Download A reversed hierarchy of active normal faults: the 6 April 2009, Mw 6.3

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This discussion paper is/has been under review for the journal Solid Earth (SE).
Please refer to the corresponding final paper in SE if available.
Discussion Paper
Solid Earth Discuss., 5, 117–134, 2013
www.solid-earth-discuss.net/5/117/2013/
doi:10.5194/sed-5-117-2013
© Author(s) 2013. CC Attribution 3.0 License.
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Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Pavia, Italy
Dipartimento della Protezione Civile, Rome, Italy
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Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
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Received: 9 January 2013 – Accepted: 10 January 2013 – Published: 17 January 2013
Correspondence to: L. Bonini ([email protected])
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Published by Copernicus Publications on behalf of the European Geosciences Union.
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A number of recent damaging earthquakes worldwide were generated by previously
unmapped, blind or hidden faults (e.g. January 2010, Haiti, Mw 7.0; September 2010–
February 2011, Darfield-Christchurch, New Zealand, Mw 7.1-6.3; October 2011, Van,
eastern Turkey, Mw 7.1). The 6 April 2009 L’Aquila earthquake (Mw 6.3) belongs to this
category as it occurred on a previously unidentified seismogenic source. As most of
the studies for investigating the earthquake potential of large continental faults are
based on strictly surficial data and investigations (e.g. Wells and Coppersmith, 1994;
Wesnousky, 2008), understanding how seismogenic slip at depth relates to surface
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1 Introduction
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Understanding the relationship between seismogenic slip at depth and surface deformation is fundamental in any seismic hazard analysis because the assessment of the
earthquake potential of large continental faults relies largely on field investigations. The
well-documented 6 April 2009, Mw 6.3, L’Aquila earthquake affords a unique opportunity to explore the relationships between the activity of the deep source and its surface
evidence.
We used available high-resolution geologic, geodetic and seismological data aided
by analogue modeling to reconstruct the geometry of the seismogenic rupture in relation with surface and sub-surface faults.
We contend that the earthquake was caused by a blind fault, controlled at depth
by pre-existing discontinuities and expressed at the surface by pseudo-primary breaks
resulting from coseismic crustal bending.
Finally, we propose a scheme for hierarchizing normal faults that explains all surface
occurrences related to blind faulting in the frame of a single, mechanically coherent,
interpretative model. Failure to appreciate such complexity may result in severe over–
or under-estimation of the local seismogenic potential.
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L. Bonini , D. Di Bucci , G. Toscani , S. Seno , and G. Valensise
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A reversed hierarchy of active normal
faults: the 6 April 2009, Mw 6.3, L’Aquila
earthquake (Italy)
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The complex structure of the Apennines results from subsequent tectonic phases. At
the onset of the Mesozoic the region was part of the African passive margin of the
Tethys Ocean; it hosted large carbonate platforms and intervening pelagic basins, subsequently broken up by Triassic-Lower Jurassic extension (Calamita et al., 2011). Since
the Cretaceous the region has evolved within the framework of the convergent motion
between the African and European plates; east to northeast-verging thrusts and associated foredeep/thrust-top basins progressed toward the Adriatic foreland up to the
Mid-Pleistocene (Patacca and Scandone, 1989) and were subsequently dissected by
strike-slip and normal faulting. Following a major geodynamic change at ∼ 800 ka, SWNE extension has become the dominant tectonic style over the core of the Apennines
(Hyppolite et al., 1994; Galadini, 1999), as demonstrated also by breakout and seismicity data (Montone et al., 2012).
The L’Aquila earthquake was generated by nearly a meter of slip over a planar,
∼ 16 km-long, 45–50◦ SW-dipping normal fault (for a summary see DISS Working
Group, 2010; Vannoli et al., 2012). The 6 April mainshock was the culmination of
a long foreshock/aftershock sequence recorded by permanent and temporary INGV
seismometers (Chiaraluce et al., 2011). Due to the high quality of available data, the
whole sequence has been the object of several investigations, resulting in over 100
papers published to date. Instrumental data, including high-resolution aftershock locations, GPS observations and DInSAR measurements based on Envisat and COSMOSkyMed data, revealed coseismic slip between 9–10 to 2–3 km depth, resulting in bowlshaped, gently-asymmetric surface subsidence up to 15–20 cm (Atzori et al., 2009;
D’Agostino et al., 2012; Fig. 1). The mainshock caused faint, discontinuous surface
fractures along a structural feature referred to as Paganica fault, with a maximum net
throw of ∼ 5 cm (Emergeo Working Group, 2010). Conversely, no genuine slip was reported along a number of steep, clearly exposed, well known, presumably active faults
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2 Tectonic and seismological setting
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deformation is a fundamental pre-requisite in any seismic hazard analysis. Although the
L’Aquila region had long been known for its high seismicity level, the 2009 earthquake
challenged the standard approach for active fault identification because (1) almost all
studies carried out prior to it inexplicably failed to identify its causative source, and (2)
presumed active faults showed partial or no reactivation.
