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Unloaded natural rock masses are known to generate seismic signals (GREEN et al., 2006;HAINZL et al., 2006;HUSEN et al., 2007;KRAFT et al., 2006). Following a 1,000 m 3 mass failure into the Mediterranean Sea, centimeter-wide tensile cracks were observed to have developed on top of an unstable segment of the coastal cliff. Nanoseismic monitoring techniques JOSWIG, 2008), which function as a seismic microscope for extremely weak seismic events, were applied to verify whether brittle failure is still generated within this unconsolidated sandstone mass and to determine whether it can be detected. Sixteen days after the initial mass failure, three small-aperture sparse arrays (Seismic Navigation Systems-SNS) were deployed on top of this 40-m high shoreline cliff. This paper analyzes dozens of spiky nanoseismic (-2.2 C M L C -3.4) signals recorded over one night in continuous mode (at 200 Hz) at very short slant distances (3-67 m). Waveform characterization by sonogram analysis (JOSWIG, 2008) shows that these spiky signals are all short in duration ([0.5 s). Most of their signal energy is concentrated in the 10-75 Hz frequency range and the waveforms display high signal similarity. The detection threshold of the data set reaches M L -3.4 at 15 m and M L -2.7 at 67 m. The spatial distribution of source signals shows 3-D clustering within 10 m from the cliff edge. The time distribution of M L magnitude does not display any decay pattern of M L over time. This corroborates an unusual event decay over time (modified Omori's law), whereby an initial quiet period is followed by regained activity, which then fades again. The polarization of maximal waveform amplitude was used to estimate spatial stress distribution. The orientation of ellipses displaying maximal signal energy is consistent with that of tensile cracks observed in the field and agrees with rock mechanics predictions. The M L -surface rupture length relationship displayed by our data fits a constant-slope extrapolation of empirical data collected by WELLS and COPPERSMITH (1994) for normal fault features at much larger scale. Signal characterization and location as well as the absence of direct anthropogenic noise sources near the monitoring site, all indicate that these nanoseismic signals are generated by brittle failure within the top section of the cliff. The atypical event decay over time that was observed suggests that the cliff material is undergoing post-collapse bulk strain accommodation. This feasibility study demonstrates the potential of nanoseismic monitoring in rapidly detecting, locating and analyzing brittle failure generated within unconsolidated material before total collapse occurs.

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3-D simulations of elastic wave propagation generated by earthquakes with magnitudes between 5.5 and 7.0 are used to parameterize strong ground motion attenuation relations for the Dead Sea Rift (DSR) graben structure. The results show that standard attenuation relations with an isotropic distance parameter are inadequate for a graben structure with a deep sedimentary trough. A new strategy is devised for the parameterization of attenuation relations in graben structures by looking at the statistical properties of 53 simulated earthquakes of variable magnitudes located at various sites along the western boundary fault of the DSR graben. An exemplary attenuation relation is designed from the synthetics for the 1 Hz spectral acceleration, modifying the Joyner-Boore-type parametrization by adding coefficients suited for three different source-to-sensor configurations: within the graben, beyond the graben and path unaffected by the graben structure.