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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!
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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.
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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.
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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.
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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
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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
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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.
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Text
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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
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Rubber base isolation foundations.
There are a range of different designs of base isolators
for earthquake resistant foundations.
Text
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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
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Text
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Japanese device – apply computer controlled counter force.
Text
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Text
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Retrofitting.
Text
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Or columns can be cased in
sheet steel, to prevent
crumbled concrete from
being able to collapse.
Text
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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
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