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.

No abstract

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.

No abstract

No abstract

S U M M A R YAnchor Bay and surrounding regions are located on the northwest coast of the island of Malta, Central Mediterranean. The area is characterized by a coastal cliff environment having an outcropping layer of hard coralline limestone (UCL) resting on a thick (up to 50 m) layer of clays and marls (Blue Clay, BC). This configuration gives rise to coastal instability effects, in particular lateral spreading phenomena and rock falls. Previous and ongoing studies have identified both lateral spreading rates and vertical motions of several millimetres per year. The area is an interesting natural laboratory as coastal detachment processes in a number of different stages can be identified and are easily accessible. We investigate the site dynamic characteristics of this study area by recording ambient noise time-series at more than 30 points, over an area of 0.07 km 2 , using a portable three-component seismograph. The timeseries are processed to give both horizontal-to-vertical spectral ratio graphs (H/V) as well as frequency-dependent polarisation analysis. The H/V graphs illustrate and quantify aspects of site resonance effects due both to underlying geology as well as to mechanical resonance of partly or wholly detached blocks. The polarization diagrams indicate the degree of linearity and predominant directions of vibrational effects. H/V curves closer to the cliff edge show complex responses at higher frequencies, characteristic of the dynamic behaviour of individual detached blocks. Particle motion associated with the higher frequencies shows strongly directional polarization and a high degree of linearity at well-defined frequencies, indicative of normalmode vibration. The stable plateau areas, on the other hand, show simple, single-peak H/V curves representative of the underlying stratification and no predominant polarization direction. These results, which will be compared with those from other experiments in the area, have important implications for the understanding of ongoing processes in geologically active and unstable coastal environments.

No abstract

The islands composing the Maltese archipelago (Central Mediterranean) are characterized by a four-layer sequence of limestones and clays. A common feature found in the western half of the archipelago is Upper Coralline Limestone (UCL) plateaus and hillcaps covering a soft Blue Clay (BC) layer which can be up to 75 m thick. The BC layer introduces a velocity inversion in the stratigraphy, implying that the V S30 (traveltime average sear wave velocity (V S ) in the upper 30 m) parameter is not always suitable for seismic microzonation purposes. Such a layer may produce amplification effects, however might not be included in the V S30 calculations. In this investigation, V S profiles at seven sites characterized by such a lithological sequence are obtained by a joint inversion of the single-station Horizontal-to-Vertical Spectral Ratios (H/V or HVSR) and effective dispersion curves from array measurements analysed using the Extended Spatial Auto-Correlation technique. The lithological sequence gives rise to a ubiquitous H/V peak between 1 and 2 Hz. All the effective dispersion curves obtained exhibit a 'normal' dispersive trend at low frequencies, followed by an inverse dispersive trend at higher frequencies. This shape is tentatively explained in terms of the presence of higher mode Rayleigh waves, which are commonly present in such scenarios. Comparisons made with the results obtained at the only site in Malta where the BC is missing below the UCL suggest that the characteristics observed at the other seven sites are due to the presence of the soft layer. The final profiles reveal a variation in the V S of the clay layer with respect to the depth of burial and some regional variations in the UCL layer. This study presents a step towards a holistic seismic risk assessment that includes the implications on the site effects induced by the buried clay layer. Such assessments have not yet been done for Malta.