Clustering characteristics in time, space and energy domains for the Vrancea, Romania intermediate-depth earthquakes

Increasing evidence shows that the oceanic lithosphere subducted beneath Vrancea region is divided into several segments. Two of these segments hosted the largest Vrancea earthquakes (Mw > 7), while other two are less seismically active. The segments are well delimited by the high-precision location of the low and moderate magnitude earthquakes. The segmentation indicates significant inhomogeneities in the seismic generation process and the related material properties, which must reveal resolvable clustering properties, as well. As shown by our time clustering analysis using the variation coefficient technique, the moderate-magnitude earthquakes (M ∼ 4) tend to occur in clusters in the lower lithosphere (h > 110 km) in contrast with the upper lithosphere (h < 110 km), where they seem to be randomly generated. The situation is opposite if we look to the seismic regime at low magnitudes: in the lower segment the process is random, while in the upper segment the process is clustered. At this stage, we may speculate a sort of metastable process (phase transformation of olivine) in the lower segment, randomly generating crack-like slips which stimulate the generation of asperity-like earthquakes (as in a percolation process). The asperity - like earthquakes (M ∼ 4) are interconnected stronger as compared with crack - like earthquakes (M < 3.6). By contrast, the low magnitude earthquakes (crack - like) are more clustered (due to fluid migration or dehydration reaction?) in the upper segment than the moderate earthquakes. On the basis of an extended earthquake catalog covering the time interval of January 1974 - April 2002, we analyze the scale-invariant clustering in space, time and energy domains in correlation with the distribution of different segments of the subducting slab. The differences outlined in the clustering characteristics are interpreted in terms of the focal mechanism, rheological properties and seismotectonic characteristics. Implications for the seismic hazard assessment are also discussed. (authors)