Earthquake Anatomy

EARTHQUAKES THEORY OF PLATE TECTONICS THE RICHTER SCALE Seismograph reading The surface of the Earth is divided Relies on a worldwide network of seismographs which record earth Distance to the focus is calculated by measuring time between the Primary and Secondary waves of the quake Japan into seven major plates and several minor ones. They move a few centimetres Eurasian Plate The biggest earthquake to hit Japan in 140 years struck the northeast coast on Friday, triggering a 10-metre tsunami that swept away everything in its path, including houses, cars and farm buildings on fire North American 23mm Plate movements amplitude a year riding on semi-molten layers of rock underneath African Plate the crust. As the plates move they pull apart or collide unleashing the powerful movements known as earthquakes The relative sizes of quakes are recorded by measuring the distance to the focus of the earthquake against the maximum amplitude of the seismograph reading Pacific Plate 24 seconds South American Plate 300 + 50 Earthquake measures 5.0 100 Australian Plate 40 50 200 30 20 TYPES OF PLATE Subduction zone 10 100 - 20 Spreading zone Rift valley Continental plates pull apart allowing crust to sink BOUNDARY Line Antarctic Plate 50 + 10 Thrust fault connecting 24 seconds Strike-slip fault 25 ---- Uncertain / diffuse boundary and 23 mm 0.5 10 Mountain building Continental plates collide forcing rock layers to fold and pile up into mountain ranges Sea floor spreading Basaltic magma rises to form new ocean floor along a fault. An undersea ridge is formed which gradually spreads as new material pushes it along its way accompanied by almost constant earthquake activity 01 Each increment on the magnitude scale is equal to a tenfold increase in the seismic wave amplitude of the quake EARTHQUAKE WAVE TYPES Volcanic Islands Spreading ocean floor is much thinner than the continental plates but can break the sea surface forming volcanic islands Primary or P wave Fast waves which travel through average crustal rocks at about 5 km per second Secondary or S wave Travel at about 3 km per second. They cannot pass through liquid or penetrate the Earth's outer core SLIPPAGE ALONG A FAULT Love wave --- Earth's crust deforms along fault lines. Friction prevents plates moving, building pressure Surface waves The slowest waves, comprising principally Rayleigh and Love waves, whose depths of penetration are dependent on their wavelengths Subduction Ocean floor is forced under Rayleigh continental crust and into the magma where it is consumed and recycled. The subduction process is accompanied by the worlds strongest quakes, measuring up to 8.9 on the Richter scale, which can heave the ocean floor by scores of metres Pressure finally wave overcomes friction, resulting in violent movement of crust Strike-slip faults Form where two plates shear past each other. The resulting earthquakes are less powerful than those in subduction zones but can be more destructive to mankind because their focus is nearer the surface Direction of wave travel Direction of rock movement Crust rebounds and finds new equilibrium ---- ----. Mantle currents Movement of tectonic plates is thought to be driven by sluggish heat currents within the mantle Earthquake focus Semi-molten mantel Sources: Earthquake / The Lie of the Land, Time Life Books / Restless Earth, National Geographic Society Books Credit: Reuters DISTANCE (MILES) S-P (SECONDS) MAGNITUDE AMPLITUDE (MM)

Earthquake Anatomy

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