Normal faulting in the forearc of the Hellenic subduction margin: Paleoearthquake history and kinematics of the Spili Fault, Crete, Greece

Accepted Manuscript Normal faulting in the forearc of the Hellenic subduction margin: Paleoearthquake history and kinematics of the Spili Fault, Crete, Greece Vasiliki Mouslopoulou , Daniel Moraetis , Lucilla Benedetti , Valery Guillou , Olivier Bellier , Dionisis Hristopulos PII:

S0191-8141(14)00121-7

DOI:

10.1016/j.jsg.2014.05.017

Reference:

SG 3069

To appear in:

Journal of Structural Geology

Received Date: 2 December 2013 Revised Date:

21 May 2014

Accepted Date: 28 May 2014

Please cite this article as: Mouslopoulou, V., Moraetis, D., Benedetti, L., Guillou, V., Bellier, O., Hristopulos, D., Normal faulting in the forearc of the Hellenic subduction margin: Paleoearthquake history and kinematics of the Spili Fault, Crete, Greece, Journal of Structural Geology (2014), doi: 10.1016/j.jsg.2014.05.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Normal faulting in the forearc of the Hellenic subduction margin: Paleoearthquake

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history and kinematics of the Spili Fault, Crete, Greece

3 4 Vasiliki Mouslopoulou1,2*, Daniel Moraetis3, Lucilla Benedetti4,

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Valery Guillou4, Olivier Bellier4, Dionisis Hristopulos1 1

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Department of Mineral Resources Engineering, Technical University of Crete, 73100, Greece

GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Section 3.1,

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Germany

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Sultan Gaboos University, College of Earth Science, PO Box 36, P.C. 123, Muscat, Oman

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Aix-Marseille Université, CEREGE CNRS-IRD UMR 34, 13545 Aix en Provence, France

13 Abstract

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The late-Cenozoic kinematic and late-Pleistocene paleoearthquake history of the

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Spili Fault is examined using slip-vector measurements and insitu cosmogenic

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(36Cl) dating, respectively. The Spili Fault appears to have undergone at least three

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successive but distinct phases of extension since Messinian (~7 Ma), with the most

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recent faulting resulting in the exhumation of its carbonate plane for a fault-length

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of ~20 km. Earthquake-slip and age data show that the lower 9m of the Spili Fault

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plane were exhumed during the last ~16500 years through a series of, at least, five

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large-magnitude

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paleoearthquakes varied by more than one order of magnitude (from 800 to 9000

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years), suggesting a highly variable earthquake recurrence interval during late

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earthquakes.

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timing

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Corresponding author. Email address: [email protected]; Tel.: ++49 3312881312; Fax: ++49 331 288 1370

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Pleistocene (CV=1). This variability resulted to significant fluctuations in the

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displacement rate of the Spili Fault, with the millennium rate (3.5 mm/yr) being

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about six times faster than its late-Pleistocene rate (0.6 mm/yr). The observed

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variability in the slip-size of the paleoearthquakes is, however, significantly smaller

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(CV=0.3). These data collectively suggest that the Spili Fault not only is active but

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it may also be one of the fastest moving faults on the forearc of the mainly aseismic

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Hellenic subduction margin.

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32 Keywords:

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paleoearthquakes, cosmogenic dating, limestone scarp, normal fault, Spili Fault, Crete.

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1. Introduction

The island of Crete is located on the forearc of the Hellenic subduction margin, the only

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currently active subduction system in Eastern Mediterranean, where the African and the

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Eurasian plates converge at rates of up to ca. 40mm/yr (Reilinger et al., 2010) (Fig. 1,

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inset). Despite the mainly aseismic character of the Hellenic subduction (~80% of the

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relative plate motion is accommodated by aseismic slip; McClusky et al., 2000; Reilinger

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et al., 2010), large earthquakes across the margin do occur (Guidoboni and Comastri,

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1997; Papadopoulos, 2011). In the proximity of Crete alone, more than 200 large

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magnitude (M>5) earthquakes have been historically and instrumentally recorded during

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the last 4000 yrs (Papadopoulos, 2011).

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A number of the recorded earthquakes may be associated with slip on the plate

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interface or faults splaying off the plate interface (Pirazzoli et al., 1996; Stiros, 2001;

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Papazachos and Papazachou, 2003; Shaw et al., 2008, Shaw et al., 2010; Papadopoulos,

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2011). The majority, however, of these events are shallow and are associated with upper

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plate faulting, either onshore or offshore Crete (e.g., Papazachos and Papazachou, 2003;

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Caputo et al., 2010; Mouslopoulou et al., 2011; Papadopoulos, 2011). A question that

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arises is how earthquakes that break the upper crust of this convergent system are

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accommodated spatially and temporally on the forearc by normal faulting? Is there a

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systematic pattern between fault systems of different orientations? What portion of the

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total plate motion is accommodated by normal faulting on the forearc?

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A means of addressing the above questions is by quantifying the timing and size of

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late Quaternary fault slip on key active normal faults on Crete. Interestingly, and despite

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the intense earthquake activity, up to date, there has been no quantitative paleoseismic

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study on any active crustal fault on Crete. This study presents the first paleoearthquake

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record that derives from a normal fault on Crete, the Spili Fault, providing constraints on

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its late Pleistocene seismic history. Paleoearthquakes were dated by measuring the

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changes in the content of the in situ

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sequentially exhumed carbonate fault plane. This approach has been developed and

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validated over the last decade on several faults globally (e.g. Benedetti et al. 2002,

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Daëron et al., 2004; Schlagenhauf et al. 2010, Benedetti et al. 2013).

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2. Geology and geomorphology of the Spili Fault

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The island of Crete forms the central section of the sub-aerially exposed forearc of the

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Hellenic subduction zone, a region that currently undergoes extension (Armijo et al.,

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1992; Fassoulas, 2001; Reilinger et al., 2010). Extension often manifests itself as normal

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faulting on the upper crust. Indeed, the island is broken up by numerous active normal

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faults of mainly NW-SE, N-S and E-W orientation that often traverse limestone country,

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forming impressive post-glacial carbonate scarps (Armijo et al. 1992; Caputo et al.,

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2010). Another manifestation of the ongoing extension is the intense microseismicity

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recorded below the island of Crete (e.g. in the upper 20 km of the crust) which exhibits

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significant oblique-normal movement as it is revealed by analysis of focal mechanisms

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recorded by temporal seismic networks (Delibasis et al., 1999; Meier et al., 2004; Becker

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et. al., 2010).

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The Spili Fault is one of the numerous normal faults of south-central Crete

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(Papavassiliou et al., 1985) (Figs. 1, 2 &3). Other prominent faults in the area include

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those of the south-dipping Asomatos and Sfakia faults in the south and the Ag. Galini,

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Klima and Ag. Triada faults, that bound the Messara depression, in the southwest

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(Papavassiliou et al., 1985) (Fig. 1). The Spili Fault strikes 300-320° (NW-SE), bounding

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the southwest margin of Kedros ridge (Figs. 2a & 3). Its surface trace extends for ca. 20

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km, defining along most of its length the boundary between various Mesozoic bedrock

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units (e.g., limestone and flysh of the Tripoli Unit on the upthrown fault section and

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schist of the Vatou Unit on the downthrown sectin) or Mesozoic units (upthrown section)

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and conglomerates of Tortonian age (downthrown section) (Angelier, 1979;

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Papavassiliou et al., 1985) (Fig. 3). The near surface dip of the Spili Fault, as measured

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on its exhumed carbonate fault plane, ranges from ~50° SW to ~80°SW (see details in the

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supplementary material).

