Seismic Hazard Assessment
for a Characteristic Earthquake Scenario:
Integrating Probabilistic and Deterministic Approaches
Antonio Emolo
with Vincenzo Convertito and Aldo Zollo
Prague, March 18, 2005
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Table of Contents
• Brief review of Probabilistic Seismic Hazard Analysis
•
•
technique
Integration of Probabilistic and Deterministic approaches to
seismic hazard
Application to the September 26, 1997, 9:40GMT, Colfiorito
(Central Italy) earthquake, MW=6.0
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PSHA basic steps (Cornell, 1968)
•
•
•
•
Seismogenetic zone
Seismicity recurrence characteristics
Earthquakes effects
Hazard evaluation
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In our proposed approach
we aim at
applying the classical PSHA technique to the single fault
case
by integrating
PSHA with a statistical - deterministic technique for
predicting strong ground motion parameters associated
with a characteristic earthquake occurring on a given
causative fault
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For doing this we need
•
•
•
•
the magnitude distribution
the seismicity rate for a single fault/magnitude earthquake
a (deterministic) tool for evaluating earthquake effects
a statistical description of (deterministic) earthquake
effects
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The Colfiorito earthquake: source parameters
fault length, L
12 km
fault width, W
7.5 km
bottom depth, zmax
8.0 km
strike, Φ
152°
dip, δ
38°
slip, λ
-118°
seismic moment, M0
1.0×1018 Nm
moment magnitude, MW
6.0
rupture velocity, vR
2.7 km/s
After Zollo et al., 1999
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The Colfiorito earthquake: simulation facts
•
•
•
•
number of simulated rupture processes: 150
investigated area: 60×60 km2
number of receivers: 64
spacing between adjacent receivers: 5 km
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The Colfiorito earthquake: simulation results
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The Colfiorito earthquake: simulation results
Simulated PGAs vs. minimum
distance from the surface fault
projection are compared with
the Sabetta and Pugliese
(1987) attenuation curve for a
magnitude 6.0 earthquake
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Coming back to the hazard integral
E (A  A0 )     f R (r ) f M (m ) pa A(m , r )  A0 m , r dm dr
RM
average seismicity
rates:
from simulation study
we do not
both
need!
characteristic
-1
c=0.00204 yrs
and
exponential models
exp=0.155 yrs-1b-value = 0.8475
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Hazard maps
in terms of PGA values having a fixed frequency of
exceedance corresponding to three return periods:
T1=1,000 yrs
T2=5,000 yrs
T3=10,000 yrs
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Hazard maps – T=1,000 yrs
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Hazard maps – T=5,000 yrs
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Hazard maps – T=10,000 yrs
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Hazard curves for selected sites
characteristic earthquake model
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Hazard curves for selected sites
exponential magnitude distribution
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I nearly forgot: are PGAs log-normally distributed?
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In conclusion
• we account for time variable (return period, time of
•
•
•
interest, …) in deterministic scenarios;
we account for source parameters (geometry, radiation
pattern, directivity, …) in PSHA approach;
due to the waveforms availability, we can consider any
ground motion parameter both in time and in frequency
domains
we can easily include site effects in the modeling if specific
transfer function was available
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Before ending…
I would like to thank
the MAGMA center and all the people
who gave me the opportunity to spend
a very useful period at the Charles University
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And finally,
that’s all
Thank you very much for your kindly attention
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Seismogenetic zone
• Each zone has uniform earthquake
•
potential
The configuration could be
point
line
area
volume
ZS9 – Meletti and Valensise, 2004
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Seismicity recurrence characteristics
Recurrence relationship (e.g., Gutenberg and Richter, 1944)
Log N (M )  a  b M
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Earthquakes effects
The ground motion level at a given site
and for a selected range of magnitude
is generally evaluated through empirical
attenuation relationships (e.g., Joyner
and Boore, 1981; Sabetta and Pugliese,
1986; …)
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Hazard evaluation
It consists in the computation of the probability of exceedance
of different levels of selected ground motion parameter A
thorough the evaluation of the hazard integral
E i (A  A0 )  i   f R (r ) f M (m ) pa A(m , r )  A0 m , r dm dr
RM
frequency of exceedance
seismic activity rate
probability
probability
of of
occurrence
exceedance
of aofgiven
a threshold
earthquake
value A0
For a given time of interest t, the probability of exceedance
can be computed as
of a given threshold
(from catalogues)
A0
having
atfor
amagnitude
distance
given distance
ininthe
therange
rrange
and magnitude
(r,
(m,
r+dr)
m+dm)
m
P (A  A0 ;t )  1  e
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E i ( A  A0 ) t
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Hazard evaluation
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Hazard evaluation
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The characteristic earthquake model
is based on the hypothesis that individual fault tend to
generate similar size (i.e., “characteristic”) earthquakes
Characteristic earthquakes occur on a fault not at the
exclusion of all other magnitude events, but with a frequency
distribution which differs from the exponential one
Several paleoseismic evidences in different tectonic
environments support the idea that geometry, mechanism and
average slip per event could be considered constant over a
large time scale
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The magnitude distribution
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The seismicity rate
It can be evaluated both for the exponential model and for the
characteristic earthquake model following the approach
proposed by Youngs and Coppersmith (1985)
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Earthquake effects
Seismic radiation emitted by an extended rupturing fault is
computed by solving the representation integral in high
frequency approximation (Aki and Richards, 1980)


 
FF 
u c ( r ; t )   G c ( r , ; t ,0)  u (; t  Tc )d

The HF Green function is computed in a flat-layered velocity
medium
The slip function is approximated by a ramp
A k-squared final slip distribution on the fault is assumed
(Herrero and Bernard, 1996)
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Statistical description of earthquake effects
In the frame of a scenario simulation associated with a
characteristic earthquake, some “low frequency” source
characteristics can be considered constant over a large time
scale in successive rupture episodes.
However the single rupture process does not repeat the same
style of nucleation, propagation and stopping even if it keeps
the mean characteristics.
With this in mind, we simulated a large number of rupture
processes occurring on the same causative fault considering
different positions of nucleation point and different final slip
distributions.
Synthetic seismograms are computed for each considered
rupture process and ground motion parameters of interest are
then evaluated through a statistical analysis
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Statistical description of earthquake effects
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