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Earthquakes: An IntroductionPresentation Transcript
Image: Port-au-Prince, Haiti 12 January 2010
See Introduction to Plate Tectonics presentation for the causes of earthquakes
An earthquake is a very sudden shaking of the ground
P-waves travel fastest. They consist of successive contractions and expansions, just like sound waves in air. The motion of the particles in the rocks that the waves travel through is parallel to the direction of the wave. S-waves are slower than P-waves. They are transverse waves, which means that the particle motion is at right angles to the direction of travel. S-waves cannot travel through air or liquids. They shake the earth up and down and side to side SEISMIC WAVES Two different types of seismic waves are generated by the sudden movement on a fault: P-waves and S-waves. A third type of seismic wave is generated by the interaction of the P- and S- waves with the surface and internal layers of the Earth. The speed of the waves depends on wave type and the properties of the rock; the denser the rock, the faster the waves travel. In the Earth’s crust P-waves travel at around 6-7 km/s, while S-waves travel at around 3.5-4 km/s.
Surface waves radiate outward from the epicentre - the point on the surface directly above the focus (hypocentre) - and arrive after the main P and S waves.
The waves of energy released by earthquakes can be recorded by instruments called seismographs
The most widely documented earthquake zone in the world is the San Andreas fault running through California. The fault is the boundary between the Pacific and North American plates. Here they form a transform margin where movement is in a slip zone as shown on the map.
Movement along the San Andreas Fault during the 1906 earthquake displaced this fence by about 1.5 metres. The gap in the fence was caused by the primary surface fracture along the fault. This view is from the Pacific Plate on the west side of the fault looking toward the North American plate on the east side. The fault line separating the two plates is shown by the arrow. What has happened here?
This young woman is standing on the San Andreas Fault
You can see clearly where the fault line is in this orchard
Check out the line of the fault in this picture
Rocks of the Pacific Plate on the left slip past rocks of the North American Plate on the right What do you notice here?
Offset at Wallace Creek, California Can you see what has happened here? What may happen to this river in time? 90 metres
The San Andreas Fault in the Carrizo Plain (California) is a showcase of classic tectonic landforms. On the previous slide can be seen the famous offset in Wallace Creek.
The creek runs out of the Temblor Range on the North American Plate from the upper right. When it hits the fault, it's diverted northwest along the trace.
The Pacific Plate, in the foreground, carries the channel with it as it moves, and the creek goes along as well.
The offset in the present drainage is about 90 metres, but further to the left we see the ancestral drainage that was abandoned about three thousand years ago when the creek cut its present channel.
Note that the San Andreas Fault and plate boundary are evident as a line of low scarps (cliffs) running horizontally through the centre of the picture.
Not only transform margins cause earthquakes ...
Earthquakes occur at collision zones . Intense compressional forces are exerted as a result of the collision. This is evident in the Himalayas where the Indo-Australian plate is moving northwards and collides with the Eurasian plate which is moving southeast. An examples of such an earthquake is the Kashmir 7.6 ‘quake on the Indian, Pakistan, Afghanistan border in October 2005, where 75,00 people died.
They also occur at destructive margins . As oceanic plates are subducted beneath continental plates the movement is irregular and sudden jots after periods of relative stability can cause earthquakes along the fault lines.
The Moment Magnitude scale and the Richter scale are both measures of the magnitude of earthquakes, where magnitude is the seismic energy released by the earthquake.
The Richter scale is the older of the two but is now being gradually superseded by the Moment Magnitude scale.
The Richter scale was developed in the 1930s by American seismologist Charles Richter. It gave scientists an objective measurement of the size of earthquakes, and served us well for many decades.
The main problem with the Richter scale is that it is a comparison of one earthquake to another rather than an absolute measurement.
Another problem was that it was developed in Southern California and conditions elsewhere in the world are not exactly comparable.
