Geophysical Journal International

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1 Geophysical Journal International Geophys. J. Int. (2011) 187, doi: /j X x Palaeoseismology of the L Aquila faults (central Italy, 2009, M w 6.3 earthquake): implications for active fault linkage Paolo A.C. Galli, 1,2 Biagio Giaccio, 2 Paolo Messina, 2 Edoardo Peronace 2 and Giovanni Maria Zuppi 2 1 Dipartimento della Protezione Civile Nazionale, Rome, Italy. paolo.galli@protezionecivile.it 2 Istituto di Geologia Ambientale e Geoingegneria, CNR-IGAG, Monterotondo Scalo, Rome, Italy Accepted 2011 September 9. Received 2011 August 22; in original form 2011 April 29 SUMMARY Urgent urban-planning problems related to the 2009 April, M w 6.3, L Aquila earthquake prompted immediate excavation of palaeoseismological trenches across the active faults bordering the Aterno river valley; namely, the Mt. Marine, Mt. Pettino and Paganica faults. Cross-cutting correlations amongst existing and new trenches that were strengthened by radiocarbon ages and archaeological constraints show unambiguously that these three investigated structures have been active since the Last Glacial Maximum period, as seen by the metric offset that affected the whole slope/alluvial sedimentary succession up to the historical deposits. Moreover, in agreement with both 18th century accounts and previous palaeoseismological data, we can affirm now that these faults were responsible for the catastrophic 1703 February 2, earthquake (M w 6.7). The data indicate that the Paganica San Demetrio fault system has ruptured in the past both together with the conterminous Mt. Pettino Mt. Marine fault system, along more than 30 km and causing an M w 6.7 earthquake, and on its own, along ca. 19km, as in the recent 2009 event and in the similar 1461 AD event. This behaviour of the L Aquila faults has important implications in terms of seismic hazard assessment, while it also casts new light on the ongoing fault linkage processes amongst these L Aquila faults. Key words: Palaeoseismology; Seismicity and tectonics; Continental tectonics: extensional; Tectonics and landscape evolution; Neotectonics. GJI Geodynamics and tectonics INTRODUCTION The 2009 L Aquila earthquake struck in a previously well-studied, and apparently well-understood, area, at least as far as active tectonics are concerned. Active faults and related fault scarps are well preserved in the carbonate mountains of Abruzzo (a region in central Italy; Fig. 1, right-upper panel), and they have been traced on maps and studied since the 1970 s (Bosi 1975; Galadini & Galli 2000; Boncio et al. 2004; Roberts & Michetti 2004). Fifty-six palaeoseismological trenches have been excavated across these faults (Galli et al. 2008), with the aim of: (1) documenting conclusively their recent activity; (2) quantifying the Late Pleistocene Holocene sliprate and (3) determining the age of the palaeoearthquakes. However, only one has been completed in the vicinity of the 2009 epicentre [Fig. 1, Mt. Marine Fault (MMF); Moro et al. 2002], and that was not on the Paganica San Demetrio fault system, that is, the causative structure of this 2009 earthquake. Moreover, this structure (Fig. 1, PSDFS; Galli et al. 2009; 2010; Boncio et al. 2010) was little known (see in Galadini et al. 2009), roughly traced (e.g. in Bagnaia et al. 1992; Galadini & Galli 2000; Boncio et al. 2004), and undocumented in terms of its recent activity, slip-rates and palaeoearthquakes. During the emergency management of the 2009 earthquake, the Italian Department of Civil Protection was involved in urgent urbanplanning studies that were aimed at the construction/reconstruction of buildings and/or the restoration of damaged structures located along suspected active faults. Thus, we started to investigate the traces of the various potentially active fault splays that are close to urban settlements, with the digging of explorative trenches across them. The first of these was excavated on the outskirts of L Aquila, across the Mt. Pettino Fault scarp (Fig. 1, MPF; 2009 May), with a second explorative trench across the Mt. Marine Fault inside Arischia village (Fig. 1, MMF; 2009 June). The data from these trenches have been compared with palaeoseismological data obtained a few years ago near the village of Pizzoli (Moro et al. 2002) that we improved through new radiocarbon dating of samples collected at the time (all 14 C dates are given as 2σ calibrated ages in Table 1). As far as the Paganica San Demetrio fault system is concerned, between 2009 April and June, we exploited a long gorge that opened during the coseismic breakage of the Gran Sasso aqueduct, from which we obtained valuable palaeoseismic data. We also dug a new trench in the southeastern sector of Paganica village (2009 June), while other data were collected in a pit dug across the Colle Enzano Fault segment (2009 April), and in an ephemeral C 2011 The Authors 1119

2 1120 P.A.C. Galli et al. Figure 1. Simplified map of Quaternary fluvial-lacustrine basins along the present Aterno River and primary active faults of the region (shaded relief from 20-m DEM). 1, Late Pleistocene Holocene eluvial alluvial deposits and alluvial fans; 2, Late Pleistocene slope-alluvial deposits; 3, Middle Pleistocene fluvial sand and gravel and equivalent slope deposits; 4, Lower Pleistocene white lacustrine silts, fan-delta gravels, slope breccias. Primary active faults of the eastern fault system (EFS; in red those investigated in this paper) are: MMF, Mt. Marine Fault; MPF, Mt. Pettino Fault; PSDFS, Paganica San Demetrio fault system; MAFS, Middle Aterno fault system; CFCCFS, Campo Felice Colle Cerasitto fault system. WFS, Western fault system; MSFF, Mt. San Franco Fault; CIFS, Campo Imperatore fault system. Focal mechanisms from Pondrelli et al. (2010). Dashed rectangle a is Fig. 2; b, Fig. 6; c, Fig. 8. T1 6 and P1 are the palaeoseismological sites described in the text. Note the complex fault array of the PSDFS, made of several right-en echelon steps, compared to the continuous fault segments of MMF and MPF. Panel A: epicentres distribution of earthquakes with M w > 5.9 in central Italy and primary active faults (from Galli et al. 2008). Empty arrows, GPS-derived extension rates (mm yr 1 ; from Serpelloni et al. 2005; D Agostino et al. 2011; Giuliani et al. 2009). Cones are the Quaternary volcanoes of the Roman comagmatic province Auctorum. NAA SAA, front of northern and southern compressive Apennine arcs; a-b-c-d are the Fucino, Umbria-Marche, Norcia and Mt. Marzano fault systems, respectively. Red box is the area of the main panel, which matches with part of the inner Abruzzo region.

