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On the estimation of earthquake recurrence along
faults and the crucial role of intermediate
magnitudes in seismic hazard assessment
O. Scotti, C. Clement and F. Bonilla Institut de
Radioprotection et Sûreté Nucléaire, Bureau
d'Evaluation du Risque Sismique BP 17 - 92262
FONTENAY-AUX-ROSES FRANCE

• Let us consider, for exemple, the Durance fault
  system in SE France
• Seismological and structural data
• length 60 km
• segments 10 km long
• seismogenic depth 5 to 10 km
• slip motion strike-slip to reverse
• slip rate 0.07 (GPS) to 0.2 mm/yr (geological
  markers)
• one Mgt6 paleo-earthquake
• four M5 historical events (every 100 years )
• few 2ltMlt3 instrumental events in the last 5
  years

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B
P
1000 years
On the difficulty of choosing the appropiate
earthquake recurrence model Let us assume that
activity rates, l, can be deduced from slip
rates using - Maximum magnitude
Mmax 4.33 0.90 log10(S) - Seismic moment
rate - Seismic moment
Mo(m)10(cMd) where v is the slip rate, S is
the surface of the fault, m the rigidity modulus.
We then apply three models available in the
literature to compute the probability density
functions for magnitude (as defined by Stewart
et al., 2001).
truncated exponential (GR), characteristic (YC)
and maximum magnitude (CE) probzility density
fonctions
The seismologiacl and structural data allows to
formulate a variety of hypotheses on the geometry
and slip rates. Here we show the resulting return
period estimates of Mgt5 events for a 10 km long
segment. In spite of the important uncertainty in
the seismological data, the key uncertainty is
the choice of the most adequate earthquake
recurrence model. The GR model, which is rarely
used in practice, leads to the highest return
period estimates in this approach.Which is the
most adequate recurrence model that best
describes Durance-type faults? GR, YC or CE?
On the difficulty of estimating activity rates
which is more reliable, catalogue data or slip
rates? Let us now use instead the historical
record, which suggests M5 events recur every 100
years along a segment of the fault. Assuming such
events can occur anywhere along the fault and
that they represent the maximum magnitude events,
then their return period on a single fault
segment can lower to 500 years. But which is the
physical reason to limit Mmax at M5? Fault
maturity? Strain rate?

• Conclusions Using the Durance fault segment as an
  example we showed
• -that it is the uncertainty in earthquake
  recurrence models that has the
• greatest impact on return period estimates
  compared to seismicl data.
• - that the large contribution to the hazard of
  low probability ground
• motions allows lower magnitude bins to contribute
  down to very low
• probability levels.
• We thus emphasize the crucial role of
  intermediate magnitude
• earthquakes in hazard assessment as well as the
  need to find
• measurable field parameters that may be injected
  in numerical models
• to help discriminate among the different
  seismicity models for faults in
• moderate seismicity regions.

