LANDFORMS AT BOUNDARIES Can you describe how each landforms are formed, give examples. (mid-oceanic ridge, trench, fold mountain, rift valleys, island arcs) 1 4 3 2 5
EARTHQUAKE WHAT & WHERE? FACTORS CAUSES CASE STUDIES PREDICTION
WHAT & WHERE?
What? A series of shocks & tremors that
result from a sudden release of pressure.
At destructive boundaries
Human activity (explosion, nuclear testing, mining, drilling & construction of dam)
*Create shock waves & pressure change in
underlying rock layers.
Results from the sudden release of energy deep underground.
This energy takes the form of a jerk or slippage along a line of weakness called a fault.
Shock waves called seismic waves radiate out in all directions from the point of slippage called the focus
These waves cause the shaking on the ground surface.
The greatest effect is felt nearest epicentre (the point on the surface immediately above the focus of the earthquake.
Active faults lies at the edge of the plates, that is why the majority earthquakes occur at plate margins.
HUMAN ACTIVITIES TRIGGER EARTHQUAKE CONSTRUCTING A DAM
Constructing a dam can increased earthquake activity:
Increase pressure exerted on a fault
Water seeping into groundwater zone & lubricating a fault.
E.g. In Zambia, 100 earthquake occurred a year as Lake Kariba
Filled up between 1958 & 1963. the earthquake stopped after
DIGGING UP COAL Underground subsidence following deep mining. E.g. earthquake in the UK coalfield areas, Stoke-on Trent.
ABILITY OF BUILDING
A COUNTRY’S WEALTH
FACTORS CONTRIBUTING TO EARTHQUAKE
The more powerful the earthquake the
more damage it will cause.
But it is not always the case, e.g. San
Francisco earthquake 1989 has a Richter
scale 7.1. but only 65 were killed,
whereas a less severe earthquake 6.8 killed
over 25 000 people in Armenia a year
Ability of building to withstand shaking .
Poor building design in Mexico city in 1985, both low-rise and high-rise building collapsed. Some buildings toppled over and others sank into the ground.
Densely populated area such as Tokyo city most vulnerable to earthquake.
In 1923, 143 000 people lost their lives and 2/3 of the city was flattened.
NATURE OF UNDERLYING GEOLOGY
Sands and clays may become liquefied (jelly-like) promoting landslips and building collapsed.
E.g. Mexico city where most of the city lies on old-lake-bed sediments were attack by earthquake in 1985.
In 1989 San Francisco earthquake, most severe damage to property was in Marina District where houses constructed on landfill material was unstable.
WEALTH OF A COUNTRY
The wealthier a country, the better able it is to build appropriately and respond effectively to earthquake hazard.
In poorer country, there have low building standards, and there have less money and emergency resources, to cope up with aftermath of an earthquake.
PLATE TECTONICS THEORY
SEISMIC GAP THEORY
Scientists have identified several precursors:
Foreshocks : occur prior to main earthquake,e.g. several earthquakes occurred in the months before 1989 San Francisco earthquake.
Geophysical changes: localised uplift and subsidence identified. E.g. Niigata earthquake in Japan in 1964 accompanied by a sudden subsidence of coastline by up to 20 cm.
Changes in electrical resistivity: Van method suggests that electrical curents become disturbed just prior to an earthquake and causes a change in resistivity. (measured)
Changes in animal behaviour prior to major earthquake, e.g. Haicheng in 1975, rats panicked, used as earthquake precursor.
PLATE TECTONIC THEORY
The theory is useful in predicting where
earthquake likely to occur.
But scientist do not know when it will occur.
Stress builds up at a constant rate but only released periodically.
As the build-up pressure is constant, it is expected the released of pressure (earthquake) occurs regularly.
By using historical records and geological evidence (i.e. fault) scientist able to plot recurrences of earthquakes and use
informaiton to suggest likelihood next earthquake that will occur.
SEISMIC GAP THEORY
Concept of elastic rebound theory has been developed into seismic gap theory.
If earthquakes along a particular fault line are plotted, gaps appear where no earthquake occurred for some time.
Pressure building up in these gaps will be released, thus these areas might be hit by future earthquake.
