unit 1 pre-exercise lecture on how earthquakes damage structures

This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the Directorate for Education and Human Resources (DUE-1245025). Questions, contact education-AT-unavco.org

IMAGING ACTIVE TECTONICS UNIT 1HOW EARTHQUAKES DAMAGE

STRUCTURES

OUTLINE

• How earthquakes cause damage• How might that damage affect society?• What do we need to know to prepare?

DAMAGE!

In what ways do earthquakes cause damage to structures, and why?

Think on your own for a minute, and then confer with your neighbor for a minute, and come up with a list.

DAMAGE!strain!

shaking• resonance• toppling• swaying

liquefaction• differential movement• sinking

landsliding• differential movement• burial

fault offset• differential movement

secondary effects• fire• flooding

NEWTON’S SECOND LAW AND INERTIA

F = m aforce = mass x acceleration

Inertial Response

Building atRest

Ground Motion

NEWTON’S SECOND LAW AND INERTIA

D’Alembert’s principle: the inertial force on a mass resisting acceleration is of equal magnitude and in the opposite direction.

Inertial Response

Building atRest

Ground Motion

YIELD STRAIN

Inertial forces cause shear strains in a structure and its components. If any element exceeds its yield strain, it will be permanently damaged.

Lateralload

Yield Point

Ductile

Brittle

Deflection

ElasticRange

TOPPLING AND ROTATION

One response of a structure to lateral inertial forces is to rotate like an inverted pendulum. If it is not secured at its base, it may topple.

Free-standing

TOPPLING AND ROTATION

On the other hand, if it is secured at its base, it must absorb the inertial force internally, possibly leading to yielding at its base.

Free-standing Fixed-base

SWAYING AND RESONANCE

Structures have their own natural frequencies, related to their dimensions and stiffness. If they are shaken at those frequencies, they will resonate.

Download video from:

http://www.iris.edu/hq/programs/%20education_and_outreach/videos#P

SITE AMPLIFICATION

Seismic waves traveling from stiff rock to softer material (e.g. from basement to a sedimentary basin) typically experience an increase in shaking amplitude. Why?

SITE AMPLIFICATION

Conservation of kinetic energy requires that amplitude of shaking increase when waves travel into less stiff materials. This can strongly affect the likelihood of building damage.

small x large = large x smallamplitude stiffness amplitude stiffness

LIQUEFACTION

Water saturated

Loose sediments

Sand injected into overlying area upon shaking

LIQUEFACTION

LIQUEFACTION

Above: Christchurch, New Zealand sand boils (2011 earthquake)

Right: Christchurch, New Zealand road affected by liquefaction

BUILDING DAMAGE FROM SHAKING

LANDSLIDES

Above: Wenchuan, China, 2008

Right: El Salvador, 2001

FAULT OFFSET

Above: Horizontal offset from 1906 San Francisco earthquake (Marin County)

Right: Vertical offset from 1999 Taiwan earthquake

TRANSPORTATION CORRIDOR DAMAGE

Above: South Napa, CA earthquake 2014

Right: Northridge, CA earthquake 1994

SECONDARY HAZARDS

SAN FRANCISCO, 1906

The worst damage in San Francisco was caused by the fire following the earthquake, not the earthquake itself. (Why?)

Port of Seattle

Major highway

EARTHQUAKE PROBLEMS!

In what ways might earthquake damage to structures (buildings and infrastructure) affect society?

Think on your own for a minute, and then confer with your neighbor for a minute, and come up with a list.

RISK VS HAZARD

Informally, people use these terms interchangeably, but in the natural disasters field, they have specific, different definitions.

Hazard refers to the probability of a particular disaster–e.g. earthquake hazard = the likelihood of a given size of earthquake occurring. Risk refers to the likely implications of such a disaster, and usually is quantified in dollars–e.g. uninsured earthquake risk in LA is ~ $100 billion.

PREPARING FOR EARTHQUAKE HAZARDS

What information might we want to have in order to characterize and prepare for future earthquake risk and hazard?

Think on your own for a minute, and then confer with your neighbor for a minute, and come up with a list.