4�

GNGTS 2014 sessione 1.1

CoSEISMIC And PoSt-SEISMIC SLIP of thE 2012 EMILIA �noRthERn ItALy�� SEISMIC SEqUEnCE InfERREd fRoM joInt InvERSIon of hIGh-PRECISIon LEvELLInG, InSAR And GPS dAtAD. Cheloni1, R. Giuliani2, N. D’Agostino1, M. Mattone2, M. Bonano3, G. Fornaro3, R. Lanari3, D. Reale3,F. Deninno4, R. Maseroli4, G. Stefanelli4

1 Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy2 Ufficio Rischio Sismico, Dipartimento della Protezione Civile, Rome, Italy3 Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche, Naples, Italy4 Sezione Geodetica, Istituto Geografico Militare, Florence, Italy

Introduction. The 2012 Emilia earthquake sequence struck the central sector of the Ferrara arc, which represents the external fold-and-thrust system of the northern Apennines thrust belt buried below the Po plain. The Ferrara arc is composed by two major fold-and-thrust systems, the Ferrara and the Mirandola systems, where the two main seismic events (the 20 May MW 5.9 and the 29 May MW 5.8) occurred.

However, seismological, geodetic and geological-based works that have been published so far have clarified neither the exact association between activated thrust segment and related mainshock, nor the fine detail of the geometry of the activated fault systems during the Emilia seismic sequence.

Fault-plane solutions of the Emilia sequence (Malagnini et al., 2012; Pondrelli et al., 2012; Scognamiglio et al., 2012; Cesca et al., 2013) obtained very similar solutions for the two mainshocks, showing low dip angles for the south dipping planes. Preliminary GPS and InSAR based uniform slip models (Bignami et al., 2012; Serpelloni et al., 2012) also indicate that the two main events occurred on S-dipping low angle fault planes, tentatively associated to segments belonging to the ITCS050 “Poggio Rusco-Migliarino” (for the May 20 event) and a segment of the ITCS051 “Novi-Poggio Renativo” (for the May 29 event) composite seismogenic sources of the Database of Individual Seismogenic Sources (DISS Working Group, 2010). Conversely, preliminary seismotectonic interpretation (Lavecchia et al., 2012) attributed both the May 20 and May 29 events to the Ferrara system. According to the authors the May 20 event ruptured the Ferrara basal thrust at depth between 3-7 km, while during the May 29 event the rupture jumped on an inner splay of the Ferrara system (assuming that the Mirandola thrust have been only subordinately involved in the sequence). Subsequent InSAR based studies (Pezzo et al., 2013; Tizzani et al., 2013) have allowed to solve also for more complicated geometries and for the variable slip distribution on the two fault segments activated during this sequence. In particular, Pezzo et al. (2013) identified two main fault planes for the Emilia seismic sequence, computing the corresponding slip distributions: the May 20 event was modelled with a variable dip geometry (with a shallower segment dipping at 40° SSW followed at depth by a 20° SSW sloping segment), corresponding to the middle Ferrara thrust, with a maximum slip of ~120 cm at 5 km depth; for the source location of the May 29 event the authors approximated the shape of the frontal Mirandola thrust by a shallower segment dipping 45° S continuing at depth in a 30° S dipping segment, with two main slipping areas (a maximum slip of ~54 cm at 6 km depth and a lower slip concentration of ~30 cm at 5 km depth on the western side of the modelled fault plane). Tizzani et al. (2013) by means of an advanced modelling based on a finite-element structural-mechanic method, proposed instead the activation of a single low angle fault segment for the May 20 event, corresponding to the inner Ferrara thrust, and the activation of three different segments of the Mirandola anticline for the May 29 event, which describe an overall listric geometry with the steeper part that dips ~65° between 5-8 km depths. Finally, by accurate aftershock location Govoni et al. (2014) defined two separate segments arranged in a left-stepping en-enchelon pattern, whose dip changes from steeper to flatter going west to east. In particular, from the cross sections across the two main shocks, they estimate the dip of the western (through the May 29 event) and eastern (through

