Paganica Fault Research Articles

The contribution of fluid geochemistry to define the structural patterns of the 2009 L’Aquila seismic source

Field investigations performed in the epicentral area within the days following the April 6, 2009 L’Aquila earthquake (M w 6.3) allowed several researchers to detect evidence of coseismic ground rupturing. This has been found along the Paganica Fault and next to minor synthetic and antithetic structures. Although a lot of geo-structural and geophysical investigations have been recently used to characterize these structures, the role of the different fault segments – i.e. as primary or secondary faults – and their geometrical characteristics are still a matter of debate. In light of this, we have here integrated data derived from fluid geochemistry analyses carried out soon after the main-shock with field structural investigations. In particular, we compared structural data with CO 2 and CH 4 flux measurements, as well as with radon concentration measurements (for other geogas concentration see Voltattorni N., this issue). Our aim was to better define the structural features and complexities of the activated Paganica Fault. Here, we show that, in the near rupture zone, “geochemical signatures” could be a powerful method to detect earthquake activated fault segments, even if they show subtle or absent geological-geomorphological evidence and are still partially “blind”. In detail, a clear degassing zone was identified just along the San Gregorio coseismic fracture zone – i.e., the surface deformation related to the “blind” San Gregorio normal fault. Indeed, CO 2 and CH 4 flux maximum anomalies were aligned along the Northern sector of the San Gregorio fault, in the Bazzano industrial area as well as at the border of the studied area (Fossa antitethic fault crossing an anti-apenninic transverse belt). The Bazzano-San Gregorio fault area also corresponds to the depocenter of the maximum coseismic deformation highlighted by DInSAR analysis (Atzori et alii , 2009). Here, maximum radon concentration values in soil gases were also found. As a whole, these results corroborates the hypothesis of Boncio et alii (2010) who suggested that the San Gregorio fault probably represents a synthetic splay of the Paganica Fault, being thus connected with the main seismogenic fault at depth. Moreover, another maximum in CO 2 flux anomaly has been measured along the Southernmost tip of the earthquake rupture zone, close to the San Gregorio village. Minor or absent soil gas and flux anomalies were instead located along antithetic structures as the Bazzano fault, while some anomalies in CO 2 flux or radon concentration in groundwater have been found within transfer zones, such as the step-over zone between the central segment of the Paganica fault and the San Gregorio fault and in the zone which separates the Paganica fault from the i) Middle Aterno Valley-Subequana Valley and ii) Barisciano-S. Pio delle Camere-Navelli fault systems. Our results corroborate the power of fluid geochemistry in investigating the structural features of active tectonic structures, being particularly helpful in discerning blind faults. More specifically, our data suggest that the youngest fault splays, as in the case of the San Gregorio fault, may represent preferential sites for degassing.

High-resolution controlled-source seismic tomography across the Middle Aterno basin in the epicentral area of the 2009, Mw 6.3, L’Aquila earthquake (central Apennines, Italy)

We present high-resolution Vp models of the Middle Aterno basin obtained by multi-scale non-linear controlled-source tomography.Seismic data have been collected along four dense wide-aperture profiles, that run SW-NE for a total length of ~6 km in the hangingwall of the Paganica-S. Demetrio Fault, source of the 6th April 2009 (Mw 6.3) L'Aquila normal-faulting earthquake. Seismic tomography expands the knowledge of the basin with high spatial resolution and depth penetration (>300 m), illuminating the Meso-Cenozoic substratum that corresponds to high-Vp regions (Vp>3500-4000 m/s).Low Vp (1500-2000 m/s) lacustrine sediments (Early Pleistocene in age) are imaged only in the SW sector of the basin, where they are up to 200 m thick and lie below coarse fluvial and alluvial fan deposits. The overall infill consists of Early Pleistocene to Holocene alluvial fan and fluvial sediments between the Paganica Fault and the Bazzano ridge, with Vp reaching 3000 m/s for the oldest conglomeratic bodies. The substratum has an articulated topography. The main depocenter, ~350 m deep, is in the SW sector of the basin south of the Bazzano ridge. Remarkably, this depocenter and the overlying thick lacustrine body match the area of maximum coseismic subsidence observed after the 2009 earthquake. In the Paganica area, Vp images unravel large steps in the substratum related to two unreported SW-dipping buried strands, synthetic to the Paganica Fault, with ~250 m associated total vertical throw. This finding has important implications on the long-term history of the Paganica-S. Demetrio Fault system, whose total vertical displacement has been previously underestimated. An additional ~250 m vertical offset along this complex Quaternary extensional structure should therefore be considered.

