The seismicity of the Adriatic Sea is mostly referred to as weak, and in the past the Adriatic microplate was considered a single, rigid aseismic block. However the area of Central Adriatic Sea bounded approximately by the Ancona-Zadar line to the north and the Gargano-Dubrovnik line to the south shows a significant seismicity.
Several earthquakes struck this area in the past, but the locations and the magnitudes of these events in the middle of the sea are highly uncertain.
Three earthquakes with Ml>5.0 occurred in the Central Adriatic in the last 20 years (in 2003, 1988, and 1986). The seismic potential of the area is higher than assumed up to now. The study in detail of the recent seismic activity can contribute to solving some open questions regarding present-day tectonics of the area.
Herak et al. (2005) analyse the earthquake sequence started in March 2003 near the small island of Jabuka in the centre of the Adriatic Sea, within the Adriatic microplate.
Their fault-plane solution for the mainshock is in good agreement with the CMT and the RCMT solutions. The fault-plane solution computed on the basis of the first motion polarity analyses indicates dip-slip faulting with a small reverse component. The pressure axis is nearly horizontal, and trending almost South-North. It is parallel with the orientation of regional compressive stress based on geological measurements (Prelogovic et al., 2003), and with the direction of movement estimated from geodetic measurements (Oldow et al., 2002).
The fault-plane solution of the Jabuka main shock is in excellent agreement with the previously mapped Jabuka-Andrija fault system, and seismic profiles showed presence of reverse structures bounded to the SW by Jabuka-Andrija fault (Herak et al., 2005).
The source's slip rate value has been updated according to the findings of geodynamic model (Kastelic and Carafa, 2012) and in the accordance with the hosting composite seismogenic fault's slip rate.
1) Is the Jabuka-Andrija fault system active as a whole? or only portion of it are active?
2) Can the activity of this fault system influence seismic hazard on the Central Adriatic islands and on the Croatian coast?
3) Can the Jabuka-Andrija fault system generate a sea-bottom rupture and a possible tsunami?
Oldow et al. (2002)
These researchers use GPS velocities to propose the velocity model for Adria, splitting in the northwestern and southeastern velocity domains. The authors divide Adria into two blocks separated by a boundary extending around the southern Tyrrhenian Sea, across central Italy, and along the Dalmatian coast of the eastern Adriatic. GPS sites in the northwestern tectonic block show little or no residual velocity relative to Europe. The southeastern block moves with Africa and shows significant N and NW directed velocity residual in the European reference frame (5-10 mm/year).
Venisti et al. (2003)
These investigators integrate different types of data to characterise the Adriatic plate. Integrated information arising from seismic tomography and gravimetry reveal large structural units within the Adria. These authors provide a model of the structural heterogeneities existing across the Adria plate, with reference to the location of the lateral transition between the northern plate sector (characterised by a thicker crust) and the southern sector (characterised by a thick transition zone).
Herak et al. (2005)
They analyse the earthquake sequence of 2003 recorded within the Adriatic microplate. The mainshock was preceded by over 150 foreshocks, and followed by over 4600 aftershocks. Due to lack of seismic stations in the epicentral area, these workers were able to reliably locate only 597 earthquakes.
These investigators compute hypocentral locations by a grid-search algorithm after seven iterations of refining hypocentres and adjusting station corrections.
Epicentres lie in a well-defined area of about 300 km2, just to the W and NW of the Jabuka island. The hypocentres dip to the NE, closely matching faults from the Jabuka-Andrija fault system, as identified on the available reflection profiles in the area. The fault-plane solution of the main shock indicates faulting caused by a S-N directed tectonic pressure, on a reverse, dip-slip fault.
They find out that the positions of hypocentres and the focal mechanisms are in excellent agreement with the previously mapped Jabuka-Andrija fault system, which is identified as an active one. They conclude that the recent seismicity of the Central Adriatic Sea is comparable to the seismicity of several earthquake-prone areas in the circum-Adriatic region.