity is about ~ 2.0 to 4.5 km/s in the Earth depending on frequency of the propagating wave.
Afroz Ahmad Shah is a research fellow at Earth Observatory of Singapore, Nanyang Technological University, Singapore.