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The recent activity on the Spili Fault is mainly evidenced by the presence of an

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often discontinuous and fresh-looking carbonate scarp that marks the base of Kedros

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ridge for a total distance of ca. 14 km (Figs. 2a & 3). The fault plane is brecciated and, at

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places, covered by thin layers of limonites, an indication of recent exhumation (Fig. 4a).

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Fault rock of up to 2m thickness is also present. On the fault plane we measured striations

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of various orientations, indicating a non-uniform slip movement through time (see section

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3 for a detailed discussion). The recent activity of the Spili Fault is further supported by

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geomorphological observations, such as the presence of numerous triangular facets along

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its length, vertically displaced streams (that often form perched valleys) and the offset of

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unconsolidated alluvial fans (Fig. 4).

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Vertical displacements (e.g. throw) were measured on the exhumed carbonate

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plane of the Spili Fault at 21 localities and range from ~20m, near its center, to ~3m

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toward its tips (Fig. 2b). Displacements are thought to reflect post-glacial activity as the

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landscape has been reset during the last glaciation (Armijo et al., 1991; Benedetti et al.,

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2002). Therefore, vertical displacements measured along the fault are assumed to have

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accrued over an approximately uniform time interval. Examination of the plot in Figure

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2b shows that the displacement profile along the Spili Fault is discontinuous with a throw

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deficit near the fault’s center (where the largest throw values were expected). We

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interpret this deficit to be a sampling artifact as measurements from the main strand of

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the Spili Fault are missing for the strike distance between 4 and 9 km; Instead, the plot

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includes throw values (e.g. 2-3m) which are measured on a splay fault that runs parallel

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to the main strand and about 200m upslope the Kedros ridge (Fig. 2a). The main Spili

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Fault is assumed to extend lower, at the base of the valley (Fig. 2a). The fact that there is

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no measurable surface trace along this stretch of the fault probably relates to natural

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erosion coupled with human modification. Indeed, the presence of flysch on the upthrown

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fault block and of schist/conglomerate on the downthrown block (Fig. 3) (Papavassiliou

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et al., 1985) reinforces this hypothesis as these formations are easily eroded, providing

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also fertile ground for agriculture. If true, the interpolation of the throw values either side

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of the central section of the fault, which is currently concealed, would produce a notional

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elliptical displacement profile (Fig. 2b). Nevertheless, to minimize potential sampling

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bias we located our paleoearthquake sites near the fault’s center and away from the

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displacement deficit fault section (Fig. 2a).

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3. Late Cenozoic deformation history of the Spili Fault

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The long-term kinematics of a fault may be explored by measuring the rake of the

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striations recorded on its exhumed plane. Striations of variable orientation recorded on a

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single fault-plane locality may result from either rotations of slip vectors during single

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earthquake events (Guatteri and Cocco, 1996) or larger scale changes in the plate

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kinematics of the study area (e.g., Mercier et al., 1991; Bellier and Zoback, 1995). The

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discrimination between the two hypotheses may be achieved by using structural

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arguments, such as the consistency of crosscutting relationships between striations of

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different generations along the slip surface. Systematic crosscutting relationships along

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fault planes often indicate a succession of distinct faulting episodes. Over the past twenty

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years numerous studies globally have utilised the inversion of fault slip measurements to

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determine paleostress fields and constrain the temporal and spatial stress changes (e.g.,

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Mercier et al., 1991; Bellier and Zoback, 1995; Shabanian et al., 2010; Mercier et al.,

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2013).

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To examine the kinematic history of the Spili Fault and determine the stress field

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responsible for various faulting episodes during late Cenozoic, we inverted slip-vector

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measurements collected along its exhumed carbonate plane. Specifically, we performed

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quantitative inversion of distinct families of slip data using the methodology originally

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proposed by Carey (1979). Overall, we collected a total of 46 slickenside measurements

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from 4 study sites located along the Spili Fault (sites: K1-K4 in Fig. 2a; Table 1 and

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supplementary material). The striations were recorded on the principal carbonate slip

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surface of the Spili Fault. All measurements were selected to derive from localities where

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the Spili Fault displaces formations of Pliocene and Pleistocene age in order to minimise

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the error that striations of older (pre-Pliocene) tectonic events would introduce. The age

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of fault striations is only approximately constrained as younger than the age of the faulted

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formations. The lower hemisphere stereo plots with the inversion results are illustrated in

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Figure 2a and Table 1. The original striae measurements, the deformation phases

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recorded at each of the four localities along the Spili Fault and the details on the

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methodology of the inversion method may be found in the supplementary to this article

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material.

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Data analysis shows clear evidence of changes in the kinematics of the Spili Fault

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during late Cenozoic. On all four outcrops, we measured two, and sometimes up to three,

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distinct slip events, as evidenced by clear crosscutting striations on fault planes (Fig. 2a,

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Table 1). We recorded striations of slightly right-lateral orientation overprinting striations

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of slightly left lateral orientation (Fig. 2a). Locally, both of these mainly dip-slip families

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of striations postdate another set of dip-slip striations related to an older phase of east-

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west trending extension (Fig 2a, locality K3).

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The oldest generation of E-W striations are clearly penetrative striations formed in

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a relatively significant depth and which, subsequently, have been crossed-cut by mainly

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thin mechanical striations resulting from surface or shallow faulting (Petit, 1987). Thus,

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since Messinian the kinematics of the Spili Fault are characterised by three distinct

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regional tectonic regimes, that is (from older to younger): Phase 1º: an old dip-slip

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movement associated with pervasive deformation that corresponds to a stress regime

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characterised by E-W extension. This phase could correspond to an older event within the

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Cenozoic collapse history of the Hellenic arc; Phase 2º: a normal movement with a

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minor oblique component of slip related to an NE-trending phase of extension; Phase 3º:

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nearly pure normal faulting, with a minor right-lateral component, in response to N-S

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extension (north-trending σ3 axis). Striations of this latter phase of deformation are today

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present on calcitic fault rock and on fault gouge, reinforcing our hypothesis that this type

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of movement is the most recent.

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Because the succession of the various generations of striations are systematic and

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consistent between different sites (Fig. 2a) coupled with the different nature of the striae

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(penetrative vs. mechanical), it is assumed that they result from changes in the stress

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regime during Late Cenozoic (magnitude and/or orientation) rather than variations in the

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slip vectors during successive slip events. The different families of crosscutting striae are

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reflected in the >40 º angular deviation of the slip rake on the Spili Fault (Fig. 2a) (e.g.

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faulting ranges from oblique to pure dip-slip). This is in good agreement with the analysis

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performed by Fassoulas (2001) in south-central Crete that suggests an older (late

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Messinian; 6-5 Ma) E-W trending phase of extension being overprinted by Quaternary

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NE and NW extensional stress regimes.

