CAPORALI_corretto_Layout 6 767 Analysis of the displacement of geodetic stations during the Emilia seismic sequence of May 2012 Alessandro Caporali*, Luca Ostini Università di Padova, Dipartimento di Geoscienze, Padova, Italy ANNALS OF GEOPHYSICS, 55, 4, 2012; doi: 10.4401/ag-6115 1. Introduction The two events of MW 6.11 and 5.96 [EMERGEO Work- ing Group 2012, this volume] that took place on May 20 and 29, 2012, respectively, in the Po Plain, and the associated seis- mic sequence, represent the response of the brittle upper crust (hypocentral depth ca. 10 km) to the compressive stress in the outer Apennine Arc. Kinematically, the motion of the Apennines that has led to a compressional regime in the Po Plain can be represented as a counterclockwise rotation, as demonstrated by the shortening of the transalpine baselines, with a rate that decreases going west [Caporali and Martin 2000]. The compressive stress field is known from borehole breakout data [Pierdominici and Heidbach 2012] and it agrees with the principal directions of the strain rate derived from global positioning system (GPS) data. The geodetic strain rate for seismic zone 912 (Dorsale Ferrarese, according to Meletti et al. [2008]) is 92.86 ±0.04 nstrain/yr, which is a relatively high value [Caporali et al. 2011]. On a more local scale, the Mirandola fault is described in the Database of In- dividual Seismic Sources (DISS; http://diss.rm.ingv.it/) under the ID ITIS107 as a possible individual source, and it has a po- sition, strike, dip, size and expected maximum magnitude [Burrato et al. 2003, Carminati et al. 2010, Scrocca et al. 2007] that are very close to the main events of the 2012 sequence. Several permanent GPS stations were in activity in the area. Using ultrarapid international Global Navigation Satel- lite Systems (GNSS) Service (IGS) orbits and the Bernese BSW 5.0 software [Dach et al. 2007] at our computing facil- ity, a fast solution for the coseismic displacement was pub- lished on the May 21, 2012, and on the May 30, 2012, for the events of May 20 and 29, 2012, respectively. The largest sig- nal was about 2 cm in the North direction at the station SGIP (San Giovanni in Persiceto), which was relatively consistent with the fault-plane solution [Devoti 2012, this volume]. In this study, we present more detailed estimates of the coseis- mic offsets at several other stations in the area, including a kinematic solution at 30 s temporal resolution. The coseismic displacements are inverted using the Okada [1985] model and a preliminary model of the two faults is obtained for each of the two main events, taking into account the available syn- thetic aperture radar (SAR) data. 2. GPS data We compute weekly network solutions for some 150 Italian permanent GPS stations using IGS/EUREF process- ing standards (http://www.epncb.oma.be/_documentation/ guidelines/guidelines_analysis_centres.pdf ). The normal equations that have been available since 1999 are then stacked, minimum constraints are imposed on EUREF Per- manent Network (EPN) Class A reference stations, and the time series are generated. The velocity map is updated weekly, which excludes stations with insufficiently long tracking logs [http://147.162.229.63/scidata/velocity_s.pdf]. This systematic procedure allows the deformation field in the national territory, in the sense of the average regional strain rate, to be continuously monitored and updated. In the case of seismic events, the procedure yields an immedi- ate detection of coseismic offsets. An ad-hoc kinematical so- lution for selected sites is then run to investigate displacements at high frequency. Figure 1 shows the vertical and horizontal coseismic offsets obtained for the May 20 and 29, 2012, events, and the corresponding time series. In both cases it clearly appears that the stations located south of the epicen- ters moved north (e.g. MOPS, SGIP), and those north of the epicenter moved south (e.g. LEGN), as would be expected for a reverse fault with strike nearly EW. Based on the routine daily analysis using double differences, we reprocessed se- lected baselines in kinematic mode, with output at 30 s. The tropospheric parameters and the ambiguities obtained in the daily processing were back-substituted into the kinematic so- lution, and 30 s estimates of the coordinates of selected sta- tions were obtained in a time window centered on the events of May 20 and 29, 2012. To minimize effects of multipath and satellite geometry, the computation was repeated for the Article history Received July 19, 2012; accepted August 22, 2012. Subject classification: Crustal deformations, GPS Geodesy, InSAR, Coseismic deformation. 