The May 2012 Emilia (Italy) earthquakes: Preliminary interpretations on the seismogenic source and the origin of the coseismic ground effects

On May 20, 2012, a ML 5.9 earthquake (T1) occurred in the Emilia-Romagna Region of northern Italy. This was preceded by a ML 4.1 foreshock on May 19, 2012, and followed by several aftershocks, including two ML 5.1 events, both on the same day. On May 29, 2012, a second strong event of ML 5.8 (T2) hit the same region, with its epicenter ca. 12 km to the WSW of the first mainshock, T1. We propose a model of the seismogenic source(s) responsible for the two mainshocks by comparing the seismic reflection profile interpretation with the available seismological and interferometric. Our results indicate that on 20th May the Ml 5.9 T1 earthquake originated along the central segment of the ESE-trending Th2 blind-thrust (Ferrara-Bondeno-Poggio Rusco ridge); the thrust-fault with a dip-slip kinematics probably propagated eastward and ruptured a c.25 km long segment without breaking the surface. Seismicity in the subsequent days was delimited NW-ward by a NE-SW trending barrier; this presumed transverse structural element may represent a pre-existing cross-structure (inherited from pre-Quaternary tectonic phases). On 29th May the rupture activated a different fault segment corresponding to the prosecution to the west of the Th2 or to the Th3 thrust. Based on the pattern distribution of the coseismic ground effects mapped in the epicentral area during two field survey campaigns, conducted after the 20 May and soon after the 29 May, we discuss the origin and location of the liquefaction features in the context of the local geological–geomorphological setting and with respect to the epicentral distance. The not-radially distributed pattern suggest that, although it is clear a primary control by the paleochannels pattern, also a “remote” tectonic control may have played a role on the location of liquefaction. We propose a possible interpretation where fissuring may have been favored by bending-moment extensional “fracturing” coseismically developed by thrust-generated flexure above the growing anticline. Finally, we provide our interpretation for the question: "Why did the mainshock ruptures not break the surface?"