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4. In situ cosmogenic 36Cl dating on the Spili fault

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The two paleoearthquake localities (sites A & B), were selected to be on the main

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carbonate fault plane and near the fault’s center (Fig. 2b). Site A is located at an elevation

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of 442m above sea level, where the fault marks the boundary between the bedrock units

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of Tripoli limestone (upthrown bock) and Tortonian conglomerate (downthrown block).

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The upthrown block forms an elongated northwest-trending limestone ridge, the base of

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which is locally mantled by fan deposits forming from the feeder streams. The fault here

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strikes 320° and has vertically displaced an alluvial fan, exposing sub-aerially a 12 m

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high and steep (on average 80° SW) section of its carbonate fault plane (Fig. 5a). The

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fault plane is brecciated and, at places, polished by repeated slip movement (Fig. 5a). At

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least 22 slickenside striations of two different generations were measured on the main

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fault-slip surface near site A revealing a dominantly normal sense of motion, with a

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minor strike-slip component (see K1 in Fig. 2a). The dip of the alluvial fan at site A is

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20-30°.

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The site B is located ca. 7 km southeast of site A, near the village of Platanes, at

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an elevation of 621m above sea level and along the main Spili Fault plane (Fig. 2a). Here,

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the fault strikes ca. 320-340° and displaces an alluvial fan, exhuming up to 20m of its

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carbonate plane (Fig. 5b). The fault dip is ca. 50-60° SW (Fig. 5b), value significantly

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lower than that recorded at Site A. The exhumed scarp at the sampling site is continuous

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for about 200m, forming a lens that ranges in height from 20m, near the center, to about

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2m laterally (Fig. 4b). The dip of the alluvial fan at site B is ~10°.

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Sampling involved extraction of carbonate rock from near-vertical (~60-80°)

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fault-scarps. The equipment used included an electrical generator, portable rock-saw,

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chisel, hammer and climbing equipment (Fig. 5). In order to measure the 36Cl content of

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the carbonate rock we removed from the exhumed fault plane a total of 102 individual

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slabs, each of which being 15cm wide and 3cm thick (at site A we sampled the lower 9m

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whereas at site B we sampled the lower 2m of the fault plane). This technique is routinely

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used for cosmogenic isotope dating (Benedetti et al., 2002; Schlagenhauf et al. 2010).

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In addition to these two new study sites on the Spili Fault, there is an earlier

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paleoearthquake site where measurements of Rare Earth Elements have been used to

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identify the number and size of the paleoearthquakes that exhumed the lower section of

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its carbonate plane (Mouslopoulou et al., 2011). This additional site is located about 1.5

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km southeast of Site A (Fig. 2a & 4c) and is used to directly compare (and calibrate) our

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results.

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4.2 Sample preparation and measurements From a total of 102 carbonate slabs we measured the chemical composition on 54

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samples (that is 41 from site A and 13 from site B). All 54 samples were crushed, sieved

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and chemically prepared to precipitate AgCl [Stone et al., 1996; Schlagenhauf et al.,

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2010]. 36Cl and Cl concentrations were determined by isotope dilution accelerator mass

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spectrometry at ASTER-CEREGE (France) and were normalized to a

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prepared by K. Nishiizumi [Sharma et al., 1990]. Replicates agreed within 5% and the

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samples were found to contain 106-107 atoms of 36Cl and 1017-1019 atoms of Cl, about 100

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times more than the measured blanks (Table 2).

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The [Cl] concentrations measured at site A are <20 ppm while the [Cl]

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concentrations measured at site B are between 40 and 90 ppm (Table 2). Further, the [Ca]

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content recorded at site A (40% mean Ca) is significantly higher than that measured at

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site B (25% mean Ca). This implies that the

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different between these two sites. Specifically, at site A the

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due to Ca spallation, 8% due to slow negative muons capture and <2% due to thermal and

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epithermal neutrons capture. At site B the36Cl production is ca. 60% due to Ca spallation,

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10% due to slow negative muons capture and about 30% due to thermal and epithermal

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neutrons capture [Schimmelpfennig et al., 2009]. This high contribution at site B of

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mechanisms other than the Ca spallation precludes the accurate determination of the 36Cl

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production at this locality. Hence, in the remaining analysis we will discuss the

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paleoearthquake history on the Spili Fault by including only data recorded at site A.

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Cl production is ca. 90%

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5. Paleoearthquake history of the Spili Fault

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5.1 Modelling of the 36 Cl data – uncertainites and limitations

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We measured and modelled the content of in situ cosmogenic

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samples extracted from the lower 9 m of the fault scarp at Site A (Fig. 2a). The total scarp

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height at this locality is 12 m but we did not sample the upper 3m as the rock face was

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weathered and covered by roots/trees (Fig. 5a). The modelling of the 36Cl concentrations

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has taken into account all geometrical characteristics of the site that could introduce

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uncertainty (such as the dip of the fault, the density and the dip of the alluvial fan, the dip

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of the older scarp section above the sampling site, the cosmic rays shielding that may be

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induced locally by the surrounding topography) and the full chemical composition of

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each sample (for details see Schlagenhauf et al., 2010). Nevertheless, we need to note that

Cl on 41 limestone

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the method of in situ cosmogenic 36Cl dating cannot resolve earthquakes that occurred on

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the fault over shorter time intervals than the recorded uncertainty (in this case up to 1500

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years, see section 5.2).

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5.2 Timing and size of paleoearthquakes on the Spili Fault

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The modelling indicates that the lower section (0-3.5 m) of the exhumed fault scarp is

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much younger than the remaining scarp (3.5 - 9m). Examination of the graph in Figure 6

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reveals two sharp discontinuities at 1 and 3.5 meters upscarp (the reference point for all

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upscarp measurements is considered to be the modern ground surface). These two

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discontinuities are interpreted to result from a minimum of two large-magnitude, ground-

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rupturing, past earthquakes. The timing of each of these earthquakes is modelled to be at

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500 (-400, +200) and 600 (±300) years BP, respectively. The uncertainties in the timing

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of these two events include a period of 400 years (from 300 AD to 700 AD) which is

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common for both earthquakes. This suggests that these events may have been highly

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clustered in time.

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Further examination of the profile in Figure 6 reveals another discontinuity at about

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6m upscarp, which is dated at 7300 (-300, +400) years BP. The large time-lag between

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the second and third identified discontinuities (earthquakes) on the Spili Fault is reflected

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in the increased content in 36Cl at six meters compared to the content measured lower on

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the scarp (the upper section of the scarp has been exposed to cosmic ray radiation for

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longer time-periods than the lower section of the scarp). This is also evident in the field,

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when one aesthetically compares the roughness of the fault plane above and below the

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upscarp height of 3.5m.

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The 4th and 5th recorded discontinuities (earthquakes) are identified at 7.2 m and 9

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m upscarp and their timing is modelled at 16300 (-100, +500) and 16500 (-500, +700)

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years BP, respectively (Fig. 6). Differences in the roughness of the fault plane above and

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below the notional line that marks the boundary between the 3rd and 4th earthquakes (at

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6m) is not identifiable here, despite the ca. 9000 years of time span that separates the two

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sections of the fault plane.