The 2009 L’Aquila earthquake struck a seismically very active portion of the Apennines chain, a region undergoing NE-SW extension at 2–3 mm yr−1 (D’Agostino et al.,
2011). Due to the combined effect of tectonic stress and gravity, extended terrains often
exhibit a remarkable complexity that makes the correct hierarchization of active faults
– or even their mere identification – extremely challenging. In the Abruzzi Apennines
(central Italy) these circumstances are worsened by the superposition of different tectonic regimes and by selective exhumation caused by vigorous regional-scale uplift at
1–2 mm yr−1 (D’Anastasio et al., 2006). As a result, the region is criss-crossed by many
5–10 km-long normal faults mapped as active features, although their limited spacing
(∼ 5 km) is enough to suggest that they cannot all reach seismogenic depth (Vannoli et
al., 2012).
Being the best documented normal faulting event worldwide, the 2009 earthquake
makes a unique case for exploring the relationships between the activity of the deep
source and its surface evidence. We used a wealth of high-resolution geologic, geodetic
and seismological data combined with analogue modeling to reconstruct the geometry
of the seismogenic rupture in relation with sub-surface and surface faults. We aimed at
devising a scheme for normal fault hierarchization that explains all surface outcomes
of seismogenic faulting in the frame of a single, mechanically coherent interpretative
model. Proper appreciation of such complexity forms the basis for a correct assessment
of the local earthquake potential.
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If the causative source of the 2009 earthquake is a blind fault, what is the origin of the
associated breaks (i.e. the Paganica Fault), and why is the observed surface throw so
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4 Blind and surface faulting: an alternative model
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Therefore, although most investigators agree on the primary character of the Paganica fault (to the point that the seismogenic source is called the same name), a direct
connection of this surface feature with the deep fault is neither warranted by the pattern
of surface strains nor by the aftershock locations, which clearly depict fault complexity at 2–3 km depth. Such conclusions are further supported by the cut-off depths of
the aftershock pattern, a rather distinctive feature of the sequence (Chiarabba et al.,
2009). The lower cut-off (9–10 km) has been interpreted either (a) as the bottom level
of the seismogenic volume (Bianchi et al., 2010) or (b) as due to the presence of a
non-optimally oriented inherited structure (Chiaraluce et al., 2011). Two competing hypotheses exist also for the upper cut-off (2–3 km): (a) the causative fault is “young” or
“poorly-developed” and as such it has not yet reached the surface, or (b) pre-existing
discontinuities control upward fault propagation. The alignment of several aftershocks
along an interpreted sub-horizontal thrust plane running at about 3 km depth above
the central portion of the seismogenic source (Chiaraluce et al., 2011) supports the
latter hypothesis (Fig. 1c). It has long been known that in any fault zone the fracture
propagation and arrest and the associated seismicity are largely controlled by the local stress field (e.g. Scholz, 2002). When a propagating fracture meets a mechanical
discontinuity, such as a weak layer or a pre-existing fault, it may stop, penetrate it, or
be deflected along it (e.g. Dyer, 1988). The structural complexity of the region hit by
the 2009 earthquake guarantees that all these options are equally likely. In their turn,
the scattered aftershocks located above the upper seismicity cut-off near the northern
end of the fault (Fig. 1c) suggest that the mainshock triggered pre-existing complex discontinuities, although with limited displacement; a hypothesis supported by the minor
post-seismic surface strain patch located above this shallow blind structure.
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The strain pattern revealed by DInSAR analyses matches well the architecture of the
intramountain basin (Improta et al., 2012) overlying the main coseismic slip patch, suggesting that the long-term tectonic strain in this area is the result of sustained slip
over the fault plane that ruptured on 6 April. It also shows the largest subsidence at
a relatively far distance from the Paganica fault (Fig. 1), consistent with limited or no
coseismic slip in the uppermost 3 km of the crust. DInSAR data also revealed sizable
postseismic strains in the region above the tip of the master fault (D’Agostino et al.,
2012; Fig. 1), in agreement with direct evidence from TLS observations (Wilkinson et
al., 2010), but limited or no postseismic slip along deeper portions of the fault. This
further suggests an effective decoupling between deep and surface deformation.