3 Palaeoseismology of L Aquila faults 1121 Table 1. Summary table of the 14 C ages of samples collected in the trenches dug across the L Aquila faults. RAD, standard radiometric; AMS, accelerator mass spectrometry (Beta Analytic Inc. laboratory, Miami, Florida). All age have been calibrated and or recalibrated by using the software Calib (Stuiver et al. 2010). Dated Measured Radioc. Age 2σ Trench Sample Analysis material (BP) Calibration Faults References PIZ3 AMS Charred material ± BP MMF Moro et al PIZ4 RAD Palaeosol ± BP MMF Moro et al PIZ7 AMS Organic sediment ± BP MMF Moro et al PIZ10 AMS Organic sediment ± BP MMF Moro et al T1 PIZ12 AMS Organic sediment 3880 ± BP MMF Moro et al PIZ14 AMS Organic sediment 3890 ± BP MMF Moro et al SPIZ5 AMS Charred material 205 ± AD MMF This paper - AMS Charred material ± BP MMF Galadini & Galli AMS Charred material ± BP MMF Galadini & Galli 2000 ARARA4 AMS Organic sediment 7840 ± BP MMF This paper T2 ARARA3 AMS Organic sediment 3470 ± BP MMF This paper ARARA1 AMS Organic sediment ± BP MMF This paper PET-05 AMS Organic sediment ± BP MPF This paper T3 PET-04 AMS Organic sediment 7730 ± BP MPF This paper PET-02 AMS Organic sediment ± 730 MPF This paper PET-C2 AMS Charred material ± 520 MPF This paper PAG-04 AMS Organic sediment ± BP PSDF Galli et al PAG-05 AMS Organic sediment 4580 ± BP PSDF Galli et al PAG-06 AMS Organic sediment 3760 ± BP PSDF Galli et al T4 PAG-07 AMS Organic sediment 5500 ± BP PSDF Galli et al PAG-C2 AMS Charred material 4910 ± BP PSDF Galli et al PAG-C4 AMS Charred material NA AD PSDF Galli et al PAG-C6 AMS Charred material 2540 ± BP PSDF Galli et al T5 TEMP-01 AMS Organic sediment 1200 ± AD PSDF Galli et al TEMP-02 AMS Organic sediment 1690 ± AD PSDF Galli et al TEMP-03 AMS Organic sediment 2380 ± BP PSDF Galli et al P1 ENZA-01 AMS Organic sediment 4020 ± BP PSDF Galli et al water-supply trench that was excavated across the breaching faults between Paganica and San Gregorio (see Fig. 1; 2009 October). These latter data have already been published (Galli et al. 2010), and will just be summarised here. OVERVIEW OF THE SEISMOTECTONICS OF ABRUZZO The NE SW extensional processes that affected the Apennine foldand-thrust belt in the Pleistocene are currently mainly focused along the axis of the chain (see Fig. 1, panel A). Their long-term geological signature is revealed by the SW-dipping normal faults that delimit the intermontane basins (Galadini & Galli 2000; Boncio et al. 2004; Roberts & Michetti 2004), while their ongoing movement has been shown by GPS velocities that do not exceed 3 mm yr 1 in the L Aquila Apennines (D Agostino et al. 2011). Here, the active faults can be roughly grouped into two main systems: one running along the chain axis (western fault system; WFS), and the other close to its eastern front (eastern fault system; EFS; Galadini & Galli 2000; see Fig. 1 and its caption). Primary faults are generally arranged in systems of 3 5 en-echelon segments, each of which is 5 20 km long (Bosi 1975; Cello et al. 1997; Galadini & Galli 2000; Roberts & Michetti 2004). With few exceptions, the entire length of each fault system does not exceed 30 km, and according to existing palaeoseismological studies, the fault systems are generally characterised by a 1 2 kyr recurrence time for M w > 6.5 earthquakes (Galli et al. 2008). In particular, GPS analyses of D Agostino et al. (2011) placed the ongoing extension mainly across the WFS, which is the one responsible for almost all of the strongest earthquakes in the Apennines (up to M w 7 events; see Galli et al. 2008). In turn, the EFS [Fig. 1, Mt. San Franco Fault (MSFF) and the Campo Imperatore fault system (CIFS)] has not produced historical earthquakes, although palaeoseismological studies show that it did slip during the Holocene (Galli et al. 2008). Considering now the historical seismicity of the L Aquila region, both moderate-magnitude and high-magnitude earthquakes have had epicentres in the Aterno valley area itself (see Fig. 1, panel A). Even if no local written sources exist that deal with earthquakes before the 14th century, bearing in mind that the most severe damage in Rome has always been due to earthquakes that originated in Abruzzo (e.g. Molin et al. 1995; Galadini & Galli 2001; Galli & Naso 2009), it is possible that some of those on record during the High Middle Ages were caused by the Aterno Valley faults. Amongst these, the strongest occurred in AD 801, which caused the collapse of some ancient buildings in Rome, and which was felt all along the whole Apennine chain, although there is no reliable epicentre known (Molin et al. 1995). The first event that seriously rocked L Aquila and the neighbouring villages was in 1349 September (M w 6.5 in the CPTI Working Group CPTI 2004), which resulted in 800 fatalities in the newly founded town (end of the 13th century). In 1461 November, L Aquila was hit again by a M w 6.4 earthquake (CPTI04), which razed to the ground many buildings and most of the churches. It rocked the same villages hit by the 2009 earthquake (Galli et al. 2009), including Castelnuovo, Onna, Poggio Picenze and Sant Eusanio Forconese [all with Mercalli- Cancani-Sieberg (MCS) intensities of 9 10]. On 1703 February 2, a devastating earthquake (M w 6.7; CPTI04) razed to the ground all of the villages spread along the upper Aterno Valley, together with L Aquila (MCS 9). Also in this case, most of the villages struck by the 1461 and 2009 events suffered serious damage (MCS 8 9),

