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C471 GEOHAZARDS
Seismic hazard:quake hazards & forecasting
C471 GEOHAZARDS
Structure of talk The range of seismic hazards
primary (e.g.ground shaking, surface rupture, landslides)
secondary (e.g. fire-following, toxic contamination)
Forecasting earthquakes definitions types of forecasting probabilistic forecasting
Earthquake prediction setting the scene
C471 GEOHAZARDS
Hazardous phenomena generated by earthquakes
Primary ground shaking ground lurching &
displacement ground settlement liquefaction landslides,
mudslides and avalanches
tsunami & seiches
Secondary structural
collapse fire-following falling material floods from dam
bursts & levée failures
C471 GEOHAZARDS
Ground shaking A number of aspects of
ground shaking affect severity of damage
peak ground acceleration
average ground acceleration
duration
Duration is critical 100 s quake would be
far more destructive than 30 s quake with similar shaking levels
Kobe (1995) - strong motion just 11 s long
lengthened to 100 s in areas of soft soilsKobe 1995
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More on duration of strong motion
Quake M affects duration much more than it affects max acceleration
Larger the M, the longer the fault rupture, and the larger the area from which seismic waves generated
Duration of shaking increases with distance from fault but intensity is less
Due to dispersion effects of seismic waves
Higher frequencies are attenuated more than lower ones
F
Intensity of shaking
Duration of shaking
Increasing distancefrom fault
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Duration and peak acceleration of strong motion
Magnitude Rupture length (km) Peak acceleration (%g) Duration (s) 5.0 9 2 5.5 5 – 10 15 6
6.0 10 – 15 22 12 6.5 15 – 30 29 18 7.0 30 – 60 37 24 7.5 60 – 100 45 30 8.0 100 – 200 50 34 8.5 200 – 400 50 37
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Local influences on ground shaking
Physical properties of soil and rock
competent bedrock may transmit peak accelerations > 2g
sands can transmit up to ~ 0.6g
gravels much higher clays only capable of
transmitting up to 0.15g
Geological structure focus waves
No simple correlation between M, acceleration,and distance from quake source
C471 GEOHAZARDS
More on ground conditions
Subsoil Average change in intensity Rock (e.g. granite, gneiss, basalt) - 1 Firm sediments 0 Loose sediments (e.g. sand, alluvium) + 1 We sediments, artificially filledground + 1.5
Ground conditions can also affectseismic intensity
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The clay problem (Mexico City 1985)
M s 7.9 quake
370 km away in Central American Trench
Little damage over intervening distance
High frequency ground motions attenuated out
Mexico City struck by low frequency ground motions close to natural vibration frequencies of underlying saturated lake-bed clays
Clays amplified motions up to 50 times compared to adjacent solid rock
Clay up to 40 m thick Damage correlated with
thicknessMexico City 1985
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Ground lurching & displacement
May be major problem close to source fault
Displacement may be vertical, horizontal or oblique
Movements may be very large
San Francisco 1906 in places horizontal
displacement of 6.5 m
Can be severely damaging for buildings close to faults
Particularly damaging for roads, railway lines, canals and pipelines that cross fault
San Francisco 1906
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Differential ground settlement
May involve uplift or subsidence
Often fault related Alaska 1964
shorelines uplifted by ~10m in places
subsided by ~2m in others
>250,000 sq km affected
May result in inundation by sea
Alaska; Izmit (1999)
Damage to ‘lifelines’
Izmit 1999
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Liquefaction
Sands and silts undergo temporary loss
of strength behave as viscous fluid
Seismic waves cause void collapse resulting in densification
Drainage of pore water cannot be achieved rapidly enough resulting in excessive pore pressures
End point is development of a QUICK condition
material behaves as heavy liquid with virtually no shear strength
Voidcollapse
Pore pressureincreasesresult
Originalsedimentstructure
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More about liquefaction When pore water pressures
exceed normal stress imposed by weight of sediment column the material behaves like a fluid
Water moves upwards from void spaces to surface forming sand boils
Particularly at risk loosely packed sands
and silts used for land fill & reclamation
Results of liquefaction foundering of buildings damage to utilities mass movements
Sand boils result fromexpelled water
C471 GEOHAZARDS
Examples of liquefaction
Niigata (Japan 1964) wholesale foundering
of apartment blocks
Kobe (Japan 1995) severe damage to
port facilities built on reclaimed land around Osaka Bay
Loma Prieta (Calif. 1989) liquefaction of sub-
surface wet sand layers in fill over 100y old caused serious damage in Marina District of SF
Niigata (Japan 1964)
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Maximum possible distances for liquefaction
M s 8.5
20 km
125 km
650 km
1500 km
M s 8
M s 7
M s 6
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Seismogenic mass movements Strong ground motion
commonly results in gravitational slides
Lituya Bay (Alaska 1958) large landslide triggered
60m surge with run-up of >500m
Liquefaction of material on a slope will trigger rapid flow failure
killed 200,000 in Kansu (China 1920) quake
Lateral spreads can be locally v.
