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Main hazards generated by seismic activity - Coggle Diagram
Main hazards generated by seismic activity
Plate boundaries & seismic activity
Mid-ocean ridge or continental rift zone:
tensional forces associated with spreading processes, faulting & rifting
0-70km shallow
Mid Atlantic Ridge
East African Rift
Subduction zones: Island arcs, Andean types:
compressional forces associated with subduction of ocean crust
0-70km shallow & 70-700km deep foci reflect depth to subducted plate
Japan
Indonesia
Andes
if plates are going downwards, likely deep
Continental collision zone:
compressional forces associated with continental collison
0-70km shallow & rare 70-700km deep
Himalayas
Conservative plate boundary:
shearing forces associated with movement / sliding of plates moving past eachother
0-70km shallow
San Andres Fault
Alpine Fault, New Zealand
Fault landscapes
Small scale features: Fault scarps:
fault scarp = a cliff or escarpment formed directly by rocks being displaced either side of a fault
foot wall and hanging wall moving apart
Small scale features: Escarpments:
escarpments = an extensive upland area with short, steep (Scarp) slope and a long, gentle (dip) slope
blind trust faults; they dont break the surface
example
: the Kaimai Range 110km long, 150m high escarpment on the North Island of New Zealand, direct result of Hauraki fault that uplifted primarily andesitic rocks from long-extinct volcanoes more than 1m years ago, fault has not been active since around 140,000 years ago
Large scale features: Rift Valleys & Fault block mountains:
Rift Valley = a valley formed by down faulting between parallel faults, examples are found on land (e.g. East African Rift Valley) and along MOR
rift shoulder = elevated areas around rift valleys (Horst)
rift basin = depressions within rift valleys (grabens)
horst = an uplifted block of land between 2 parallel faults
graben = a block of land bounded by parallel faults in which the block has been downthrown, producing a distinctive structural valley with a steep-sided fault scarp on either side
Examples:
Thingvellir Rift Valley in Iceland, along MAR, grows 1cm every year, Lake Thingvallavatn is the largest lake in Iceland formed as a result of volcanic activity around the rift
East African Rift Valley, small normal fault (scarps)
Basin and Range Province, USA/Mexico
Large scale features: Mountain ranges:
mountain ranges formed from thrust faulting
example
: Himalayan mountain range
Large scale features: Strike-slip fault zones:
example:
San Andres Fault
Fold mountains:
earthquakes are associated with the formation of entire mountain chains such as the Himalaya–Karakoram Range in Asia
the northward drift of India into Eurasia and the subsequent continental collision led to a complex pattern of folding and faulting of rocks
world’s highest and most impressive fold mountains: 96 of the world’s 109 peaks of over 7000 m are located here
to the north of the Himalaya–Karakoram Range lies the Tibetan Plateau
averaging 4500m above sea level it covers an area of 2.5 million km2 (10x the size of the UK); major fault systems are evident in the rocks and these indicate considerable movement.
entire region is tectonically active, as the 2008 (8.0 Mw) and 2013 (7.0 Mw) earthquakes in Sichuan province and the 2015 (7.0 Mw) event in Nepal demonstrated
Rift valleys:
rift valleys along mid-ocean spreading ridges, in East Africa and elsewhere, evidence of effects of earthquakes on morphology of the Earth’s surface
inward-facing fault scarps or escarpments of rift valleys mark the location of faults caused by tension and compression within the crust
rift valleys on the continents are altered by weathering and erosion, over time fault scarps are worn away, blending into the landscape, they can even disappear under accumulated sediments
How earthquakes create escarpments:
strain energy during plate movement causes stress to build up in rocks
when the strain energy becomes too great the rock strength is exceeded and this leads to failure and fault formation
the built up strain energy is released in the form of seismic shock waves generating an earthquake
along a fault, a fault block moves down relative to another leaving a cliff face exposed on the surface, locating the position of the fault plane this cliff is known as a fault scarp and the landform is an escarpment
these landforms are associated with tension (divergent boundaries) and compression (convergent boundaries) within the crust
How earthquakes create rift valleys:
rift valley is formed on a divergent plate boundary, a crustal extension or spreading apart of the surface
strain energy during extension plate causes stress to build up in rocks, when the strain energy becomes too great the rock strength is exceeded and this leads to failure and fault formation
the built up strain energy is released in the form of seismic shock waves generating an earthquake
tensional forced exceed the rock strength and parallel normal faults form orthogonal to the stretching direction
where there are two parallel faults one block moves up relative to another forming a horst; this leaves two steeply sided escarpments on either side
the down faulted block is known as a basin /graben.