001-260 volume 1 48 24-10-2014 16:52:48

GNGTS 2014 sessione 1.1

4�

the May 20 event) fault segments to be approximately 70° and 45° respectively, associating the activated segments to the Mirandola system and to the intermediate thrust of the Ferrara system respectively. The authors also suggested that the observed discrepancy between the relocated afterschock alignments on the Mirandola thrust and the most of the fault-plane and geodetic solutions computed for the May 29 event indicating low dip angle of this thrust segment, could be explained by supposing that the May 29 event struck at the deeper, flatter portion of a fault segment of listric geometry, with most of the afterschocks occurring on the shallow, steeper part. Finally, the possible contribution of the early post-seismic deformation following the two mainshocks has not been assessed yet.

Therefore, although the works that have been published so far have described the first-order geometrical and kinematic features of the thrust segments which were responsible for the two mainshocks, some additional complexities and discrepancies to this first-order scenario can be summarized as follows:

1) Some disagreement exists between the May 20 earthquake/fault association. Whichsegment of the Ferrara thrust system was activated during the May 20 mainshock? Theintermediate or the innermost thrust?

2) May 29 earthquake/fault association. Most of the studies associate this event to theMirandola thrust, however alternative hypothesis have attributed the May 29 mainshockto the westernmost and innermost thrust of the Ferrara system.

3) There is a discrepancy between fault planes solutions for the May 29 event and relocatedaftershock alignments. Could this discrepancy explained assuming a listric geometry ofthe Mirandola thrust? Do the geodetic data require a listric geometry?

4) What is the contribution of the post-seismic deformation in the 2012 Ferrara seismicsequence?

In this study, to address some of the open issues outline above, we present the preliminary results of the joint inversion of four different independent geodetic datasets (Fig. 1): about 67 km of high-precision levelling lines in the mainshocks epicentral area, including coseismic and early post-seismic deformation (first 12 months); repeated radar acquisitions covering the epicentral region are available for one RADARSAT-2 (RSAT2) interferogram calculated from two descending acquisitions on 30 April and 17 June 2012 pair and covering both the two

Fig. 1 – Time line of coseismic and post-seismic intervals covered by the levelling lines, the InSAR interferograms and GPS displacements used in the present study. The red dashed lines represent the MW 5.9 May 20 and MW 5.8 May 29 events. The black dashed line indicates our nominally post-seismic starting epoch, starting 5 days after the May 29 mainschock.

001-260 volume 1 49 24-10-2014 16:52:49

�0

GNGTS 2014 sessione 1.1

main earthquakes (May 20 and 29, 2012) and for a COSMO-SkyMed (CSK) interferogram obtained from two descending acquisitions on May 27 and June 4, 2012 pair, encompassing only the May 29, 2012 event; multitemporal DInSAR measurements of the CSK data between June 4, 2012 and May 7, 2013, including about 1 one year of post-seismic deformation; GPS data describing coseismic and post-seismic deformations. This allows us to reduce ambiguities associated with each dataset and to better constrain the earthquake/fault associations, the geometrical features and the slip distributions of the two activated thrust faults. In addition, preliminary post-seismic deformation models will be presented for a better understanding of the strain relaxation processes in this region.

The 2012 Emilia seismic sequence. The May 20 MW 5.9 mainshock, occurred near the city of Finale Emilia, was preceded by a ML 4.1 foreshock 3 h earlier (Scognamiglio et al., 2012). Following the mainshock an intense afterschock sequence occurred, with several events of magnitude M > 5. On May 29 a MW 5.8 located about 12 km SW of the May 20 mainshock occurred near the city of Mirandola. Moment tensor solutions of the main events (Pondrelli et al., 2012; Scognamiglio et al., 2012) exhibit thrust faulting with E-W oriented nodal planes, consistent with the direction of shortening shown in the interseismic geodetic data (Serpelloni et al., 2006; Devoti et al., 2011).

The relocated aftershock distribution delineates a E-W trending structure extended for about 50 km, defining two separate segments with different dip from west to east. Govoni et al. (2014) estimate the dip of the western (the Mirandola thrust) and eastern (the Ferrara thrust) fault segments to be about 70° and 45° respectively.