Constraining primary surface rupture length along the Paganica fault (2009 L’Aquila earthquake) with geological and geodetic (DInSAR and GPS) data

Part of this work has been carried out within the ASI-SIGRIS project, funded by the Italian Space Agency and Istituto Nazionale di Geofisica e Vulcanologia

Changes on groundwater flow and hydrochemistry of the Gran Sasso carbonate aquifer after 2009 L’Aquila earthquake

The earthquake that struck L'Aquila on April 6 2009 (Mw 6.3) directly affected the Gran Sasso aquifer. Co-seismic and post-seismic changes in groundwater discharge and in hydrochemistry, possibly induced by the earthquake, were observed. Spot and monitoring measurements of the spring discharge, of water table level and of the main physico-chemical parameters of spring waters (T, pH, electrical conductivity, major ions and 222Rn) were thus carried out to determine the effects of the L'Aquila earthquake on groundwater at regional and local scale, to be compared with available data collected since the 1990s.Short- and mid-term effects have been observed in the groundwater flow at recharge and discharge areas. The following short term effects have been observed: i) the sudden disappearance of some springs located along the surface trace of the Paganica Fault; ii) an immediate discharge increase of the Gran Sasso highway tunnel drainages (+20%) and of other springs (+10%); iii) a progressive increase of the water table elevation (+1m) at the boundary of theGran Sasso aquifer during the following month; iv) a sudden lowering of the water table in the recharge area. Similar post-seismic effects have been recorded in the following 20 months, when spring discharge and water table remain higher than the pre-seismic ones in discharge zones.A conceptual model of the earthquake consequences on the Gran Sasso aquifer is proposed herein. The short-term hydrologic effects registered immediately after the mainshock have been caused by a pore pressure increase related to elasto-static aquifer deformation.Apart from the contribution of seasonal recharge observed in 2009-10, mid-term effects observed in the 20 months following the mainshock suggest that there was a change in groundwater hydrodynamics.Supplementary groundwater that flows toward aquifer boundaries and springs in discharge areas reflects a possible increase in hydraulic conductivity in the recharge area. This increase is probably related to fracture cleaning and/or dilatancy.Additional monitoring including hydrochemical data allows a refinement of the proposed model. The outcomes of the hydrochemical spot sampling of the pre-seismic (2001-2007), post-seismic (April 2009) and after-seismic (July and September 2009, may 2010) periods, give the following insights: i) post-seismic groundwater of Tempera spring group was more mineralised and richer in 222Rn than the pre-seismic one; ii) transient changes in pH and calcite saturation index involve the whole aquifer, moving from Tempera springs and spreading from the recharge to discharge areas, causing changes in groundwater hydrochemistry; iii) post-seismic gradual return to previous hydrochemical equilibrium.Both hydrodynamic and hydrochemical observation converge towards a non-permanent increase of the bulk hydraulic conductivity in the aquifer portion close to the Paganica Fault (recharge area and local discharge zone). This fact has caused a lowering of the water table and of the calcite saturation index in recharge areas and simultaneously an increase of water table and flow rate in discharge zones.Complete interpretation of both quantitative and hydrochemical data allows to determine the long-term consequences of this earthquake on the groundwater flow of the Gran Sasso carbonate massif.