If it occurs on an oceanic divergent plate boundary a MOR is formed (e.g. Thingvellir)
If it occurs on continental crust a rift Valley can be created (E.g. East African Rift Valley)
Earthquake hazards secondary
Ground failure
Liquefaction:
the process by which sediments and soils lose their mechanical strength from a sudden loss of cohesion, the material temporarily transformed into a fluid as a result of being violently shaken during an earthquake
causes loss of strength of water saturated soils due to rapid increases in stresses on the rock - muddy & sandy soils are more prone to failure
when an earthquake strikes an area with surface materials of fine-grained sands, alluvium and landfill with a high water content, the vibrations can cause these materials to behave like liquids
happens as normal pressure can maintain strength or hardness because of friction from grains touching, but force from an earthquake causes water to increase in pressure, breaking friction in the grains and fill in the spaces (structure lost), causing liquefaction
Impacts:
materials lose their strength, slops such as river banks collapse and structures tilt and sink as their foundations give way
often causes damage to buildings, bridges, dams, highways, pipelines, and other critical elements of infrastructure
most liquefaction damage is associated with ground failure i.e. permanent lateral and vertical deformation
sand layers can slide, rips in ground surface or uneven settling of building foundations, sand can push up through the ground
lateral movement, uneven ground, damages structures
building tilts and sinks as soil stability declines
mapping sediments can help predict or mitigate impact
Mass movements: e.g. landslides & avalanches:
both ground shaking and liquefaction can cause slope failure
landslides occur due to earthquakes, volcanic eruptions, flood events eroding the slope toe or wildfires that make land more vulnerable to landslides in the future
Factors increasing severity:
deforestation and heavy monsoon rains, so that even small tremors can trigger
Landslide degree of devastation factors e.g.:
speed of onset
speed of movement
slope angle
ground saturation levels
volume of material within the slide.
Impacts:
movements of soil and rocks on slopes can also block rivers
these natural dams create temporary lakes, can threaten areas downstream with catastrophic floods were the dams to fail
a landslide on slopes above a reseviour, the displacement of water and the waves generated could weaken and overtop the dam
Tsunamis:
underwater earthquakes can cause the sea bed to rise vertically, displaces the water above, producing powerful waves at the surface which spread out at high velocity from the epicentre
underwater landslides can also displace water and create tsunami waves
submarine or island-based volcanoes can also cause
Factors increasing severity:
local height of a tsunami affected by shape of the coastline & sea bed, depending on coastal zone relief, tsunamis can spread variable distances inland
Impacts:
low height (< 1 m) and very long wavelength (up to 200 km) in ocean but wave height increases greatly as they approach the shore and enter shallow water
before the wave breaks, water in front of the wave is pulled back out to sea (drawdown)
tsunami wave rushes in as a wall of water that can exceed 25m height
underwater landslides when a large volume of rock is shaken and slides downslope, water is dragged in behind it from all sides and collides in the centre, can generate a tsunami wave which radiates outwards - wave may not have enough power to cross oceans but locally damaging
Factors determining the destructiveness of tsunami:
1) Wave energy:
water displacement can form waves with wavelengths > 100km; longer wavelength, greater volume of water, more energy conserved
deep water, no energy lost to frictional drag with the seabed
as it approaches the shore, the water becomes shallower, forcing circular wave motion into an elliptical form, heightens until no longer be maintained and breaks (called shoaling, causes destructive flooding)
2) Shape of coastline:
Indented coastlines with long, narrow bays concentrate energy on the bay head due to a funneling effect as the wave travels up the bay (Anchorage,1964)
Irregular coastlines and offshore islands can set up interference patterns in the waves which, when they coincide perfectly, can accentuate the waveform.
