It is clear that the introduction of the shallow low-velocity layer has a major impact on the wavefield. In this section we look at the underlying reasons for that impact, using 2-D full wavefield simulations and sensitivity kernels. We ask the question: What part of the velocity model is the wavefield sensitive to, for any given time window in our seismogram? Using ideas from seismic inversion [Tarantola, 1984, 1987, 1988] time reversal mirrors [Fink, 1997] and adjoint methods [Talagrand and Courtier, 1987]; Tromp et al.  give an integrated theoretical framework for the calculation of such sensitivities. In this work we implement the expressions given by Tromp et al. . Two numerical simulations in the velocity model are required: (1) a “regular” simulation, where a source is input and a synthetic seismogram is calculated at the recording station of interest, and (2) an “adjoint” simulation where the part of the seismogram under investigation (i.e., the time window of interest from the seismogram) is extracted, time reversed and used as an input source (the adjoint source). The adjoint source is located at the receiver location from the first simulation, and recorded at the location which was occupied by the source in the first (regular) simulation. Individual sensitivity kernels for P and/or for S waves can then be calculated by looking at the interaction between the regular and adjoint wavefields [e.g., Tromp et al., 2005]. We calculate P and S wave 2-D kernels for both ECPN and ECZM stations, in both a homogeneous and a layered (400 m thick near surface low-velocity layer) model, for source depths of 120 m and 1800 m. All simulations include topography, taken as a 2-D cut of the 3-D DEM, through stations ECPN, ECZM and the summit. The source time function for the regular simulation is the same as in the 3-D simulations, Figure 2. P wave (S wave) sensitivity kernels are calculated by integrating the product of the divergence (curl) of the regular and adjoint wavefields, over the time interval of interest. Figure 12 shows the seismograms for the regular simulation. As was seen in the 3-D simulations, the near surface low-velocity layer significantly distorts the wavefield, particularly for ECZM station, located on the flank. P wave and S wave sensitivity kernels for station ECZM for the shallow source are shown in Figures 13 and 14 respectively. These sensitivity kernels indicate the area of the velocity model that contributes to the wavefield which is arriving at the station, within the seismogram time slice under investigation. The “polarity” of the sensitivity is related to the polarity of the field divergence (P waves) or curl (S waves). For our purpose, the magnitude of the sensitivity is the most important parameter. It is important to note that these sensitivity plots are not analogous to regular wave propagation snapshots, but are more instructive as they map seismogram arrivals precisely onto model properties, and tell us which areas of the model are “sampled” by the wavefield for specific times in the seismic wave train, recorded at a specified station. For finite frequencies the wavefield can “see beyond” the station, even in a homogeneous model (e.g., Figure 14, second row on the left, 7–10 s). This effect will be more pronounced for longer-period data. The sensitivity kernels allow us to better quantify the underlying causes of the extended wave trains seen in Figure 12, for the layered model. Comparing Figures 13 and 14 it is clear that most of the seismogram is composed of S-type wave motion. Figure 12 also demonstrates that vertical and horizontal components are approximately 90° out of phase, indication the presence of Rayleigh waves. Even for long-wavelength (LP) events one might intuitively expect that waves will be trapped in the low-velocity layer; what is not so obvious in advance is the importance of this phenomenon and the extensive “footprint” of the sensitivity. For example, Figure 14, first row on the right, 0–7 s demonstrates that the first 0–7 s of the wavefield arriving from the summit to a flank station is primarily controlled by the combined effects of near-summit velocity and topographic structure. Perhaps surprisingly for a source located close to the summit, the structure on the opposing flank, on the side facing away from the station, also makes a significant contribution to the signal arriving at ECZM, located approximately 6 km from the summit. In seismology, “reverberations” are often attributed to site effect, which is clearly not the case in this example. Although the signals for the layered model in Figure 12 looks reverberatory, the underlying cause is not local to the station. Comparing the left and right sides of the second row in Figures 13 and 14, it becomes clear that what appear to be stations related reverberations in Figure 12 are in fact trapped waves sampling large regions of the near-surface edifice. This point is even more visible in Figure 15 for summit station ECPN. For the 10–15 s time slice (Figure 15, third row on the right) the seismogram comprises contributions from across the surface of the entire edifice of Mount Etna. This is also true for P waves in the case of ECPN (Figure 16) which are much more prevalent for the summit station, relative to the flank station, further indicating the complexity and spatial variability of the wavefield. Even without resorting to kernel analysis, it can be seen from Figure 12 that the flank station recording is severely distorted by the near surface effects. In the above examples, the seismic source lies within the low-velocity layer, which will be the case for “shallow” LP events. Though less commonly recorded, deep LPs are likely to lie below layers with such low velocities. Figure 17 shows S wave kernels for ECZM station, for a source at 1800 m depth, 1400 m below the base of the low-velocity layer. Although the effects are not as pronounced as in the case of the shallower source, relative to the contribution made by the entire edifice the near surface layer still makes a disproportionate contribution to the wavefield arriving at the station.