From surface geology to aftershock analysis: Constraints on the geometry of the L’Aquila 2009 seismogenic fault system

The aim of this study is the definition of the geometry and structural style of the extensional fault system involved during the L'Aquila 2009 seismic sequence. The surface tectonic setting of the late Quaternary faults in the epicentral areas (i.e., the Paganica, Mt. Gorzano and Montereale faults in northwestern Abruzzo) was defined by integrating data from the literature with original field work. The geometry of the faults at depth was interpreted using high-resolution aftershock distributions (based on a dataset covering 6 April to 27 December 2009, Ml ≥?1.9). The 3D fault model was reconstructed through 39 sections variously oriented across the epicentral area and represented in a map view as depth contour lines.Consistent geometric and kinematic correlations between the geological and seismological data were observed. The seismic sequence reactivated four well-distinguished fault sources differing in geometry, size and the degree of involvement. In order of importance, these sources may be ranked as follows: 1) the SW-dipping Paganica fault system activated during the 6 April mainshock (Mw 6.1/6.3) from the surface to a depth of ~10.5 km (along-strike length, L, 21 km; surface rupture length 13 km); 2) the NE-dipping hidden Ocre source activated during the strongest aftershock (9 April, Mw 5.4/5.5) at depths between 11 km and 16 km; 3) the WSW-dipping Mt. Gorzano fault activated by four relevant aftershocks (6 to 13 April, Mw ~5.0 to 5.5) at depths between 6 km and 12 km; 4) the WSW-dipping Montereale fault system and its continuation into the S-dipping Mt. San Franco fault activated at depths between 6 km and 10 km in a large number of minor events with Mw up to 3.6.The hypocentre of the 6 April main event is sited at a depth of ~9.5 km close to the intersection line between the SW-dipping Paganica fault and the S-dipping Mt. Stabiata fault, suggesting that the fault plane complexities (in this case, the sharp bend of the fault strike) may represent a favoured site for earthquake nucleation. The L'Aquila 2009 seismic sequence did not reactivate the entire surface of the involved faults. Nevertheless, on the basis of other geological and seismological constraints, the overall fault extent is schematically reconstructed and parameterised. Some considerations related to the structural style of the intra-Apennine Quaternary extensional structures and their patterns at depth are advanced.

3D displacement maps of the 2009 L’Aquila earthquake (Italy) by applying the SISTEM method to GPS and DInSAR data

Terra Nova, 25, 79–85, 2013AbstractWe present an application of the novel SISTEM approach, to obtain the dense 3D ground deformation pattern produced by the April 6, 2009, Mw 6.3 L’Aquila earthquake. This event, characterized by a SW‐dipping normal fault with thousands of foreshocks and aftershocks located in the depth range 5–15 km, is the most destructive to have struck the Abruzzo region since the major 1703 seismic sequence. The surface deformation, revealed by the SISTEM through the integration of GPS with interferometric measurements from the ENVISAT and ALOS satellites, shows a deformed area extending towards SE along the Aterno valley, in agreement with seismological and other geodetic observations. We inverted the SISTEM results using an optimization algorithm based on the genetic algorithm, providing an accurate spatial characterization of ground deformation. Our results improve previous kinematic solutions for the Paganica fault and allow identification of additional faults that have contributed to the observed complex ground deformation pattern.

Control of pore fluid pressure diffusion on fault failure mode: Insights from the 2009 L'Aquila seismic sequence