3) Relief of coastline:
cliffs present natural barrier to tsunami, but low-lying land allows seawater to incur for several km, (e.g. Sendai, Japan, 2011)
4) Presence of natural defences:
coral and mangroves act as natural defences, dissipating wave energy as large surface areas
tourist developments removed much mangrove vegetation, exacerbated the impact in Indian Ocean 2004
5) Demography:
old and young most vulnerable due to lack of mobility and strength
2004, 80% of deaths were female; men fishing at sea, whilst women domestic roles & looking after children at home on the coast
6) Lack of experience:
2004 tsunami occurred at holiday time, many foreign visitors - 2,500 tourists died, 50% German and Swedish
globalisation and emigration have led to local communities losing their history of past events
7) Lack of, or inadequate, warning systems and evacuation plans:
Indian Ocean no tsunami warning system, thought region couldn't produce such a powerful earthquake
2005 the Indian Ocean Tsunami Warning System (IOTWS) began to be constructed, cannot be predicted but modelling scenarios a vital preparation tool
Case study – Sichuan 2008:
May 2008, earthquake M7.9, eastern rim of Tibet overrides Sichuan basin, creating Longmen Shan mountain range
compressional forces so great, in places crust rose 9m up a shear face
epicentre area 300x10km here >15,000 secondary landslides, rock falls and debris flows resulted in ~20,000 deaths
earthquake energy concentrated at the top of slopes, maximising their impact, landslides pulled material down whole slope
tertiary flooding occurred, 33 lakes created by landslide dams, causing flooding both upstream initially, then downstream flash flooding when dams breached
earthquake caused >10,000 potential rock falls in deforested areas
lag time may make it difficult to link the rock falls to the original earthquake, may prevent villages from claiming reconstruction funds if cannot be proven to be
Impacts of the Sichuan earthquake:
85,000 deaths, approx. 1/3 from landslides (5,300 children died in collapsed schools not built to building codes)
375,000 injured
4.8 million homeless
$146 billion to rebuild/develop region
15,000 landslides & 10,000 potential rock falls
33 lakes with debris dams >10m deep
Case study - Boxing Day 2004 (Indian Ocean) tsunami:
tsunami followed 1 of 4 largest earthquakes ever recorded (M9.2), Indian plate subducted under Burma plate, causing a 1,600km fault rupture in a nearly north-south orientation, with the majority of slip concentrated in the southernmost 400km of the rupture
primary hazard no deaths or damage but the tsunami was one of most catastrophic disasters recorded, 238,000 deaths in the following 5hrs
greatest release of energy in an east-west direction; Sumatra and Sri Lanka hardest-hit
Banda Aceh 31,000 deaths in first hour
Bangladesh, low-lying coast, benefited from the earthquake proceeded more slowly in the northern rupture zone, reducing energy of water displacement
huge contrasts in destruction locally
one of the few coastal areas to evacuate ahead was on the Indonesian island of Simeulue, as close to epicentre as Banda Aceh
Island folklore told of an earthquake & tsunami in 1907, so the islanders fled to inland hills after the initial shaking and before the tsunami struck; population 78,000, only 7 deaths.
Earthquake hazards primary
Ground shaking:
the vibration of the ground due to the passage of seismic waves
ground is shaken violently both up and down, and side to side (simultaneously) - vertical and horizontal movement of the ground
The severity depends on:
1) magnitude
2) depth of focus
3) duration and intensity
4) distance from the epicentre
5) local geology
earthquakes have one magnitude but intensities different from the epicentre
further from epicentre = less intense
intensity amplified by weak ground e.g. bayed 5x more than bedrock
deep the focus the less intense
Impacts:
infrastructure damage
settlement (lowering of the ground, also called subsidence)
liquefaction
landslides
Ground displacement/faulting and rupture:
caused by movements along faults and rupture of the ground surface
vertical and horizontal movement of the ground
Impacts:
lateral and vertical displacement along faults causing block uplift & subsidence
formation of fault scarps
rupture of power lines & gas pipelines
damage to road, rail & transport networks
displacement of surface drainage
Measuring earthquakes
Types of seismic waves:
Body waves:
can travel through the whole earth
compressional (P Wave), rocks are deformed by change of volume, alternate periods of compression and expansion, the fastest seismic waves (6-8km/s), these are the first waves recorded after an earthquake, Primary
shear (S Wave), rocks are deformed by change of shape, alternate periods of sideways movement, typical speed of 3.5km/s, slower waves so arrive after P waves, Secondary
both P & S waves create high frequency vibrations > 1Hz and shake low buildings
Surface waves:
body waves turn into surface waves if/when they reach the surface
restricted to and guided by surface
travel at Half the speed of P waves
most of the damage near epicentre linked to these whether movement vertical or horizontal
Love Wave, shake the ground at right angles to the direct of movement
Rayleigh Wave, have a rolling motion
Love & Rayleigh waves have lower frequencies and cause high buildings to vibrate
What does a seismogram measure?
the trace of ground motion in a graphical form
amplitude of the waves, measures the amount of shaking
P waves arrive first, then S waves, then surface waves
Mercalli scale:
measures the intensity or damage caused by an earthquake as a result of ground shaking
Qualitative measure:
divided by Guiseppi Mercalli, as a 10 point scale but later modified to 12 points
measures intensity or damage
depends on opinions of observers and eye witness reports (subjective)
I = detected only by sensitive instruments, XII = damage total; waves seen on ground surface lines of sight and level distorted, objects thrown up in air
Intensity maps & isoseismal lines:
intensities are recorded on a map and plotted as lines of equal intensity - isolines or isoseismal lines
isoseismal lines on maps are usually annotated with Roman numerals, linked to the Mercalli scale
Richter scale:
measures the energy released for an earthquake
Quantitative measure:
Charles Richter constructed a diagram to show peak ground motion versus distance from the epicentre to create the first magnitude scale
a logarithmic scale with each whole value of 1 on the scale the amount of energy increases by a factor of 31.7
increase in the amplitude of the wave is increased by 10x for the whole number
if we known the distance from the epicentre & the amplitude then magnitude can be calculated on the nomogram
energy released is in tonnes of TNT equivalent
e.g. Chile (1960), around 9.5, equivalent to almost 123 trillion lb energy release (TNT)
Seismic Moment Magnitude (Mw):
Quantitative measure:
moment magnitude is a total measure of the total energy released by an earthquake
it is more accurate than the Richter scale
for lower strength earthquakes, Richter & Mw are similar, however for larger earthquakes the Richter scale ends to underestimate them (although not always), Mw is more accurate
it is based on the area of the fault that has rusted, it described something physical about the earthquake
it is calculated by:
area of the fault's rupture surface (rupture area) X distance the ground moved during the fault (fault movement, slip) X rigidity
magnitude = 2/3log (seismic moment) = -10.73
What causes earthquakes?
