Earthquake triggers panic over repeat Tsunami
A huge earthquake just miles away from the centre of the tremor that caused the Boxing Day disaster caused
widespread panic across South Asia today over fears that it would unleash a deadly tsunami.
The US Geological Survey (USGS) said that the earthquake, measuring 8.7 on the Richter scale, struck at 1609
GMT off the coast of Sumatra.
The earthquake appeared to have been caused by the same seismic fault as that in December, and to have taken
place just a few hundred miles south.
The epicentre was located about 205 kilometres (125 miles) west of Sibolga on Sumatra and 245 km (150 miles)
southwest of the Sumatran city of Medan in waters about 30km (18.6 miles) deep.
The December 26 (Boxing day) earthquake measured 9.0 and triggered a tsunami that killed more than 273,000
people around the Indian Ocean.
It was caused when the Indian-Australian tectonic plate slid under the Philippine plate in a "subduction zone"
750 miles long and more than 300 miles wide. The movement was 16½ yards at a depth six miles below the
seabed.
Researchers at the University of Ulster have recently given warning of earthquake and tsunami-related activity in
the region.
Their results, published in the March 17 edition of Nature, indicated an increase in the stress on both of the fault
zones due to the movement of the initial quake in December
Times Online - By Mark Sellman - March 28, 2005
Seismic waves
A typical seismometer recording of an earthquake is shown in Figure 1.
 Primary (P) waves cause the first tremors. These are longitudinal waves which make the material which
they pass through vibrate to and fro as they push and pull on it. They are faster than S or L waves.
 Secondary (S) waves arrive a few minutes later, causing more tremors. They travel slower than P waves.
These are transverse waves that shake the material they travel through from side to side.
 Long (L) waves arrive last to cause the main shock. These travel along the crust only, making the surface
move violently up and down as well as to and fro. They are slower than P or S waves.
Handy Hint : P waves push and pull. S waves shake side to side. L waves go the long way round.
The Earth’s structure has been deduced from seismic waves. The pattern of waves received by a seismometer
depends on what the waves have passed through inside the Earth. Waves are either reflected or refracted when
they travel from one type of material into another.
 When an earthquake occurs, its epicentre can be located by comparing seismometer readings from different
parts of the world.
 The structure of the Earth has been deduced as a result of analysing seismometer readings.
Figure 1 : A seismogram.
Keyscience – Physics – 2001 by Jim Breithaupt
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Questions :
1) What is the difference between longitudinal waves and traverse waves ? You can
draw raw schemes to explain.
2) The seismic waves can be refracted when they travel from one type of material
into another. What does refraction mean ?
3) The epicentre was 650 km far from the seismometer which recorded figure 1.
Calculate the speed of the S-wave.
4) Explain how the structure of the Earth can have been deduced as a result of
analysing seismometer readings.
5) Explain the link between tsunami and earthquake. Which one do you think is the
faster wave ?
6) What were the consequences of the last year December tsunami in Asia ?
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