The MW 6.13 L'Aquila earthquake ruptured the Paganica fault on 2009/04/06 at 01:32 UTC, and started a strong sequence of aftershocks. For the first four days, the region north of the hypocenter of the main quake was shaken by three large events (MW ∼ 5.0) that ruptured different patches of the Monti della Laga fault (hereafter “Campotosto”). In our hypothesis, these aftershocks were induced by a dramatic reduction in the fault's shear strength due to a pulse of pore fluid pressure released after the L'Aquila main earthquake. Here we model the time evolution of the pore fluid pressure northward from the main hypocenter. We show that, during the sequence, the Campotosto fault failed in multiple episodes, when the specific patches/asperities underwent fluid pressure‐related strength reductions of 7–10 MPa. Although such drops in strength are very large in amplitude, the contribution of other weakening mechanisms (perturbations of the Coulomb shear stress, and/or dynamic stresses induced by passing seismic waves) cannot be ruled out by our observations. However, the Coulomb shear stress variations either had negative amplitudes down to −0.2 MPa (i.e., tended to inhibit further seismic activity), or had very small positive amplitudes (<0.05 MPa). Paleoseismological evidence supports the hypothesis that larger events (MW 6.5–7) have occurred on the Paganica fault [EMERGEO Working Group, 2009], whereas Lucente et al. [2010] concluded that an important migration of pore fluids characterized the preparatory phase of the L'Aquila main shock. Consequently, the MW 6.13 L'Aquila earthquake may be analogous, at a larger scale, to one of the three Campotosto largest aftershocks. The complex behavior observed for the L'Aquila‐Campotosto fault system seems to be common to other seismogenic structures in the Central Apennines (e.g., the Umbria‐Marche fault system), and need to be taken into consideration for the assessment of seismic hazard.

Fault-trapped waves depict continuity of the fault system responsible for the 6 April 2009 MW 6.3 L'Aquila earthquake, central Italy

We investigate fault-trapped waves observed at a permanent broad-band station (FAGN) installed on the San Demetrio Fault, about 20km southeast of L'Aquila. This fault has the same strike of the Paganica Fault which was responsible for the MW 6.3, 6 April 2009 earthquake. The two faults display an en-echelon pattern with a few km offset. We have found that events causing efficient trapped waves are clustered at the northwestern and southeastern bottom ends of the ruptured Paganica fault plane. The efficiency of trapped waves at FAGN, which is located about 5km far from the ruptured fault plane, indicates that the two faults are linked at depth. This suggests that fault segments in the study area can be part of a longer and continuous fault system which controls the seismic hazard of the region. Moreover, we have found that the two earthquake clusters generating the most efficient trapped waves occur in portions of the fault system with the highest fluid pressure.

Space‐time distribution of afterslip following the 2009 L'Aquila earthquake

The inversion of multitemporal DInSAR and GPS measurements unravels the coseismic and postseismic (afterslip) slip distributions associated with the 2009 MW 6.3 L'Aquila earthquake and provides insights into the rheological properties and long‐term behavior of the responsible structure, the Paganica fault. Well‐resolved patches of high postseismic slip (10–20 cm) appear to surround the main coseismic patch (maximum slip ≈1 m) through the entire seismogenic layer above the hypocenter without any obvious depth‐dependent control. Time series of postseismic displacement are well reproduced by an exponential function with best‐fit decay constants in the range of 20–40 days. A sudden discontinuity in the evolution of released postseismic moment at ≈130 days after the main shock does not correlate with independent seismological and geodetic data and is attributed to residual noise in the InSAR time series. The data are unable to resolve migration of afterslip along the fault probably because of the time interval (six days) between the main shock and the first radar acquisition. Surface fractures observed along the Paganica fault follow the steepest gradients of postseismic line‐of‐sight satellite displacements and are consistent with a sudden and delayed failure of the shallow layer in response to upward tapering of slip. The occurrence of afterslip at various levels through the entire seismogenic layer argues against exclusive depth‐dependent variations of frictional properties on the fault, supporting the hypothesis of significant horizontal frictional heterogeneities and/or geometrical complexities. We support the hypothesis that such heterogeneities and complexities may be at the origin of the long‐term variable behavior suggested by the paleoseismological studies. Rupture of fault patches with dimensions similar to that activated in 2009 appears to have a ≈500 year recurrence time interval documented by paleoseismic and historical studies. In addition to that, paleoseismological evidence of large (>0.5 m) coseismic offsets seems to require seismic events, recurring every 1000–2000 years, characterized by (1) multisegment linkage, (2) surface ruptures larger than in 2009, and (3) complete failure of the 2009 coseismic and postseismic patches.

