Abstract. Landslide phenomena involve the northern coast of Malta, affecting in particular the urban area of Xemxija. Limestones overlying a clayey formation represent the shallower lithotypes that characterize the surficial geology of this area, where lateral spreading phenomena and rockfalls take place.Ambient noise records, processed through spectral ratio techniques, were analysed in order to characterize the dynamic behavior of the rock masses affected by the presence of fractures linked to the landslide body existing in the area. Experimental spectral ratios were also calculated after rotating the horizontal components of the seismic signal, and a direct estimate of the polarization angle was also performed in order to investigate the existence of directional effects in the ground motion.The results of the morphologic survey confirmed the existence of large cliff-parallel fractures that cause cliff-edge and unstable boulder collapses. Such phenomena appear connected to the presence, inside the clay formation, of a sliding surface that was identified through the interpretation of the noise measurement data. The boundaries of the landslide area appear quite well defined by the pronounced polarization effects, trending in the northeastern direction, observed in the fractured zone and in the landslide body in particular.

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A set of high quality aftershock data recorded on a closely spaced network of seismographs at Wellington, New Zealand, has enabled the determination of apparent wavefront speeds and azimuths controlled by the subducting Pacific plate. The former were very high (8.7 f 0.2 km s-') and the latter were shifted by as much as 10" eastward of the source-receiver line, which made an angle of about 12" downdip with the strike of the subducted slab beneath the North Island.The wavefront characteristics are modelled in terms of head-wave propagation along a curved refractor with a cylindrical geometry, using both a simple geometrical approach as well as a 3-D ray tracing program. With this structure, it is not possible to explain both the high apparent velocity as well as the large azimuthal anomaly, without the presence of an unusually high P-wave velocity of around 9.0 km s-'within the upper mantle of the subducted Pacific plate. Such high velocities, although reported previously in the North Island, are very hard to reconcile with known upper mantle rock velocities. Other alternatives were sought to explain the apparent slowness. In particular, modifications in the slab curvature along strike, according to proposed models, were found to contribute to the high wavefront speeds, but very high intrinsic P-velocities (8.8 km s-l) are still required. From slowness vector data, it is not possible to constrain the depth at which such velocities have to exist; however, traveltime considerations limit them to depths between the base of the subducted oceanic crust to about 12 km deeper.

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