Geodetic dataset. High-precision levelling data. The high-precision levelling measurements were obtained from first-order double precision levelling surveys during different campaign periods. The pre-seismic levelling surveys, performed by a private company with the aim of planning a new regional highway, took place in April 2011, just one year before the seismic sequence. The post-seismic data were collected between April and May 2013, one year after

the occurrence of the mainshocks (May 20 and 29, 2012) of the seismic sequence by the Istituto Geografico Militare (IGM). They consists of a levelling line oriented in a east-west direction, connecting two levelling lines of the first-order of the IGM. The total length of measured lines is 67 km, with 500 sites and a mean benchmark density of 7.4 benchmark/km. Post-earthquake levelling measurements followed high-precision levelling standards (Bomford, 1971) with mean errors well within the maximum tolerance admitted for first-order surveys.

Since the IGM measurements started about one year after the mainshocks, the resulting displacements include coseismic and initial post-seismic deformation. The benchmark 52, west of the epicentres of the seismic sequence, close to Ferrara and outside the area of significant deformation according

Fig. 2 – Geodetic sites in the Ferrara arc area: high-precision levelling lines (inverted triangles), GPS stations (black and grey squares) and RSAT-2 data sampled points. Solid lines are major faults of the area; solid red lines indicate the Ferrara arc system. The red stars indicate the May 20 and 29 mainshocks, while the green stars represent M > 5 events.

001-260 volume 1 50 24-10-2014 16:52:51

GNGTS 2014 sessione 1.1

�1

to InSAR and GPS results (Serpelloni et al., 2012; Pezzo et al., 2013; Tizzani et al., 2013) has been selected as reference benchmark for the levelling lines.

The levelling lines describe the uplifted area of the hanging-wall blocks of the faults of the Ferrara arc on the basis of 92 measurements (Fig. 2). In particular the hanging-wall uplift is well constrained on the hanging-wall of the westernmost system, the Mirandola system, where the uplift reaches maximum values of about 18 cm. This main uplifted area is in good agreement with the elongated E-W displacement observed by InSAR data (Fig. 2) near the May 29 epicenter, where previous works (e.g., Pezzo et al., 2013) observed a maximum displacement of ~14 cm. However, westwards of Mirandola city, where the lobe of InSAR displacement field tends to decrease, we observe an additional amount of uplift. This extra uplifted region could be due to the combined effects of the occurrence of the greatest aftershocks (M > 5) and early post-seismic deformation. On the easternmost part of the levelling lines, near Finale Emilia, the observed smaller peak of uplift well correlates with the eastern termination of the InSAR lobe of deformation near the May 20 epicenter.

InSAR data. In this work we used SAR data acquired with RADARSAT-2 and COSMO-SkyMed satellites. In particular, we used the April 30, 2012/June 17, 2012 pair, acquired from RADARSAT-2 (RSAT2) descending orbit to measure the cumulative coseismic ground displacements due to the two main shocks (May 20 and 29) and the May 27, 2012/June 4, 2012 pair, acquired from COSMO-SkyMed (CSK) descending orbit to obtain the coseismic displacement field related to the May 29 earthquake.

To measure the post-seismic deformation we used the radar surface displacement time series generated by applying a SBAS DInSAR processing algorithm to the data acquired from COSMO-SkyMed constellation between June 4, 2012 and May 7, 2013. This temporal interval starts 5 days after the May 29 main shock and includes about one year of post-seismic deformation (Fig. 1).

The cumulative coseismic deformation field along the RSAT2 line-of-sight presents two main lobes due to the May 20 and 29 events, with a maximum displacement of about 15 cm corresponding to a sensor-target range decrease (Fig. 2). On the other hand, the CSK interferogram yielded only a main lobe due to the May 29 event, with a maximum displacement

of about 8 cm towards the satellite. The discrepancies between the different interferograms are due to the different look angles and orbits. The displacement pattern in the CSK post-seismic interferogram, covering about one year of post-seismic deformation, shows one major lobe of decreasing line-of-sight displacements towards the satellite, with a maximum value of about 2-3 cm, that approximately appears to overlap with the westernmost termination of the area of the May 29 coseismic displacements (Fig. 2), slightly shifted to the west (Fig. 3). This uplifted region appears to correlate with the area in which we observe some amount of additional uplift from levelling measurements.