4 1122 P.A.C. Galli et al. including again Poggio Picenze, San Gregorio, Sant Eusanio Forconese, Paganica, Onna and Castelnuovo, where it reached MCS 10. For the 1703 earthquake, coeval historic accounts described the opening of long chasms along the foothill facing the northern Aterno River Valley, between Pizzoli and Arischia (see Fig. 1; Lorenzani 1703; Uria de Llanos 1703; De Carolis 1703; Baglivi 1710; Antinori, 18th century; Cappa 1871). These are interpreted today as surface faulting phenomena along the Mt. Marine Fault. As will be shown below, this has been confirmed through the palaeoseismological trench that was dug across this fault. OVERVIEW OF THE QUATERNARY BASIN GEOLOGY AROUND L AQUILA Since the Early Pleistocene, the intermontane basins of Abruzzo have hosted isolated lakes that were fed by a centripetal drainage network (Messina & Galadini 2004; Messina et al. 2007). Starting from the Middle Pleistocene, due to large-scale uplift, most of these were drained off through regressive erosion, with the exception of a few where normal fault activity (or exceptional travertine growth) preserved their damming. The geological evolution of the entire Aterno Valley area is characterized by a progressive downward embedding of forms and deposits that has been driven by the continuous deepening of the hydrographic network. This process has also been controlled by the activity of the tectonic structures that delimit the southwestern slopes of the carbonate ridges, and in particular by those faults that we have investigated for this study: namely the Mt. Marine Fault in the north, the Mt. Pettino Fault near L Aquila and the Paganica San Demetrio fault system in the south. In the northern Aterno Valley, that is, in the hangingwall of both the Mt. Marine and Mt. Pettino faults (Fig. 1), the lower continental deposits consistof>60 m thick conglomerates and alternating silty sands and gravels (potentially Pliocene; Messina et al. 2001; Bosi et al. 2003; Messina et al. 2003) that dip north and northeast. They are capped by >30 m thick fluvial sandy-silty gravels, with reversed magnetic polarity (Lower Pleistocene; Messina et al. 2001). In the Scoppito area (Fig. 1), the finest levels contain Arkidiscon meridionalis vestinus (Azzaroli 1983), which grade upwards to gravels (Madonna della Strada fm.), and well stratified breccias (San Marco Breccias), with reversed magnetic polarity (Messina et al. 2001). During the Middle Pleistocene, sandy-clayey silts concluded the fluvial cycle, which evolved upwards to lacustrine conditions (Messina et al. 2001; Bosi et al. 2003; Messina et al. 2003). This upward cycle was ended by the sedimentation of >25 m thick subhorizontal fluvial gravels, and sandy lenses with abundant Late Pleistocene volcanic material (small scoria, pyroxenes, biotite; Messina et al. 2001; Bosi et al. 2003; Messina et al. 2003). These grade into the thin fluvial sandy gravels of the present Aterno River, interfingering with slope deposits and alluvial fans (Upper Pleistocene; Bosi et al. 2003; Bosi et al. 2004), which were mainly controlled by climatogenic processes (Giraudi et al. 2011) and which formed the cores of our trenches. In turn, south of L Aquila, at the hangingwall of the Paganica Fault and at the hangingwall and footwall of both the San Gregorio and San Demetrio faults, the continental terms of the Quaternary basin of Paganica-San Demetrio-Barisciano were grouped by Bosi & Bertini (1970) and Bertini & Bosi (1993) into two main fluviolacustrine complexes. Then recently, Galli et al. (2010), Giaccio et al. (2010) and Messina et al. (2011) proposed a reorganization of the stratigraphic framework into three main fluvio-lacustrine depositional units, which were partly encountered in our trenches: (1) An Early Pleistocene complex (Matuyama reversed polarity epoch; ca Ma; Giaccio et al. 2010; Speranza, personal communication, 2010; Messina et al. 2011), which includes more than 100 m of whitish carbonate lacustrine silts (San Nicandro fm. of Bertini & Bosi 1993) that are interfingered with a deltaic gravelsand complex (up to 30 m thick), which includes lacustrine foreset (Valle Orsa fm.) and fluvial-topset (Valle dell Inferno fm.) deposits; an alluvial fan (Valle Valiano fm.) and slope-derived limestone breccias with a characteristic pink matrix (Fonte Vedice fm.). (2) A ca. 60-m-thick Middle Pleistocene fluvio-lacustrine complex (i.e. spanning ca ka; Galli et al. 2010) that is entirely entrenched in the former unit, which is made up of volcanic, rich silty sands and, subordinately, gravels (San Mauro fm.). (3) An Upper Pleistocene complex that is mainly represented by the Paganica alluvial fan and other minor comparable systems. These three depositional units are always separated by erosional surfaces and/or palaeosols. In particular, a distinctive palaeosol that is often stratigraphically associated with an idiosyncratic tephra layer occurs in-between the deposits of the Lower Pleistocene and Middle Pleistocene units. The chronological boundaries of this succession have been constrained by palaeomagnetic measurements, tephra analyses, 230 Th/ 234 U measurements, and several 14 C datings (D Agostino et al. 1997; Galli et al 2010; Giaccio et al. 2010; Messina et al. 2010). In particular, the fingerprinting of four welldated tephra layers (Tufo pisolitico di Trigoria, ca. 560 ka; Pozzolane Rosse, ca. 460 ka, Tufo Rosso a Scorie Nere, ca. 450 ka; Tufo di Villa Senni ca. 360 ka) within the sediments of the Middle Pleistocene fluvio lacustrine unit allow the dating of this latter to between ca. 560 ka and 350 ka (see Galli et al. 2010, for details). PALAEOSEISMIC ANALYSES ACROSS THE MT. MARINE FAULT The Mt. Marine Fault is the main segment of the Upper Aterno fault system described by Galadini & Galli (2000). It bounds the SW slopes of the homonymous carbonate ridge between the villages of Barete, Pizzoli and Arischia (from NW to SE; Figs 1 and 2), where it shows a prominent fault scarp that is carved into highly cataclasised rock (Fig. 2, lower panel). The footwall is made up of Mesozoic carbonates that are mantled by sparse remnants of slope breccias (Sant Antonio breccias, sensu Bosi et al. 2003), both of which are faulted against Late Pleistocene layered slope deposits (e.g. see Blumetti 1995). On the assumption that the basal fault scarp approximates the surficial expression of post-last Glacial Maximum (LGM) fault slip (Dramis 1983; Tucker et al. 2011), its average height can provide the equivalent slip-rate. For instance, in the area where Galadini & Galli (2000) sampled and dated the Sant Antonio breccias hanging in the footwall (see Fig. 2, sampling site of BP and , BP ages. 2σ, 14 C recalibrated age), we measured a vertical offset of ca. 18mand 30 m, respectively, which yielded a vertical slip-rate of ca mm yr 1. This can be seen in the geological profile made across the fault scarp (Fig. 3A), which shows the scarp height and the minimum vertical offset of the slope surface that occurred after the breccias deposition (see quoted 14 C age in Fig. 2). Between Pizzoli and Arischia, which are SW of the main basal fault, a subtle scarp runs through the slope deposits. This scarp matches with a synthetic splay of the Mt. Marine Fault, and a trench was dug by Moro et al. (2002), who provided the first reliable palaeoseismological data of this structure (Fig. 2, T1). We present here other, new and unpublished, interpretations and datings relating