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285 5.3 Earthquake and fault parameters

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Data show that the Spili Fault is active and has accrued 9 m of displacement over a

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period of ~16500 years (Figs 6 & 7). From this we derive an average late Pleistocene

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fault displacement rate of about 0.6 mm/yr (Fig. 7). The data also suggest that each

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identified paleoearthquake on the Spili Fault produced co-seismic slip that ranges from 1

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to ~2.5 m (Figs. 6 & 7). Using the Wells and Coppersmith (1994) equation for normal

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faulting that relates the size of the coseismic slip with the earthquake magnitude

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(M=6.61+0.71*log(MD)), where MD is the maximum displacement per earthquake

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event), we find that these displacements were produced by earthquakes ranging in

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magnitude between M6.6 and M6.9. Earthquakes of such magnitude and coseismic

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displacements of that size (1-2.5 m) are in conflict with fault lengths of ca. 20 km [i.e.

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based on Wells and Coppersmith (1994), smaller slip- and earthquake-size would be

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expected on the Spili Fault]. However, this discrepancy between various fault parameters

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is often observed in paleoearthquake studies (e.g., Berryman et al., 1998; Begg and

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Mouslopoulou, 2010; Schlagenhauf et al., 2011) and is mainly due to the large scatter

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associated with the Wells and Coppersmith (1994) dataset, the incomplete record of the

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total (subsurface) fault length, the intersection/termination of the fault under investigation

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with/against other neighboring faults and/or the amalgamation of more than one events in

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the measured displacement. Regarding the latter, we need to note that the vertical

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separation between two successive discontinuities on the

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assumed to result from one paleoearthquake; however, it may represent slip produced by

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a number of smaller earthquakes clustered on the fault over periods < 600-1500 years

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(this is the range of the uncertainty on our measurements). Thus, the earthquake

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magnitudes associated with each individual earthquake here, are maxima.

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Cl-profile of Figure 6 is

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Earthquakes of magnitude >M6 may be highly damaging. Therefore it is important

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to discuss the frequency with which they may be accommodated by the Spili Fault. To

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achieve this we estimate the earthquake recurrence interval (RI), which is the period of

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time between two successive paleoearthquakes on an individual fault. The RI can be

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estimated using two different methods: 1) directly from the earthquakes recorded on the

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fault-scarp (observed average RI) or 2) calculated from the mean single event

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displacement (SED) and the long-term displacement rate on the fault (estimated average

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RI). The methods are independent from one another and collectively provide a powerful

318

means of estimating recurrence intervals and their variability.

TE D

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Based on the timing of the five most recent earthquake events on the Spili Fault that

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319

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310

320

were recorded at site A, and the associated uncertainties, we derive the following

321

observed RI’s (from the most recent to the oldest pair of earthquakes): 0-100 years, 6100-

322

7400 years, 8500-9800 years and 800-1000 years. Thus, from these values we estimate

323

the average observed RI on the Spili Fault being ~ 4200 years.

14

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The value of the calculated average recurrence interval is based on the cumulative

325

displacement of a dated feature (fault-scarp in our case) that has been offset by multiple

326

earthquakes. The slip per event (D) is typically estimated from the maximum or average

327

slip documented during single earthquakes. The average D for the Spili Fault is ~2m.

328

Knowing that the displacement rate (DR) on the fault is ~0.6 mm/yr, we derive the

329

calculated average recurrence interval based on the equation: RI(calc) = D/DR = ca. 3300

330

years. The above data suggest that the calculated average RI (~3300 year) on the Spili

331

Fault is, within uncertainties, comparable to the observed RI (~4200 years) that resulted

332

by dating individual earthquakes. This is encouraging because it suggests that our

333

sampling window (e.g. 16500 years) may have been large enough to overcome the short-

334

term variability that is associated with the phenomenon of earthquake occurrence (e.g.

335

temporally clustered earthquake events or long periods of no earthquakes)

336

(Mouslopoulou et al., 2009; Nicol et al., 2009).

TE D

M AN U

SC

RI PT

324

The paleoearthquake data that derive from the Spili Fault show that, on average, the

338

paleoearthquake slip is less variable than the repeat time between successive earthquakes.

339

The variability of an earthquake parameter may be quantified by the coefficient of

340

variation (CV); that is, the standard deviation divided by the mean value. The CV

341

associated with the slip during the five most recent paleoearthquakes on the Spili Fault is

342

~0.3. That shows a relatively low variability and a tendency towards a more

343

‘characteristic’ earthquake slip. The CV associated with the timing between successive

344

paleoearthquakes is 1, indicating a nearly random earthquake occurrence. This is

345

rationalised if we consider that the average time between successive paleoearthquakes on

346

the Spili Fault varied by more than one order of magnitude (from less than 800 to more

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337

15

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than 9000 years), suggesting a highly variable earthquake recurrence interval during late

348

Quaternary. This variability leads to significant fluctuations in the displacement rate of

349

the Spili Fault, with the millennium rate (3.5 mm/yr) being about six times faster than its

350

longer term late Pleistocene rate (0.6 mm/yr) (Fig. 7). The observed variability may also

351

relate to the fact that the Spili Fault is not spatially isolated; at least another four normal

352

faults occur within its immediate neighborhood (Fig. 1). Although these faults do not

353

appear to be hard linked in map-view, their geometric arrangement and their proximity to

354

one another (in some cases <5km), implies that their tectonic and paleoearthquake

355

histories may be interrelated and, therefore, interdependent (Nicol et al., 2006). In other

356

words, the paleoearthquake history of the Spili Fault may only be fully evaluated and

357

understood when it is examined in conjunction with its seismic activity on its neighboring

358

faults as it has been observed on several other fault systems globally (Nicol et al., 2006;

359

Mouslopoulou et al., 2012; Benedetti et al. 2013).

TE D

M AN U

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347

Let us now place our results into a broader context: that of the Hellenic subduction

361

margin, on the forearc of which the Spili Fault is located. The subduction of the African

362

Plate beneath the Plate of Eurasia occurs mainly aseismically and only a small fraction

363

(up to 20%) of the relative plate motion (~40 mm/yr) is being accommodated by

364

earthquakes (that is about 7-8 mm/yr) (Shaw et al., 2008; Reilenger et al., 2010). Given

365

that a significant amount of this seismic strain is accommodated by occasional large

366

subduction earthquakes (e.g. Stiros, 2001; Shaw et al., 2008), the remaining has to be

367

accommodated by slip on upper plate crustal faults, and especially on these faults whose

368

strike is oriented favourably to the subduction margin south of Crete (e.g. NW-SE and

369

NE-SW). On the island of Crete there are at least a dozen of active normal faults, with

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360

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half of them being oriented favourably with respect to the subduction margin (Caputo et

371

al., 2010; Mountrakis et al., 2012). The measured displacement rate on the Spili Fault

372

(0.6 mm/yr) coupled with the fact that it is orientated approximately parallel to the active

373

margin, implies that it may accommodate a significant amount of the seismic strain

374

across this section of the Hellenic margin.