Resolving vertical and east‐west horizontal motion from differential interferometric synthetic aperture radar: The L'Aquila earthquake

Analysis of surface coseismic displacement has already been obtained for the 6 April 2009 L'Aquila (central Italy) earthquake from differential interferometric synthetic aperture radar (DInSAR) data. Working jointly on ascending and descending DInSAR data makes for a step forward with respect to published preliminary estimates: we process data in order to retrieve a continuous displacement pattern, both in the vertical and horizontal directions, the latter being limited to the eastward component because of the low sensibility of the SAR images used to resolve northward motion. Our analysis provides new insights on the horizontal component of displacement, obtaining a clear picture of eastward displacement patterns over the epicentral area. This result is noteworthy, as until now little information has been available on horizontal displacement following normal‐fault events in the central Apennines (Umbria‐Marche, 1997, and L'Aquila, 2009), given the lack of dense GPS networks, the only available source of horizontal displacement data in this area. Inverted fault characteristics from such data also show noteworthy differences compared to previous studies, localizing the Paganica fault as the causative fault for the earthquake.

Fault geometry, coseismic-slip distribution and Coulomb stress change associated with the 2009 April 6, Mw 6.3, L’Aquila earthquake from inversion of GPS displacements

The 2009 April 6, Mw= 6.3 L’Aquila earthquake occurred within a complex system of NW-SE trending normal faults in the Abruzzi Central Apennines (Italy). We analyse the coseismic deformation as measured by >70 global positioning system (GPS) stations, both from continuous and survey-mode networks, providing unprecedented details for a moderate normal faulting earthquake in Italy from GPS measurements. We use rectangular, uniform-slip, dislocations embedded in an elastic, homogeneous and isotropic half-space and a constrained, non-linear optimization algorithm, to solve for the best-fitting rectangular dislocation geometry and coseismic-slip distribution. We use a bootstrap approach to investigate uncertainties in the model parameters and define confidence bounds for all the inverted parameters. The rupture occurred on a N129°E striking and 50° southwestward dipping normal fault, in agreement with geological observations of surface breaks along the Paganica fault. Our distributed slip model exhibits a zone of relatively higher slip (>60 cm) between ~1.5 and ~11 km depth, along a roughly downdip, NW-SE elongated patch, confined within the fault plane inverted assuming uniform-slip. The highest slip, of the order of ~1 m, occurred on a ~16 km2 area located at ~5 km depth, SE of the mainshock epicentre. The analysis of model resolution suggests that slip at depth below ~5 km can be resolved only at a spatial scale larger than 2 km, so a finer discretization of different asperities within the main patch of coseismic-slip is not allowed by GPS data. We compute the coseismic Coulomb stress changes in the crustal volume affected by the major aftershocks, and compare the results obtained from the uniform-slip and the heterogeneous-slip models. We find that most of the large aftershocks occurred in areas of Coulomb stress increase of 0.2-13 bar and that a deepening of the slip distribution down to a depth greater than 6 km in the SE part of the fault plane, in agreement with the inverted slip model, can explain the deepest, April 7, Mw 5.3 aftershock.

