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SEISMIC CyCLE MEASUREMEntS fRoM SAR IntERfERoMEtRy: IntER-, Co- And PoSt-SEISMIC dEfoRMAtIon ASSoCIAtEd to thE 2012 EMILIA SEISMIC SEqUEnCEG. Pezzo, C. Tolomei, S. Atzori, S. Salvi, J.P. Merryman BoncoriIstituto Nazionale di Geofisica e Vulcanologia, CNT, Roma, Italy

Introduction. In this study we present some preliminary results concerning the measurement and modeling of the earthquake cycle from SAR interferometry, in a portion of the Po Plain affected by a seismic sequence during 2012. The SAR data availability for the study area spanning from 1992 to 2013 offers a precious opportunity to measure the surface deformation related to different phases of the seismic cycle. In fact the ground deformation due to the tectonic activity are one of the most important tools to improve the knowledge of the deep mechanisms governing the cycle. This can be subdivided in three main different phases: interseismic, coseismic and postseismic (e.g. Scholz and Kato, 1978). The interseismic phase can be again subdivided into a purely interseismic step and a preseismic one, but the state of knowledge concerning the latter is still vague (e.g. Deng et al.,1998). Soon after the seismic event the post-seismic deformation occurs; two phases can be discriminated: short and long-term deformation. The first one is related to afterslip and/or pore pressure readjustments, which take place over periods ranging from a few hours to a few months after the earthquake and can be observed by using SAR methods; in contrast, the second one is related to the viscoelastic relaxation occurring in the lower crust and upper mantle, ranging from months to years after the earthquake (e.g. Segall, 2002), and it is difficult to measure with SAR methods. Moreover, due to the very low velocity

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and spatial correlation it can be confused with interseismic signals or orbital errors. Thus, measurements of ground movements occurring on the different time scales of the earthquake cycle, from the nearly instantaneous deformation caused by seismic dislocations (Massonnet et al., 1993), to the slow strains of the interseismic phase (Wright et al., 2001) can be obtained using interferometric methods: DInSAR (Differential Interferometric Synthetic Aperture Radar) (Massonnet and Feigl, 1995) and MAI (Multi Aperture Interferometry) (Scheiber and Moreira, 2000; Bechor and Zebker, 2006) are suitable for the coseismic displacement, whereas multitemporal InSAR methods (Ferretti et al., 2001; Berardino et al., 2002; Hooper, 2007) with an accuracy of up to one millimeter per year over time periods of several years (Casu et al., 2006), are suitable for small displacements over large temporal spans.

We perform a large SAR data analysis focusing on the portion of the Emilia region affected by a seismic sequence during 2012. This area is located in the Po plain, which represents the east–west continental collisional boundary between the subducting Adria plate to the north, and the overriding Northern Apennine block to the south (Picotti and Pazzaglia, 2008). The 2012 seismic sequence occurred in the central Po plain tectonic domain, characterized by arcuate blind thrust systems and related growth folds located below the Plio-Quaternary sedimentary cover (Pieri and Groppi, 1981; Pieri, 1983; Consiglio Nazionale delle Ricerche, 1992; Burrato et al., 2003), as also highlighted by several seismic sections (Boccaletti and Martelli, 2004). Based on geomorphological analysis, subsurface geology, seismicity, and present day stress field, many authors proposed the current activity of the frontal part of the Northern Apennine accretionary wedge (Meletti et al., 2000; Valensise and Pantosti, 2001; Burrato et al., 2003; Scrocca et al., 2007; Boccaletti et al., 2010). Active fold growth in this area is describe by Burrato et al. (2003), which observe the northward migration of the Po River and other smaller watercourses during the Late Quaternary. Recent horizontal GPS measurements (Serpelloni et al., 2006; Devoti et al., 2011) suggest ~1 mm yr-1 of shortening, proving the active thrusting of the area. The Emilia sequence took place in an area of low historical seismicity. Since at least the year 1000 A.D. this area has never experienced earthquakes of magnitude larger than ML ~ 4.7 (1574 Finale Emilia earthquake, Rovida et al., 2011). Larger magnitude earthquakes occurred west and east of the 2012 epicentral area. The coseismic deformation of two 2012 mainshocks, occurred respectively on May 20, 2012, Ml 5.9, and May 29, Ml 5.8, and were investigated analyzing COSMO-SkyMed and Radarsat-1 surface displacements as well as GPS observations (Pezzo et al., 2013). The major finding of this work can be summarized as follows: i) The location of the coseismic deformation measured by InSAR data corresponds to part of the Mirandola and Ferrara folds, located under the Po alluvial plain. This evidence supports the long-term geomorphic analyses that attribute to the growth of the same folds the wide northward bend of the Po river course and the deviation of the Secchia and Panaro rivers (Burrato et al., 2003); ii) The SAR data inversion results suggest that the main activated faults were the Ferrara thrust (May 20 event) and the Mirandola one (May 29 event); iii) displacement map analysis shows an aseismic deformation that the is well modeled by slip on the eastern side of the frontal Mirandola thrust, later activated during the May 29 mainshock; this aseismic pattern reasonable occurred in the time span between the two mainshocks.