GPS data. Coseismic and initial post-seismic displacements have

Fig. 3 – Geodetic sites in the Ferrara arc area. The circles show the CSK post-seismic data points, while the white triangles the distribution of levelling benchmarks. Other symbols as in Fig. 2.

001-260 volume 1 51 24-10-2014 16:52:54

�2

GNGTS 2014 sessione 1.1

been recorded by permanent GPS stations managed by different private and public institutions located around the epicentral area (Figs. 2 and 3). Most of the GPS sites are located in the far field, whereas only few stations were available for the epicentral area. A complete description of the processing details and strategies are given by Serpelloni et al. (2012). We extend the GPS time series to the acquisition interval of the CSK and levelling data set. We modelled site positions as the sum of a coseismic offset and a time-dependent function, which reproduces coseismic displacement and post-seismic response (e.g., Langbein, 2008). The model equation contains parameters that describe the time-dependent signal characterised as follows:

where 0iy is the pre-seismic position, iA is the coseismic offset, t is the time after the

earthquake, H is the Heaviside step function, iB is the exponential decay amplitude and τ is the exponential decay time for the i-th component. We calculate an additional coseismic offset associated with the May 29 Mw 5.8 mainshock. Time dependent post-seismic deformation is modelled only for the May 29 mainshock. In this way we assume that the contribution of post-seismic deformation of the May 20 event is minimal, as verified by Pezzo et al. (2013) from a CSK ascending interferogram spanning the period May 22-26.

The coseismic displacement field for the May 20 event presents the largest horizontal displacements at Finale Emilia (MO05) which moved to the SSW by ~3 cm, at SERM station that moved southwards by ~1.5 cm and at SERM station that moved NNE-wards by ~2 cm. The other GPS sites, located around the epicentral area, show horizontal displacements < 1 cm. For the May 29 event, all GPS horizontal displacements converge toward the area of the mainshock, with displacements smaller than for the May 20 event (< 1 cm).

The preliminary analysis of the post-seismic GPS time series shows that few stations (e.g., SERM, SGIP and SBPO) underwent a progressively decaying motion well-reproduced by a time-dependent function typical of post-seismic evolution. In particular the north components of SERM and SBPO sites move southward, while the north component of SGIP station shows a movement towards north (with horizontal displacements of ~1-2 cm covering ~1 year of post-seismic deformation).

Preliminary results of the inversion of geodetic data. In this section we illustrate the results of the inversion for distributed slip on a pair of fault planes corresponding to the Ferrara and Mirandola thrust faults. We combine all geodetic datasets, that is, levelling measurements, InSAR interferograms and GPS data, to simultaneously infer the coseismic and post-seismic slip distributions on the fault planes that ruptured in the two mainshocks. Before modelling, the interferograms were downsampled using a resolution-based resampling technique (Lohman and Simons, 2005), with the highest density of data points close to the source of deformation.

We compute the Green’s functions, which relate unit slip on individual fault patches to surface displacements at individual points using dislocations in an elastic, homogeneous and isotropic half-space (Okada, 1985). The Ferrara and Mirandola thrust faults are thus discretized into smaller patches. We explore different possible geometries (e.g., listric versus straight geometry) and dip angle of the rupture planes, defined by moment tensor solutions, aftershock distribution and mapping studies, in order to better define the segments of the Ferrara arc that were activated during the two mainshocks and aftershock sequence.

