Ambient noise and ERT data provide insights into the structure of co-seismic rock avalanche deposits in Sichuan (China)

AbstractThe post-seismic history of the 2008 Mw7.9 Wenchuan earthquake shows that marginally stable deposits of large co-seismic landslide dams can pose persistent debris flow hazards for the downstream areas. Here, we combine analyses of single-station recordings of ambient noise with electrical resistivity tomography (ERT) surveys to explore the potential of drawing information on structure and geometry of the deposit of a large rock avalanche triggered by the Mw 7.9 2008 Wenchuan earthquake, which dammed the Yangjia stream in the Sichuan Province (China). The substantial thickness and heterogeneity of this kind of deposits limit the application of standard geophysical techniques, like active seismic surveys, which require highly energetic sources and long linear geophone arrays to reach adequate investigation depths. Passive single-station methods, relying on ambient noise recordings to determine site resonance properties, controlled by the contrast between soft surface layers and a stiffer substratum, offer the opportunity of investigating subsoil properties down to larger depths. In particular, we use a recently developed technique, which isolates the contribution of Rayleigh waves to ambient noise and draws information on sub-soil properties from the inversion of Rayleigh wave ellipticity curves plotted as function of frequency. In this framework, the ERT data can support the ellipticity curve inversion, typically affected by highly non-univocal solutions, by providing constraints for defining of the thickness of the uppermost surficial layers. The results allowed inferring the overlap of different layers within the 2008 rock avalanche deposit, as well as estimating lateral variations in their thickness and S-wave (Vs) velocities

Liquefaction damage potential for seismic hazard evaluation in urbanized areas

The liquefaction susceptibility of granular soils under seismic actions is commonly estimated by means of the liquefaction safety factor and recently by the potential index also. Since its original formulation the potential index has been developed and modified according to both deterministic and probabilistic approaches in order to draw liquefaction microzonation maps. In this study a new approach to potential index definition is proposed in order to relate the liquefaction potential prediction to the loss of bearing capacity for shallow foundation. Such new method has been used to estimate the so called liquefaction damage potential PDL at Barletta site, located in Puglia Region, where strong seismic events may occur. © 2011 Elsevier Ltd

Characteristics of the low energy seismicity of central Apulia (Southern Italy) and hazard implications

The central part of the Apulia region, in southern Italy, has been generally considered practically free from significant level of seismicity, but historical documentation, geological indicators and recent instrumental observations suggest that the activity of local minor tectonic structures could have been “masked” (and partly also induced) by that of major seismogenic structures located in the neighbouring regions. A revision of the central Apulia seismicity characteristics was conducted considering its space and time distribution, energy release rate and focal mechanisms, in view of possible hazard implications. To better constrain the seismicity rates inferable from the set of available historical data, special attention was paid to the declustering of a catalogue of low energy events (magnitude < 3.5) instrumentally detected in about 20 years: a new declustering procedure, useful for cases like to the one at hand, was purposely devised taking into account the peculiarity of local seismicity characteristics and the limitations of the available database. The results obtained by combining instrumental and historical data show that this area is affected by a rather sporadic seismicity, likely associated to a general tensional regime and possibly stimulated by the interaction with Apenninic and northern Apulia seismogenic activity. Even though less energetic, the local seismicity contributes to increase the moderately damaging shaking probability due to the activity of seismic sources located in the near areas, so to justify the adoption of at least a minimum level of caution in relation to the local definition of seismic protection measures

Combined analysis of seismic and gravimetric data in the Adriatic plate

The presence of a structural heterogeneity between the northern part of the Adriatic plate (Central Mediterranean) and its southernmost sector was hypothesised by different authors and also confirmed recently by 3D velocity models obtained through surface wave tomography and local earthquake tomography. However, the seismological data leave some uncertainty about the location of the lateral limits of this heterogeneity and about its "structural" meaning. The analysis of the Bouguer anomalies observed on a profile running from NW to SE along the Italian side of the plate has allowed to obtain a laterally improved and better constrained integrated model of the area. In particular, the gravimetric data have been interpreted through a 2(1/2)D density modelling developed adopting the seismic tomography model as starting point: the obtained results support the hypothesis of a 10 km thinning of the crust from NW to SE, compensated by the presence of a thick transition zone under the crust in the southern part of the Adriatic plate. The location of the boundary of this lateral heterogeneity was better defined, being marked by an increase of the Bouguer anomalies. The estimated density contrasts of the transition zone with respect to the contiguous layers of lower crust and mantle (about +0.5 and -0.1 g/cm3, respectively) and the comparison with the corresponding velocity contrasts, suggest that the material constituting this transition zone might have a composition more similar to that of the mantle. © 2004 Elsevier B.V. All rights reserved

Tomographic Study of the Adriatic Plate

4Two well-known methodologies have been used, for the first time, to derive a detailed and reliable lithospheric model of the Adriatic Plate, consistent with the thus far available data: non linear inversion of phase and group velocity dispersion curve obtained from surface-wave tomography, and body-waves tomography. To carry out the body-waves tomography, about 16500 P phases and 2000 S phases from 1219 seismic events, 73 seismic stations located at the border of the investigated region and a vertically heterogeneous starting model derived from the nonlinear inversion of average dispersion measurements, are used. The 3-D velocity model of the upper lithosphere, obtained from the combined analysis, shows a rather clear structural anomaly on the northeastern side, where the crust is thicker, and an uprising of the top of the lid in the northern part of the plate with a very thin, if any, transition zone from crust to mantle.nonemixedVENISTI N.; CALCAGNILE G.; PONTEVIVO A.; PANZA G.Venisti, N.; Calcagnile, G.; Pontevivo, A.; Panza, Giulian