Surface Faulting of the 6 April 2009 Mw 6.3 L'Aquila Earthquake in Central Italy

This paper documents evidence of surface faulting associated with the 6 April 2009 moderate-sized earthquake (ML 5.8, Mw 6.3) in the central Apennines of Italy, which caused major damage to the town of L’Aquila and its surroundings. Coseismic surface ruptures were mapped for a minimum distance of 2.6 km along the Paganica fault, a fault still poorly investigated relative to the other active faults nearby, which bound much wider range fronts. Surface rupture length (SRL) and maximum displacement parameters (2.6 km minimum and 10–15 cm, respectively) are in agreement with what is expected for an Mw 6.3 event in the Italian Apennines tectonic environment. Different viewpoints exist on the amount of SRL and the number of activated faults. We propose a pattern of sympathetic and secondary slip on an array of faults around the master seismogenic structure. Past seismicity and evidence for larger Holocene offsets on this and other capable faults nearby prove that the 2009 event is not a good reference event for assessing the seismic hazard of the region. Nevertheless, the 2009 L’Aquila earthquake once more confirmed the importance of detailed geological studies for a proper seismic hazard assessment, and it clearly illustrates the need to pay attention to moderate events and supposedly minor active faults. Indeed, this type of earthquake is rather frequent in the whole Mediterranean region and is potentially much more destructive than in the past, due to the expanding urban centers and infrastructures inside their epicentral regions and even right above the traces of capable faults.

Evidence for surface faulting events along the Paganica fault prior to the 6 April 2009 L'Aquila earthquake (central Italy)

[1] We performed paleoseismological investigations at four sites across the normal Paganica fault (PF) (source of the 2009 Mw 6.3 L'Aquila earthquake), with the goal of reconstructing the rupture history and of contributing to the evaluation of the maximum event expected along the PF. We recognized five distinct surface faulting earthquakes (including the 2009) in the trenches. The age of the penultimate event is consistent with the 1461 earthquake; the third event back occurred around 1000 AD. The two oldest events have larger uncertainties and occurred in the interval 760 BC–670 AD and 2900–760 BC, respectively. The along-strike vertical displacement for each paleoevent has a limited variability consistently with the fairly homogeneous slip observed in 2009 along the northern part of the rupture. Conversely, the throws change between distinct events and range between 0.15 m in 2009 (maximum estimate) and close to 0.4 (lower bound estimate) in earlier events. These paleorecords and the high fault escarpments imply that earthquakes larger than 2009 occurred on the PF, with implications for the level of hazard. Recurrence intervals also reflect a change with time, the average interval before ∼1000 AD is longer compared to that after this date. Two events occurred in the 2000–4000 years preceding ∼1000 AD, while three events occurred since ∼1000 AD. The age uncertainties affecting the interpreted events prevent the evaluation of a unique value for interevent interval; the older events appear closely spaced in time or far apart depending on the upper or lower boundary of the age interval. We tentatively assign an average interevent time of ∼500 years for the three youngest events, whereas the time elapsed between the previous ones could be larger, in the order of 1000–2000 years. We calculate a late Pleistocene dip-slip rate for the PF of 0.2–0.4 mm/yr, consistent with 0.25–0.5 mm/yr for the early Pleistocene. Using age and throw of individual events, we calculate a similar late Holocene average dip-slip rate of ∼0.3–0.4 mm/yr. This suggests that the portion of the PF where the 2009 continuous surface faulting occurred has fairly a constant average slip release since late Pleistocene. Finally, we discuss different rupture scenarios and alternative models of occurrence compatible with our data and their variability.

InSAR data as a field guide for mapping minor earthquake surface ruptures: Ground displacements along the Paganica Fault during the 6 April 2009 L'Aquila earthquake

On 6 April 2009, a moderate earthquake (Mw = 6.3; Ml = 5.8) struck the Abruzzo region in central Italy, causing more than 300 fatalities and heavy damage to L'Aquila and surrounding villages. Coseismic surface effects have been thoroughly documented by timely field surveys as well as by remote sensing analyses of satellite images. The outstanding quality of geological, seismological, geodetic, and interferometric synthetic aperture radar (InSAR) information arguably represents the best ever data set made available immediately after a moderate seismic event. Based on this data set, we aim at testing the capability of coupled geological and InSAR data to map surface faulting patterns associated with moderate earthquakes. Coseismic ground ruptures have been mapped at a scale of 1:500 in the whole epicentral area. Traces of surface ruptures have been inferred from linear phase discontinuities identified in the interferogram. A very good agreement between the two methods resulted in the characterization of the main surface rupture along the Paganica fault. The same approach applied to ground ruptures hypothesized along other capable fault segments provided more questionable results. Thus, the combined field and InSAR approach appeared useful for detecting continuous surface ruptures exceeding 1 km in length and showing displacements greater than a few centimeters. These are the typical faulting parameters for moderate earthquakes (6.0 < Mw < 6.5) in central Apennines. For continuous ground cracks shorter than a few hundred meters and/or that show displacements smaller than 1–2 cm, the described approach may be less helpful, most probably due to the limited resolution of the data.