damaging Alaska 1964; damaged
200 bridges
Lateral spreadSan Francisco 1906
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Lituya Bay (Alaska 1958)
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Other major seismogenic landslides
Huascaran (Peru 1970)
Sherman glacier(Alaska 1964)
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Seiches & tsunami
Seiches: oscillations set up in enclosed bodies of water by distant quakes
Observed in English lakes in 1950 due to Assam (India) quake
May cause damage to retaining walls of reservoirs and flooding
Tsunami: common feature of
submarine earthquakes; enhanced by submarine landslides
also from sub-aerial seismogenic landslides
Hilo (Hawaii) 1946
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Some notable seismogenic tsunami
Year Origin Run-up height (m) Damage to Comments 1755 Eastern Atlantic 5 - 10 Lisbon (Portugal) Reached Caribbean 1868 Peru & Chile > 18 South America
Hawaii Detected in New Zealand
1896 Honshu (Japan) 24 Sanriku coast (Japan) 26,000 killed 1908 Messina (Italy) 5 Messina & Reggio
Calabria >8,000 killed
1946 Aleutian Islands (Alaska)
17 Hawaii >150 killed
1964 Alaska 8.5 Crescent City (California)
>100 killed
1993 Hokkaido (Japan) 20 Okushiri Island >200 killed 1994 Java (Indonesia) 11 Malang >200 killed 1998 Offshore PNG >10 Aitape ~2,500 killed
submarine landslide
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Destructive capacity of tsunami High velocities
700 - 800 kph in deep water
impact velocities 50 kph or more
Long wavelengths 150 - 250 km
Wave period tens of minutes
Run-up heights in excess of 20m
Ocean-basin extent possible
Multiple waves in a tsunami ‘wave train’
Aitape (PNG) 1998
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Tsunamigenic earthquakes
<10% of submarine quakes generate tsunami
Magnitude 6.5 or greater Focal depth < 50km Most destructive have
depths of < 25 km Vertical uplift of large
area of sea bed Link between average
run-up and quake size May be strongly focused Submarine landslides
enhances tsunami potential
Submarine slide simulation(PNG 1998)
C471 GEOHAZARDS
Tsunami magnitude scale (Japan)
Richter Magnitude Tsunami Magnitude Maximum run-up (m) 6 - 2 < 0.3 6.5 - 1 0.5 – 0.75 7 0 1 – 1.5 7.5 1 2 – 3 8 2 4 – 6 8.25 3 8 – 12 8.5 4 16 – 24 8.75 5 > 32
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Structural collapse
‘Buildings not earthquakes kill people’
Structural collapse depends upon
how well buildings constructed
how well maintained magnitude, duration &
acceleration of strong ground motion
the ground response (type & character of underlying soil & rock)
distance from epicentre
Pancake collapse of concretebuilding (Gujarat 2001)
C471 GEOHAZARDS
More on structural collapse Structural collapse primarily
due to inflexible response of buildings
low tensile and shear strength
high rigidity low ductility low capacity for
redistributing loads
Particularly poor adobe (mud bricks) and unreinforced masonry
Common problem: not ‘tying’ walls and floors together
Lack of enforcement of codes
Collapse of rock and cementbuildings (Gujarat 2001)
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Solutions to structural collapse
Design buildings to dissipate vibrational energy by inelastic (plastic) response at key locations (beam-column joints)
Aim is to have buildings that are strong and flexible; and that yield but don’t collapse
Ensure and enforce appropriate building codes
Continuing problem in developing countries
C471 GEOHAZARDS
Earthquake loss-susceptibility by construction type
VI VII VIII IX X Adobe 8 22 50 100 100 Unreinforced masonry (NSD) 3.5 14 40 80 100
Reinforced concrete frames (NSD) 2.5 11 33 70 100 Reinforced masonry (NSD) 1.5 5.5 16 38 66 Steel frames (seismic design) 0.4 2 7 20 40
Reinforced masonry (high-quality seismic design) 0.3 1.5 5 13 25
NSD = Non-seismic design
Modified Mercalli Intensity
Average damage (%)
C471 GEOHAZARDS
Fire-following quake
Major problem in 20th century quakes
San Francisco 1906 Tokyo 1923
More recently Kobe 1995 Factors contributing to fires
wooden buildings narrow streets inadequate water provision fractured gas mains &
power lines car fires discarded cigarettes overturned stoves and
water heaters chemical and petroleum
leaks and spills strong winds
San Francisco 1906
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Fire following Japanese quakes
Great Kanto quake (Tokyo-Yokohama) 1923
killed at least 140,000 tens of thousands burnt
to death
Great Hanshin quake (Kobe) 1995
fires started in old, cramped parts of city
many wooden buildings 146 fires started 23,000 homes destroyed
Tokyo today ~ 1 million wooden
homes
Kobe
Tokyo
C471 GEOHAZARDS
Floods & dam bursts
New Madrid area liquefaction during
1811-12 quakes caused banks of Mississippi to fail
Levee failure could be a problem in next quake
Lower San Fernando dam (California)
failed during quake in 1971
12 s of strong shaking peak acceleration of ~
0.5g upstream section of
dam collapsed but held - just!