We take as a starting point the fault geometry of the main active thrusts proposed by Boccaletti et al. (2011), which by using morphotectonic, geological-structural, and stratigraphic analysis compared with the current seismicity of the region, suggested that the external part of the northern Apennines is characterised by presence of major systems of Quaternary compressive structures corresponding to the Emilia, Ferrara and Adriatic Fold systems buried below the Po Plain. These structures have a typical arcuate shape in map view (Figs. 2 and 3), giving rise to the Emilia and Ferrara folds (Pieri and Groppi, 1981). In particular, in the aftershock region, the Ferrara arc is structured in two major fold-and-thrust systems, the Ferrara system in the NE

001-260 volume 1 52 24-10-2014 16:52:55

GNGTS 2014 sessione 1.1

�3

and the Mirandola system in the southwest located in a more internal position. Furthermore, the Ferrara system is composed of an innermost segment, a middle segment and outermost segment. We use the dip of the fault segments proposed by the most recent work on accurate relocated aftershock (Govoni et al., 2014) which estimate the dip of the Mirandola and of the Ferrara thrusts to be approximately 70° and 45° respectively in the upper portion of the thrust. We have also tried dip angles inferred by moment tensor solutions (Pondrelli et al., 2012; Scognamiglio et al., 2012), which conversely suggested a lower dip angle for the Mirandola thrust. More complicated geometry (i.e., listric geometry) as proposed by Cesca et al. (2013) and Govoni et al. (2014), were also tested while looking at the misfit to the data.

Preliminary inversion results show that the inferred spatial distribution of coseismic slip related to the May 20 event is in good agreement with previous inversions of geodetic data (e.g., Pezzo et al., 2013), showing in the centre of the fault a main coseismic patch (maximum value around 100 cm) at a down-dip depth between 5-7 km on the NE fault (the Ferrara thrust). In addition on the eastern side a lower slip concentration (~20 cm) is located at deeper depths. As regards the May 29 event, the modelled slip distribution shows one main coseismic patch in the central part of the fault with a maximum slip of ~50 cm at ~8 km depth, well correlated to the MW 5.8 May 29 event. The inversion of post-seismic deformation, assuming that the observed post-seismic displacements are due to afterslip on the fault plane related to the May 29 mainshock, yields to a main patch of afterslip located westwards and up-dip of the main coseismic patch, suggesting that afterslip was triggered at the edges of the largest coseismic asperity.

ReferencesBignami C. et al.; 2012: Coseismic deformation pattern of the Emilia 2012 seismic sequence imaged by Radarsat-1

interferometry. Ann. of Geophys.,Ann. of Geophys., 55, 789-795, doi:10.4401/ag-6157.Boccaletti M., Corti G. and Martelli L.; 2011: Recent and active tectonics of the external zone of the Northern

Apennines (Italy). Int. J. Earth Sci., 100, 1331-1348, http://dx.doi.org/10.1007/s00531-010-0545-y. Bomford G.; 1971: Geodesy. Oxford University Press, New York (731 pp.).Cesca S., Braun T., Maccaferri F., Passatelli L., Rivalta E. and Dahm T.; 2013: Source modelling of the M5-6 Emilia-

Romagna, Italy, earthquakes (2012 May 20-29). Geophys. J. Int., 193, 1658-1672, http://dx.doi.org/10.1093/gji/ggt069.

Devoti R., et al.; 2011: Evidence of large scale deformation patterns from GPS data in the Italian subduction boundary. Earth Planet. Sci. Lett., 311, 3-4, doi:10.1016/j.epsl.2011.09.034.

DISS Working Group; 2010: Database of Individual Seismogenic Sources (DISS), Version 3.1.1: A compilation of potential sources for earthquakes lager than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/.

Govoni A., Marchetti A., De Gori P., Di Bona M., Lucente F.P., Improta L., Chiarabba C., Nardi A., Margheriti L., Agostinetti N.P., Di Giovanbattista R., Latorre D., Anselmi M., Ciaccio M.G., Moretti M., Castellano C. and Piccinini D.; 2014: The 2012 Emilia seismic sequence (Northern Italy): Imaging the thrust fault system by accurate aftershock location. Tectonophysics, 622, 44-55, http://dx.doi.org/10.1016/j.tecto.2014.02.013.

Langbein J.; 2008: Noise in GPS displacement measurements from Southern California and Southern Nevada. J. Geophys. Res., 113, B05405, http://dx.doi.org/10.1029/2007JB005247.