Which seismic hazard level and scenarios for such
faults?
Assuming a 10 km long and 10 km deep vertical
fault segment and a slip rate of 0.07 mm/yr,
seismic hazard is calculated for a site located
at a distance of 10 km, using the activity rates
and recurrence models discussed above.
1E-2
Hazard levels and hazard scenarios vary strongly
due to uncertainty in both recurrence models and
activity rate estimates of intermediate magnitude
events
GR
1E-3
CE
YC
1E-4
Mmax5.0
Annual occurrence
1E-5
1E-6
Attenuation relationship
Abrahamson and Silva (1997)
Reverse faulting 3
s
1E-7
0
50
100
150
200
Peak Ground Acceleration (cm/sec2)
Deagregation of hazard for a 10-5 annual
probability level
Benedicto A.  1996  Modèles tectono-sédimentaire
s de bassins en extension et style structural de
la marge passive du Golfe du Lion (partie nord),
sud-est de la France. PhD thesis, Univ.
Montpellier, 235 pp Ghafiri A. (1995).
Paléosismicité de failles actives en contexte de
sismicité modérée application à l'évaluation de
l'aléa sismique dans le Sud-Est de la France.
Thèse de Doctorat, Univ. Paris XI, 337 pages J.
P. Stewart, S.J. Chiou J., D. Bray R.W., Graves,
P. G. Somerville and N. Abrahamson (2001) Ground
Motion Evaluation Procedures for
Performance-Based Design. PEER Report 2001/09.
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On the estimation of earthquake recurrence along
faults and the key role of intermediate
magnitudes in estimatic seismic hazard O. Scotti,
C. Clement and F. Bonilla
The 4th International Workshop on
Statistical Seismology 9-13 January 2006
Japan
Institut de Radioprotection et Sûreté Nucléaire,
Bureau d'Evaluation du Risque Sismique BP 17 -
92262 FONTENAY-AUX-ROSES FRANCE
Consider, for example, the seismological and
tectonic setting of the Durance fault system in
SE France
Length Depth Slip motion Slip rate (mm/yr) 5 Segments
60 - 90km 5-10 km reverse to strike-slip 0.07 (GPS) to 0.2 (geological) 10 km long (geophys./cartog.)
Seismicity Location
Mgt6 1 9000-25000 yrs old Segment 3
M5 4 in the last 400 yrs Segment 3
2ltMlt3 lt 10 in 5 yrs Mostly on segments 2,3,5
Seismicity models for a given moment rate and
geometry
Choosing the appropriate earthquake recurrence
model GR,YC and/or CE? Input data Geometry of
segment 3 (M6 vertical fault), slip rates (v)
and a regional b-value Given Seismic moment
rate and Seismic moment
Mo(m)10(cMd) (S segment surface m rigidity
modulus isegment 3 Mmagnitude) the activity
rate (l) can be estimated by distributing the
moment rate following the GR, YC and CE
recurrence models. For acceptable depth and slip
rates hypothesis, resulting return period
estimates vary by 50 to 80 a given recurrence
model, Uncertainty in the choice of the
recurrence model, however, reaches 100.
truncated exponential (GR) characteristic (YC)
maximum magnitude (CE)
Which model is more compatible with the
seismicity catalogue?
Return period (years) of Mgt5
Less than 10 events of 2ltMlt3 have occured in 5
yrs along segment 3, 2 and 5 4 M5 historical
events and one M6 paleoearthquake on segment 3
segment 1 and 4 appear to be inactive. Although
the seismicity record may not be sufficiently
long to capture the steady-state behaviour of the
entire fault system, the seismic behaviour of
segment 3 appears more compatible with a GR-type
recurrence model. Assuming the entire fault is
active, should we apply the GR model to the
other segments as well? Is this behaviour typical
of low strain-rate faults? Alternatively, is the
absence of seismicity evidence of their
CE-character?
Are there parameters other than seismicity that
can be considered as indicative of each
recurrence model?
Conclusions Using the Durance fault as an
example we showed that 1. Using catalogues to
estimate return periods of earthquakes in regions
of moderateto-low seismicity is delicate and
subjected to the hazard of the catalogue
window. 2. On the other hand, return periods of
intermediate magnitude events estimated through
slip rates and fault geometries depend strongly
on the choice of the recurrence model. 3. Segment
3 appears to be more GR-like, but what can be
said of the other segments? The GR model is
the least commonly used recurrence model in
seismic hazard assesment studies. Why? How to
discriminate among the different seismicity
models for Durance-type faults? Which are the
key parameters? It is important to provide
answers to these questions because 3.
Intermediate magnitude events contribute to the
hazard down to very low probabilities due to the
strong contribution of high epsilons. 4.
Scenarios resulting from GR-models are very
different than those resulting from YC or CE
recurrence models.
Impact of recurrence models and on seismic hazard
estimations?
Example of hazard curves for segment 3. Hazard is
calculated for a 10 km distant site using the
attenuation relationship of Abrahamson and Silva
(1997), three recurrence models with Max6.5 and
2,09e14 Nm/yr.
Uncertainty in recurrence models, as well as in
the estimation of observed activity rates
combined with ground motion variability lead to
great differences in hazard estimates and in
the scenarii that contribute the most to the
hazard.
Deagregation of hazard for a given target level
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Sismicité instrumentale sur la FMD
Benedicto, 1996 (modified) Décalage de la base du Miocene depuis 20 ma Durance 0.08-0.14 (v)
Hippolyte Dumont, 2000 Décalage verticale de la base du chevauxhement de la nappe de Digne (6ma) Valavoire (NE Sisteron) 0.1 (v)
Cushing et al., 1997 Décalages de cours deau Manosque 0.05-0.22 (h)
Baroux, 2000 Etude géomorphologique, modélisation de la déformation de lanticlinal de Manosque Manosque fold 0.11 (v)
Ghafiri, 1995 Tranchée de Paléosismicité à Valvéranne, séisme entre 25 et 9 ka Manosque 0.07-0.13 (v)
Siame et al. 2001 Datation de terrasse en utilisant les isotopes cosmogéniques Manosque 0,02 0,11 (v)
Nockey,2005 GPS permanent depuis 4 ans St-Michel-lO. - Ginasservis 0,05 (h)
Localisations 3D 1999-2004
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2. Potentiel sismique de la FMD