RI PT

370

SC

375 5.4 Comparison with the REE method

377

The fault plane locality where the REE study (Mouslopoulou et al., 2011) took place is

378

situated only 1.5 km to the southeast of Site A. Therefore, we would expect to record

379

roughly comparable slip-sizes during individual earthquake ruptures. Indeed, data on

380

Figure 7b show that the slip-sizes of the five most recent earthquakes identified at Site A

381

(starting from the most recent: 1m, 2.5m, 2.5m, 1.2m and 1.8) are overall comparable to

382

those derived from the nearby REE-site (starting from the most recent: 2.7m, 2.5m, 2m,

383

1.7m and 1.3m) (Mouslopoulou et al., 2011). Specifically, the variability for the four

384

older events ranges from zero (penultimate event) to 29%. Interestingly, the most recent

385

earthquake shows significant variability between the two sites, that is ~62%. Such large

386

earthquake-slip variability over short lateral distances has been previously recorded on

387

individual faults, either on surface ruptures of modern earthquakes (Beanland et al., 1989;

388

Rockwell et al., 2002; Shaw, 2011; Rockwell, 2013) or on paleoearthquakes (e.g.

389

Schlagenhauf et al., 2011; Benedetti et al., 2013).

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376

390 391

5.5 Comparison with the historic record on Crete

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Despite the abundance of historic earthquakes on Crete and the identification of the main

393

active faults on the island, the causal link between them is poorly known. Some studies

394

(Jusseret and Sintubin, 2012; Jusseret et al., 2013) have tried to establish a methodology

395

that would relate systematic archaeological effects to historically recorded earthquake

396

destruction and, further, to specific faults on Crete; a promising attempt that should be

397

further pursued.

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392

At least 109 earthquakes of M>5 have been historically recorded on, or proximal to,

399

Crete during the period 1200-1900 AD, when the island of Crete suffered successively

400

the Venetian and the Ottoman occupation (Papadopoulos, 2011). The majority of these

401

109 events are interpreted to be shallow events, most likely generated by crustal faults

402

located either onshore or offshore Crete (Papadopoulos, 2011). About seven of these

403

events are recorded to have triggered significant damage in the city of Rethymno (and

404

nearby villages) and/or south-central Crete, proximal to the Spili Fault. Moreover, four of

405

these earthquakes were assigned magnitudes between M6 and M6.5 (Papadopoulos,

406

2011), values that roughly match the size of the recorded, by this study, paleoearthquakes

407

on the Spili Fault. Despite this encouraging record, it is not possible to safely correlate

408

any of the above individual historic earthquakes to the paleoearthquakes recorded on the

409

Spili Fault. This is mainly due to the large uncertainties associated with the timing of

410

each identified paleoearthquake, which ranges from 600 to 1500 years.

M AN U

TE D

EP

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411

SC

398

Another interesting repercussion of our study is that the Spili Fault did not rupture

412

during the Minoan Period (ca. 4000-3500 yrs BP) (Fig. 7). Previous studies (e.g. Monaco

413

and Tortorici, 2004) had considered the Spili Fault as the most likely candidate for the

414

double destruction of the Minoan Palace at Phaistos (Fig. 1) at 3700 and 3450 BP. Here,

18

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415

we reject this hypothesis as the Spili Fault not only did not rupture during the Minoan

416

period but our data show that it remained quiet for nearly 6500 years, prior to the last

417

millennium (Fig. 7).

419

RI PT

418 6. Conclusions

Slip vector analysis on the Spili Fault suggests that at least three distinct phases of

421

extension were present on south-central Crete since Messinian. The most recent stress

422

regime is associated with N-S extension and is responsible for the paleoearthquake

423

activity recorded on the Spili Fault. Indeed, the first paleoseismic study on Crete reveals

424

that the Spili Fault has ruptured intermittently during the last 16.5 kyr, producing 5 large

425

magnitude earthquakes. Specifically, the Spili Fault accommodated two earthquakes

426

within the last millennium, accruing a total of 3.5 m of throw, while it remained quiet for

427

the preceding ca. 6.5 kyr. The two older (4th and 5th) earthquakes recorded on the fault

428

occurred at 16 and 16.5 kyr BP, confirming the tendency of the fault to accommodate

429

earthquakes in clusters which are separated by long periods of seismic quiescence (the

430

coefficient of variation of its earthquake recurrence interval is 1). Variable earthquake

431

recurrence produces fault displacement rates that range by more than 6 times (from 0.6

432

to 3.5 mm/yr) when averaged during various time intervals. Late Pleistocene

433

displacement rates on the Spili Fault (0.6 mm/yr) suggest that upper plate faulting in the

434

forearc of the Hellenic subduction margin may accommodate a larger proportion than

435

previously thought of the ~7-8 mm/yr seismic slip across the margin. It also suggests

436

that the Spili Fault may be one of the fastest moving faults on the Hellenic forearc.

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420

437

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Acknowledgements

439

We acknowledge the Latsis Foundation in Greece for partially supporting this work,

440

Charalambos Fassoulas for introducing us to the Spili Fault and John Begg for numerous

441

inspiring discussions. We also acknowledge Didier Bourlès, Maurice Arnorld, Georges

442

Aumaître and Karim Keddadouche at ASTER-CEREGE for the AMS measurements. Jim

443

Tesson is acknowledged for the data modelling.

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Figure captions

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Figure 1: Aster (20m) Digital Elevation Model of western Crete. The Spili and other

635

principal active normal faults in western Crete are illustrated by red solid lines (ticks

636

indicate downthrown side). Yellow and grey filled circles indicate paleoearthquake

637

studies along the Spili Fault performed by 36Cl and REE analysis, respectively (see Fig. 2

28

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for details). The three stars illustrate the only M>5 earthquakes that occurred on, or

639

proximal (<10 km) to, western Crete since 1964. The number inside each star indicates

640

the epicentral depth. The main modern and ancient cities in the area are indicated by

641

black circles and triangles, respectively. Sp= Spili Fault, As= Asomatos Fault, AG= Agia

642

Galini Fault, K=Klima Fault, AT=Agia Triada Fault, Sf= Sfakia Fault, R=Rethymno,

643

Ch=Chania, Ph=Phaistos, G=Gortina. Faults are adopted from Caputo et al. (2010) and

644

Mountrakis et al. (2012). Inset: Map illustrating the Hellenic subduction zone and the

645

location of the island of Crete. The study area is enclosed within the box.

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Figure 2: a) The surface trace of the Spili Fault extends NE-SW (partly marking the base

648

of the Kedros ridge). The solid faultline indicates sections of the fault that the carbonate

649

plane is exhumed whereas the dashed line depicts sections where the fault trace is

650

concealed or unclear. The paleoearthquake sites are indicated by yellow (36Cl) and grey

651

(REE) circles, respectively. The four fault-kinematic sites are indicated by white circles.

652

Lower hemisphere stereonets for each kinematic locality (K1-K4) are illustrated (the

653

numbers within each stereonet indicate the locality and the deformation phase recorded;

654

see text for details); b) Displacement (throw) profile along the Spili Fault. Displacements

655

derive from localities along the solid line only (where the fault plane is exhumed). The

656

two paleoearthquake sites A and B of this study and the REE-site of Mouslopoulou et al.

657

(2011) are indicated on the profile.

659

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AC C

658

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Figure 3: Geological map the study area. Map modified from Papavassiliou et al. (1985)

660

29

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Figure 4: Geomorphologic and kinematic markers indicating recent activity on the Spili

662

Fault: dip-slip striations on limonite layers on the exhumed fault plane (a); the exhumed

663

fault plane at site B (b) and at the REE site (c); the Spili Fault displacing an alluvial fan

664

(d). Note the normal drag on the fan deposits.

665

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661

Figure 5: Sampling the exhumed carbonate plane of the Spili Fault for cosmogenic

667

dating at Site A (a) and Site B (b).