Performance of the Italian strong motion network during the 2009, L’Aquila seismic sequence (central Italy)

On April 6, 2009, the town of L’Aquila in the Abruzzo region (central Italy) was struck by a seismic event at 01:32 (UTC), of magnitude MW = 6.3. The mainshock was followed by a long period of intense seismic activity and within seven days after the mainshock there were seven events of magnitude MW ≥ 5 that occurred from April 6 to April 13. This long seismic sequence was characterized by a complex rupture mechanism that involved two major normal faults of the central Apennines: the Paganica and the Gorzano faults. The strong-motions of the mainshock were recorded by 64 stations of the Italian Strong-motion Network (RAN) operated by the National Civil Protection Department (DPC). Six stations of a local strong-motion array were working in NW L’Aquila suburb area. One of them, located at about 6 km from the Paganica fault surface tip-line, set up in trigger mode, recorded continuously for more than 20 min the mainshock and the aftershocks. Besides the mainshock, the RAN stations recorded in total 78 foreshocks and aftershocks of ML ≥ 3.5, during the period from January to December 2009. The corresponding waveforms provide the most extensive digital strong ground motion data set ever recorded in Italy. Moreover, the 48 three-component observations of events of magnitude MW ≥ 5, recorded at a distance less than 15 km from each of the major involved faults, provide a significant increasing of near-field records available for the Italian territory. Six days after the mainshock, the strong-motion dataset, referred to preliminary locations of the events with ML ≥ 4.0, was made available on the DPC web site (http://www.protezionecivile.it/) and at the same time it was delivered to the ITACA database (http://itaca.mi.ingv.it/ItacaNet/). This dataset has been used by many authors in scientific papers and by engineers, geophysicists and geologists for professional technical works. In this paper, the present-day available strong-motion signals from the L’Aquila sequence and the performance of the Italian strong-motion network in terms of the number and quality of recorded data, the geometry and data transmission system are described. In addition the role of the temporary network that represents an extension of the permanent Italian strong-motion network, supporting the emergency response by civil protection authorities and improving the network coverage has been evaluated.

Shallow subsurface structure of the 2009 April 6 Mw 6.3 L’Aquila earthquake surface rupture at Paganica, investigated with ground-penetrating radar

P>The shallow subsurface structure of the 2009 April 6 M-w 6.3 L'Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210 degrees and 228 degrees across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23-0.30 mm yr-1, consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be similar to 33 ka. This compares with published values of 0.4 mm yr-1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr-1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3-5 mm yr-1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard.

Coseismic and initial post-seismic slip of the 2009Mw6.3 L’Aquila earthquake, Italy, from GPS measurements

SUMMARY Here we report the preliminary results of GPS data inversions for coseismic and initial afterslip distributions of the Mw 6.3 2009 April 6 L’Aquila earthquake. Coseismic displacements of continuous and survey-style GPS sites, show that the earthquake ruptured a planar SW-dipping normal fault with ∼0.6 m average slip and an estimated moment of 3.9 × 1018 Nm. Geodetic data agree with the seismological and geological information pointing out the Paganica fault, as the causative structure of the main shock. The position of the hypocentre relative to the coseismic slip distribution supports the seismological evidence of southeastward rupture directivity. These results also point out that the main coseismic asperity probably ended downdip of the Paganica village at a depth of few kilometres in agreement with the small (1–10 cm) observed surface breaks. Time-dependent post-seismic displacements have been modelled with an exponential function. The average value of the estimated characteristic times for near-field sites in the hanging-wall of the fault is 23.9 ± 5.4 d. The comparison between coseismic slip and post-seismic displacements for the first 60 d after the main shock, shows that afterslip occurred at the edges of the main coseismic asperity with a maximum estimated slip of ∼25 cm and an equivalent seismic moment of 6.5 × 1017 Nm. The activation of the Paganica fault, spatially intermediate between the previously recognized main active fault systems, suggests that strain accumulation in the central Apennines may be simultaneously active on distinct parallel fault systems.