5 Palaeoseismology of L Aquila faults 1123 Figure 2. Simplified geological map of the southern slope of the Mt. Marine area (DTM base, 3 poinst m 2, bare-earth airborne Light Detection and Ranging, LiDaR). T1 2 trenches described in the text. 1, trace of geological profile in Fig. 3(A). Stars recalibrated 14 C ages of samples in Galadini & Galli (2000). The camera symbol indicates the point of view for the lower-panel photo. Lower panel, helicopter view looking north of the Mt. Marine fault scarp (segment Pizzoli-Barete in Fig. 1). White arrows indicate the fault scarp. to the long southern wall of this trench, together with those obtained in a second trench excavated inside Arischia village, across the southeastern tip of the fault scarp (T2). Generally speaking, this fault scarp has been extensively modified by bimillenarian human activities, and it has also been eroded and buried by frequent alluvial fan floods (i.e. surface run-off, and/or sheet erosion). T1 was opened by Moro et al. (2002) across an apparently undisturbed strip of the scarp, and was more than 30 m long and ca. 3 m deep. It exposed colluvial and alluvial deposits of the Upper Pleistocene (post 40 kyr), as well as a couple of palaeosols, all of which were displaced by four faults (see Fig. 4, unpublished log of the SE trench wall). In particular, in Fig. 4, faults A and B displaced the post 40 kyr succession (visible in the central part of the trench) against historical Holocene deposits, which also provided the most striking palaeoseismological data. The oldest units outcropping in the horst structure between faults A and D are fine alluvial gravels and sands, which alternate with two dark palaeosols (Fig. 4, units 19a and 21) that developed on the colluvial deposits of a thick tephra level (Fig. 4, unit 23), and which were tentatively correlated to the Campanian Ignimbrite (ca. 40 ka according to De Vivo et al. 2001). Both of these palaeosols have radiocarbon ages that match with the Campanian Ignimbrite ( BP and BP; Table 1), although they appear reversed, probably because unit 19a is the pedogenised colluvium of unit 21 (i.e. sample PIZ3 is actually a bulk of unit 21; see Table 1). These deposits are tectonically tilted uphill and were displaced by faults C and D,

6 1124 P.A.C. Galli et al. Figure 3. Geological profile across the Mt. Marine and Mt. Pettino rock fault scarp (A B, respectively), and across the apex of a recent alluvial fan (C, Mt. Pettino; see Figs 2 and 6 for location). D, view of the trench T3 across the Mt. Pettino Fault (rfs, rock-fault-scarp; the others labels as in Fig. 7). against alluvial sands and gravel with an age of BP (Fig. 4). Faults A and B displaced the entire succession against late Holocene and historical colluvia, the latter of which contained pottery shards (Fig. 4, units 6 and 4), although these are not suitable for precise age determination. As the radiocarbon dating of units 4 and 7 (Fig. 4, fault B) also provided an inconclusive age (Moro et al. 2002; their age is probably that of the parent material from which these colluvial units originated), to obtain an analytical palaeoseismic constraint, we have now dated a charcoal fragment that was sampled at the time, from the uppermost faulted unit (Figs 4 and 6a, fault A). The age of this sample ( AD; Fig. 4) predates the most recent earthquake that ruptured the Mt. Marine Fault, which was the 1703 February 2, earthquake (M w 6.7); that is, the only possible historical candidate for surface faulting in this area. This is the first time the 1703 event has been so tightly constrained in a palaeoseismic trench (see also Galli et al. 2005). The vertical offset of the bottom of unit 6b (assumed to fit to unit 6d in the footwall of fault A; Fig. 4) is ca. 1.5 m, whereas that of unit 6a (Fig. 4, bottom) is at least 0.9 m (i.e. unit 6a is eroded in the footwall of fault A; Fig. 4). Therefore, we believe that another surface rupture occurred before 1703; that is, within the historical period during which unit 6 was deposited, as shown also by the offset of the bottom of unit 6a by a secondary fault splay. A rough indication for the Late Pleistocene slip-rate of the fault is provided by the offset of the uppermost palaeosol (Fig. 4, unit 19a), which was encountered at the very bottom of the deepest pit made in the hangingwall before the trench filled in. Considering the slight tilting of the footwall succession and that the top of unit 19b is truncated by erosion (i.e. by unit 6d in Fig. 4), the calculated vertical offset of 13 m must be considered a minimum value, giving the relative slip-rate of ca. 0.4 mm yr 1 (for the past 35 ka). However, by summing this value with that calculated for the basal fault scarp (0.6 mm yr 1 ), this gives a vertical slip-rate of ca. 1 mm yr 1 over the entire structure. T2 is instead located inside Arischia village, in a highly anthropised area behind the San Benedetto church (12th 18th century). The excavation exposed mainly fine, well-stratified, alluvial fan gravels (Fig. 5, unit 7) that are carved inside the brownish colluvia (Fig. 5, unit 8 in the footwall), which have faulted against similar gravels capped by a dark-brown palaeosol (Fig. 5, unit 5 in the hangingwall). The fault scarp has been deeply reworked by secular agricultural and historical building activities, which has erased the entire Tardiglacial-Holocene succession, and also scraped away the modern deposits. Nevertheless, the 14 C dating of samples has made it possible to obtain some constraints regarding the age of the faulted succession. Indeed, the fan gravels were deposited massively at the end of the LGM, during a few, consecutive, alluvial sheet floods, as shown by the age of the top of the colluvia that they have buried (Fig. 5, unit 8, BP), and by the age of the palaeosol that has developed over them (Fig. 5, unit 5, BP). During this short period, the fault ruptured at the surface (minimum offset, ca. 1 m), as shown by the presence of unit 6 (Fig. 5), a faulted colluvial wedge that was built up mainly by the same fan gravels in an orange matrix that was successively sealed by palaeosol 5. Due to the above-mentioned erosional hiatus, there is further palaeoseismological information only for more recent times. Unit 4 is an orange deposit that has been packed along the fault zone (Fig. 5), and assuming that it represents the infilling of a chasm that opened during a faulting event along the main shear plane, its 14 C age ( BP, bulk) will predate the age of one of the palaeoearthquakes generated by this structure. This deposit is then capped by unit 3, which is made of fine angular gravel, as colluvia from the scarp carved into unit 7 (Fig. 5). By comparing some parts of the deposits inside unit 2, we believe that unit 3 is

