Download DECivil - Departamento de Engenharia Civil, Arquitectura e

Module: Seismicity and seismic risk
DECivil
(for civil engineers)
Mário Lopes
([email protected])
Departamento de Engenharia Civil,
Arquitectura e Geo-Recursos do Instituto
Superior Técnico, Lisboa
Bolonha, 3-6 March 2014
INDEX of CONTENTS
1 – The origin of earthquakes
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 Plate tectonics
 The Earth inner structure
 Focus and epicenter
 Seismic waves
 Scales of Magnitude and Intensity
 Seismicity
2 – Natural effects, consequences
and mitigation
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2.1 - Seismic effects on the Earth
surface
 Fault movements
 Soil vibrations
 Soil effects – landslides, liquefaction,
subsidence
 Tsunamis
 Fires and Contaminations
2.2 – Global impact of earthquakes
 Types of earthquake effects: direct and
indirect effects
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 Earthquakes of the past
 Comparison with impact of other natural
and technological catastrophes
 The future – earthquake simulators.
 Risk = hazard x exposition (people and
economic assets exposed to seismic
damage) x vulnerability
2.3 – Mitigation actions
 Fault movements
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 Soil vibrations
 Soil effects – landslides, liquefaction,
subsidence
 Tsunamis
 Fires and Contaminations
 Civil Protection
3 – Characterization of seismic
actions
 Seismicity
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 Probabilistic definition of the seismic
action
 Definition of the seismic action for
structural applications
 Annual probability of ocurrence. Return
period
 Quantifications of other seismic effects
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Map of epicenters
Earth´s plates and the epicenters of some of the 30 000
earthquakes recorded over a period of six years
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Press and Siever, 1994
Continental drift theory
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Latha, 2001
Origin of earthquakes
In the lithosphere (10 km to 100 km), the crust and
external part of the mantle
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Internal Earth structure
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(Press and Siever, 1994)
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Latha, 2001
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Latha, 2001
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Latha, 2001
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Main faults in Italy
(Buratti, 2013)
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Focus and epicenter
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(Lai, 2013)
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Rays of light reflecting when
crossing borders between air
and water. Seismic waves
behave in a similar manner
when crossing boundaries
between different materials
Press and Siever,1994
Seismic waves radiate from the focus in
many directions
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P waves – vibration of the soil in the direction
of propagation
S waves – vibration of the soil perpendicular
to the direction of propagation
Dziewonski and Anderson, Phys.
Earth Planet Intern., Vol 25, 1981
Velocity of P and S waves
Surface waves: Rayleigh and Love
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Surface waves are confined to the vicinity of the surface,
therefore their energy spreads less than body waves (P
and S). Therefore they tend to produce the highest
accelerations at larger distances from the epicenter.
Surface waves: Rayleigh and Love
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(Lai, 2013)
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Determination of the epicenter
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Seismic scales
Magnitude – measure of the energy released at the
origin of the earthquake
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- It is determined from instrumental recordings
- Depends only on characteristics of the earthquake
(size of the rupture area and displacement between
sides of the fault)
Intensity – measures the way an earthquake is felt at
a given location
- Determined by the damage produced and human
perception
- It depends on characteristics of the earthquake
(energy released at the origin, epicentral distance,
geology and site effects) and of the built environment
(vulnerability, type and amount of buildings)
Scales of Magnitude
Richter magnitude:
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it is a logaritmic scale. Each
degree corresponds to a variation of 1/32 in the energy
released at the origin of the earthquake. It is associated
to the displacement amplitude on a Wood-Anderson
seismograph at a certain distance of the epicenter. It
tends to be less reliable at the higher magnitudes.
Moment magnitude:
it is calculated from the
seismic moment 𝑀0 as 𝑀𝑤 = 2 3 𝑙𝑜𝑔10 𝑀0 -6,07. The
seismic moment 𝑀0 is calculated as 𝑀0 = 𝜇𝐴 𝐷, in which
 is the shear stiffness of the crust, 𝐴 is the rupture area
and 𝐷 is the relative displacement between faces of the
fault.
Scales of Intensity
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Modified Mercalli Scale: qualitative description of
damage and human perception. Degrees I to XII
Mercalli Intensity
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Description
I
Felt by very few people; barely noticeable.
II
Felt by a few people, especially on upper floors.
III
Noticeable indoors, especially on upperfloors, but may not be recognized as an earthquake.
IV
Felt by many indoors, few outdoors. May feel like heavy truck passing by.
V
Felt by almost everyone, some people awakened. Small objects moved. trees and poles may shake.
VI
Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may
be slightly damaged.
VII
Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly built
structures. Some walls may fall.
VIII
Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage
to poorly built structures. Some walls collapse.
IX
Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked
noticeably. Wholesale destruction. Landslides
X
Most masonry and frame structures and their foundations destroyed. Ground badly cracked.
Landslides. Wholesale destruction.
XI
Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves
seen on ground.
XII
Total damage. Waves seen on ground. Objects thrown up into air.
European Macroseismic Scale
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There are 12 degrees. Up to degree IX the intensity is
defined as a function of:
a) How the earthquake is felt by people
b) Effects on soil and objects
c) Damage on constructions
Degrees X, XI and XII are characterized only as a
function of damage on the constructions.
In order to charaterized damage on buildings, these are
divided in vulnerability classes, identified by the letters A
to F, as a function of the material and characteristics of
seismic resistance. For masonry and reinforced concrete
buildings damage is defined as a function of the degree
of destruction, according to a graphic definition and
description.
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Isoseismals maps
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Benavente, Portugal, 1909
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Faial, Azores, 1998
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(Lai, 2013)
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Duration
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The duration of an earthquake is the time during which the energy is
released. It influences the duration of seismic motions. The ones of large
duration have larger number of cycles, what may influence the damage
caused by the earthquake. The repetition of cycles may affect the
accumulation of interstitial pressure in saturated loose sand deposits
(liquefaction) and the degradation of the stiffness of structural elements.
This effect is variable, being larger on the elements under large shear
forces than in elements whose nonlinear behaviour is predominantly due
to flexural deformations.
Response spectrum
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