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Seismic waves can be measured by seisometers. For example the 'San Andreas Fault Observatory at Depth' where seisomoters have been placed in an L-shaped bore hole accross the faultline at Parkfield.
Earthquakes can be measured by their magitude/intensity on the Richter Scale. The Richer Scale measues the energy released by the earthquake and is a logarithmic scale (an increase in magnitude of 1 = tenfold increase in energy released) .
Earthqukes can aso be measured by the Mercalli Scale which records the level of damage caused by an earthquake based on observations. For example, level 1 on th Mercalli Scale accounts fo earthuakes which are detected by seisometers but barely felt, whereas level 12 records earthqukes which cause extensive damage.see more
Radioactive deacay in the Earth's core creates extremely high temperatures which heat the lower mantle. When radioactive decay is concentrated, local thermal currents are generated in the lower mantle. Plumes of magma rise from the lower mantle and burn through the Earth's lithosphere to create volanic activity on the Earth's surface. An example of a hotspot Hawaii.The hotspot remains stationary whilst the overlying plate moves. Using the example of Hawaii, the Pacific Plate is moving North-West at a rate of 5-10cm per year. This leaves a chain of extinct volcanoes which are no longer fed by the source of magma. The extinct volcanoes put pressure on the plate and therefore subside and become seamounts. Bends in the island chain show changes in the direction of plate movement. Hotspots are proof of Wagners theory of plate tectonics and plate movement.see more
Seismic waves are shock waves released by the rupture of rock duing an Earhquake. Seismic waves radiate from the focus of the earthquake and travel through rocks.
Primary waves (P-waves) are compressional waves and therefore vibrate in the direction of travel. P-waves are the fastest seismic waves and reach the Earth's surface first. P-wavs are high frequency and can tavel through the Earths mantle and core.
Secondary waves (S-wavs) vibrate at right angles to the direction of travel and are the second fastest waves, traveing at half the speed of P-waves.
Surface Lowe waves travel near to the Earth's surface and are the slowest waves but cause the most damage.
Raleigh waves occur in complicated, low frequency, rolling motions, spreading out from the earthquake epicentre.
Seimic waves are measued on seismographs an have helped to discover the internal structure of the Earth. This is due to the fact that S-waves can travel through solid mantle but not through the fluid crust and therefore refract upon reaching the core. However, P-waves can tavel through both solid and fluid and can therefore be measured on the opposite side of the epicentre.see more