Coseismic ground deformation of the 6 April 2009 L'Aquila earthquake (central Italy, Mw6.3)

We provide field data of coseismic ground deformation related to the 6 April Mw 6.3 L'Aquila normal faulting earthquake. Three narrow fracture zones were mapped: Paganica‐Colle Enzano (P‐E), Mt. Castellano‐Mt. Stabiata (C‐S) and San Gregorio (SG). These zones define 13 km of surface ruptures that strike at 130–140°. We mapped four main types of ground deformation (free faces on bedrock fault scarps, faulting along synthetic splays and fissures with or without slip) that are probably due to the near‐surface lithology of the fault walls and the amount of slip that approached the surface coseismically. The P‐E and C‐S zones are characterized by downthrow to the SW (up to 10 cm) and opening (up to 12 cm), while the SG zone is characterized only by opening. Afterslip throw rates of 0.5–0.6 mm/day were measured along the Paganica fault, where paleoseismic evidence reveals recurring paleo‐earthquakes and post‐24.8 kyr slip‐rate ≥ 0.24 mm/yr.

Evidence for surface rupture associated with the Mw 6.3 L’Aquila earthquake sequence of April 2009 (central Italy)

Terra Nova, 22, 43–51, 2010 Abstract An earthquake of Mw = 6.3 struck L’Aquila town (central Italy) on 6 April 2009 rupturing an ∼18-km-long SW-dipping normal fault. The aftershock area extended for a length of more than 35 km and included major aftershocks on 7 and 9 April and thousands of minor events. Surface faulting occurred along the SW-dipping Paganica fault with a continuous extent of ∼2.5 km. Ruptures consist of open cracks and vertical dislocations or warps (0.1m maximum throw) with an orientation of N130°–140°. Small triggered slip and shaking effects also took place along nearby synthetic and antithetic normal faults. The observed limited extent and small surface displacement of the Paganica ruptures with respect to the height of the fault scarps and vertical throws of palaeo-earthquakes along faults in the area put the faulting associated with the L’Aquila earthquake in perspective with respect to the maximum expected magnitude and the regional seismic hazard.

Strong foreshock signal preceding the L'Aquila (Italy) earthquake (<i>M</i><sub>w</sub> 6.3) of 6 April 2009

Abstract. We used the earthquake catalogue of INGV extending from 1 January 2006 to 30 June 2009 to detect significant changes before and after the 6 April 2009 L'Aquila mainshock (Mw=6.3) in the seismicity rate, r (events/day), and in b-value. The statistical z-test and Utsu-test were applied to identify significant changes. From the beginning of 2006 up to the end of October 2008 the activity was relatively stable and remained in the state of background seismicity (r=1.14, b=1.09). From 28 October 2008 up to 26 March 2009, r increased significantly to 2.52 indicating weak foreshock sequence; the b-value did not changed significantly. The weak foreshock sequence was spatially distributed within the entire seismogenic area. In the last 10 days before the mainshock, strong foreshock signal became evident in space (dense epicenter concentration in the hanging-wall of the Paganica fault), in time (drastic increase of r to 21.70 events/day) and in size (b-value dropped significantly to 0.68). The significantly high seismicity rate and the low b-value in the entire foreshock sequence make a substantial difference from the background seismicity. Also, the b-value of the strong foreshock stage (last 10 days before mainshock) was significantly lower than that in the aftershock sequence. Our results indicate the important value of the foreshock sequences for the prediction of the mainshock.