GNGTS 2017 - 36° Convegno Nazionale

74 GNGTS 2017 S essione 1.1 ones. The horizontal components of ground acceleration are rotated in steps �� δθ� �� of one degree in the span [0, 2�� π]. From a system oriented to geographical coordinates (N-E) we create a set of ray parameter oriented horizontal acceleration traces {a_R(��θ��), a_T( �� )}, where �� θ� is the unknown Back azimuth. Provided the plane wave assumption and linear elasticity we know that vertical rotation and transverse acceleration (Aki and Richards, 2009; Cochard et al. , 2006) should show in the seismograms as the same waveform scaled by the frequency dependent phase velocity C(f). Given the last assumption we use the Wavelet coherence tool (WTC) (Grinsted et al. , 2004) to obtain time- frequency maps of correlation between the vertical rotation rate �� Ω�� _z and the transverse accelerations set {a_T( ��)}, derived after the previous step of traces rotation. The result of this processing is an array of correlation values C(�� θ�,t,f) that are function of the time, the frequency and the rotation step �� δθ� of the seismometer horizontal components. This representation will allow us to have a time-frequency estimation of the back azimuth. This Fig. 1 - The experiment location inside the “Laboratori Nazionali del Gran sasso” (top right) and the picture of the Gingerino Ring Laser Gyroscope (bottom right), in the center of the granite frame is located the broadband seismometer. In the large background picture we see the events used (see Tab. 1) for this study, the epicenters, and for the first ten events of Tab. 1 we provide the relative focal mechanisms. Fig. 2 - The back azimuth analysis in different frequency bands for the Visso M 5.9 main shock, In this plot the red color means that the correlation is equal to one i.e. the waveforms are identical and in phase. On top of the figure the superposition of rotation rate (red) and transverse acceleration (black).