The scale is logarithmic - see next slide
Richter TNT for Seismic Example Magnitude Energy Yield (approximate) -1.5 6 ounces Breaking a rock on a lab table 1.0 30 pounds Large Blast at a Construction Site 1.5 320 pounds 2.0 1 ton Large Quarry or Mine Blast 2.5 4.6 tons 3.0 29 tons 3.5 73 tons 4.0 1,000 tons Small Nuclear Weapon 4.5 5,100 tons Average Tornado (total energy) 5.0 32,000 tons 5.5 80,000 tons Little Skull Mtn., NV Quake, 1992 6.0 1 million tons Double Spring Flat, NV Quake, 1994 6.5 5 million tons Northridge, CA Quake, 1994 7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weapon 7.5 160 million tons Landers, CA Quake, 1992 8.0 1 billion tons San Francisco, CA Quake, 1906 8.5 5 billion tons Anchorage, AK Quake, 1964 9.0 32 billion tons Chilean Quake, 1960 10.0 1 trillion tons (San-Andreas type fault circling Earth) 12.0 160 trillion tons (Fault Earth in half through center, OR Earth's daily receipt of solar energy)
A newer measurement of the magnitude of earthquakes is the Moment Magnitude scale, which is designed to overcome the problems of the Richter scale.
This means that the Moment Magnitude is not comparative, and it is applicable equally everywhere in the world. It is also completely independent of the type of equipment used to measure the earthquake.
This new way of measuring the magnitude of earthquakes is now commonly used by seismologists.
Moment Magnitude Scale
In order to make a scale comparable to the well-known Richter scale, seismologists invented the (Seismic) Moment Magnitude Scale, which gives magnitudes roughly equivalent to the Richter magnitudes.
Like the Richter scale it too is uses a logarithmic scale
The numbers generated by the two scales are usually very similar. For example, the earthquake in Northridge California in 1994 measured 6.4 on the Richter and 6.7 on the Moment Magnitude scale.
When reporting the magnitude of an earthquake, many news organisations simply report the earthquake as having a magnitude of [number], without stating whether the Richter or Moment Magnitude scale is being used. For most people, the difference is unimportant. The bigger the number, the worse the earthquake.
Intensity is the measure of impact. Whilst magnitude needs specialist equipment to record it (seismographs) intensity can be assigned a value according to the felt responses of the population.
These are then assigned a value from 1 to 12, the most commonly used such scale being the European Macroseismic Scale (EMS).
I. Not felt
Not felt, even under the most favorable circumstances.
II. Scarcely felt
Vibration is felt only by individual people at rest in houses, especially on upper floors of buildings.
The vibration is weak and is felt indoors by a few people. People at rest feel a swaying or light trembling.
IV. Largely observed
The earthquake is felt indoors by many people, outdoors by very few. A few people are awakened. The level of vibration is not frightening. Windows, doors and dishes rattle. Hanging objects swing.
The earthquake is felt indoors by most, outdoors by few. Many sleeping people awake. A few run outdoors. Buildings tremble throughout. Hanging objects swing considerably. China and glasses clatter together. The vibration is strong. Topheavy objects topple over. Doors and windows swing open or shut.
VI. Slightly damaging
Felt by most indoors and by many outdoors. Many people in buildings are frightened and run outdoors. Small objects fall. Slight damage to many ordinary buildings; for example, fine cracks in plaster and small pieces of plaster fall.
EMS - a measure of intensity
Most people are frightened and run outdoors. Furniture is shifted and objects fall from shelves in large numbers. Many ordinary buildings suffer moderate damage: small cracks in walls; partial collapse of chimneys.
VIII. Heavily damaging
Furniture may be overturned. Many ordinary buildings suffer damage: chimneys fall; large cracks appear in walls and a few buildings may partially collapse.
Monuments and columns fall or are twisted. Many ordinary buildings partially collapse and a few collapse completely.
X. Very destructive
Many ordinary buildings collapse.
Most ordinary buildings collapse.
XII. Completely devastating
Practically all structures above and below ground are heavily damaged or destroyed.
Why does the impact of earthquakes vary?
