Earthquakes Prof. David Alexander Global Risk Forum Davos
The causes of seismicity:-
tectonic activity caused by
stresses in the earth's crust
volcanic activity resulting in stress
caused by the injection of magma into the crust (volcanosiesmicity).
A typical year of earthquakes (1996) Magnitude
Seismology: the study of elastic (seismic) waves The properties of waves:
wavelength and frequency
(number per unit of time)
amplitude (vertical distance
from crest to trough)
crest AMPLITUDE trough WAVELENGTH (per unit time = frequency) PHASE DIFFERENCE
Strong motion of the ground:-
frequencies present in the waves
maximum amplitude of the waves
dispersion if wave energy with
distance from the point of generation
Bracketed duration: the period of time in which seismic shaking exceeds a predefined level. Background seismicity: small tremors that occur constantly during periods of quiescence between major earthquakes.
the margins of tectonic plates
(e.g. subduction zones)
zones of geological faults under stress
areas of volcanic activity
and geothermal fluxes.
Geological faults (extensive planes that divide rock units):-
normal faults (extensional movement)
inverse (compressional movement)
Types of geological fault Overthrust fault Reverse fault Right-lateral normal fault (oblique slip) Normal fault Fault line Hanging wall Footwall Dip Right-lateral fault (strike-slip)
The elastic rebound theory of H.F. Reid:
hypocentre (focus): small area in the
crust where the earthquake begins
epicentre: the point on the surface
directly above the hypocentre
deep focus earthquakes:
hypocentre buried at least 80 km
shallow focus earthquakes:
hypocentre <50 km, often <20 km.
The elastic rebound theory of H.F. Reid
Types of seismic wave:- Body waves: P, primus, compression or pressure waves - longitudinal S, secundus, shear waves - transverse Surface waves: L, Love S, Stoneley R, Rayleigh C, channel R waves travel at 92%of the speed of S waves, which travel at 58%of the speed of P waves, etc.....
The main kinds of seismic wave Primus (pressure) 'P' waves Body waves Secundus (shear) 'S' waves Love 'L' waves Surface waves Rayleigh 'R' waves
LOVE WAVES EPICENTRE RAYLEIGH WAVES FAGLIA SHEAR WAVES PRESSURE WAVES HYPOCENTRE FRONT OF THE WAVES
Instruments for measuring seismic waves:-
a continuous trace
accelerometers only function
when there is strong motion (which triggers them to record) Seismic monitoring instruments, synchronised to different wavelengths, must have an optimum combination of robustness and sensitivity.
Magnitude was originally defined as the amplitude of the largest seismic wave at a nominal 100 km from the epicentre
Richter scale: local magnitude, ML,
no longer used because it is inaccurate at high magnitudes
various scales for S-waves
Moment Magnitude scale, MW,
the world standard.
In earthquakes energy expenditure is proportional to magnitude: the points on the magnitude scale have a logarithmic relationship in terms of energy release - thus: M8= 31.6 x M7
Relationship between magnitude and other variables
World seismic activity over a typical eight-year period Number of earthquakes Magnitude
Location of epicentres and reconstruction of the macroseismic field:-
three seismic stations can
locate an epicentre on the basis of the radial distance computed from the travel times of different kinds of wave
the isoseismsof the macroseismic
field are based on the relationship between the tremors and damage.
5 minutes Time S waves with different frequences Distance 1000 km Epicentre Slowing down of P and S waves with distance from epicentre
stn. 1 stn. 2 epicentre stn. 3 Localisation of epicentre on the basis of the arrival times of seismic waves
Epicentres and faults located for the Bam, Iran, earthquake of 26 Dec. 2003.
Effect of different geological materials in amplifying seismic waves
Extract from a seismic microzonation map showing different shaking potentials
A typical sequence of aftershocks after a major seismic event (Northridge, California, 1994) Number of earthquake per day all others m<5 Days after the earthquake
Colfiorito earthquake swarm, central Italy September - December 1997 Days after earthquake Days after earthquake Associated seismic intensities Days after earthquake
MCS intensities for 23-11.1980 southern Italian earthquake M6.8
NEW ZEALAND Intensities with a probability of 50% of returning within 50 years Return periods of earthquakes with intensities of at least MM=VI
Comparative data on four earthquakes
Consequences of Earthquakes - and significant issues -
Construction failure poses the greatest threat to life in earthquakes
Structural integrity: a building's physical adequacy for its intended purpose Failure: total or partial collapse, or the destruction or non-functionality of a building.
Seismic damage to buildings is a function of:-
seismic energy expenditure:
magnitude duration of strong motion maximum acceleration
distance from epicentre
(soft sediments amplify waves)
effect of construction type
regularity of building form
state of maintenance of building.