• 1. Paléosismologie
• 2. Dimension de la faille
• Segmentation
• Géométrie 3D
• Enracinement ?
• 3. Vitesse de la faille

Représentation simplifiée du système de failles
FMD support de modélisation - quantification
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Exemples détudes permettant de quantifier la
vitesse de la faille
2. Potentiel sismique de la FMD
Auteur Marqueur/méthode Lieu Vitesse mm/a

• 1. Paléosismologie
• 2. Dimension de la faille
• Segmentation
• Géométrie 3D
• Enracinement ?
• 3. Vitesse de la faille

Benedicto, 1996 (modified) Décalage de la base du Miocene depuis 20 ma Durance 0.08-0.14 (v)
Hippolyte Dumont, 2000 Décalage verticale de la base du chevauxhement de la nappe de Digne (6ma) Valavoire (NE Sisteron) 0.1 (v)
Cushing et al., 1997 Décalages de cours deau Manosque 0.05-0.22 (h)
Baroux, 2000 Etude géomorphologique, modélisation de la déformation de lanticlinal de Manosque Manosque fold 0.11 (v)
Ghafiri, 1995 Tranchée de Paléosismicité à Valvéranne, séisme entre 25 et 9 ka Manosque 0.07-0.13 (v)
Siame et al. 2001 Datation de terrasse en utilisant les isotopes cosmogéniques Manosque 0,02 0,11 (v)
Nockey,2005 GPS permanent depuis 4 ans St-Michel-lO. - Ginasservis 0,05 (h)
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CONCLUSION
Sismicité régulière modérée M 5 à 5,5 Vitesse
de glissement par différentes approches de
lordre de 1/10e mm/an Faille segmentée (10- 20
km) Magnitude gt 6,5 Conditionnée par la
profondeur dinitiation de la rupture Période de
retour des séismes majeurs de lordre de 10000
ans. ! On ne connaît pas le comportement des
failles lentes. Si toute la faille casse, la
magnitude peut atteindre 7. Période de retour
plus longue
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Let us consider, for example, the Durance fault
system in SE France Seismological and tectonic
setting
Length Seismogenic depth (km) Slip motion Slip rate (mm/yr) Number of Segments
60 km Geologic cross-section 5 km ? Seismicity 5-10 km Geologicalreverse Instrumental s/s GPS 0.07 Geological 0.2 5 geophysical lines 10 km long

Seismicity Location
Mgt6 1 based on paleo Segment 3
M5 4 in last 400 yrs Segment 3
Mlt3 less than 10 in 5 yrs All segments
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