Investigation on directional effects of Vrancea subcrustal earthquakes

This paper evaluates possible directional effects from strong ground motions recorded during the last four significant subcrustal earthquakes produced in the Vrancea seismic zone (Romania) in August 1986 (M (W) = 7.1), May 1990 (M (W) = 6.9 and 6.4) and October 2004 (M (W) = 6.0). Several measures of the horizontal component of the ground motion given in the literature (Boore et al., 2006; Boore, 2010) are computed and are related to the geometric mean of the as-recorded horizontal components. ANOVA method is applied in order to quantify the influence of the earthquake magnitude and of the soil class on some strong ground motion parameters (e.g. Arias intensity, PGV/PGA, or mean period T (M)). The study of the directional effects is performed using the distribution of I (JMA) and of the rotation angle corresponding to a measure of the horizontal component of the ground motion (RotD100) and the results reveal a visible directional pattern for I (JMA), while in the case of RotD100 the directional patterns are less visible.