2012 EMILIA EARTHQUAKES CAPORALI AND OSTINI 768 F ig u re 1 .T op : C os ei sm ic d is pl ac em en ts (l ef t: h or iz on ta l; ri gh t: v er ti ca l) o f th e pe rm an en t G P S st at io ns fo r th e M ay 2 0, 2 01 2 (b lu e ar ro w ) a nd M ay 2 9, 2 01 2 (r ed a rr ow ) e ve nt s. P A D O (r ed d ot ) w as o ne o f th e E P N C la ss A p er m an en t G P S st at io ns u se d as r ef er en ce in th e le as t s qu ar es a dj u st m en t. T he lo ca ti on o f th e ep ic en te rs is b as ed o n th e pr el im in ar y so lu ti on b y G FZ (h tt p: // ge of on .g fz -p ot sd am .d e/ eq in fo /l is t. ph p) . B ot - to m : T im e se ri es a t da ily in te rv al s of t he s ta ti on s SG IP a nd L E G N , w hi ch a re o n op po si te s id es o f th e fa u lt s an d w er e m os tl y af fe ct ed b y th e co se is m ic d is pl ac em en ts . T he a ss oc ia te d di sc on ti nu it ie s fo r th e M ay 20 a nd 2 9, 2 01 2, e ve nt s ar e m ar ke d by a g re en li ne . 769 EARTHQUAKE GEODESY OF THE EMILIA 2012 SEQUENCE F ig u re 2 . T im e se ri es o f L E G N a nd S G IP a t 30 -s t im e re so lu ti on , f or t he M ay 2 0 an d 29 , 2 01 2, e ve nt s. T he a rr ow s in t he S G IP t im e se ri es m ar k po ss ib le c re ep p ha se s of s om e 40 m in d u ra ti on b ef or e th e sh oc k. T he x ax is is in h ou rs U T C s in ce m id ni gh t of t he r es pe ct iv e da y. same time window one day earlier, with the same baselines and satellites, which generated a reference time series that was not affected by the seismic motion, but with very simi- lar systematic effects related to the satellite geometry and multipath [Bock 1991, Choi et al. 2004]. These time series, shifted by 3 min 56 s, were subtracted from the time series cen- tered at the epochs of the two seismic events, which resulted in a mitigation of high and low frequency noise (sidereal fil- ter). Figure 2 shows the time series of stations SGIP and LEGN after the removal of the common noise. The high frequency changes of the nominal coordinates that were visible in the unfiltered time series are now removed, but it is as yet too early to conclude whether they are a geophysical signal or they are noise. The ramp in the north component of station SGIP on May 20 and 29, 2012, is very clearly resolved. We speculate a sort of creep movement that initiated roughly 40 min be- fore the respective shocks, but evidence based upon such small amounts of data cannot be claimed. 3. Modeling To model the GPS data, we used the Coulomb 3.20 software developed by the U.S. Geological Survey [Lin and Stein 2004, Toda et al. 2005], complemented with a mod- ule for numerical computation of partial derivatives, set-up for the normal equations and parameter adjustment by weighted least squares. The data available for the inversion are unfortunately few. The first check we did was to verify that a source model was relatively compatible with the ob- served horizontal displacements of the GPS stations. This was based on the published fault-plane solution, on the em- pirical relations of Wells and Coppersmith [1994], and on the data published in the DISS for the Mirandola fault. We tested the option of north-verging faults, although it ap- pears most logical from the geological sections that south- verging faults are more likely. The fault in both events was initially considered a rec- tangle of length (L) × width (W) of 12 × 9 km, with an in-plane slip of 0.5 m, mostly in the reverse (up-dip) direction. This yielded a seismic moment of 1.62 × 1018 Nm, or MW 6.07, as- suming (µ = 30 GPa). However, the publication of the InSAR data based on the Radarsat 1 and, in part, the Cosmo Skymed satellites [Salvi et al. 2012, this volume] made it clear that the structures of both faults, and particularly the May 29, 2012, event, were somewhat different from that implied by the seis- mological data. In addition, a rather large uplift of some 0.12 m was reported in the central parts of the faults, which would have been incompatible with the original model, and in particular with a slip of only 0.5 m. We constrained the seismic moment to be equal to the seismologically derived value and started to investigate an al- ternative source model, with smaller L and W, and a slip in the range of 0.7 m to 0.9 m. The result is described in Figure 3 (right), which shows that the GPS data that is intended as the sum of the displacements of the May 20 and 29, 2012, events, still fits this new model. The vertical uplift along the strike of the two faults is consistent with that obtained from the InSAR data. The epicenters of the two main events, how- ever, do not coincide with the centers of the faults, but are slightly offset to the east (May 20) and to the west (May 29). The measured GPS displacements are systematically larger than the computed ones, perhaps due to monument instabil- ity, although in the same direction. Figure 3 (right) also shows how the reverse faulting of the May 2012 events is consistent with the deformation regime implied by long-term GPS data. The solutions independently presented in this volume by Bi- gnami et al. [2012] and Serpelloni et al. [2012] appear qualita- tively consistent with our proposed, preliminary solution. 