The principle used to establish the location of an earthquake is similar to deducing the distance to a lightning bolt from the time interval between the lightning flash and the sound of thunder. The faster travelling P wave reaches the seismograph in almost half the time taken by the slower S wave. The P wave can therefore be likened to the flash of lightning and the S wave to the roar of thunder. The time interval between the arrival of P and S therefore increases with distance the waves have travelled away from the earthquake source. If you count the time in seconds between when you feel the P wave and then the S wave, and multiply this time by 8 or 10, you will have a rough estimate in kilometres of how far away the earthquake occurred 2:45 PM
22.8.03
The Secretary Island area near Doubtful Sound, the site of today’s (22 August) Magnitude 7.1 earthquake, had a magnitude 6.7 earthquake in 1993, and magnitude 6.1 earthquakes in 1988 and 1989. 2:26 PM
15.8.03
Bulging volcanoes As a mass of hot molten rock (magma) moves toward the surface of a volcano, it may cause the ground surface to bulge or tilt. Scientist can detect these changes with satellite and electric distance-measuring equipment. Ground tilting can be measured by precise surveying. At White Island a precise levelling network was established in 1966 on the floor of Main Crater to measure changes in crater height 2:25 PM
8.8.03
How hot is lava? The temperature of molten rock will vary depending on the type of rock. The hottest lava cools to form the rock known as basalt - this is the type of lava that erupts from the volcanoes of Hawaii. When it emerges from below ground, basalt lava glows yellow-white and is usually at a temperature of about 1,200°C. Once lava has cooled to below 750°C it cannot flow readily.
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1.8.03
Aftershocks Following a major earthquake aftershocks may continue for days, weeks, or even months. During the first day following the 1987 Edgecumbe earthquake (magnitude 6.3) there were about 83 aftershocks. About 421 aftershocks greater than magnitude 3 were measured over a period of 142 days following the quake.
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