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3 Seismotectonic model
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lying in the epicentral area: e.g. Pizzoli, Pettino, Stabiata and Bazzano faults (Emergeo
Working Group, 2010).
The large density of faults, coupled with a limited historical earthquake record, is
perhaps at the basis of a sort of “L’Aquila paradox” (Valensise, 2009). In a study based
on a mixed geological-statistical approach (Akinci et al., 2009) the area of the future
2009 earthquake was given a very high probability of experiencing strong ground shaking in the following 50 yr. This conclusion was supported by the many seismogenic
sources included in the input model; ironically though, the corresponding surface primary active faults did not include the Paganica fault because of its weak geomorphic
expression. For the same reason the Paganica fault received little attention in studies
carried out prior to 2009, some of which referred to it as “uncertain or buried” (Geological Map of Italy, 2009) while others did not map it at all (Galadini and Galli, 2000).
What is the basis for this “reversed hierarchy”, such that the main seismogenic fault
lies hidden beneath a blanket of clearly visible yet substantially harmless faults? What
are its potential implications for seismic hazard assessment? Can we derive from this
earthquake a general rule to be used in other normal faulting environments?
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Our analysis of seismogenic faulting in the L’Aquila area revealed an unprecedented
complexity in the interaction between coseismic slip and pre-existing structural features. In particular, it suggested that the main surface coseismic rupture, the Paganica
fault, is likely a result of surface bending rather than the direct prolongation of the
seismogenic fault. Under these circumstances the length of the faulted zone and the
extent of surface slip would be controlled more by the rheology of shallow deposits
than by slip at depth; this would ultimately prevent the surface rupture parameters
from being used to derive the earthquake magnitude using empirical relationships (e.g.
Wells and Coopersmith, 1994), both for the current event and for earthquakes detected
through paleoseismological trenching. This conclusion justifies both the hidden nature
of the earthquake causative fault prior to 2009 and the multiple divergent proposed
surface rupture models (see Vittori et al., 2011; Vannoli et al., 2012, for a review). We
believe the L’Aquila earthquake illustrates well the nature of the interaction between
the seismogenic rupture and other pre-existing structures at depth (e.g. buried thrust
planes), the partial reactivation of pre-existing surface faults (e.g. Bazzano, Mt. Stabiata
and Pettino faults), and the occurrence of pseudo-primary surface ruptures (Paganica
Fault).
In summary we contend that the “unusual” 6 April 2009 L’Aquila earthquake in fact
illustrates a common style of complex tectonic deformation, implying that the number of
hidden seismogenic faults in Italy – and probably elsewhere – is larger than previously
thought. Driven by the current tectonic regime such faults break through a highly complex upper crust, interacting in various ways with the existing structural fabric. This may
result in limitations of their size, reactivation of older faults and generation of new surface breaks. What is absolutely crucial for the geological reconnaissance work is that in
the field these highly diverse faults may exhibit a reversed hierarchy, the most obvious
being the least relevant to fault-based seismic hazard assessment and vice-versa.
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5 Conclusions
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limited in size? In view of its implications for the identification and correct hierarchization of active faults in earthquake-prone areas, we chose to test this circumstance with
a simple analogue experiment. Scaled experimental models have been widely used to
understand and document the evolution of faults and related folds for decades (e.g.
Withjack and Schlische, 2006; Miller and Mitra, 2011). We designed a model specifically aimed at detecting brittle structures developed above a blind normal fault (Fig. 2).
Our technique follows well established rules (e.g. Shellart, 2000; Bonini et al., 2011)
aimed at reproducing the brittle behavior of upper crustal carbonate rocks (up to 3 km
depth) similar to those that exist in the L’Aquila region. As a reference natural rock we
selected a fractured limestone having the following properties: cohesion 15 MPa, co◦
efficient of internal friction 0.53, angle of internal friction 27.9 (Handin, 1969). For the
analogue material we used dry quartz sand having the following mechanical properties:
cohesion 230 Pa, coefficient of internal friction 0.88, angle of internal friction 41◦ . The
−4
experiment took place in a normal gravity field with a geometric scale ratio of 2 × 10
(1 cm equals 0.5 km in nature).
We reproduced an asymmetric basin made up of analogue material layers using a
flexing plate. The folding shape was fixed to reproduce the pattern of coseismic deformation (Fig. 1), with a basin depocenter located at the same relative position as that
shown by DInSAR data at L’Aquila (Fig. 2b). During the experiment, a forced-fold developed above the tip of the assumed blind fault; as a result of bending, a series of crestal
fractures, some showing measurable vertical throw, formed where the tensile stress is
maximum, that is to say along the hypothetical up-dip prolongation of the main seismogenic fault (Fig. 2b) but not in connection with the fault itself. Overal these structures
are reminiscent of the bending-moment faults observed in nature, a rare but mechanically feasible phenomenon in normal fauting environments. All the result reproduce
well the data and knowledge available for the Paganica fault. Moreover, our analogue
models show the mechanical feasibility of a long-lived buried extensional master fault
with disconnected secondary normal faults lying along the same hypothetical plane.
