Post-seismic relaxation and earthquake triggering in the southern Adriatic region

Abstract

SUMMARY An attempt at quantifying post‐seismic relaxation triggered by decoupling earthquakes along the eastern thrusting border of the Adriatic plate (southern Dinarides) is carried out by finite element modelling, with a model constituted by an elastic lithosphere riding on a viscous asthenosphere. In particular, it is investigated the possibility that the above phenomenon is responsible for the fact that in the last two centuries most major earthquakes in the southern Dinarides (MS > 6) have been followed, within a few years, by intense, mainly tensional, earthquakes in southern Italy, i.e. the zone lying on the opposite margin of the Adriatic plate. This analysis has been applied to the last example of the supposed seismic interrelation, i.e. the triggering 1979 April 15 Montenegro event (MS= 6.7) and the presumably induced 1980 November 23 Irpinia earthquake in the southern Apennines (MS= 6.9). Results indicate that the strain induced in the southern Apennines by the triggering event has significant amplitude, since it largely exceeds the effect of earth tides, and the principal stress axes are consistent with those of southern Apenninic earthquakes. The order of magnitude of the time delay between the Montenegro and Irpinia events (1.6 yr) could be explained by assuming that earthquake triggering is most probable when the highest values of the induced strain rate reach the southern Apennines. In particular, this interpretation predicts the observed time delay when a model diffusivity of 400 m2 s−1 is assumed. The constraints that this diffusivity value may pose on the structural and rheological features of the crust–upper‐mantle system in the study area are discussed. It is shown that the effects of the Montenegro event on the present velocity field are comparable to, though systematically lower than, the velocities suggested by geodetic observations in the Italian region. This suggests that geodynamic interpretations of geodetic data given without taking into account possible transient effects on the kinematic pattern, as those related to post‐seismic relaxation, may be incorrect. Experiments carried out by tentatively simulating the presence of subducted lithosphere along the western margin of the Adriatic plate as a lateral variation of diffusivity, have shown that this structural feature may emphasize E–W tensional strains in the southern Apennines.

A model of deep crustal fluid flow following the Mw = 8.0 Antofagasta, Chile, earthquake

“…For example, a Maxwell time t = h/G, with t = 50 days and a shear modulus for lower continental crust of G = 40 GPa, leads to a dynamic viscosity h of around 1 Â 10 17 Pa s for the lower crust. This is 1 -2 orders of magnitude smaller than expected for the lithosphere [Pollitz and Bürgmann, 1998;Viti et al, 2003]. That is, the timescale of our problem (50 days) is 1 or 2 orders of magnitude too short to be related to viscous behavior of a relatively cold, lower continental crust.…”

“…[31] Postseismic viscous relaxation processes [Pollitz and Bürgmann, 1998;Viti et al, 2003] can be excluded because of the short timescale of the observation. For example, a Maxwell time t = h/G, with t = 50 days and a shear modulus for lower continental crust of G = 40 GPa, leads to a dynamic viscosity h of around 1 Â 10 17 Pa s for the lower crust.…”

A Statistical Investigation on a Seismic Transient Occurred in Italy Between the 17th and 20th Centuries

“…It is also reinforced by analogy with the eruptive activity of Vesuvius, although the physical nature of the relationship is not fully clarified. The synchronous behavior of large areas or areas that are very distant from each other has been observed for earthquakes in Italy (Viti et al 2003;Mantovani et al 2010;Bragato 2014) and elsewhere (Bath 1984a, b;Tsapanos and Liritzis 1992;Liritzis et al 1995;Bragato and Sugan 2014), as well as for volcanoes (Marzocchi et al 2004). The explanatory theories for the phenomenon are of different nature.…”

“…Some models rely on the earthquake-to-earthquake interaction through different mechanisms of stress transfer (Freed 2005). Among them, those based on post-seismic viscoelastic relaxation explain triggering at distances of the order of 100 of kilometers with delays of some years (Viti et al 2003). Another class of models involves transient crustal deformation at the level of entire plates or portions of plates (Kasahara 1979).…”

Periodicity of Strong Seismicity in Italy: Schuster Spectrum Analysis Extended to the Destructive Earthquakes of 2016

“…In Europe, the SHARE European Earthquake Catalog (Stucchi et al 2012) represents an attempt to revise and integrate the various catalogs adopting common standards, but the quality of the overall common catalog is far from being comparable with that of the Italian one (especially for what concerns the minimum magnitude of completeness). Nonetheless, a few studies (Mantovani and Albarello 1997;Viti et al 2003;Mantovani et al 2010) have found some degree of synchronization (although with no periodicity) between the strong earthquakes occurring on the two sides of the Adriatic Sea, in correspondence with the borders of the Adriatic microplate (Southern Apennines on west and Southern Dinarides-Albanides on east).…”