4. Conclusion The geodetic data available for the Emilia 2012 seismic sequence are few but valuable, in that they can discriminate displacements attributable to the individual events of May 20 and May 29, 2012. The InSAR data are also of moderate quality. The Radarsat 1 interferogram refers to the combined events, and the Cosmo Skymed only to the event of May 29. Merging the GPS and SAR datasets appears to fill the mutual gaps and enables a reasonable source model to be outlined. Taking into account the available information on the InSAR and CMT solutions, we conclude that the data are consistent with the elastic model defined in Table 1. CAPORALI AND OSTINI 770 Coordinates Slip Dip Strike Rectangular fault MW Date Longitude (˚E) Latitude (˚N) Right lateral (m) Reverse (m) (˚) (˚) Depth of lower corner (km) Length (km) Width (km) (ddmmyyyy) 11.35 44.83 0.19 0.85 35 118 12 10 6 6.06 20052012 11.20 44.83 0.12 0.70 35 91 11 9 6 5.97 29052012 Table 1. Central coordinates and characteristics of the rectangular faults, with implied moment magnitude for the two main events, assuming a shear modulus µ = 30 GPa. The 1 formal uncertainties are 0.05˚ for the geographical coordinates, 0.05 m for the slip vector, 3˚= for the dip and strike angles, 2 km for the depth, length and width. 771 EARTHQUAKE GEODESY OF THE EMILIA 2012 SEQUENCE F ig u re 3 .L ef t: B es t- fit m od el o f th e G P S an d R ad ar sa t 1 (d es ce nd in g or bi t) d at a fo r th e to ta l d is pl ac em en t as so ci at ed w it h th e tw o m ai n ev en ts . C on to u rs (m m ) a re p re di ct ed fr om t he e la st ic m od el a nd s ca le d to ac co u nt f or t he li ne -o f- si gh t an gl e, a nd a re c om pa re d w it h th e pa tt er n of t he in te rf er om et ri c fr in ge s re su lt in g fr om t he p as se s of M ay 1 2 an d Ju ne 5 , 2 01 2. R ed /b lu e ar ro w s re pr es en t co m pu te d/ m ea su re d di s- pl ac em en ts ( cu m u la ti ve f or t he M ay 2 0 an d 29 e ve nt s) o f th e pe rm an en t G P S st at io ns . D is pl ac em en t m ap c ou rt es y of T R E T el er ile va m en to E u ro pa . R ig ht : P ri nc ip al d ir ec ti on s of t he g eo de ti c st ra in r at e co m - pu te d fo r ea ch o f th e 36 s ei sm ic z on es o f M el et ti e t al . [ 20 08 ] [ fr om C ap or al i e t al . 2 01 1] . C om pr es si on is in r ed a nd e xt en si o n in b lu e. T he r ec ta ng le s ho w s th e st u dy a re a fo r th e 20 12 s ei sm ic s eq u en ce in E m ili a. C M T s ol u ti on s ar e ta ke n fr om t he I N G V Q u ic k C M T C at al og . Acknowledgements. This study is supported by a Research Grant from the University of Padova entitled Monitoring from space and interpreting surface deformations in seismic areas. We are grateful to Ufficio Cartografico of Regione Veneto for the support to the stations of the Permanent GPS Network used in this study. We thank Dr. A. Ferretti and Ms. S. Del Conte for kindly making available the georeferenced InSAR displacement maps of Radarsat 1. We thank Dr. A. Ganas and an anonymous Reviewer for valu- able comments which helped to improve the first draft of the manuscript. References Bignami, C., P. Burrato, V. Cannelli, M. Chini, E. Falcucci, A. Ferretti, S. Gori, C. Kyriakopoulos, D. Melini, M. Moro, F. Novali, M. Saroli, S. Stramondo, G. Valensise and P. Van- noli (2012). Coseismic deformation pattern of the Emilia 2012 seismic sequence imaged by Radarsat-1 interferom- etry, Annals of Geophysics, 55 (4); doi:10.4401/ag-6157. Bock, Y. (1991). Continuous monitoring of crustal deforma- tion, GPS World, 2 (6), 40-47. Burrato, P., F. Ciucci and G. Valensise (2003). 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Am., 84, 974-1002. *Corresponding author: Alessandro Caporali, Università di Padova, Dipartimento di Geoscienze, Padova, Italy; email: alessandro.caporali@unipd.it. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. 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