Earthquake definition:
An earthquake is a release of stress (in the form of energy or shock waves) that has been built up in the Earth as a result of tension, compression or shearing (resulting from plate tectonics).
Why do earthquakes occur?
Forces on rocks
Stress - application of a force(s) due to plate tectonics forces
Strain - change in shape & volume of a rock as a result of stress (called deformation)
The greater the stress the greater the strain - and likelihood of failure.
If the forces on a rock exceed the rock's strength then they will fail - leading to fault movement and shock waves (earthquakes).
Elastic rebound theory:
an explanation for how energy is spread during earthquakes.
as rocks on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded.
at that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.
Elaborated:
rock bodies are subjected to tectonic forces acting in
opposite directions and the putting the rocks under a stress, process called deformation
the rocks are slowly deformed and put under strain, they bend and change shape
the energy applied to the rocks is stored as strain energy
the deformation continues until the strain overcomes the rock strength and it fractures and snaps into a new position
two parts of the rock move relative to each other and there is displacement along a fracture or fault
rocks quickly spring back to an undeformed (unstrained) original shape
in the process of rupturing strain energy is released in the form of seismic waves radiating outwards in all directions
Types of stress on plate margins:
compression = pushing together causes upwards and/or downwards movement where they meet
tension = pulling apart, stretching, develops rift valleys
shearing = moving against eachother, grind past, horizontal displacement
Moving plates: relative velocity and direction of plate motions:
strain rate = earthquake potential (EQP)
EQP high at both Islands arcs & Andean type convergent plate boundaries (e.g. Peru-Chile trench)
continent-continent collision zones (Himalayas)
high at conservative plate margins (San Andres Fault)
EQP decreases with distance from boundaries, indicating strain is highest at shallow levels in subduction zones
moderate EQP along MORs & rifts
Deforming rocks: stress & strain in rocks:
deformation of rocks due to stress can lead to a change in shape & volume
the formulation of cracks and fractures is referred to as rock failure
Effects of deformation:
folding and faulting
folded rocks eventually fall leading to formation of a fault, this releases the built up stress as fault movement and shock waves
major fold types in order of increasing compression/strain: syncline, strong anticline, folding, overfold, isocline, recumbent fold, nappe, thrust
Type of fault:
foot wall = the block of rock below the fault line
hanging wall = the block of rock above the fault line
the type of fault formed during deformation is dependent on the stress directions
normal fault (dip-slip fault), where the foot wall moves upwards (apart) - extensional stress
reverse fault (dip-slip fault), where the hanging wall moves upwards (together) - compressional stress
thrust fault - compressional stress
strike slip fault (left-lateral or right-lateral), vertical fractures where the blocks move horizontally - lateral shear stress (wrenching)
Where do earthquakes occur?
earthquakes occur as linear concentrations that define the boundaries between plates (these are the Earth's seismic zones)
they occur at different depths (0-70km shallow) (70-700-km deep)
some occur within plate boundaries / are not related to boundaries
some areas have no earthquakes (aseismic zones - plate interiors)
deepest earthquakes occur on subductive plate boundaries
Earthquake terminology
RUPTURE
- the area along a fault that fails
FAULT SCARP
- a surface feature caused by fault movement
The
EPICENTRE
is the point immediately above the focus on the earths surface
FOCUS
- the location where the stress is released.
FAULT
-a plane/surface of movement caused rock failure of rocks under stress
A series of seismic
SHOCK WAVES
originates from the focus and travels in all directions
Shallow focus:
surface down to about 70 km
shallow quakes occur in cold, brittle rocks resulting from the fracturing of rock due to stress within the crust
very common, many releasing only low levels of energy, although other high-energy shallow quakes are capable of causing severe impacts
Deep focus:
70 to 700 km
deep quakes are poorly understood
with increasing depth, pressure and temperatures increase to very high levels
minerals change type and volume, which may contribute to a release of energy
may be that water has a role in releasing energy but scientists continue to evolve their ideas about these less frequent but often powerful quakes