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22/01/2017 Most earthquakes come from tectonic plate boundaries (but not all). EARTHQUAKES – a Hazard for society, but also evidence of what is beneath the Earth’s Surface. By Frank Nicholson Huyton U3A Average of 27 Earthquakes every day! Most earthquakes are due to fault slippage. Stress begins, the fault is “locked” Stress builds, deformation continues, maybe foreshocks Fault slips, stress released, earthquake occurs. Seismic waves dissipate with distance and a fault scarp may be formed (the fault is usually already present). SCALES FOR THE INTENSITY OF EARTHQUAKES Mercalli Scale. Uses descriptive damage on a scale of 1-12 e.g. Mercalli III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognise it as an earthquake. Standing motor cars may rock slightly. Vibration like passing of truck. Duration estimated. e.g. Mercalli VI Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of cracked plaster or damaged chimneys. Damage slight. e.g. Mercalli X Some well built wooden structures destroyed; most masonary and frame structures with foundations destroyed; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed over banks. Note the focus of the earthquake and the epicentre (point on the surface vertically above). SCALES FOR THE INTENSITY OF EARTHQUAKES Mercalli Scale. Early scale – updated version still used for damage comparisons but not widely used now. Richter Scale. Uses measured seismic wave amplitude and distance from epicentre to estimate the ENERGY of the earthquake. It is a logarithmic scale and one unit increase is a 30 times increase in energy. Still widely used – most used by news reports. Now the Moment Magnitude Scale (MMS) is used by some seismic professionals, especially United States Geological Survey (USGS). It uses area of fault plane that slipped, amount of slip and shear strength of the ground. It is a better quantitative measure of earthquake size. The MMS values have been designed to give figures for most damaging earthquakes that are similar to Richter values. Values for very extreme earthquakes may be up to one unit higher. 7 on the Richter is definitely a major earthquake. 6 may be! 1 22/01/2017 P and S Waves as they would act on a long coiled spring. Portable seismometer. P S LR Typical short period seismic record. Different waves can be identified P Wave (= Primary Wave. But easy to remember action as push waves). S Wave (= Secondary Wave. But easy to remember action as shake waves). P travel fastest and S waves a little slower. Both P & S waves have relatively short periods (fast vibration). L & R are SURFACE Waves. L stands for Love and R for Rayleigh – the two technical names. Their movements are more complex rolling motions in different planes. However they travel more slowly and have longer periods. A typical seismometer array at a measurement station will have 3 short period seismometers mounted to particularly receive short wave components which are North-South, East-West, and Vertical. Also 3 long period seismometers with similar orientations. Seismic waves travelling in an earth of uniform composition. “Much of our knowledge of the internal constitution of thee Earth has come from the study of the seismic waves generated by earthquakes.” S waves are simplest. They cannot pass through liquids. There is a zone across the Earth from a large earthquake where no direct S waves are received. This is evidence for a liquid core to the Earth. e.g. The depth of focus of an earth quake is determined by the difference in arrival time of the direct P wave, and the arrival time of the reflected P wave from the surface (pP wave) – necessarily a longer path. P-pP time difference is a measure of the focus depth. P waves pass through the liquid core. However refraction at the solid/liquid boundary means there is a circular P-wave shadow zone. 2 22/01/2017 Travel paths of seismic body waves (P & S waves). The difference between time of arrival of P & S Waves can be used to determine the distance of the Earthquake from the seismic station. With 3 seismic stations that determine distance to an earthquake by P&S wave arrival times, the position of an earthquake can be found. First motion of seismic waves from a strike-slip fault, giving compressional or tensional movement in different quadrants. With information from several seismic stations fault movement direction can be determined (also atomic explosions obvious – in every direction the seismic records start with compression). The Mohorovicic discontinuity (the Moho) at the base of the crust, was discovered by recognising that waves from nearby earthquakes are reflected from it (by Andrija Mohorovicic). The depth to the Moho was estimated from the travel times of the reflected earthquakes. The mantle begins below the crust. Cross-section of the South American subduction zone near latitude 22° S. Triangles represent the locations of stratovolcanoes. Shaded circles represent earthquake focci, shaded in depth groups. Deep earthquake focci, organised in zones, gave strong clues about plate tectonics and subduction. Generalised diagram of the crust and upper mantle. The thickness of the asthenosphere varies. 3 22/01/2017 Haiti Earthquake 2010 Richter earthquake magnitude 7.0 Port-au-Prince capital Dense population (2 million) 300,000 deaths 180,000 homes destroyed 1.5 million homeless Plate tectonics is facilitated by the rigid lithosphere plates moving on the much hotter aesthenosphere which is able to be deformed plastically. 19 million m3 of rubble 1.5m living in temporary camps 5000 schools destroyed Nepal 2015. Earthquakes 25 April and 12 May. 8,787 dead. Over 500,000 homes destroyed. 8 million of the country’s 28 million people badly affected. Cost to the economy estimated as $7 billion (including tourism). Plus at least $8.5 billion needed for rebuilding. Nepal Earthquake Nepal earthquake. 