GNGTS 2017 - 36° Convegno Nazionale

76 GNGTS 2017 S essione 1.1 very effective in identifying the regional SH onset, this is traditionally an hard task since this onset is often buried in the coda signal and is again based on a real ground motion measure. A more quantitative and statistically consistent analysis of the back azimuth for the entire event database is given in the next steps. The C(������� ����� �� ���������� ��� ����� ������ �� ���� ��� θ� ����� �� ���������� ��� ����� ������ �� ���� ��� ,t,f) array is calculated for every event. We find the maxima of correlation in a time window that goes from the beginning of the S-coda to the end of the surface waves phase. The obtained values are binned in histograms and the distribution is modeled with a gaussian function (KDE Gaussian). We apply this processing to all the events and we resume the analysis by plotting the estimated Back azimuth and the theoretical one for the entire set in the left panel of Fig. 3. In the right panel of Fig. 3 we represent also the polar histogram of the misfits and the relative gaussian kernel modeling of the distribution. Conclusions. The theoretical BAZ is just an indication of the possible direction of the wave field. In a complex topography like the Gran Sasso region and specially in an underground environment at 1400 m from the free surface is not trivial whether the seismic waves should follow the theoretical BAZ. Nevertheless for teleseisms in (Simonelli et al. , 2016) we measured a misfit of five degrees after the Love waves onset at periods longer than 10 seconds. From the analysis on this entire data set we can state that we observe a 10 degrees systematic misfit that can be compatible with the orientation error of the seismometer or can be due to a structural effect. A future measure with a triaxial fiber optic gyroscope, used as a gyroscopic compass will allow us to orient our instrument and measure the previous orientation with a precision lower than 0.1 deg. We tried a cluster analysis in order to check if the misfit could be dependent on the events parameters of Tab. 1 and on the S/N ratio but the result does not show any clear dependence. In conclusion an average misfit of ������� � �� � �� ���� �� ��������� θ������ � �� � �� ���� �� ��������� misf. = 10 ± 18 deg. is observed. References Aki, K. and Richards, P. G. (2009). Quantitative Seismology. University Science Books. Belfi, J., Beverini, N., Bosi, F., Carelli, G., Cuccato, D., Luca, G. D., Virgilio, A. D., Gebauer, A., Maccioni, E., Ortolan, A., Porzio, A., Saccorotti, G., Simonelli, A., and Terreni, G. (2017). ���� ����������� �������� ������������� Deep underground rotation measurements: Gingerino ring laser gyroscope in gran sasso. Review of Scientific Instruments, 88(3):034502. Cochard, A., Igel, H., Schuberth, B., Suryanto, W., Velikoseltsev, A., Schreiber, U., Wassermann, J., Scherbaum, F., and Vollmer, D. (2006). Rotational motions in seismology: Theory, observation, simulation. In Teisseyre, R., Majewski, E., and Takeo, M., editors, Earthquake Source Asymmetry, Structural Media and Rotation Effects, pages 391–411. Springer, New York. Grinsted, A., Moore, J. C., and Jevrejeva, S. (2004). Application of the cross wavelet transform and wavelet coherence to geophysical time series. ��������� ��������� �� ����������� ���������������� Nonlinear Processes in Geophysics, 11(5/6):561–566. Simonelli, A., Belfi, J., Beverini, N., Carelli, G., Virgilio, A. D., Maccioni, E., Luca, G. D., and Saccorotti, G. (2016). First deep underground observation of rotational signals from an earthquake at teleseismic distance using a large ring laser gyroscope. Annals of Geophysics, 59(0). Fig. 3 - The theoretical BAZ ad the measured one for all the events indexed as in Tab. 1 (left). Misfit distribution and the relative gaussian KDE modeling in solid red line (right).