7 Figure 4. Sketch of southern wall of trench 1 across the Mt. Marine basal fault splay. Note that the last rupture occurred in historical times, as indicated by the faulting of deposits containing modern pottery shards (fault B, right-hand side) and after AD (antithetic fault A, left-hand side), that is on 1703 February 2, M w 6.7 earthquake. Left lower panel, fish-eye view looking SE of the fault A area. Right lower panel, fish-eye view looking SE of the fault B area. Palaeoseismology of L Aquila faults C 2011 The Authors, GJI, 187, C 2011 RAS Geophysical Journal International 1125

8 1126 P.A.C. Galli et al. Figure 5. Sketch of trench 2 across the SE tip of the Mt. Marine basal fault splay, inside the Arischia village. Note that the last rupture affected deposits containing modern pottery shards and thus could be related to the 1703 earthquake. Note also the anthropic reworking of the upper units, which produced repeated geometrical depression (Unit 2 2a) and obliterated the original stratigraphical and tectonic features. Upper left-hand side, view looking north of the fault zone. the basal part of unit 2, which, in turn, is a strongly reworked unit that has been affected by ploughing and vineyard pits, and it is very rich in modern pottery shards (common tile and vase fragments) of a generic post-renaissance period (L. Scaroina, personal communication, 2010). As unit 3 has undoubtedly faulted against unit 7, if it really matches with the undisturbed portion of unit 2, then the displacement occurred during the modern epoch; that is, in the 1703 earthquake (so before the recent agricultural workings devastated the uppermost fault architecture). PALAEOSEISMIC ANALYSES ACROSS THE MT. PETTINO FAULT The Mt. Pettino Fault is the southern segment of the Upper Aterno fault system (Galadini & Galli 2000). It bounds the SW slopes of the homonymous carbonate ridge, where it shows a fault scarp carved into cataclasised rock (Fig. 6); this is faulted against Late Pleistocene layered slope deposits. Considering that the basal fault scarp has an average height of ca. 10 m (e.g. see Fig. 3D), the post-lgm vertical slip-rate should be in the order of 0.6 mm yr 1. Due to high-precision topographic levelling, we also measured 2.5 m of offset of the top surface at the apex of a currently inactive fan that is crossed by the Mt. Pettino Fault (Fig. 3C); this reasonably represents the cumulated displacement of the fault in recent times. On the assumption that this fan is mainly related to the cold climate-driven processes of hill-slope weathering that occurred during the above-mentioned Neoglacial phase (4.2 kyr BP in the mountains in central Italy, according to Giraudi et al. 2011), then the rough vertical slip-rate in the late Holocene would be ca. 0.6 mm yr 1. It needs to be stressed that due to the secular agricultural workings and to the 20th century reforestation, the fault scarp has also been deeply reworked in this area (e.g. terraced). This means that the deposits of the scarp toe have often been removed and/or disturbed, which hampers any conclusive palaeoseismological analyses. T3 was 15 m long and up to 5 m deep, and it exposed mainly slope angular fine gravels (Fig. 7, unit 7) that are faintly stratified and mantled by a dark-brown palaeosol (Fig. 7, unit 6) with sparse carbonate clasts, which is truncated upwards by other layered slope debris deposits (subangular gravels in an ochre sandy matrix; Fig. 7, units 4 3). The palaeosol was buried after BP, which is around the same period observed in the Arischia trench. In turn, other bulk 14 C datings in the deeper portions of this palaeosol have suggested that its parent material was an older colluviated palaeosol (ca. 40 ka; see Fig. 4, unit 19a and 21 in T1). These are probably the same as those that we found here that were trapped and dragged upwards along the fault plane (Fig. 7, unit 9). This entire succession is faulted against highly cataclasised carbonate dolomite rock (Fig. 7, unit 10; see also Figs 3B and D), which constitutes the 20-m-thick damage zone of the fault, while

9 Palaeoseismology of L Aquila faults 1127 Figure 6. Simplified geological map of the SW slope of the Mt. Pettino area. (DTM base, 3 poinst m 2, bare-earth airborne Light Detection and Ranging, LiDaR). T3, trench described in the text. 2 3, trace of geological profiles in Fig. 3(B) (C). Lower panel, aerial view looking east of the Mt. Pettino fault scarp. Note the high urbanization of the near-fault pediment. only the present soil seals the fault planes. Post-LGM faulting here is testified by the displacement of palaeosol 6, which was sealed by the yellowish sands of unit 4 (which is probably a colluvial wedge that originated from the loose cataclasite exhumed from the free-face; Fig. 7) well before BP. Indeed, due to erosive processes, we can affirm analytically that the surface faulting is certain to have occurred after BP, which is the age of the parent material from which colluvial unit 5 originated; that is, a colluvial wedge that itself testifies to the faulting event. In turn, this unit has been successively faulted and dragged against the fault gorge (Fig. 7, unit 8, argillified cataclasite). Although we do not know when the last surface rupture occurred, the abrupt thickening of the present soil in the hangingwall (Fig. 7, unit 1, which is actually a poorly pedogenised colluvium) suggests a very recent age. PALAEOSEISMIC ANALYSES ACROSS THE PAGANICA SAN DEMETRIO FAU LT S The Paganica-San Demetrio fault system is made up of a dozen fault segments, with each one km long, and with all of these arranged in a dextral en-echelon geometry (Figs 1 and 8). The stepover between the main segments ranges from between a few dozen metres up to km. NNW SSE breaching faults that cross relay zones have been recognized between segments with high stepover