International audienc

[1] Single-station H/V curves from ambient noise recordings in Boumerdes (Algeria) show smooth bumps around 1 and 3 Hz. A complementary microtremor study, based on two 34 and 134-meter aperture arrays, evidences that these bumps are indeed real peaks produced by two strong V S contrasts at 37 and 118 meters depth, strongly smoothed by very high S-wave attenuation in the two sedimentary layers. These two H/V bumps, observed over a broad area, are meaningful and reveal the importance of Q in S-wave velocity modeling from microtremor array data processing. It also appears that Tertiary rocks should be, at least in some cases, taken into account, together with the Quaternary sediments, to explain single-station H/V frequency peaks, and therefore that considering only the first 30 m of soil for V S amplification evaluation, as usually recommended, sometimes leads to flaky results by artificially eliminating non-explained low-frequency peaks from the analysis.

International audienceWe present three geomorphologic and geological phenomena that have occurred in Algeria in recent years: (i) the Bab El Oued mudflow on 11 November 2001, which claimed several hundred lives, (ii) a soil collapse induced by sand liquefaction triggered by the Boumerdes earthquake (M w = 6.8) on 21 May 2003, and (iii) landslides that are threatening Constantine city, for which a hazard map is presented using a qualitative approach. We briefly describe and analyze these natural disasters, and in the first two cases propose the application of geophysical techniques such as ambient noise recordings and electrical imagery to help evaluate their extent and potential threat. Finally a landslide hazard map of Constantine is proposed

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