VULNERABILITY NATURAL HAZARD RISK OF DISASTER Sudden events and chronic issues Past recurrence intervals Future probability Speed of onset Magnitude Duration Areal extent Exposure, sensitivity and resilience of: Population Economy Land use and development Infrastructure and critical facilities Cultural assets Natural resources Source: Geography for EDEXCEL by Digby et al (Oxford University Press) EARTHQUAKES VENN DIAGRAM The connection between natural hazard events and locational site vulnerability
Magnitude - amount of seismic energy which remains unchanged with distance from the earthquake. Greater the magnitude the greater the potential impact
Intensity - the degree of shaking at a given place and thus an indication of impact. This decreases with distance from the earthquake epicentre .
Depth - focal point of the earthquake. The shallower the focus the greater the impact
Surface land use . A city is likely to be more vulnerable than a remote rural area where few if anyone lives. The density of the population will also have an influence.
State of economic development . Poor regions less likely to have earthquake proof buildings in place and emergency services may be inferior
Why does the impact of an earthquake vary from place to place and from time to time?
Time of day . In cities ‘rush hour’ is likely to lead to more devastation on the roads and railways, whereas mid morning in the CBD hundreds of thousands will be in their offices. At night the CBD will be empty but the suburbs will be at a greater risk.
Time of year . What are normally quiet sparsely populated areas, in the tourist season they may be densely populated. The Boxing Day tsunami caught thousands of tourists in Thailand for example.
Landscape characteristics . Some areas become vulnerable to secondary events like land slides (where land is steep and unstable), avalanches (in ski resorts), tsunamis (for example where high magnitude earthquakes strike offshore) or liquefaction where land is unstable (as in Mexico City 1985 when 10,000 died, most because of the collapse of buildings due to liquefaction). NB seismic shaking can turn water-saturated sand into a liquid mass that will not support buildings.
Emergency services . Some areas are well served by emergency services so people can be rescued but others are more remote of too poor for such services and therefore rescue is less likely.
Earthquakes in UK
Between 200 and 300 earthquakes are detected and located in the UK, by the British Geological Survey annually.
Although distant from the nearest plate boundary, the Mid-Atlantic Ridge, earthquakes occur as crustal stresses within the tectonic plates occurring on pre-existing fault planes.
The risk from these earthquakes is not insignificant and must be considered when engineering for sensitive installations such as nuclear power stations and gas terminals.
The North Sea earthquake of 7 June 1931, with a magnitude of 6.1ML and with an epicentre offshore 120 km NE of Great Yarmouth, is the largest known earthquake in the UK.
The felt area encompassed most of Britain, E of Ireland, the Netherlands, Belgium, N France, parts of NW Germany, Denmark and SW Norway.
Damage in Britain was reported from 71 different places, with the strongest effects at Filey, where the top of a church spire was rotated. Hull was also affected, with most of the damage affecting chimneys and plaster. A factory roof is reported to have collapsed at Staines (Surrey) and rocks or cliff collapse occurred at Flamborough Head and Mundesley, Norfolk.
The earthquake was felt by a number of vessels in the North Sea and a woman in Hull died of a heart attack, apparently as a result of the earthquake! What would have been the impact had the epicentre been on the mainland?
The Colchester earthquake of 1884, with a magnitude of 4.6 ML, was the most damaging earthquake in the UK for several centuries. It had an EMS (see next slide) of 8.
There was considerable damage to churches, including the top of a spire falling, falling masonry from roofs, falling turrets and parapets.
Damage to residential properties included shattering of brick walls, and chimney falls, often through roofs.
It was caused by the rupture of an old fault line in the East Midlands.
There were reports of mainly minor damage such as tiles off roofs, breakages inside the homes and brick walls collapsing.
The 5.2 earthquake that struck just before 0100 GMT on 27 February 2008 near Market Rasen, Lincolnshire, was the biggest earthquake to hit the UK for 25 years (Nefyn, North Wales, 5.4, July 1984).
Many of our school’s AS geography field trippers were awoken by this when staying in Overstrand on the north Norfolk coast.