Lavecchia G., De Nardis R., Cirillo D., Brozzetti F. and Boncio P.; 2012: The May-June 2012 Ferrara arc earthquakes (northern Italy): structural control of the spatial evolution of the seismic sequence and of the surface pattern of coseismic fractures. Ann. Geophys.,Ann. Geophys., 55, 533, 540, http://dx.doi.org/10.4401/ag-6173.

Lohman R.B. and Simons M.; 2005: Some thoughts on the use of InSAR data to constrain models of surface deformation: noise structure and data downsampling. Geochem. Geophys. Geosyst.,Geochem. Geophys. Geosyst., 6. Q01007, http://dx.doi.Q01007, http://dx.doi.org/10.1029/2004GC000841.

Malagnini L., Herrmann R.B., Munafò I., Buttinelli M., Anselmi M., Akinci A. and Boschi E.; 2012: The 2012 Ferrara seismic sequence: regional crustal structure, earthquake sources, and seismic hazard. Geophys. Res. Lett., 39, http://dx.doi.org/10.1029/2012GL053214.

Okada Y.; 1985: Surface deformation due to shear and tensile faults in a half-space. Bull. Seism. Soc. Am., 75, 1135-1154.

Pezzo G., Boncori J.P.M., Tolomei C., Salvi S., Atzori S., Antonioli A., Trasatti E., Novali F., Serpelloni E., Candelazzo G., Boncori J.P.M., Tolomei C., Salvi S., Atzori S., Antonioli A., Trasatti E., Novali F., Serpelloni E., Candela L. and Giuliani R.; 2013: Coseismic Deformation and Source Modeling of the May 2012 Emilia (Northern Italy) Earthquakes. Seism. Res. Lett., 84, 645-655, doi:10.1785/0220120171.

Pieri M. and Groppi G.; 1981: Subsurface geological structure of the Po Plain (Italy). C.N.R., Prog. Fin. Geodinamica,C.N.R., Prog. Fin. Geodinamica, Pubbl., 414, 1-13.

001-260 volume 1 53 24-10-2014 16:52:55

�4

GNGTS 2014 sessione 1.1

Pondrelli S., Salimbeni S., Perfetti P. and Danecek P.; 2012: Quick regional centroid moment tensor solutions for thement tensor solutions for the Emilia 2012 (northern Italy) seismic sequence. Ann. Geophys., 55, doi:10.4401/ag-6146.

Scognamiglio L., Margheriti L., Mele L., Tinti F.M., Bono A., De Gori P., Lauciani V., Lucente F.P., Mandiello A.G., Marcocci C., Mazza S., Pintore S. and Quintiliani M.; 2012: The 2012 Pianura Padana Emiliana seismicsequence: locations, moment tensors and magnitudes. Ann. Geophys., 55, 549-559, http://dx.doi.org/10.4401/ag-6159.

Serpelloni E., Anzidei M., Baldi P., Casula G. and Galvani A.; 2006: GPS measurement of active strains across the Apennines. Ann. Geophys.,Ann. Geophys., 49, 319-329.

Serpelloni E. et al.; 2012: GPS observations of coseismic deformation following the May 20 and 29, 2012, Emilia seismic events (northern Italy): data, analysis and preliminary models. Ann. of Geophys.,Ann. of Geophys., 55, 759-766, doi:10.4401/ag-6168.

Tizzani P., Castaldo R., Solaro G., Pepe S., Bonano M., Casu F., Manunta M., Manzo M., Pepe A., Samsonov S., Lanari R. and Sansosti E.; 2013:osti E.; 2013: New insights into the 2012 Emilia (Italy) seismic sequence through advanced numerical modeling of ground deformation InSAR measurements. Geophys. Res. Lett., 40, 1971-1977, doi:10.1002/grl.50290.

Ventura G. and Di Giovanbattista R.; 2013: Fluid pressare, stress field and propagation style of coalescing thrusts from the analysis of the 20 May 2012 ML 5.9 Emilia earthquake (Northern Apennines, Italy). Terra Nova, 25, 72-78, http://dx.doi.org/10.1111/ter.12007.

001-260 volume 1 54 24-10-2014 16:52:56