Analysis of the displacement of geodetic stations during the Emilia seismic sequence of May 2012

The two events of Mw 6.11 and 5.96 [EMERGEO Working Group 2012, this volume] that took place on May 20 and 29, 2012, respectively, in the Po Plain, and the associated seismic 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 Individual Seismic Sources (DISS; http://diss.rm.ingv.it/) under the ID ITIS107 as a possible individual source, and it has a position, 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 Satellite Systems (GNSS) Service (IGS) orbits and the Bernese BSW 5.0 software [Dach et al. 2007] at our computing facility, a fast solution for the coseismic displacement was published on the May 21, 2012, and on the May 30, 2012, for the events of May 20 and 29, 2012, respectively. The largest signal 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 coseismic 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 synthetic aperture radar (SAR) data. […]