668 36

SC

666

669

Figure 6: Modelling (green bars) of the

670

from the lower 9 m of the Spili Fault at site A. Each horizontal line represents a

671

discontinuity that is interpreted to result from at least one paleoearthquake. The timing

672

and slip-size of each earthquake is indicated on the graph. b) About 1.5 km southeast of

673

site A, Mouslopoulou et al. (2011) identified, using the REE-method, that a minimum of

674

five paleoearthquakes exhumed the lower ca. 10 m of the fault plane. The two methods

675

compare favourably.

EP

676

TE D

M AN U

Cl concentrations (black circles) that derive

Figure 7: Plot illustrating the displacement (throw) accumulation on the Spili Fault

678

during the last 16.5 kyr. Each step corresponds to an earthquake event. The fault

679

displacement rate averaged over various time intervals is indicated. The Minoan period is

680

highlighted.

681

AC C

677

682

Table 1: Table illustrates the results of the fault kinematic inversions that characterise the

683

Plio-Quaternary stress regimes proximal to the Spili Fault, central Crete. See the

30

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supplementary electronic material for further details on the inversion method utilised

685

[e.g., Carey, 1979]. Site numbers refer to locations in Figure 2a. N= number of fault slip

686

pairs considered for stress calculation; stress regimes are all extensional (horizontal σ2,

687

σ3); Q=quality: A, constrained; B, poorly-constrained solutions. For the fault data

688

populations comprised of less than four well-distributed fault directions, a “fixed”

689

solution [Bellier and Zoback, 1995] was applied, in which the principal stress axes are

690

fixed to lie in horizontal and vertical planes; these results are identified by a star

691

associated to the site label – example: K4*. CH indicates the local (site) chronology (slip

692

generation). All angles are in degrees.

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SC

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684

693

Table 2: The Ca, Cl and 36Cl concentrations measured on each sample as a function of

695

fault scarp height.

AC C

EP

TE D

694

31

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Latitude (°N)

K1 K1

24,54 24,54

35,22 35,22

K2 K2 K3 K3 K3 K4* K4*

24,54 24,54 24,54 24,54 24,54 24,54 24,54

35,22 35,22 35,21 35,21 35,21 35,21 35,21

N

Q

CH

Kinematic event 2 Stress axis (trend/plunge) R σ1 σ2 σ3 135/90

316/0

226/0

0.58

N

Q

CH

13

A

1°

Kinematic event 3 Stress axis (trend/plunge) R σ1 σ2 σ3

176/88

1°

NE-SW

E-W

1° 2°

NE-SW NE-SW

3

B

267/0

357/2

0.78

N

Q

CH

7

A

2°

N-S

2°

N-S

3°

1°

127/90

278/0

8/0

0,74

10

A

2°

Lithology Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone

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(°E)

Kinematic event 1 Stress axis (trend/plunge) R σ1 σ2 σ3

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Longitude

SC

Site

Table 1: Table illustrates the results of the fault kinematic inversions that characterise the Plio-Quaternary stress regimes proximal to the Spili Fault, central Crete. See the supplementary electronic material for further information on the inversion method utilised [e.g., Carey, 1979]. Site numbers refer to locations in

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Figure 2a. N= number of fault slip pairs considered for stress calculation; stress regimes are all extensional (horizontal σ2, σ3); Q=quality: A, constrained; B, poorly-constrained solutions. For the fault data populations comprised of less than four well-distributed fault directions, a “fixed” solution [Bellier and Zoback, 1995] was applied, in which the principal stress axes are fixed to lie in horizontal and vertical planes; these results are identified by a star associated to

AC C

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the site label – example: K4*. CH indicates the local (site) chronology (slip generation). All angles are in degrees.

31

TE D

% CaO 55% 55% 55% 56% 56% 55% 55% 55% 56% 56% 55% 55% 56% 55% 55% 55% 54% 54% 55% 54% 55% 57% 56% 55% 55% 56% 56% 56% 56% 56% 56% 55% 56% 56% 55% 56% 56% 56%

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SC

±% 1.8% 2.3% 2.4% 2.0% 2.1% 1.8% 2.3% 2.2% 1.9% 1.8% 0.4% 0.3% 0.3% 0.4% 0.4% 0.4% 0.3% 0.4% 0.4% 0.6% 0.4% 0.3% 0.3% 0.4% 0.4% 0.4% 0.4% 0.4% 0.5% 0.3% 0.3% 0.3% 0.4% 0.5% 0.3% 0.4% 0.4% 0.4%

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% Ca 39.5% 39.6% 39.5% 40.1% 39.8% 39.5% 39.6% 39.2% 39.7% 40.0% 39.5% 39.2% 39.9% 39.1% 39.0% 39.4% 38.9% 38.9% 39.0% 38.6% 39.5% 40.4% 39.9% 39.3% 39.6% 39.8% 39.7% 39.8% 39.8% 40.0% 39.8% 39.2% 39.9% 40.0% 39.1% 39.8% 39.9% 39.7%

EP

Sample ID HC-2 HC-4 HC-6 HC-08B HC-10 HC-12 HC-14 HC-16 HC-18 HC-20 HC-21 HC-23 HC-25 HC-27 HC-29 HC-31 HC-33 HC-35 HC-37 HC-39 HC-41 HC-43 HC-45 HC-47 HC-49 HC-51 HC-53 HC-55 HC-57 HC-59 HC-61 HC-63 HC-65 HC-67 HC-69 HC-71 HC-73 HC-75

AC C

Site A

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40.0% 40.1% 39.9% -

0.4% 0.4% 0.3% -

56% 56% 56% -

PL 01B PL 03B PL 05B Pl-5 PL 07B PL 09B PL 11B Pl-11 PL 13B PL 15B Pl-15 PL 17B PL 19B BK PLB

24.7% 24.6% 25.2% 24.3% 25.1% 24.3% 24.9% 24.5% 25.1% 26.0% 25.5% 27.0% 28.6% -

2.0% 2.0% 2.0% 1.1% 2.0% 2.0% 2.0% 1.1% 2.0% 2.0% 1.3% 2.0% 2.0% -

34% 34% 35% 34% 35% 34% 35% 34% 35% 36% 36% 38% 40% -

EP

TE D

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SC

RI PT

HC-77 HC-79 HC-81 BK HC 1 BK HC 2

AC C

Site B

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± ppm 1 1 1 0 0 1 1 1 1 1 0 0 0 0 1 0 0 1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0

SC

[Cl] ppm 11 14 10 8 8 12 13 16 14 16 5 9 15 12 13 8 8 7 11 20 15 9 15 16 21 18 10 8 17 21 4 3 3 3 2 4 4 6

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±% 4 6 2 4 1 0 2 0 0 0 1 1 1 1 4 1 1 6 4 1 1 3 1 1 1 1 1 1 2 1 1 3 7 1 3 1 2 3

EP

AC C

35Cl/37Cl 16 13 17 27 21 15 14 12 13 12 35 24 15 18 18 25 25 27 19 12 16 24 15 15 12 14 20 27 14 13 40 52 62 63 65 49 40 33