4 22/01/2017 Particularly in more advanced countries with common earthquake frequency e.g. California and Japan. Nepal earthquake. TSUNAMI GENESIS Canada – advice for emergencies: For Earthquakes: DROP COVER HOLD Drop down to the floor - Get under something (e.g. strong table or desk until earthquake is definitely over. wait Mexico Hotel – Earthquake advice (after Fire Alarm etc info) in typical reinforced concrete frame hotel: shelter in a door frame!. • The phenomenon of ‘drawdown’ before tsunami arrives. Boxing Day Tsunami 2004 • Remember this – it could save your life. DON’T be tempted to examine exposed sea bed, pick up fish etc. QUICKLY MAKE FOR HIGH GROUND. The area affected by the Boxing Day Tsunami 5 22/01/2017 Country where deaths occurred Confirmed Estimated Injured Missing Displaced Indonesia 130,736 167,799 n/a 37,063 500,000+ Sri Lanka 35,322 35,322 21,411 n/a 516,150 India 12,405 18,045 n/a 5,640 647,599 Thailand 5,395 8,212 8,457 2,817 7,000 Somalia 78 289 n/a n/a 5,000 Myanmar (Burma) 61 400–600 45 200 3,200 Maldives 82 108 n/a 26 15,000+ Malaysia 68 75 299 6 n/a Tanzania 10 13 n/a n/a n/a Seychelles 3 3 57 n/a 200 Bangladesh 2 2 n/a n/a n/a South Africa 2 2 n/a n/a n/a Yemen 2 2 n/a n/a n/a Kenya 1 1 2 n/a n/a n/a n/a n/a n/a 1,000+ ~184,167 ~230,273 ~125,000 Madagascar Total ~45,752 Sri Lanka Thailand • Victims taken by surprise despite a lag of up to several hours. • At the time – no tsunami warning system for the Indian Ocean. ~1.69 million India Japan Tsunami 2011 Indonesia • Earthquake magnitude 9.0 • One of the biggest ever recorded Tsunami travel times Japan (Tohoku) Tsunami 2011 Epicentre 70km east of Japan Tsunami waves up to 38.9m Waves travelled 10km inland Nuclear accidents – 3 explosions. $300 billion cost 6 22/01/2017 Unexpectedly high water surge 15,148 deaths 5304 injured 8881 missing 125,000 buildings damaged or destroyed Over 60’s accounted for 65% More than 100 tsunami evacuation sites inundated • Houses and cars washed away • A tsunami-tossed boat rests on top of a building….. Ideas originating from China – on web site in India. In practice reliable earthquake warning is very rarely possible, though there is the one striking example! Text This was the 1975 Haicheng earthquake. At the time observation of unusual animal behaviour was given prominence as an earthquake indicator. Unfortunately one year later 1 million were killed in Tanshan in China when no indicators were observed. Many factors affect the damage and loss of life produced by an earthquake. The size of the earthquake, and the distance from the epicentre. The depth of the focus is also important. A shallow focus earthquake under a city will do MUCH more damage. The types of structures are also important. Foundations, materials, (wood, brick, masonary, concrete etc), building frame, quality of construction, building height etc. The substrate is important. Soil, unconsolidated sediment etc all may easily yield or even liquify, causing structures to collapse. At longer distances short period waves lose their effect, but long period shaking will affect taller buildings with a natural long period – as Mexico City 1985 when buildings 6-16 stories high were worst affected by a distant earthquake. Now many designs/modifications to reduce damage. 7 22/01/2017 Text Text Buildings are normally designed to support vertical loads but earthquakes can put strong lateral forces onto a building. One way to make a simple structure more resistant to the lateral forces from an earthquake is to tie the walls, floor, roof, and foundations into a rigid box that holds together when shaken by a quake. “The most dangerous building construction, from an earthquake point of view, is unreinforced brick or concrete block.” Quote from an educational site on earthquake proofing buildings. Many of the following ideas are from India & Pakistan web sites – applicable to cheaper construction, not just costly buildings. Text Effect of isolating the base of a building subject to a sideways ground movement. Text 8 22/01/2017 Rubber base isolation foundations. There are a range of different designs of base isolators for earthquake resistant foundations. Text Text Text Text 9 22/01/2017 Important for masonry or brick buildings. Bands are made from reinforced concrete (most successful), or wood (bamboo may be used), or reinforced brick. Hold the building together at especially critical levels, and reduce building collapse. Text Text Text Text Text 10 22/01/2017 Text Text Text Japanese device – apply computer controlled counter force. Text Text 11 22/01/2017 Text Text Retrofitting. Text Text Or columns can be cased in sheet steel, to prevent crumbled concrete from being able to collapse. Text 12 22/01/2017 26 August – Central Italy, Earthquake magnitude 6.2 struck 50+ miles NE of Rome. Town of Amatrice severely damaged & nearby settlements – 297 dead. 2,500 aftershocks. 26 October Earthquake magnitude 6.1 in the same general area, followed by another 6.6 on 30 October. Much damage. Wednesday 18 January 2017 Aquila – same region as Amatrice 10.25am Earthquake magnitude 5.3, magnitude 5.7 49 mins later, magnitude 5.6 11 mins later, magnitude 5.2 3 hours later. Many aftershocks. Evening of 18 Jan – Rigopiano Hotel in Farindola destroyed by avalanche, with probably 28 dead. The avalanche was most likely triggered by earthquake. •Just after midnight on 14 November, 2016, a 7.8 magnitude earthquake struck New Zealand’s South Island 55 miles north east of Christchurch. •Later a 6.8 magnitude aftershock, one of 313 aftershocks recorded within 13 hours. •Two people died, damage estimated at $2 billion and tens of thousands of landslides. •A gauge in the town of Kaikoura measured a wave over 8ft tall as a tsunami hit the east coast. Geographical Association Newsletter 17 Nov 2016. How would YOU react to this? New Zealand Earthquake 14 November 2016. That’s all for today Folks 13