Lower San FernandoDam (California) 1971
C471 GEOHAZARDS
Forecasting earthquakes Forecasting is not prediction
less precise based upon analysis of earthquake return periods rather than
identification of pre-cursor y signs
Active faults or fault segments do not rupture in a random manner
they have characteristic return periods (or at least return period envelopes)
these reflect strain accumulation along the fault and the capacity of the fault to resist strain up to a given characteristic point - for that fault or fault segment
There are complications: Rupture will not occur according to a rigid timetable - there is a
return period envelope rather than specific date Strain may be released by one large quake or a number of smaller
ones (e.g. Marmara Sea south of Istanbul) this has implications for risk assessment
C471 GEOHAZARDS
San Andreas example
Prior to 1906 M 8.25 San Francisco quake ~ 3.2m displacement across fault in 50 years
Post-quake rebound on the fault was ~ 6.5m
Amount of time for strain released in quake to accumulate
(6.5/3.2) x 50 ~100 y
Return period until next comparable quake = 100y
Assumes uniform strain
accumulation quake did not alter fault properties
C471 GEOHAZARDS
Problems with forecasting
Forecasts only as good as the available catalogues
Historical catalogues good for well studied regions such as California, Japan, Europe, China
Poor for regions of low frequency-high magnitude seismicity
Cascadia subduction zone New Madrid Jamaica Western Europe
Catalogues need to go back further; requires geological studies Cascadia subd. zone
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The Seismic Gap concept
Defined as an area in an earthquake-prone region where there has been a below average level of seismic energy release
The 1989 Loma Prieta quake filled a gap that had been aseismic since 1906
Other gaps exist in Aleutian arc (Alaska) south of Istanbul Tokyo southern California
Istanbul seismic gap
C471 GEOHAZARDS
Seismic intensity forecasting
Other parameters can be usefully forecast than just timing of a quake
Forecasting seismic intensity at a particular site is vital for:
siting structures such as dams, schools, hospitals & emergency centres
constructing seismic hazard maps
Requires detailed information on geology, ground conditions
Seismic intensity forecast map - Tokai (Japan)
C471 GEOHAZARDS
Probabilistic forecasting
Most useful way of expressing a forecast of a future quake is in terms of probabilities
Most people are familiar with probabilities as a result of gambling
Example from San Francisco area (Bolt, 1999) 5 quakes > M = 6.75 in 155 y between 1836 & 1991 if events are random, another quake of 6.75 can be
expected in 155/5 y = 31 y with high probability
Problem: quakes not entirely random. On a particular fault system may be clustered (due to stress transfer) or follow certain trends
Alternative method of probabilistic forecasting is based on the ELASTIC-REBOUND model
Based upon estimates of strain accumulation across fault
C471 GEOHAZARDS
Strain measurement and forecasting
Geological mapping undertaken to define active fault segments
Assumption made that a discrete segment will rupture in one go
As Seismic moment links magnitude with rupture length this gives measure of maximum expected earthquake
Relationship between M s and fault rupture length L: M s = 6.10 + 0.70 log L
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Calculating probabilities
Next: determine slip history of each segment
Calculate strain accumulation rate for each segment
Slip history for fault segment can then be plotted against time
As slip is related to quake magnitude allows recurrence intervals between quakes greater than a given magnitude to be determined
Amountof slip
TimeMagnitude6
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The quake probability histogram
Quakefrequency
Recurrencetime
T1 T2
Construct histogram showing No. of quakes that occur with each specified recurrence time
Most probable recurrence interval is that which divides histogram into two equal areas
If time since last quake in the magnitude range is T1, the probability of the next quake occurring in T1 - T2 years = ratio of red area to yellow area
As recurrence time T2 increases ratio approaches 1 and a quake becomes virtually certain
The more consistent the recurrence time the better the forecast
T1 T2
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The quake probability histogram & the San Andreas
Suited to California & San Andreas fault system because active faults exposed at surface
Enables displacements to be measured easily and strain to be monitored
Method crucially depends on constraining well the number of potentially destructive quakes in historic time and their ages
For more discussion of problems see Bolt (1999) p228 - 229)
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Predicting earthquakes A highly controversial issue in seismology Involves giving a precise warning about the
timing and size of a future quake Reliant upon the occurrence of pre-cursory signs
in advance of a quake Method must be shown to be repeatable in order
to be of any use In a zone of high seismicity, any prediction is
going to have greater than chance than zero of being right
On the other hand - a prediction that is not fulfilled ensures that the method is invalid
C471 GEOHAZARDS
Proposed earthquake precursors
Changes in seismic velocities
Crustal deformation Groundwater changes Gas release Atmospheric effects Anomalous animal
behaviour Changes in magnetic
and electrical properties of the rocks
the so-called VAN method