values, whereas the overlap of the main segments is generally of m. Moreover, at least in the Paganica area, there are four or more synthetic, and one antithetic, splays (Fig. 8), which have ruptured at the surface differently in time and space (Galli et al. 2010). This internal fault architecture matches the surficial breaks (and/or surface faulting) pattern observed for the 2009 earthquake (e.g. see the Fig. 5 in Galli et al. 2010; Giocoli et al. 2011). Several trenches have been dug across the Paganica-San Demetrio fault system (Galli et al. 2010; Moro et al. 2010; Cinti et al. 2011), and we have obtained reliable palaeoseismological data from two different sites along the main Paganica splay (Figs 1 and 8, T4, T5), one in the relay ramp between the Paganica and San Gregorio faults (Fig. 1, T6), and one on the Colle Enzano Fault segment (Figs 1 and 8, P1). The detailed stratigraphical descriptions of these excavations were reported in Galli et al. (2010; trenches T1 3 and pit P1), while we further analyse the main results here. P1 was excavated across the NW tip of this fault system (Fig. 8, Colle Enzano segment). Here, under the present gravely soil (Fig. 9A, unit 1; which faulted in the 2009 earthquake) there is a wedge of massive subangular clasts that is rich in an organic brownish sandy matrix (Fig. 9A, unit 2, bulk age yr BP) that had been deeply dragged along the rock fault plane also before the 2009 surface faulting. As the 14 C age of this colluvium is that of the parent material from which it originated (i.e. its depositional age is younger than its analytical date), it just predates

10 1128 P.A.C. Galli et al. Figure 7. Sketch of southern wall of trench 3 across the Mt. Pettino Fault. Due to the truncation of the top layers, it is only possible to affirm that the last rupture occurred well after BP. the deposition of unit 2. We can only argue that this happened after the phase of slope stability and pedogenesis that was recorded in the central Apennines until 4 5 ka. This was during the Neoglacial period (Giraudi et al. 2011), although it is not possible to define a more precise time interval. However, faulting occurred repeatedly after this period, as shown by a bronze fibula (6th century BC) that was trapped along the fault plane and was sampled inside a trench dug near to our pit (Pizzi et al., in preparation). T4 was 25 m long and 3 6 m deep and is at the apex of an alluvial fan that was fed by a small ephemeral stream (Fig. 9B). The trench exposed Lower Pleistocene alluvial fan gravel in the footwall that was faulted against alternating Late Pleistocene Holocene slope debris, colluvia and alluvial fan deposits. Apart from the multiple faulting of the pre-lgm succession, the most reliable data that we want to stress here is the displacement of a colluvium that was deposited between 2760 and 2560 BP and at least AD (Fig. 9B, unit 3; minimum offset of the bottom is 0.6 m across fault A), which was sealed after a strong erosional phase by a very recent colluvium that contains several common, contemporary pottery shards (Fig. 9B, unit 2). The time-span of this very recent surface faulting (which was actually one or more after AD) can be narrowed down due to the data obtained by Moro et al. (2010) in a trench 200 m from T4 (Fig. 8, Tb), on a secondary splay that branches from the main fault. Here, the uppermost faulted unit (with ca. 0.2 m offset) contained a sample that was dated to AD, thus indicating that the last earthquake occurred after this period. T5 (Fig. 9C) matches with the median sector of a long gorge that was opened by the 2009 coseismic Gran Sasso aqueduct rupture, and it shows the faulting of Middle Pleistocene alluvial gravels Figure 8. Shaded relief view (DTM base, 3 poinst m 2, bare-earth airborne Light Detection and Ranging, LiDaR) of the Paganica San Demetrio fault array in the Paganica village area. T4 5 and P1 are palaeoseismological sites discussed in the text. Ta-c are palaeoseismological sites studied by Cinti et al. (2011) and Moro et al. (2010) (Fig. 9C, unit 12) capped by Late Pleistocene palaeosol and colluvia (Fig. 9C; 8 6) against Holocene slope deposits (Fig. 9C, 6 3). The fault investigated is not the only one that was exposed by the gorge along the fault scarp (see Boncio et al. 2010; Galli et al. 2010; Cinti et al. 2011), but in our interpretation it is the one that ruptured more frequently during the late Holocene. The main data from this trench are the identification and dating of the last surface

11 Palaeoseismology of L Aquila faults 1129 Figure 9. Palaeoseismic evidences collected along the Paganica San Demetrio fault system (for further details see Galli et al. 2010). Panel A, sketch of the pit (P1 in Fig. 1) dug across the Colle Enzano fault segment; note the dragging of the entire succession against the carbonate fault plane, where a 9-cm offset was measured due to the 2009 earthquake (Galli et al. 2010). Panel B, sketch of the SE wall of the palaeoseismological trench excavated in the southern side of Paganica village (T4 in Fig. 1). Note the faulting of the entire sedimentary succession, with the exception of units 1, 2. Panel C, Sketch of the NW wall of the gorge opened by the Gran Sasso aqueduct rupture, to the northern side of Paganica village (T5 in Fig. 1). Also note the faulting of the entire sedimentary succession, with the exception of units 1 2. Panel D, Evidence of recent faulting across one of the secondary segments of the PSDFS (T6 in Fig. 1). The sketch shows the fault scarp crossed by a water-supply trench: 1, present soil, reworked by ploughing; 2, light-brown to reddish sandy colluvia, with abundant etherometric clasts; 3, carbonate gravels; 4, deeply altered reddish palaeosol; 5, coarse carbonate gravels. C 2011 The Authors, GJI, 187, C 2011 RAS Geophysical Journal International