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3 6 3 3 4

4 5 5 -

0 0 0 -

9 8 8 7 7 6 6 5 6 6 6 6 6 511

2 1 2 0 2 2 2 1 1 1 0 1 2 2

43 54 49 43 57 82 72 75 77 72 68 89 91 -

2 2 2 2 3 4 3 4 3 2 3 3 5 -

SC

M AN U

TE D EP AC C

RI PT

46 36 41 312 295

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SC

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36Cl at / g rock 53740 53905 46114 51627 52177 54860 48860 37883 33287 31894 52289 62223 72001 75619 82338 90727 108039 136640 152708 164272 159338 172477 178675 195647 204573 225649 230347 246929 240763 263236 267700 287788 273602 285345 268896 271381 283715 283520

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±% 5.9 6.3 4.9 4.6 4.6 4.9 5.9 4.8 6.5 6.8 4.4 4.5 4.2 3.9 11.2 3.9 3.1 9.5 3.7 3.1 3.0 3.9 2.7 2.9 3.0 2.6 2.5 2.5 7.7 3.6 3.3 3.4 4.4 3.1 5.5 2.6 3.6 2.5

EP

AC C

36Cl/35Cl 1.20E-13 1.14E-13 1.11E-13 6.49E-14 1.22E-13 1.15E-13 1.08E-13 9.02E-14 8.79E-14 8.39E-14 6.17E-14 6.86E-14 7.42E-14 8.23E-14 8.53E-14 1.02E-13 1.22E-13 1.57E-13 1.64E-13 1.59E-13 1.63E-13 1.82E-13 1.82E-13 1.92E-13 1.91E-13 2.19E-13 2.49E-13 2.66E-13 2.39E-13 2.32E-13 3.20E-13 3.45E-13 3.32E-13 3.38E-13 3.21E-13 3.22E-13 3.33E-13 3.29E-13

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3.7 4.1 2.8 20.9 30.2

292442 283710 286884 -

5.57E-14 6.04E-14 5.35E-14 9.46E-14 5.25E-14 5.05E-14 4.69E-14 7.35E-14 4.90E-14 4.72E-14 7.86E-14 4.24E-14 4.56E-14 7.29E-15

5.4 4.5 4.8 5.2 4.2 5.5 5.9 4.6 4.5 4.2 6.5 4.6 4.8 14.9

79816 96117 77673 77866 81353 96611 82605 82292 92798 83074 84731 84395 96656 -

AC C

EP

TE D

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SC

RI PT

3.44E-13 3.24E-13 3.33E-13 1.38E-15 6.67E-16

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SC

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Height (cm) 188 169 149 129 109 89 69 50 30 10 205 225 245 265 285 305 325 345 365 385 405 425 445 465 485 505 525 545 565 585 605 625 645 665 685 705 725 745

TE D

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%RSD 11 12 11 5 10 10 12 12 16 17 5 5 4 4 11 4 3 10 4 3 3 4 3 3 3 3 3 3 8 4 3 4 5 3 6 3 4 3

EP

AC C

Incertitude 6119 6578 4926 2821 5075 5448 5896 4617 5289 5405 2410 2897 3124 3032 9326 3627 3494 13159 6020 5238 4951 7041 5020 5868 6468 6218 6127 6523 18805 9833 9290 10090 12382 9379 15182 7488 10649 7661

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4 4 3 -

765 785 805 -

3776 3701 3346 7126 3209 4137 3743 7203 3523 2879 9597 3062 4357 -

5 4 4 9 4 4 5 9 4 3 11 4 5 -

197 177 156 156 136 116 96 96 76 55 55 35 16 -

AC C

EP

TE D

M AN U

SC

RI PT

11100 12174 8553 -

AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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900

800

SC

700

M AN U

600

500

PL

TE D

400

HC

EP

300

AC C

200

100

0 0

50000

100000

150000

200000

250000

300000

350000

RE

EC

E

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G

SC

H c

ni le el

Ch

M5.5 /1973

AC C

Major cities Major ancient cities Paleoseismic sections (36 Cl)

EP

Paleoseismic section (REE)

Figure 1

M AN U

TE D

2454m

M5.6/1994 80

Sp

Sf

Fig. 2

Ps

24

ilo

56

m

As AG

rit

is

K

Ph

N AT

20 km

in

10

g ar

M6.1 1972

African Plate

R

Ori

L 50

M

Θάλασσα Κρήτης

a efk

Eurasian Plate

G

Messara ba

sin

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K1

K1

K1-2

2

3

a

K3 K2

1

1

2

K4

K3-1

3

K1-3

3 K4-3

K3-2

K2-2

2

K2-3

ili F

1

RI PT

K3-3

Sp

aul

t

Ke

dr

REE

As

M AN U

A

SC

2

os

17

B

N

3

m

rid

ge

Fig

.3

Sp

ili F

au

lt

b

25

NW

AC C

EP

5 km

1

50

TE D

1140m

K4 K4-2

Spili Fault

Throw (m)

20 ) 15 m ( w 10 o r h T 5

A

SE

B

REE

Displacement deficit

splay fault

0 -2 0

Figure 2

0 2

42

46 68 Distance along strike (NW-SE)

Distance along strike (km) (NW-SE)

8 10

10 12

12 14

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553000

3897000

549000

oli

ic ss ra sts Ju chi s

ip Tr REE

RI PT

Site A

lime

sto

lom

er

ate

ch

EP

N

ys

TE D

Site B

ng

Figure 3

M AN U

Fl

co

AC C

3892000

es

h

lit

sc

io

y Fl

ic ss ra sts Ju chi s ph

tone

imes

sL Pindo

P co liong Ple lo is m to er ce at n e e To rto ni an

O

SC

ne

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Dip-slip striations

RI PT

b

SC

a

M AN U

Fault scarp at site B d

AC C

EP

TE D

c

Fault scarp at the REE site

Figure 4

Normal drag on alluvial fan

b

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M AN U

SC

RI PT

a

AC C

EP

TE D

Figure 5

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a

RI PT

1.8 m

8

1.2 m

SC b

EQ-1

EQ-2

EQ-3

EQ-4

EQ-5

2.7m

2.5m

2m

1.7m

1.3m

3

1.5

S1

Modern soil

4

REE ( i

m)

M AN U

4.5

2.5 m

TE D

Fault scarp height (m)

6

S2

S3

S4

S5

9

10

0

EP

2.5 m

AC C

2

-1.5 -1

2 5 Cl atom/g rock x 10

36

Figure 6

1

2

3

4

5

6

7

Fault scarp height (m)

1m

0 0

0

4

8

SC

RI PT

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Minoan period

6 3.5 mm/yr

4

EP

2 0 0

2

4

6

8

10

Time (kyr BP)

Figure 7

yr

m/

m 0.6

TE D

8

AC C

Cumulative Slip (m)

M AN U

Slip accumulation through time

10

12

14

16

18

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Research Highlights:

RI PT

First quantitative paleoearthquake study on Crete. The Spili Fault ruptured a minimum of five times over the last 16500 years. Displacement rate and earthquake recurrence interval on the fault highly variable. Two large earthquakes in the last millennium call for seismic risk re-evaluation. Spili Fault may be one of the fastest moving faults on the Hellenic forearc.

