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Seismicity
Seismicity
Causes
of EQs
Distribution
Primary
Soil liquefaction,
Landslides/Avalanches,
Effects on people and
the built environment
Mag &
Freq
Effects
Secondary
Tsunamis
Causes of Earthquakes
• As the crust of the Earth is mobile, there tends to be a slow
build up of stress within the rocks.
• When this pressure is suddenly released, parts of the surface
experience an intense shaking motion that lasts for just a few
seconds.
• This is an earthquake.
Causes of Earthquakes
• The point at which the pressure of an earthquake is released occurs within the
crust and is known as the focus.
• The point immediately above the focus, on the Earth’s surface, is known as
the epicentre.
• The depth of the focus is important and 3 categories of earthquake are
recognised:
1. Shallow focus (0-70km deep) these tend to cause the greatest damage and
account for 75% of all the earthquake energy released.
2. Intermediate focus (70-300km deep)
3. Deep focus (300-700km deep)
• Seismic waves radiate from the
focus rather like the ripples in
water when a rock is thrown into a
pond.
• Fig 1.26
• Primary (P) waves travel faster
and are more compressional,
vibrating in the direction in which
they are travelling. They are also
refracted when they pass through
a medium.
• Secondary (S) waves travel at half
the speed of P waves and shear
rock by vibrating at right angles to
the direction of travel. They will
also only travel through a solid.
• Surface (L) waves travel the slowest and near to the ground surface. Some
surface waves shake the ground at right angles to the direction of wave
movement and some have a rolling motion that produces vertical ground
movement.
P and S waves travel through the interior of the Earth and are recorded on a
seismograph. Studying earthquakes and the seismic waves they generate has
made it possible to build up a picture of the interior of the Earth.
Distribution
• The vast majority of earthquakes occur
along plate boundaries (fig 1.27), the most
powerful being associated with destructive
margins.
• A conservative margins, the boundary is
marked by a fault, movement along which
produces the earthquake.
• Perhaps the most famous is San Andreas
fault, California (the boundary between the
North American and Pacific plates).
• In reality, the San Andreas system consists
of a broad complex zone in which there are
a number of fractures of the crust. (fig 1.28)
• Some earthquakes occur away from plate
boundaries and are associated with the
reactivation of old fault lines.
• 23 Sept 2002, UK midlands, earthquake measuring
4.8 on the Richter Scale. Epicentre: Dudley, West
of B’ham
• It is believed that the cause was movement along
an old fault line known as the Malvern lineament.
• It has been suggested that human activity could
also be the cause of some minor earthquakes.
Examples are the buildings of large reservoirs in
which the water puts pressure on the surface
rocks, or subsidence of deep mine workings.
• Newcastle, Australia (1989) earthquake
measuring 5.6 on the Richter Scale and has been
attributed to deep coal mining in the region.
• 13 people dead, 160+ in hospital, thousands of
buildings damaged
• Economic losses equivalent to almost A$5 billion,
3.4% of Australia's Gross Domestic Income (GDI)
or 80% of the nation's GDI per capita growth.
• Just a coincidence? Read article – highlight and
annotate as per your own notes.
Mag & Freq
• Magnitude: measured on two scales: The Richter
Scale, which is a logarithmic scale which means
that an event measured at 7 on the scale has an
amplitude of seismic waves ten times greater
than the one measured at 6 on the scale.
• The energy release is proportional to the
magnitude so that for every unit increase in the
scale the energy released increases by
approximately 30 times.
Mag & Freq
• The Mercalli scale measures the intensity of the event and
its impact.
• It is a 12 point scale that runs from Level I (detected by
seismometers but felt by few people, approx 2 on the
Richter scale) to Level XII (total destruction and ground
visibly shaking, approx. 8.5 on Richter scale).
• Seismic records enable earthquake frequency to be
observed, but these records only date back to 1848 when
an instrument capable of recording seismic waves was first
developed.
The effects of Earthquakes
• Ground Shaking
• Severity of this is dependent on: magnitude,
distance from epicentre and local geological
conditions.
• Mexico, 1985, seismic waves were amplified
several times by the ancient lake sediments
upon which the city is built.
1. Soil Liquefaction
When violently shaken, soils with high water content lose their mechanical
strength and start to behave like a fluid.
2. Landslides/avalanches
Slope failure as a result of ground shaking.
3. Effects on people and
the built environment
Collapsing buildings, destruction of road systems and other forms of
communications, destruction of service provision such as gas, water,
electricity transmission systems, flooding, disease, food shortages,
disruption to the local economy.
Some of the effects on the human environment are short term; others
occur over a long period and will depend to a large extent on the ability of
the area to recover.
4. Tsunamis
4. Tsunamis
• In the open ocean tsunamis have a
very long wavelength (sometimes
more than 100km) and a low wave
height (under 1m).
• They travel quickly at speeds
greater than 700km/h – some
tsunamis take less than a day to
cross the Pacific Ocean.
• On reaching shallow water
bordering land, they increase
rapidly in height.
4. Tsunamis
• Often the first warning for coastal populations is the wave trough in front of the
tsunami which causes a reduction in sea level known as a drawdown.
• Behind this comes the tsunami itself which can reach heights in excess of 25m.
• The event usually consists of a number of waves, the largest not necessarily being
the first.
4. Tsunamis
When a tsunami reaches land, its effects will
depend upon:
1. The height of the waves and the distance
they have travelled
2. The length of the event that caused the
tsunami
3. The extent to which warnings can be/were
given
4. Coastal physical geography, both offshore
and in the coastal area
5. Coastal land use and population density
4. Tsunamis
• The wave will wash boats and wooden coastal
structures inland and then the backwash may
carry them back out to sea.
• Both the water itself and the debris that it carries
cause drowning and injuries.
• The effects of most tsunamis are felt at least 500600 m inland depending on the coastal geography.
• Buildings, roads, bridges, harbour structures, trees
and even soil are washed away.
• Tsunamis generated by the explosion of volcano
Krakatoa in 1883 are estimated to have drowned
more than 35,000 people and produced waves that
travelled around the world, the highest being over
40m.
4. Tsunamis
• Around 90% of all tsunamis are generated within the Pacific basin and are
associated with the tectonic activity taking place around its edges.
• Most are generated at convergent plate boundaries where subduction is
taking place, particularly off the Japan-Taiwan island arc (25% of all events).
• Since the devastating tsunami of Dec 2004 (case study), the area has been
affected by at least two major tsunamis:
1. July 2006, south Java coast, generated by an earthquake of mag 7.7 on
the Richter scale 180km offshore – 600 deaths
2. April 2007, Solomon islands – at least 15 deaths
4. Tsunamis
• The geological evidence indicates that huge tsunamis have affected
areas such as the Mediterranean basin (e.g. the Santorini eruption
around 1450BC) and the North Sea area.
• Around 7250BP* the Storegga slide caused by huge submarine debris
slides off Norway, produced tsunamis more than 6m high in Scotland
and other areas bordering the N Sea.
• It is believed that these tsunamis continued across the Atlantic to
affect the coastlines of Spitsbergen, Iceland and Greenland.
*BP = before present, standard practice is to use 1 January 1950 as commencement date of the age scale, reflecting the fact
that radiocarbon dating became practical in the 1950s