12 1130 P.A.C. Galli et al. faulting(s), through both 14 C and archaeological determinations. Indeed, the bottom of unit 6 (dated BP, as with the bottom of the similar unit 3 in T4; Fig. 9B) was displaced by ca. 0.8 m, whereas unit 3, which is also faulted, contains terracotta invetriata a fuoco pottery shards of the 17th 18th century. Therefore, one or more large events have occurred in the past two millennia, with the last one certainly after the age of pottery (i.e. the 1703 earthquake). The previous event can be tentatively indicated by the age of the colluvial wedge that was trapped and refaulted between the fault splays (Fig. 9C, unit 5), and that has been dated AD. As far as this event is concerned, Galli et al. (2010) hypothesised that it fits with the 801 AD earthquake that is known to have struck the central Apennines and Rome. It is worth noting that Cinti et al. (2011) identified a comparable event (ca. between AD and AD) in the trench that they dug across a secondary splay of the Paganica Fault (Fig. 8, Ta). Analogously, according to the data gathered by Moro et al. (2010) in another trench dug 300 m from T6 (Fig. 8, Tc), a large faulting event occurred well after the 2nd century AD, and prior to AD. Finally, T6 (Fig. 9D) involved one of the breaching faults that crossed the relay zone between Paganica and San Gregorio, where we also observed surficial breaks in the 2009 earthquake (Galli et al. 2010). Here, coarse alluvial gravels are mantled by a reddish, altered sandy-silty palaeosol (i.e. the above-mentioned Middle Pleistocene alluvial and pedomarker pair; Fig. 9D, units 5 4) and are faulted against loose and chaotic slope deposits, with large boulders along the segment underlined by the fault scarp. Also here, unit 2 (Fig. 9D) is very recent, as it contains many pottery shards, and tile and brick fragments. In particular, a pottery shard ( invetriata verde type) found in the upper part was dated to the 16th 18th century AD (as determined by Ilaria De Luca, Gianfranco De Rossi, Luigi Scaroina, Istituto Nazionale di Archeologia e Storia dell Arte, written communication). Analogous to T5, this allows the conclusion that this fault segment also ruptured in modern times; that is, in the 1703 AD earthquake. DISCUSSION The first conclusive result obtained from the seven palaeoseismological sites presented above is that all of the faults investigated have ruptured repeatedly during the Holocene and through historical times (Fig. 10). This conclusively confirms the hypothesis in Moro et al. (2002) as far as the Mt. Marine Fault activity is concerned, whereas it is new evidence for the Mt. Pettino Fault. Moreover, by comparing the individual pieces of evidence for each segment, it emerges that all of these faults ruptured together at least once on 1703 February 2 (M w 6.7). This is a matter of record for the Mt. Marine and the Paganica faults (as shown by T1 2 and T5 6; Fig. 1), whereas it is assumed for the Mt. Pettino Fault, where we have only a remote post-quem term. As we have not established narrow time-spans for previous event horizons across the Mt. Marine and Mt. Pettino faults, we do not know whether the three faults ruptured together before 1703, or if the Mt. Marine Mt. Pettino faults also slipped separately in the past (e.g. in the poorly known 1349 earthquake). However, as the 2009 earthquake taught us that the Paganica San Demetrio fault system can rupture also alone causing M w 6.3 earthquakes we can reasonably affirm that all of these faults can rupture separately or together in time, which will result in differently sized earthquakes. This means that the indication of surface faulting that occurred during the 1703 and 2009 events along the Paganica San Demetrio Figure 10. Graph showing the radiocarbon and archaeological constraints for the different historical earthquakes documented in trenches across the Mt. Marine and Mt. Pettino faults (MMF and MPF) and Paganica San Demetrio fault system (PSDFS, to be compared with Fig. 11). Triangles span the 2σ interval of each date with reference to a faulting event (right-hand side, post quem term; left-hand side, ante quem term). Pottery symbol, common tile and vase shards (iv, invetriata verde pottery). Grey bars, earthquake year (dotted where uncertain). Black dotted bars are the surface faulting observed de visu, that is, from historical accounts (1703) or direct survey (2009). Arrows indicate a far post/ante quem. fault system has strong implications in terms of the seismogenic behaviour of the entire L Aquila fault system. As can be seen from Fig. 11, as well as the 2009 L Aquila event [see highest intensity datapoint distribution (HIDD), upper-right panel], it appears reasonable that the Paganica San Demetrio fault system was also responsible for the 1461 earthquake (M w 6.4; Fig. 11, see HIDD, upper-left panel); this can thus be considered as a twin of the 2009 earthquake (see also Galli et al. 2009; 2010; Tertulliani et al. 2009). Both of these earthquakes were probably generated by contemporary ruptures of several of the segments that form this en-echelon system, along a length of ca. 19 km. On the other hand, during the stronger 1703 event, the HIDD fills the entire hangingwall of the Mt. Marine, Mt. Pettino and Paganica San Demetrio fault system (Fig. 11, lower panel). Thus, we can hypothesise that the 1703 earthquake was generated by a ca. 30-km-long fault rupture, which matches with the empirical relationship of fault length versus magnitude (e.g. in Wells & Coppersmith 1994; Galli et al. 2008). If this interpretation is reliable, we can conclude that the Paganica San Demetrio fault system can rupture independently, to cause M w < 6.4 earthquakes, and which will occur perhaps every ca. 0.5 kyr (e.g. from 1461 to 2009; Fig. 10). Although these events induce a measurable surficial deformation, their traces are not clearly identifiable in palaeoseismological trenches, because of

13 Palaeoseismology of L Aquila faults 1131 Figure 11. Highest intensity data-point distribution for the three most recent major historical earthquakes in the L Aquila region (1461, 2009, and 1703; I s > 8 MCS scale) compared to the active faults system of the area. Besides the clustering of the maximum intensities in the hangingwall of the causative faults, the palaeoseismological data from trenches across the Paganica San Demetrio fault system (PSDFS) indicate that this system ruptured in 1703, together with the Upper Aterno FS (UAFS; namely, Mt. Marine and Mt. Pettino faults, MMF and MPF, respectively). On the other hand, the 1461 and 2009 events were generated only by the PSDFS. Note the peak of intensity (9 10 MCS) at Castelnuovo, which is a clear example of local seismic amplification in every earthquake (see the strong peak in the Horizontal/Vertical Spectral Ratio HVSR obtained from 65 M l > 3 earthquakes and 15 min. of noise recording by Gallipoli et al. 2011; upper-right corner of lower panel).

