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Loess is a meta-stable, cemented assemblage of mainly silt and clay-sized particles of low plasticity.When dry it behaves like a brittle material, but when wetted up the fabric rapidly collapses. Unique geomorphological features include extensive surface erosion, soil piping (loess 'karst'), catastrophic landslides, and widespread collapse (hydro-consolidation). The Chinese Loess Plateau is a more or less continuous drape of thick loess covering some 440,000 km 2 . It isone of China's regions that is most prone to geohazards. This paper reviews advances in the research related to loess geohazards, drawing particular attention tothe need to apply research findings to recent, very large (mega-)construction projects in loess terrain such as the Mountain Excavation and City 2 Construction in Yan'anlevelling 78 km 2 for urban expansion, the Lanzhou New District creating 246 km 2 , and large engineered interventions in the landscape for gully control and land reclamation such as those in Shaanxi and Gansu generating agricultural land covering an area of some 8,000 km 2 . These projects are in response to increasing pressures to facilitate expansion of urban centres, their interconnecting infrastructures and their agricultural support systems. It is argued that,where proper application of scientific knowledge for engineering control (e.g. density, drainage)of these new landscapes is absent, these project generate a substantial, and costly geohazard legacy for future generations.

Abstract.Recent observations have shown that slope response to seismic shaking can be characterised by directional variations of a factor of 2-3 or larger, with maxima oriented along local topography features (e.g. maximum slope direction). This phenomenon appears influenced by slope material properties and has occasionally been detected on landslide-prone slopes, where a down-slope directed amplification could enhance susceptibility to seismically-induced landsliding. The exact conditions for the occurrence of directional amplification remain still unclear and the implementation of investigation techniques capable to reveal the presence of such phenomena is desirable. To this purpose we tested the applicability of a method commonly used to evaluate site resonance properties (Horizontal to Vertical Noise Ratio -HVNR or Nakamura's method) as reconnaissance technique for the identification of site response directivity. Measurements of the azimuthal variation of H/V spectral ratios (i.e. between horizontal and vertical component) of ambient microtremors were conducted in a landslide-prone study area of central Italy where a local accelerometric network had previously provided evidence of directivity phenomena on some slopes. The test results were compared with average H/V spectral ratios obtained for low-to-moderate earthquakes recorded by the accelerometric stations. In general, noise and seismic recordings provided different amplitudes of spectral ratios at similar frequencies, likely because of differences in signal and instrument characteristics. Nevertheless, both kinds of recordings showed that at sites affected by site response directivity major H/V peaks have orientations consistent (within 20 • -30 • ) with the direction of maximum shaking energy. Therefore, HVNR appears to be Correspondence to: V. Del Gaudio (delga@geo.uniba.it) a promising technique for identifying seismic response directivity. Furthermore, in a comparative test conducted on a slope mantled in part by a deep-seated landslide we detected spectral peaks with orientations close to the maximum slope direction, whereas no evidence of directivity was found outside the slide boundaries. This indicates the influence of the landslide body on seismic response directivity.