In the Tangshan (China) earthquake of July 1976 (magnitude 7.6), of 352 multi-storey buildings:
4 (1%) maintained their
177(50%) collapsed completely
85(24%) collapsed partially
86(25%) were severely damaged.
Knowledge must be gained of how to:-
buttress buildings temporarily
repair buildings permanently
design and construct
Antiseismic design utilizes:-
experience of a building's
hypotheses about a building's
design techniques to combat
weaknesses in buildings induced by earthquakes.
The largest urban seismic risk is in Istanbul and Tehran Major earthquakes represent an enormous urban search-and-rescue (USAR) challenge
The international relief system is hugely expensive and inefficient $1,000,000 per life saved!
in the 20th century 46 lethal
earthquakes caused 128,000 deaths
a damaging quake once in 24-56 months
greatest risk is in the
22,000 historical town centres
70% of the population lives in
municipalities classified seismic; 40% in 2,965 highly seismic ones
35.3% of homes in earthquake zones
are anti-seismically constructed.
DISASTER VICTIM Search and rescue UNINJURED INJURED Medical assistance Public health measures IMPROVEMENT OF CONDITION WORSENING OF CONDITION HEALTHY INFECTED Medical aid HEALTHY IMPROVEMENT OF CONDITION WORSENING OF CONDITION Mortuarial and funeral services DEATH DISEASES INJURIES
Injuries Emergency room injuries Hospital admissions No damage to buildings Damage to buildings Source: Linda Bourque, UCLA
In the Italian Irpinia-Basilicata earthquake of 1980 hospitals collapsed. In the El Salvador earthquake of 1986 hospitals collapsed. In the El Salvador earthquake of 2001 hospitals collapsed. In the Bam, Iran, earthquake of 2003 hospitals collapsed. ....haven't we learnt anything at all?
"At Olive View Medical Center, two buildings collapsed in 1971, and three people died, including two patients on life-support systems that failed when auxiliary generators did not start. The third was an ambulance driver who was crushed by a falling wall. Olive View was an 888-bed hospital then. It had only been open a month when the quake hit. Because of extensive damage, the hospital was rebuilt, with attention to strengthening it against any future quake. But it was much smaller. Now it has a capacity of 377 patients." [Sylmar, California, history file]
Of the 58 fatalities caused by building damage, 50 occurred in hospitals. The worst damage to medical facilities occurred at the Veterans Administration Hospital in Sylmar where two large buildings collapsed. Even though the hospital site was right on the edge of the heavily urbanized San Fernando Valley, it took one hour and 22 minutes before a fire department helicopter happened to spot the collapses and send help. The reason for such a delay? The phones didn’t work, the hospital’s radio was in one of the collapsed buildings, and the first message orally delivered by a hospital staff member to a nearby government facility was confused with an already received report of damage to a different nearby hospital. [Reitherman 2004]
Seismic hazards to hospitals:-
loss of structural integrity
and building functionality
landsliding and subsidence
loss of trained personnel
loss of medical and surgical capacity
loss of supplies (including utilities)
and failure of supply chains
Hospitals Seismic design level Fully operational Operational Life safe Near collapse Collapse Unaccaptable performance Unaccaptable performance Rare 60%g Unaccaptable performance Very rare 80%g
surgical and curative (fixed or
diminished in the short term).
ensuring the structural integrity
of the medical facility
ensuring continuity of
services and medical supplies
ability to cope with very
large surges in demand
protecting personnel and
their immediate families
protecting lifelines that enable the
injured to access medical care.
seismic integrity surveys
(structural and non-structural)
emergency planning (...testing,
revising, diffusing plans...)
estimate medical, surgical,
pharmaceutical and logistical needs
earthquake casualty estimation.
Internal emergency plan Site of the disaster Other hospitals and clinics in the area Health facility network emergency plan External emergency plan The three foci of medical emergency planning
Disaster planning for the medical centre Disaster planning for the system of medical centres Disaster planning for the external environment Disaster in the medical centre Disaster in the system of medical centres Co-ordinated EMS Disaster plans Disaster in the external environment
Helicopter links Ambulance routes Emergency bus transportation Telecommunications links T1 T2 Primary triage point Secondary triage points Pulmonary specialists Hospital I Hospital II T2 Staging area T2 T1 Burns unit Disaster T2 Secondary treatment centre Incident command post Incident commander Mortuary Emergency operations commander Emergency operations centre Next-of-kin Coroner
IMPACT ON THE COMMUNITY RISK OF EARTHQUAKE Characteristics of the event: magnitude, duration, location of epicentre, level ofground acceleration Buildings, drinking water, sewerage, public transport, public buildings, hospitals, fire stations, etc. Local geological characteristics Population distribution and density Aftershocks Socio-economic characteristics RISK OF EARTHQUAKE Landslides Previous experience of risk Floods Casualties, economic damage; type, costs times of reconstruction Fires Level of community preparedness