14 1132 P.A.C. Galli et al. the little surficial offset (see Cinti et al. 2011, for an alternative point of view). On the other hand, the Paganica San Demetrio fault system can also rupture together with the Mt. Marine and Mt. Pettino faults, which generates earthquakes of M w 6.7, with the offsets great enough to be seen in the palaeoseismological record (e.g. 1703; Fig. 10, T1 6 in Fig. 1; see also Moro et al. 2010). We have also indicated that a possible ancestor of the 1703 event might have been that which is supposed to have occurred in the late 1st millennium AD in the Paganica Fault trenches (Fig. 10, T4 5, Ta, Tc in Fig. 8), that is, the event that we tentatively associated with the AD 801 earthquake. If this hypothesis is true, the rough return time for this class of event on the Mt. Marine and Paganica San Demetrio fault system would be ca. 0.9 kyr. From a structural point of view, if we accept the scenario suggested by the palaeoseismic analyses relating to the contemporary rupture of the Mt. Marine and Paganica faults (at least during M w 6.7 events), it could be asked whether the Mt. Pettino Fault has a mechanical role here. Indeed, as can be seen from Fig. 11 (lower panel), it is clear that this fault falls entirely in the hangingwall of the (newly forming?) system that is made up by the Mt. Marine Paganica faults. This particular fault architecture suggests an upcoming passive mechanism for this structure that, conversely, had a fundamental role during the structuring of the Lower-Middle Pleistocene basin of the Aterno valley (see deposits distribution in Fig. 1). In turn, the proximity (less than 4 km) of the SE and NW tips of the Mt. Marine and Paganica San Demetrio faults, respectively, suggests a progressive and future hard linkage of these faults in the Mt. Pettino Fault footwall (Fig. 11, dotted ellipse in lower panel). The persistence at depth of a barrier between the Mt. Marine and Paganica San Demetrio faults has been tentatively detected by an analysis of receiver functions at two permanent broad-band seismic stations in the 2009 earthquake epicentral area (Bianchi et al. 2010). This barrier might be represented by a 4 6-km-thick, high S-wave velocity body (V s 4.2 km s 1, absent in the southeastern fault portion, where the rupture propagated), which fits with the area of the few aftershocks. This barrier is also anticorrelated with the maximum slip patches of the 2009 earthquake (e.g. Cheloni et al. 2010), and during the 2009 mainshock, it stalled the propagation of the northwestward rupture. Conversely, it was probably broken, or passed over, during the 1703 February 2 event, which allowed the rupture of the entire Mt. Marine Paganica fault system. CONCLUSIONS The results from the seven palaeoseismological excavations summarized in this work are from unpublished, published and revisited data, and they have allowed us to ascertain conclusively the post- LGM, Holocene and historical activity of the structures investigated along the Aterno River valley, from the Mt. Marine Fault in the NW, to the Paganica San Demetrio faults in the SE. Activity of the Mt. Pettino Fault was documented during the Holocene, but we lack analytical data for the historical period. From the cross-correlating of these data with the highest macroseismic effects distribution of the historical earthquakes, we suggest that the fault systems studied can rupture separately in time and space, to generate differently sized earthquakes. For instance, in the cases of the 1461 and 2009 events (with the slip limited to the Paganica San Demetrio fault system), magnitude was 6.3/6.4, whereas in 1703 (with the slip of the Mt. Marine-Paganica San Demetrio fault systems) it was 6.7. These thus depended on the break/passing over or not of the barrier that separates the Mt. Marine Fault and the Paganica San Demetrio fault system at depth. The recurrence time for these M w 6.3 earthquakes on the Paganica San Demetrio faults would be of the order of 0.5 kyr (based on historical data), whereas it would be of the order of 0.9 kyr for events with M w 6.7, for the contemporary rupture of the Mt. Marine and Mt. Pettino faults (palaeoseismological and historical data). From this point of view, the elapsed time for the lower magnitude earthquake class on the Paganica San Demetrio fault is obviously now 2 yr, whereas it is 308 yr for the higher magnitude class along the entire L Aquila system. The largest vertical slip-rate was calculated for the Mt. Marine Fault, where it might have reached 1 mm yr 1 in the post-lgm period, whereas it should be in the order of 0.6 mm yr 1 and 0.5 mm yr 1 for the Mt. Pettino and Paganica San Demetrio faults, respectively. For the en-echèlon relationships amongst the different segments of these faults, the fault length of the Mt. Marine Fault is approximately 14 km, whereas for the Mt. Pettino Fault this is ca. 10km. Considering the overlap between the two respective tips, together they reach ca. 20 km. In turn, the Paganica San Demetrio fault system reaches 19 km, that when added to these previous faults becomes more than 30 km, which is the fault rupture length that in our interpretation generated, at depth, the M w 6.7 earthquake of 1703 February 2. In the latter case, it appears that the Mt. Pettino Fault had a role as a synthetic splay of the main fault system that has developed in its footwall, and that is probably concluding the process of hard linkage between the northwestern and southeastern tips of the Paganica and Mt. Marine faults, respectively (the area of the 2009 earthquake high V s velocity barrier). The awareness that this segmented fault system can rupture entirely or partially, and thus generate differently sized earthquakes while also alternating the possible combinations of any fault rupture, should inspire the choice of the future methodological approach for seismic-hazard assessment in Italy, which is still mainly anchored to a probabilistic, Cornell (1968) setting. Indeed, the seismogenetic behaviour described is not only typical of the L Aquila faults; it has also been documented for the fault system near Fucino (a in Fig. 1A; Galli et al. in press), for the Umbria Marche Apennine faults (b in Fig. 1A; Galadini et al. 1999), for the Norcia fault system (c in Fig. 1A; Galli et al. 2005), and for the Mt. Marzano Fault (d in Fig. 1A; southern Italy; Galli et al. 2011). Therefore, this situation should be considered as the rule, rather than the exception, along the whole Apennine chain. ACKNOWLEDGMENTS Gian Maria Zuppi passed away unexpectedly two weeks after the submission of this manuscript: we guess that he will read the Journal from elsewhere. We thank F. Galadini, M. Moro and A. Sposato, who participated to the T1 analysis in 2001 November, together with E. Falcucci, S. Gori, B. Quadrio and J.A. Naso, who also discussed our results in the new trenches. A. Moretti kindly informed us of the unearthing of the Arischia Fault. We thank L. Scaroina for his professional and friendly archaeological support. We are indebted with S. Piscitelli and A. Giocoli for the geoelectrical investigation of the fault. The criticisms of two anonymous referees strongly improved our initial version of the manuscript, and we wish to thank them for their careful analytical assessment.