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IV. Pre-existing surface faults (e.g. Bazzano and Pettino faults): they formed during previous deformation phases; in our case they correspond mostly to faults
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III. Newly-formed surface faults: they are directly generated by coseismic deformation due to crustal bending over the uppermost portion of the master fault (e.g.
Paganica fault) or to stress concentration near its upper tip. As such they are expected to (a) occur near the upward prolongation of the deeper master fault, thus
simulating primary surface faulting, (b) be restricted to the middle of the master
fault, where slip is usually large and bending consequently is tightest. Bending
moment faults are also expected to nucleate at the surface and to extend downward up to a depth controlled by the bending geometry. They may cause sizable
and somehow unpredictable surface faulting hazard.
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II. Pre-existing subsurface faults (e.g. the low angle thrust showed in orange in
Fig. 3c): includes faults generated during previous deformation phases. They may
act passively and serve as segment boundaries, effectively limiting the magnitude
of earthquakes generated by the master faults, or they can be themselves the
locus of large aftershocks. As such they may play a limited role in the assessment
of ground shaking hazard. The same role may be played by generic mechanical
discontinuities within the host-rocks.
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I. Seismogenic master fault (e.g. the L’Aquila earthquake master fault): includes all
faults capable of generating earthquakes of M ≥ 5.5. They may or may not reach
the surface and cause primary surface faulting, depending on the amount of slip
at depth and/or the presence of favorably oriented, pre-existing discontinuities in
the host rocks (see II.). They are the main players in the assessment of ground
shaking hazard.
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Based on these observations, corroborated by the experience gained from a number
of other cases (DISS Working Group, 2010), we propose a new hierarchization scheme
of active normal faults in structurally complex areas (Figs. 3 and 4). Our scheme uses
four categories, listed in descending order of relevance for seismic hazard assessment:
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Full and proper appreciation of this hierarchy, to be achieved by blending surface, subsurface, geomorphic, structural and seismological data, is mandatory for a correct assessment of the nature and extent of the associated seismic hazard.
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Acknowledgements. This work was financially supported by Regione Lombardia, INGV and
Presidenza del Consiglio dei Ministri – Dipartimento della Protezione Civile (DPC). The views
and conclusions contained in this paper are those of the authors and should not be interpreted
as necessarily representing official opinions and policies, either expressed or implied, of the
Italian Government.
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dependence on probabilistic seismic-hazard maps and deaggregation for the Central Apennines, Italy, B. Seismol. Soc. Am., 99, 585–610, 2009.
Atzori, S., Hunstad, I., Chini, M., Salvi, S., Tolomei, C., Bignami, C., Stramondo, S., Trasatti,
E., Antonioli, A., and Boschi, E.: Finite fault inversion of DInSAR coseismic displacement of the 2009 L’Aquila earthquake (central Italy), Geophys. Res. Lett., 36, L15305,
doi:10.1029/2009GL039293, 2009.
Bianchi, I., Chiarabba, C., and Piana Agostinetti, N.: Control of the 2009 L’Aquila earthquake,
central Italy, by a high-velocity structure: A receiver function study, Geophys. Res. Lett., 115,
B12326, doi:10.1029/2009JB007087, 2010.