CASE STUDY Draw a concept map for any two case studies: (effects, causes, contributing factors and lessons learned)
INDIAN EARTHQUAKE, MAY 1997 (HAZARD, NAGLE,G.,P:17)
KOBE JAPAN JAN 1995 (HAZARD, NAGLE,G., P:19-23)
WHERE & HOW
Volcanoes are openings in the earth’s crust through which magma, molten rock and ash can erupt onto the land.
Where & How
At destructive margin
At a destructive margin one plate dives beneath the other.
Friction causes it to melt and become molten magma.
The magma forces its way up to the surface to form a volcano to the side of the actial plate margin. (ocean trench)
A number of volcanoes may reach the surface to form a string of islands called an island arc .
E.g. Mt. Unzen, Japan(Pacific/Eurasian)
Istead of forming volcanic islands, volcanoes occur within
a range of mountain. E.g. Nevado del Ruiz
In places where a plate is particularly thin,
magma may be able to escape to the
Such a place called a hot spot.
A shield volcano will be formed.
Where two plates are moving apart, new magma can reach the surface through the gap.
Volcanoes forming along this crack create a submarine mountain range called an ocean ridge.
Seismometers record tiny earthquakes that occur as the magma rises
Chemical sensors to measure increased sulphur levels, normally low but will increase before and during eruption.
Lasers to detect the physical swelling of the volcano.
Ultra sound to monitor low frequency waves in magma, result fro the surge of gas and molten rock under surface.
Hazards associated with volcanic eruptions
GLACIAL OUTBURST FLOODS
Lava flows Lava flows extend beyond 10 km from volcanic crater. Their speed of flow is determined by silica content of lava. Silica poor lava is very fluid and flows faster and much thicker A lava flow may destroy farmland, buildings and lines of communications, but rarely love of live.
Fine-grained ash together with larger pyroclastic (fire rocks) may be ejected into the air during a violent eruption.
The sheer weight of deposited ash can cause the roofs of buildings to collapse
Air thick with hot ash lead to the asphyxiation of humans and animals.
Ash combined with water to form mudflows called lahars.
Ash increase supply of condensation nuclei in lower atmosphere promoting formation of water droplets and increase rainfall.
The strong smell of sulphur in the air.
Other gases can be emitted including carbon dioxide and cyanide.
The cloud of incandescent gas, ash and rocks.
This temperature reach up to 800 C and speed of 200 km/hr.
E.g. Mt. Unzen (Japan) in 1991, triggered by collapse of part of the volcanic summit.
Two reasons for development:
Rain bringing soot and ash back to ground and washing the debris downhill.
Heat from a volcano melting snow and ice-the resulting flow of water picks up sediment.
A lahar can travel at speeds over 30km/hr and may exceed 15 m in height.
It can destroy houses or whole settlements, ruin communications, bury crops, animals and people.
GLACIAL OUTBURST FLOODS
Volcano erupts beneath an ice cap, a vast amount of ice can be melted, and when water escapes, large amount of water and sediment will move down mountainsides onto plains below.
E.g. in 1996 an eruption beneath the Vatnajokull glacier in Iceland melted, led to water 50 m deep in Grimsvotn volcanic crater.
Abnormally high sea waves generated by ground shocks such as earthquakes and volcanic eruptions.
E.g. 1883 Krakatau eruption created waves up to 35 metres high.
These immense waves swept along coasts of Java and Sumatra killing over 36 000 people.
Tsunami are often responsible for causing more loss of life and damage to property than more direct effects of volcanic eruptions.
This is because they affect low-lying, heavily populated areas and they are unprepared.
EXPOSURE (HAZARD MAP)
Erecting barriers to divert free-flowing lava away from valuable land or property, e.g. widely used in Hawaii.
A lave flow can be bombed to break it up.
Cooling the lava fronts with water to encourages lithification, e.g. island of Heimaey (iceland) in 1973.
The danger of building collapse can be avoided to some extent by encouraging appropriate building design involving sloping roofs to discourage the formation of thick and weighty deposits of ash.
A map indicating areas at greatest risk from eruption.
Advances in radar enabled digital elevation models (DEMs) to be produced.
These volcanic models can be used likely paths of lava and pyroclastic flows.
Case studies MT. ST HELENS, 1980 (HAZARD, NAGLE, G, P:27) NEVADO DEL RUIZ, COLUBIA 1985 (HAZARD, NAGLE, G. P:31-32)