AC C

EP

TE D

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SC

• • • • •

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Supplementary electronic material for JSG article

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Normal faulting in the forearc of the Hellenic subduction margin: Paleoearthquake history and kinematics of the Spili Fault, Crete, Greece Vasiliki Mouslopoulou1,2*, Daniel Moraetis3, Lucilla Benedetti4,

1

Department of Mineral Resources Engineering, Technical University of Crete, 73100, Greece

GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Section 3.1, Germany

M AN U

2

SC

Valery Guillou4, Oliver Bellier4, Dionisis Hristopulos1

3

Sultan Gaboos University, College of Earth Science, PO Box 36, P.C. 123, Muscat, Oman

4

Aix-Marseille Université, CEREGE CNRS-IRD UMR 34, 13545 Aix en Provence, France

Inversion of fault-slip data

TE D

Methodology: Inversion method and data separation

AC C

EP

To determine the late Cenozoic states of stress proximal to the Spili Fault, south-central Crete, we performed a quantitative inversion of distinct families of fault slip data measured at individual sites along the fault (Table 1 and Fig.2a), using the method originally proposed by Carey (1979). This fault kinematics inversion method computes a mean best fitting deviatoric stress tensor from a set of striated faults by minimizing the angular deviation (misfit angle) between a predicted slip-vector and the observed striation (Carey, 1979). The inversion results include the orientation (azimuth and plunge) of the principal stress axes (σ1>σ2>σ3, corresponding to maximum, intermediate and minimum stress axis, respectively) of a mean deviatoric stress tensor as well as a “stress ratio” R = (σ2-σ1)/ (σ3-σ1). This linear parameter (R) describes relative stress magnitudes ranging from 0 to 1 (e.g., Carey, 1979; Mercier et al., 1991; Bellier and Zoback, 1995, and references therein). Data separation

The main assumption in all fault kinematics inversion schemes is that slip responsible for the striation occurs on each fault plane in the direction and sense of the shear stress resolved on the fault plane. Therefore, a distinct stress deviator (σ1, σ2, σ3, and R) can only produce one slip direction on a given fault plane. In the field, often more than one generation of striae are detected on a fault plane and/or at the same measurement locality. This may be due to (1) multiple slip within single event, (2) changes in slip *

Corresponding author. Email address: [email protected]; Tel.: ++49 3312881312; Fax: ++49 331 288 1370

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directions due to changes in the fault strength or local boundary conditions, or (3) temporal changes in regional state of stress due to distinct tectonic events that are representative for changes in boundary conditions in the scale of plate tectonics. In a regionally homogeneous stress field, the two first ones are expressed as local heterogeneities of the regional stress field and are easily distinguishable from the regional changes in the state of stress. However, the separation of fault slip data from different tectonic events remains the main problematic subject in the inversion of fault kinematics. In the present study, distinct datasets were separated using direct evidence that is, the relative chronology of successive fault movements expressed in crosscutting relationships between different fault slip-vectors and/or fault planes.

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The fault kinematic data have been measured in post-Pliocene age deposits. Such a measurement strategy eliminated the slip evidence of pre-Pliocene tectonic events in our data sets and consequently, the inconvenient effects of paleo tectonics on fault kinematic analysis were discarded. In spite of this systematic geological selection of data, in almost all measured outcrops, we observed always two, and sometimes three, slip events evidenced by clear crosscutting striations on fault planes (Fig. 2a). Fault slip populations comprised of 3 different tectonic events, which could be manually separated into appropriate data sets (Table 1). For the fault data populations, a fixed solution (Bellier & Zoback, 1995) was applied, in which the principal stress axes are fixed to lie in horizontal and vertical planes. In this fixed inversion one requires only two independent fault sets examining compatibility of fault planes with a homogeneous regional stress field or stress tensors deduced from nearby sites in the same tectonic context, these unconstrained deviatoric stress tensors were only served for deducing the direction of principal stress axes and not for R value analysis.

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The stress ratio, R = (σ2-σ1)/ (σ3-σ1), describes the relative stress magnitudes of the calculated mean deviatoric stress tensors. The significance of stress-ratio in interpreting inversion results was already discussed by previous authors (e.g., Bellier & Zoback, 1995).

Easting

Northing

Azimuth (°)

Dip (°)

Dip sense

Pitch (°)

Pitch sense

Movement

K1

24,54

35,22

135

71

W

85

S

N

K1

24,54

35,22

134

72

S

88

E

N

K1

24,54

35,22

136

73

W

89

S

N

K1

24,54

35,22

137

73

W

85

N

N

K1

24,54

35,22

136

73

W

89

N

N

K1

24,54

35,22

142

76

W

78

S

N

K1

24,54

35,22

142

75

W

87

S

N

K1

24,54

35,22

130

55

S

79

W

N

K1

24,54

35,22

128

51

S

82

W

N

K1

24,54

35,22

125

52

S

K1

24,54

35,22

125

52

S

K1

24,54

35,22

134

52

S

K1

24,54

35,22

131

57

S

SC

K1

24,54

35,22

131

57

K1

24,54

35,22

170

K1

24,54

35,22

5

K1

24,54

35,22

80

K1

24,54

35,22

80

K1

24,54

35,22

50

K1

24,54

35,22

50

K1

24,54

35,22

K1

24,54

35,22

K2

24,54

35,22

K2

24,54

35,22

K2

24,54

35,22

K3

24,54

K3

24,54

K3

24,54

K3

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Site ID

W

N

60

E

N

85

W

N

45

E

N

S

78

E

N

85

W

60

S

N

55

W

58

S

N

25

N

92

PV

N

25

N

152

PV

N

35

N

80

PV

SN

35

N

155

PV

N

TE D

M AN U

85

73

W

70

S

N

130

68

S

89

E

N

162

58

W

50

S

N

164

54

W

80

S

N

164

54

W

54

S

N

35,21

144

58

W

65

S

N

35,21

145

56

W

62

S

N

35,21

2

52

W

60

S

N

EP

138

AC C

Spili Fault

ACCEPTED MANUSCRIPT

24,54

35,21

178

50

W

65

S

N

24,54

35,21

65

65

N

35

S

S

24,54

35,21

66

66

N

36

S

S

24,54

35,21

1

48

W

60

S

N

24,54

35,21

162

45

W

85

PV

N

K4

24,54

35,21

138

56

S

70

S

N

K4

24,54

35,21

132

52

S

73

S

N

K4

24,54

35,21

128

50

S

78

E

N

K4

24,54

35,21

136

59

S

80

E

N

K4

24,54

35,21

136

62

S

70

S

N

K3 K3 K3 K3

Deformation phases 2° & 3°

Deformation phases 2° & 3°

Deformation phases 1°, 2° & 3°

Deformation phases 2° & 3°

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Easting

Northing

Azimuth (°)

Dip (°)

Dip sense

Pitch (°)

Pitch sense

Movement

K4

24,54

35,21

118

54

S

86

E

N

K4

24,54

35,21

136

60

W

80

S

N

K4

24,54

35,21

105

51

S

68

W

N

K4

24,54

35,21

105

50

S

60

W

N

K4

24,54

35,21

105

50

S

85

E

N

K4

24,54

35,21

104

50

S

65

W

N

K4

24,54

35,21

104

50

S

84

E

N

K4

24,54

35,21

124

54

S

85

E

N

SC

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Site ID

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Table illustrating the main kinematic attributes of the Spili Fault as measured at four localities along its exhumed plane.