The short-term coseismic deformation observed during the seismic sequence together with inter- and post-seismic velocities showed in this work, may provide the opportunity to harmonize all geological long-term observations with the present day rates.

Data availability. The available SAR dataset includes several overlapping COSMO-SkyMed image beams (Stripmap mode) as well as ERS and Envisat datasets for both ascending and descending tracks covering almost the entire Emilia Region (for details see Tab.1). We measured ground deformation rates adopting the SBAS technique (Berardino et al., 2002) for multi-temporal SAR Interferometry, using the Sarscape© software.

We used both ascending and descending COSMO-SkyMed image datasets spanning the June, 2012- December, 2013 time interval to study the post-seismic deformation, as well as

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ASAR and ERS image datasets spanning from 1992 to 2010 for the inter-seismic period.The SBAS algorithm allows us to follow the temporal evolution of the deformation through

displacement time-series, and to analyze its spatial distribution over the temporal span covered by the SAR acquisitions.

We performed several SBAS processing runs for the SAR datasets, concatenating two or three consecutive frames acquired from the same track, where available, to obtain a wider coverage and to improve the estimation of InSAR orbital errors. It is important to notice that, at the time of writing, a couple of image datasets (the most external ones with respect to the mainshocks) have not yet been processed. Finally, we cross-validated the different ground velocity maps by comparing the results of independent processing runs in the overlapping areas. We mosaicked the retrieved mean ground velocity maps basing on the mean value of the overlapping area (not considering in this step the local subsidence areas). Then, we validated the obtained results making a comparison with the available CGPS benchmarks. To this end, we estimated and subtracted the plane minimizing the differences between CGPS (re-projected along the ascending and descending LoS, respectively) and InSAR velocities.

Tab. 1 - available SAR dataset for COSMO-SkyMed image beams (Stripmap mode), ERS and Envisat sensors for both ascending and descending geometries covering almost the entire Emilia Region.

Sensor Orbit Track-Frame Temporal Span N° of images N° of pairs

COSMO-SkyMed Ascending 0� west �/�/12-2�/12/13 2� ��

COSMO-SkyMed Ascending 0� east �/�/12-1�/11/13 22 �0

COSMO-SkyMed Ascending 04 13/�/12-23/12/13 2� ��

COSMO-SkyMed Ascending 03 30/�/12-14/�/13 1� 4�

COSMO-SkyMed Ascending 10 4/�/12-14/12/13 34 12�

COSMO-SkyMed Ascending 11 2�/�/12-2�/12/13 3� 2�1

COSMO-SkyMed Descending 03 20/�/12-20/12/13 2� ��

COSMO-SkyMed Descending 0� �/�/12-1�/12/13 2� ��

ERS Descending 1��-2�0� 30/�/�2-�/�/�� 3� 134

ASAR Descending 1��-2�0� 2�/1/03-21/�/10 33 13�

Results. Ground deformation during the interseismic and preseismic phase. The mosaicked ASAR and ERS mean velocity maps relative to the time span 1992-2010 show several clear patterns ascribable to different phenomena (Fig. 1A). We observe some strong subsidence patterns (red colours) likely due to the strong human water pumping carried out in the Po Plain; these patterns are mainly concentrated where the soft sediment thickness is greater. A relative uplift pattern with respect to the Po Plain is present along the entire Apennines front, due to the general subsidence trend of the plain. Finally we observe a rough correspondence between the growth anticline and the positive velocity values (Fig. 1B). Vice versa, negative velocity patterns correspond to the locations of the tectonic synclines

