Application of Surface wave methods for seismic site characterization

Surface-wave dispersion analysis is widely used in geophysics to infer a shear wave velocity model of the subsoil for a wide variety of applications. A shear-wave velocity model is obtained from the solution of an inverse problem based on the surface wave dispersive propagation in vertically heterogeneous media. The analysis can be based either on active source measurements or on seismic noise recordings. This paper discusses the most typical choices for collection and interpretation of experimental data, providing a state of the art on the different steps involved in surface wave surveys. In particular, the different strategies for processing experimental data and to solve the inverse problem are presented, along with their advantages and disadvantages. Also, some issues related to the characteristics of passive surface wave data and their use in H/V spectral ratio technique are discussed as additional information to be used independently or in conjunction with dispersion analysis. Finally, some recommendations for the use of surface wave methods are presented, while also outlining future trends in the research of this topic

Do trench sediments affect great earthquake occurrence in subduction zones?

Seismic energy release is dominated by the underthrusting earthquakes in subduction zones, and this energy release is further concentrated in a few subduction zones. While some subduction zones are characterized by the occurrence of great earthquakes, others are relatively aseismic. This variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity. Previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties: age of the subducting lithosphere and convergence rate. These two properties do not explain all the variance in maximum earthquake size. I propose that a third subduction zone property, “trench sediments”, explains part of the remaining variance in maximum earthquake size. Subduction zones are divided into two groups: (1) those with excess trench sediments, and (2) those with horst and graben structure at the trench. Thirteen of the 19 largest subduction zone events, including the three largest, occur in zones with excess trench sediments. About half the zones with excess trench sediments are characterized by great earthquake occurrence. Most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age-rate correlation. Two notable exceptions are the Oregon-Washington and Middle America zones. Overall, the presence of excess trench sediments appears to enhance great earthquake occurrence. One speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer, which at elevated temperature and pressure, forms a homogeneous and strong contact zone between the plates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43133/1/24_2004_Article_BF00874629.pd

Severe postoperative complications decrease overall and disease free survival in pancreatic ductal adenocarcinoma after pancreaticoduodenectomy

International audienceBACKGROUND:Postoperative complications influence overall and disease free survival after pancreaticoduodenectomy for pancreatic ductal adenocarcinoma is still a matter of debate and controversy.METHODS:The outcome of 942 consecutive patients, from the multicentric study of the French Association of Surgery, between January 2004 and December 2009 was analyzed. Perioperative data, including severe complications (grade III and above), were used in univariate and multivariate analysis to assess their influence on overall and disease free survival. Recurrence and its location were investigated as well.RESULTS:Median overall and disease free survival were 24 and 19 months respectively. Postoperative complications occurred in 444 patients (47%) with 18.3% of severe complications. On multivariate analysis, severe complications, positive lymph node status and R1-R2 resection were independent prognostic factors for both overall and disease free survival. The median overall survival decreased from 25 to 22 months (p = 0.005) and disease free survival from 21 to 16 months (p = 0.02) if severe complications occurred. Severe complications were independent prognostic factor of recurrence (p < 0.001).CONCLUSIONS:Severe complications significantly alter both overall and disease free survival and are an independent factor of recurrence