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A
S1
Mt. Stabiata fault
B
S1
Pizzoli fault
Paganica fault
L’Aquila
Paganica
San Gregorio fault
42° 20'N
Mw 6.3
06042009
(1:32 UTC)
Mt. Pettino fault
42° 20'N
S2
S2
-14
-10
S1
42° 15'N
Bazzano fault
Monticchio Fossa fault
S2
km
km
CROSS SECTION 2
POST- AND COSEISMIC DEFORMATION PROFILE
POST- AND COSEISMIC DEFORMATION PROFILE
0 cm
0 cm
10 cm
10 cm
POST-SEISMIC DEFORMATION PROFILE
POST-SEISMIC DEFORMATION PROFILE
0 cm
0 cm
10 cm
COSEISMIC DEFORMATION PROFILE
10 cm
COSEISMIC DEFORMATION PROFILE
0 cm
0 cm
10 cm
10 cm
San Gregorio fault
SW
Mt. Pettino fault
Mt. Stabiata fault NE
10
5
0
10
5
0
CROSS SECTION 1
SW
0
5
5
Paganica fault
NE
Bazzano fault
|
0
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Mw 5.4
07042009
(17:47 UTC)
|
-6
-2
S2
Mw 4.9
07042009
(9:26 UTC)
42° 15'N
S1
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13° 30'E
13° 20'E
13° 30'E
13° 20'E
Mw 4.9
06042009
(2:37 UTC)
10
km
C
5
10
15
20 km
D
5
10
15
20 km
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Fig. 1. (A) Map view of the L’Aquila area showing the location (red stars) and focal mechanism of the largest events of the 2009 sequence (Scognamiglio et al., 2010). Blue dashed
lines are contours of the elevation changes observed between 4 and 12 April 2009 (D’Agostino
et al., 2012). (B) Mapped active faults, shown by black lines; those shown in red were reported reactivated in 2009 (Emergeo Working Group, 2010). S1, S2 indicate the traces of cross
Sects. 1 and 2. (C) and (D) cross sections, showing dominantly coseismic (D’Agostino et al.,
2012) (4 April 2009–12 April 2009; blue dashed line), post seismic (D’Agostino et al., 2012)
(12 April–5 October 2009; green dotted line) and cumulative elevation changes (orange dotted
line) measured along the section trace. In all figures black dots indicate relocated aftershocks
(Chiarabba et al., 2009; Chiaraluce et al., 2011).
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10
km
|
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SETUP
6 cm
granular material (quartz sand)
flexing plate
|
theoretical position
of the master fault
STEPPER
MOTOR
RESULTS
crestal normal faults
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A
|
flexing plate
theoretical position
of the master fault
STEPPER
MOTOR
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Fig. 2. Analogue modelling setup and results. (A) Sketch of the experimental modelling apparatus. The granular material (quartz sand) is deformed above a flexing plate pulled by a stepper
electric piston. A dashed line indicates the theoretical position of the master (seismogenic)
fault. (B) Detail of the results of the experiment; thick black lines represent newly formed faults
(Category #3 faults, see text).
|
B
Discussion Paper
depocentre
CROSS SECTION 2
Category #4
Category #3
Mt. Stabiata fault
SW
NE
Mt. Pettino fault
0
Category #4
SW
San Gregorio fault
Bazzano fault
Paganica fault
NE
0
Category #4
10
km
Category #1
42° 08' 19,90" N
13° 15' 04,71" E
SW
Il Morrone
5
A
Mt. Orsello
Ocre Mts.
10
km
Category #1
Bazzano fault
AQU station Paganica fault
B
Mt. Prena
C
42° 29' 52,50" N
13° 46' 46,06" E
NE S VELOCITY (km/s)
0
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Category #2
|
Category #2
5
0
Sedimentary
Successions
|
2,5
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CROSS SECTION 1
Crystalline
Upper Crust
Crystalline
Upper Crust
10
km
Anisotropic
layer with
negative
anisitropy
10
km
Mw 6.3
Lower Crust
Thrusts
Extensional Faults
5
Lower Crust
Seismogenic fault
1.5 2.5 3.5 4.5
|
|
S1
Discussion Paper
SURFACE MAPPED FAULTS AND RUPTURES
San Gregorio fault
Category #4
Mt. Stabiata fault Bazzano fault Monticchio-Fossa fault
Paganica fault
Category #3
Pettino fault
NNW
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133
S2
SSE
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0
|
Fig. 3. (A) and (B) are Sect. 1 and Sect. 2 across two portions of the seismogenic fault. Red
lines delineate the mainshock rupture. Orange lines identify secondary blind faults whose activity was triggered by the mainshock. A blue line marks the Paganica fault. Brown lines describe
the geometry of the active faults not directly connected to the main seismogenic fault. (C) Regional cross-section showing the main tectonic features of the area (modified after Satolli and
Calamita, 2008; Vezzani et al., 2009). The main seismogenic rupture is shown in red along
with the focal mechanism and rupture nucleation. The small diagram to the right shows the S
velocity model (black dashed line) for seismological station AQU from Bianchi et al. (2010).
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Crystalline
Upper Crust
5
Category #2
-5
|
Category #1
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20 km
|
-10
600
400
200
-15
km
5
10
15
Fig. 4. Down-dip section along the L’Aquila master fault (strike N135E; dip 50◦ to the SW),
showing the mainshock location (red star); black dots are aftershocks located within 0.5 km
of the fault plane (Chiaraluce et al., 2011); red dashed lines show the coseismic slip pattern
(contours in mm; Atzori et al., 2009) and the location of the secondary faults (orange line and
ellipses). The box above shows the categorization of surface faults and their relative location
along strike of the master fault.
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