A detailed analysis of microearthquake waveforms recorded at Mt. Etna shows, with some exceptions, that first pulse of events with magnitude M≥2.5 is consistent with the presence of a slow initial phase which precedes a phase of high-speed rupture propagation. This is in agreement with experimental results, theoretical models and laboratory experiments which considered gradually increasing rupture velocity or slip. In order to investigate the dependence of the pulse shapes on the azimuth we analyze recordings for six earthquakes (M 2.7--3.7) with well constrained focal mechanisms computed by a high number of observations (21--27). For each earthquake, we observe that the values of the slow phase duration (tsip) and those of the high-speed rupture duration (tris), compared over the focal sphere, exhibit a spatial distribution which seem to depend strictly on the type of rupture mechanism. Moreover, waveforms of very closely located earthquakes do not exhibit similar characteristics in the pulse shapes, which would exclude that the waveforms of analyzed earthquakes are influenced drastically by attenuation effects and that the slow initial phase is the result of the propagation path. These and other evidence suggest that: i) the nucleation phases are mainly a source effect, and ii) directivity effects must be adequately considered on pulse width measurements. ¿ 2000 American Geophysical Union