GNGTS 2019 - Atti del 38° Convegno Nazionale

472 GNGTS 2019 S essione 2.2 SH-mode excites only the horizontal motion in the direction of the valley elongation (called longitudinal), the SV-mode excites both the horizontal motion in the direction transversal to the valley and the vertical motion. 2D modes are characterized by a constant frequency across the valley, a gradual decay of amplification from the center to the edges and in-phase motion across the valley. We will use the terms transversal and longitudinal to indicate the motion perpendicular and parallel to the valley axis and we therefore refer to the SH- and SV- modes as longitudinal and transversal modes respectively. Identification of 1D and 2D resonance on experimental measurements. We recorded about 60 single-station tremor measurements along three profiles, two of which located across theAdige valley (NS1 and EW1) and the third one in the eastern part of the Bolzano basin (NS2; Fig. 1d). All the recordings were acquired with two microtremor recorders Tromino (MoHo srl, Italy) in order to cross-check the results, during several field campaigns in winter, spring and summer 2019. Each instrument was oriented with its axes parallel to the main morphological axes of the basin. Because the Adige valley has a roughly EW orientation on the western side of the basin and a NS orientation on the southern side, we chose to orient the instrument with its NS axis always parallel to the geographical NS direction. This proved to be fundamental in order to identify 2D effects that are expected to differ along the longitudinal and transversal directions. The H/V curves were combined in cross-sections in order to better correlate the peaks observed on each single curve. Most curves exhibit clear H/V peaks at frequencies between 0.3-2 Hz in the two profiles across the Adige valley and between 0.5-3 Hz in the other profile. All three profiles show the H/V peak frequencies decaying towards the center, but with different patterns in terms of spatial variation of frequency and features of the H/V peaks. The H/V peaks above 0.4 Hz, which are observed within the basin (along profile NS2) and on the edges of the profiles across the Adige valley, vary in frequency along the profiles suggesting a correlation with the changing depth of the bedrock (Fig. 2a). These peaks have the known features of the resonance peaks generated by stratigraphic 1D resonance, specifically, a local minimum in the vertical spectral component (Fig. 1b; Castellaro, 2016; Molnar et al. , 2018). Conversely, in a number of measurements located in the central part of the Adige valley, H/V peaks occur at constant frequencies around 0.35 Hz (Fig. 2a) and are generated by local maxima in the two horizontal spectral components at about 0.3 and 0.4 Hz in the longitudinal and transversal components respectively (Fig. 1,2b). In addition, the peak in the longitudinal component has always higher amplitude (in terms of spectral velocity) compared to the transversal one. It is interesting to note that, because the instrument was always oriented in the same direction but the axis of the Adige valley turns of about 90° at the intersection with the Bolzano basin, the pattern of the horizontal components is symmetric for the two profiles located across it (NS1 and EW1; Fig. 1a,c). The local peak with lower frequency is observed on the NS component along the EW1 profile and on the EW component along the other profile and vice versa for the other peak. This also indicates that the directional behavior is not an instrumental effect. This pattern is observed recurrently in all measurements acquired in the central 1.5 km of the Adige valley (Fig. 2b). Moreover, the amplitude of these peaks is maximum at the center of the valley and a gradually decays towards the edges. This holds for the H/V amplitudes (at 0.35 Hz) and for transversal/V (at 0.4 Hz) and longitudinal/V (at 0.3 Hz) amplitudes separately, the longitudinal amplitude being the highest (Fig. 2c). The frequency and amplitude features observed for H/V curves recorded within the Adige valley suggest that the longitudinal peak at 0.3 Hz and the transversal peak at 0.4 Hz are generated by the longitudinal and transversal 2D fundamental resonance modes of the valley. On the edges of the Adige valley and within the Bolzano basin we observe instead the evidences of 1D resonance, which is marked by spatial variation of the frequency and by the shape of the H/V peak.