Ground deformation during the postseismic phase. In the mosaicked mean velocity maps we observe similar deformation patterns for both the ascending (Fig. 2A) and the descending (Fig. 2B) data: positive values (approaching to the sensor) are present in the epicentral area, reaching about 20 mm yr-1. Moreover, as observed for the interseismic velocities, there is a rough correspondence between the growth anticline and the positive velocity values. Vice versa, negative velocity patterns correspond to the locations of the tectonic synclines. Thus, there is a general agreement between the preseismic and postseismic deformations, albeit for the afterslip pattern located in the epicentral area, where higher values are located. Local subsidence occurs

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Fig. 1 – A) mosaicked ASAR and ERS mean velocity maps relative to the time span 1992-2010. The measurement are in descending line of sight (LoS): positive values (blues colours) are approaching to the sensor; vice versa for negative ones (red colours). B) the same map of panel A is superimposed on the geo structural model of Italy (Bigi et al., 1983).

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Fig. 2 – mosaicked mean velocity maps in ascending (A) and descending (B) geometries. The measurement are in line of sight (LoS): positive values (blues colours) are approaching to the sensor; vice versa for negative ones (red colours). In panel C and D we show the time series calculated in the epicentral area of the 2012 seismic sequence for the ascending and descending measurements respectively.

due to fluid migration and compaction of the Po plain sediments. Analyzing the post-seismic time series we observe a quasi-exponential afterslip deformation pattern in the epicentral area for both the ascending (Fig. 2C) and the descending (Fig. 2D) maps.

Preliminary post-seismic source modeling. We modelled the retrieved post-seismic deformation using the seismic source obtained for the co-seismic modelling (Pezzo et al., 2013), using the elastic dislocation model of Okada, 1985. We obtain a good agreement between observed and modelled velocity field (Fig. 3B), as shown in the residual velocity maps (Fig. 3C). We inverted only for the slip distribution, using the same geometric and kinematic parameters the co-seismic case. The retrieved slip distribution (Fig. 3D) shows a similar pattern compared to the co-seismic one (Fig. 3E).

Conclusions• InSAR operational monitoring of the earthquake ground deformation during the coseismic

and post-seismic periods is today feasible using SAR satellite constellations as COSMO-SkyMed, thanks to their short repeat pass (down to 1 day).

• Ground velocities and displacement time series obtained from multi-temporal InSAR

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analysis give an accurate quantitative information which can be used to support the geological interpretation of the tectonic and fluid-related processes occurring in the post-seismic phase.

• The retrieved interferometric results allow us to well reproduce the post-seismic ground deformation. The latter is recognizable in a small area close to the mainshock epicenters and remains above the measurement noise floor for ~6-7 months after the mainshock occurrence.

• The study of the pre-seismic (inter-seismic) phase by means of ERS1/2 and ASAR-Envisat data covering the 1992-2010 temporal interval allow to identify an interseismic deformation pattern due to the thrusts related anticline growth below the Po plain sediments

• The Emilia seismic sequence represents a very interesting case for the study of the complete seismic cycle because of the geodetic, geological and seismological data availability.

Acknowledgements. G Pezzo and J.P. Merryman Boncori are funded by the INGV-ESA CHARMING project. Results carried out using COSMO-SkyMed® PRODUCTS, © ASI (Italian Space Agency) – provided under license of ASI in the framework of the S3 Project “Short term earthquake prediction and preparation” (DPC-INGV, 2013). The ERS and Envisat images are provided by ESA (European Space Agency) under the CAT.1P 5605. The InSAR processing was carried out using the Sarscape© software (Sarmap, CH). We thank Dr. Paolo Riccardi and Dr. Alessio Cantone (Sarmap, CH) for the fruitful collaboration and useful suggestions during the SAR data processing. We also thank Dr. Enrico Serpelloni (INGV-Bo) for providing GPS solutions. The SRTM DEM is a product of the NASA Space Shuttle Mission.

Fig. 3 – Observed (A); modelled (B) and residuals (C) velocity maps relative to the post-seismic ascending dataset. The maps are in line of sight (LoS): positive values (blues colours) are approaching to the sensor; vice versa for negative ones (red colours). In D and E we show the slip distributions obtained respectively from the post seismic and coseismic ground velocity field inversions.

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