Source identification for situational awareness of the August 24th 2016 Central Italy event ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/AG-7233 1 Source identification for situational awareness of the August 24th 2016 Central Italy event CHRISTIAN BIGNAMI*, CRISTIANO TOLOMEI, GIUSEPPE PEZZO, FRANCESCO GUGLIELMINO, SIMONE ATZORI, ELISA TRASATTI, ANDREA ANTONIOLI, SALVATORE STRAMONDO, STEFANO SALVI (1) (1) Istituto Nazionale di Geofisica e Vulcanologia *christian.bignami@ingv.it Abstract On August 24, 2016, at 01:36 UTC a ML 6.0 earthquake struck a portion of the Central Apennines between the towns of Norcia and Amatrice. The epicentre was located near the town of Accumoli. Prompt Synthetic Aperture Radar (SAR) acquisitions and the available scientific knowledge of the area allowed to elaborate a first interpretative framework of the ongoing seismic sequence only 30 hours after the mainshock and a second analysis, complete of several Interferometric SAR (InSAR) data within two weeks. Through the in- version of InSAR data, we found that the seismogenic structure is oriented NNW-SSE and extends about 20 km between the towns of Norcia and Amatrice with a width of about 10 km. The retrieved slip reaches a maximum value of more than 1.2 m, and stops at a depth of about 4 km. Preliminary fault slip inversions suggest two main patches of co-seismic deformation located NW and SE of the hypocenter. I. INTRODUCTION entral Apennines are known to be one of the most seismically hazardous area in It- aly [http://zone- sismiche.mi.ingv.it/mappa_ps_apr04/ita- lia.html]. The region surrounding the Monti della Laga, was struck by several earthquakes in historical times. In 1627 and 1639, the area was affected by two events with estimated mag- nitudes of about 5.3 and 6.2 respectively. A cen- tury later, in 1672, another Mw 5.3 hit the same area, followed in 1703 by a strong event of mag- nitude higher than 6 [Rovida et al., 2016]. The 24th of August 2016, at 01:36 UTC an Mw 6.0 earthquake hit an extensive portion of this area. C ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/AG-7233 2 The epicentre is located near the village of Ac- cumoli, between the towns of Norcia and Ama- trice, at a depth of 8.00.2 km. The mainshock was followed by an M 5.3 earth- quake at 02:33 UTC, about 12 km NW from the first event, and it was the only M>5 aftershock registered. The aftershocks sequence evolved mainly to the NW towards Norcia and the Sibil- lini Mountains. In this paper, we show the scientific produc- tions obtained during the emergency phase. A first report was sent to the DPC (Italian Civil Protection Department) the 25th of August (doi: 10.5281/zenodo.60938, in Italian). As soon as new SAR (Synthetic Aperture Radar) images from COSMO-SkyMed (CSK) and Sentinel-1A (S1A) became available, prompt data analysis and new fault inversions led to model InSAR (Interferometric SAR) images, with different Line of Sight (LOS) and orbits, producing a ro- bust slip distribution just fourteen days after the mainshock. II. FIRST INSAR RESULTS The first post-event SAR image was acquired the 24/08/16 by ALOS-2 (L-band instrument of the Japan Aerospace Exploration Agency, JAXA). The first InSAR (Interferometric SAR) analysis was performed considering three pre- event (01/07/15, 09/09/15, 27/01/16) and one post event (24/08/16) ALOS-2 satellite acquisitions. Among the three co-seismic interferograms, the most coherent and with minimum atmospheric influences, is the 09/09/15-24/08/16 (Id-1, Table 1). Even if it includes the effects of all the seis- micity encompassing the period, most of the de- formation is due to the Mw 6.0 mainshock (24/08/16 1:36 UTC) and possibly the Mw 5.3 greatest aftershock (24/08/16 2:33 UTC). The measured surface displacement field was sub- sequently modelled in a first instance by means of a non-linear inversion providing the param- eters (strike, dip, and rake) for a uniform slip fault plane. The heterogeneous slip was then obtained, by using a linear inversion algorithm exploiting the fault parameters previously ob- tained [e.g., Atzori et al., 2009] and a prelimi- nary model of the seismogenic fault was al- ready released the day after the mainshock (doi: 10.5281/zenodo.60938). III. INSAR ANALYSIS AND FAULT MODELLING The InSAR analysis benefitted of a large num- ber of multi-frequency images from different satellites having different characteristics and spatial resolutions: ALOS-2 (JAXA) operating at L-band, S1A (from the European Program Co- pernicus) at C-band, CSK (Agenzia Spaziale Italiana, ASI), at X-band. The European Space Agency (ESA) also provided Sentinel-1B (S1B) data (the twin satellite of S1A, with the same sensor on board), although the satellite was still in the commissioning phase, and therefore not fully operative. With S1A and S1B images we were able to process cross-interferometric pairs, with a temporal baseline of 6 days only (respect to the 12 days revisiting time of the single satel- lite). Table 1 shows the resulting co-seismic in- terferometric SAR pairs and the sensors’ main The authors want to thank ASI for providing COSMO-SkyMed data during the emergency and ESA for furnishing Sentinel-1B images even if the satellite is still in commissioning phase. ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/AG-7233 3 characteristics, while some of the calculated in- terferograms are shown in Figure 1. All the interferograms were unwrapped to re- trieve the deformation field caused by the earthquake. The resulting displacement maps from all these interferograms show very similar patterns, characterised by a maximum displace- ment value of about 20 cm away from the re- spective sensor LOS. Note that the incidence an- gle of the data is similar for all the available data, apart from the different orbits. To estimate 3D surface displacement maps, we applied the SISTEM (Simultaneous and Integrated Strain Tensor Estimation from geodetic and satellite deformation Measurements) method [Gug- lielmino et al., 2011a] to the available co-seismic GPS [INGV Working Group “GPS Geodesy”, 2016] and to three InSAR data with the wider spatial coverage of the epicentral area. In partic- ular, we used the two S1 (ascending and de- scending) and the descending ALOS-2 InSAR data, reported in Table 1 with Id-3, Id-4, and Id- 7, respectively. The SISTEM’s outputs are re- ported in Figure 2. Table 1: Co-seismic interferometric pairs available and processed. S1 is Sentinel-1, CSK is Cosmo-SkyMed. IWS means Interferometric Wide Swath. Id Sensor Acquisition mode Interferometric Pair Wavelength [cm] Perpendicular baseline [m] Orbit Incidence angle [deg] Id-1 ALOS-2 StripMap 09092015 24082016 23.6 -198 Ascending 36.6 Id-2 S1 IWS 20082016 26082016 5.56 105 Descending 39 Id-3 S1 IWS 21082016 27082016 5.56 79 Descending 39 Id-4 S1 IWS 15082016 27082016 5.56 32 Ascending 39 Id-5 CSK StripMap 20082016 28082016 3.1 101 Descending 30.6 Id-6 S1 IWS 22082016 28082016 5.56 -29 Ascending 39 Id-7 ALOS2 StripMap 25052016 31082016 23.6 88 Descending 32.9 ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/AG-7233 4 Surface deformations revealed by SISTEM, show a predominant vertical deformation pat- tern (2 to 3 times higher than the horizontal ones) with a maximum value of -0.25 m. Thanks to these 3D maps, it would be - hopefully - pos- sible to identify further structures activated during the seismic sequence. The vertical component confirms the surface subsidence with the typical "spoon" shape, as already observed in past earthquakes in the Ap- ennine (e.g., Mw 6.3 L’Aquila earthquake, 2009). It extends for about 20 km along the NNW-SSE direction. The maximum displace- ment value is in correspondence of the town of Accumoli. Figure 2. SISTEM results: East, North, and Up components of the displacement field are reported, respectively. The dashed box refers to the inverted fault plain. Figure 1. Examples of InSAR wrapped maps from: A) S1 ascending pair (Id-4 in Table 1); B) CSK interferograms. ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/AG-7233 5 With respect to the first report, a new solution was delivered after all the interferograms listed in Table 1 were processed. InSAR data used in the inversion procedure consist of about 19500 measurements obtained by sampling 5 un- wrapped interferograms (Id-1, Id-3, Id-4, Id-5, and Id-7 in Table 1) and 107 measurements from the CGPS [INGV Working Group "GPS Geodesy", 2016; Cheloni et al., 2016]. The results of such inversion are shown in Figure 3. The re- trieved source model consists of a fault plane oriented NNW-SSE. The slip spans an area of about 20 km length, and a width of 10 km, and stops at a depth of about 4 km. This area is lo- cated between the towns of Norcia and Ama- trice. The fault slip is characterised by two main patches, one located between Amatrice and Ac- cumoli, and one between Accumoli and Norcia. The maximum slip values are about 1 m for both patches. Note that a relatively large slip is also present in the southern part of the fault plane, and it is associated to some uncertainties on the data. Comparison of the fault plane with the relocated aftershocks distribution [Michele et al., 2016] shows a good agreement between the structures constrained by the two different analysis. Note that, in this preliminary result, two main patterns of seismicity are visible. One pattern is approximately laying around the fault plane, while a second one seems identify an antithetic fault East of Norcia village. The last feature is still under investigation. In order to show how the slip of the mainshock modified the stress loading of the region, we computed the Coulomb Failure Function [CFF, e.g., Harris et al., 1998] on nearby fault planes as listed in the DISS [DISS Working Group, 2010] data-base. In particular, we imaged the Figure 3. Modelling results. a) Co-seismic fault slip distribution from the InSAR data modelling. The mainshock and the M 5.3 aftershock are reported with the red pentagon, while the M<5 aftershocks are shown with black dots. b) 3D view of the fault below the topographic surface. The view is from SSE along the strike direction (SSE-NNW, section A-A’ in panel (a)). The red dots are the largest events (M≥4) and black dots are the M<4 aftershocks. ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/AG-7233 6 CFF values on six fault planes surrounding the mainshock. Positive CFF variations can pro- mote subsequent slip on adjacent faults, while negative CFF values in principle inhibit it. Our results show a decrease of CFF value on the fault planes West of the main fault, while a pos- itive increment of its value is present on the two planes aligned with the Amatrice fault. In par- ticular, a value relatively high compared with the mean stress drop on an earthquake of this size, is present in the northern portion of the Gorzano fault. IV. CONCLUSION We showed how, during the emergency phase of an earthquake, the use of InSAR geodetic data and their inversion for seismic source re- trieval can play an important role for support- ing civil protection authorities. The prompt availability of a large dataset of SAR images and the fast deliver of displacement modelling and fault slip distribution demonstrated that In- SAR, thanks to the present day SAR constella- tion (S1, CSK, ALOS-2, etc.) can rapidly issue scientific information. A quite robust solution of InSAR data inversion was provided only few hours after the main event. Indeed, thanks to ALOS-2 data, the INGV team responsible for the analysis of the geodetic data was able to process and deliver a first seismic source model just one day after the mainshock occurrence. This first estimation was improved few days later, when additional SAR images, acquired at different wavelengths and different viewing geometries, allowed to better constrain the fault parameters. 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