Are megaquakes clustered?

We study statistical properties of the number of large earthquakes over the past century. We analyze the cumulative distribution of the number of earthquakes with magnitude larger than threshold M in time interval T, and quantify the statistical significance of these results by simulating a large number of synthetic random catalogs. We find that in general, the earthquake record cannot be distinguished from a process that is random in time. This conclusion holds whether aftershocks are removed or not, except at magnitudes below M = 7.3. At long time intervals (T = 2-5 years), we find that statistically significant clustering is present in the catalog for lower magnitude thresholds (M = 7-7.2). However, this clustering is due to a large number of earthquakes on record in the early part of the 20th century, when magnitudes are less certain.Comment: 5 pages, 5 figure

Rfam: updates to the RNA families database

Rfam is a collection of RNA sequence families, represented by multiple sequence alignments and covariance models (CMs). The primary aim of Rfam is to annotate new members of known RNA families on nucleotide sequences, particularly complete genomes, using sensitive BLAST filters in combination with CMs. A minority of families with a very broad taxonomic range (e.g. tRNA and rRNA) provide the majority of the sequence annotations, whilst the majority of Rfam families (e.g. snoRNAs and miRNAs) have a limited taxonomic range and provide a limited number of annotations. Recent improvements to the website, methodologies and data used by Rfam are discussed. Rfam is freely available on the Web at http://rfam.sanger.ac.uk/and http://rfam.janelia.org/

Deformation and localization in earthquake ruptures and stick-slip instabilities

The dynamic earthquake problem spans a broad range of length scales, from microscopic grain contacts through faults that are hundreds of kilometers long. A major goal of dynamic earthquake modeling is to develop friction laws that capture the small scale physics and that can also be used to model fault scale rupture. However, friction laws used in studying earthquake rupture are often simply fits to data, and give little physical insight into the rupture process. The goal of this work is to develop a model for the deformation of amorphous materials such as granular fault gouge, and to investigate the dynamics of instabilities at larger scales. The model is based on Shear Transformation Zone (STZ) Theory, a microscopic physical model for plastic deformation in dense amorphous materials such as fault gouge, granular materials, glasses, foams, and colloids. STZ Theory captures fracture and deformation features that are observed in numerical simulations, and remains tractable for modeling friction at larger scales. STZ Theory ties fault weakening to the evolution of an effective temperature, which quantifies the configurational disorder in the gouge and serves as the dynamic state variable in STZ Theory. STZ Theory predicts logarithmic rate dependence and that the length scale for frictional evolution increases with increasing average strain rate, which are observed in laboratory experiments. Additionally, STZ Theory captures the spontaneous formation and growth of narrow shear bands in the fault gouge. Shear bands within a layer of gouge are observed in many studies of faulting, which indicates that resolving the dynamics of shear banding is important for capturing the small scale physics during earthquake slip. At the scale of frictional interfaces, we investigate the role of strain localization for stick-slip instabilities in an elastic block slider system. We perform a linear stability analysis to predict the critical value of the spring stiffness when steady sliding becomes unstable, and verify our results through numerical integration. We find that when a shear band forms, steady sliding becomes unstable at a larger spring stiffness. We also investigate the implications of STZ Theory and strain localization in dynamic earthquake simulations. We compare STZ Theory without strain localization, Dieterich-Ruina (DR) friction, and linear slip-weakening (SW). The dynamic rupture governed by STZ Theory accelerates more rapidly to the limiting wave speed, exhibits a decreased peak slip rate, and transitions to supershear rupture at a lower initial shear stress than equivalent ruptures with DR or SW friction. For dynamic ruptures where a shear band does form, strain localization alters fault behavior because localization is a mechanism for dynamic weakening. The dynamic weakening of strain localization increases the slip rate during rupture, and also increases the stress drop. We also show that strain localization occurs below seismogenic depths where constitutive properties are rate strengthening due to slip propagating down dip from the seismogenic zone. Our results indicate that the small scale physics occurring within the gouge can have a large scale impact on the dynamics of friction and the propagation of slip on earthquake faults

Constraining depth range of S wave velocity decrease after large earthquakes near Parkfield, California

International audienceWe use noise correlation and surface wave inversion to measure the S wave velocity changes at different depths near Parkfield, California, after the 2003 San Simeon and 2004 Parkfield earthquakes. We process continuous seismic recordings from 13 stations to obtain the noise cross-correlation functions and measure the Rayleigh wave phase velocity changes over six frequency bands. We then invert the Rayleigh wave phase velocity changes using a series of sensitivity kernels to obtain the S wave velocity changes at different depths. Our results indicate that the S wave velocity decreases caused by the San Simeon earthquake are relatively small (~0.02%) and access depths of at least 2.3 km. The S wave velocity decreases caused by the Parkfield earthquake are larger (~0.2%), and access depths of at least 1.2 km. Our observations can be best explained by material damage and healing resulting mainly from the dynamic stress perturbations of the two large earthquakes

Evolution of genes and genomes on the Drosophila phylogeny

Comparative analysis of multiple genomes in a phylogenetic framework dramatically improves the precision and sensitivity of evolutionary inference, producing more robust results than single-genome analyses can provide. The genomes of 12 Drosophila species, ten of which are presented here for the first time (sechellia, simulans, yakuba, erecta, ananassae, persimilis, willistoni, mojavensis, virilis and grimshawi), illustrate how rates and patterns of sequence divergence across taxa can illuminate evolutionary processes on a genomic scale. These genome sequences augment the formidable genetic tools that have made Drosophila melanogaster a pre-eminent model for animal genetics, and will further catalyse fundamental research on mechanisms of development, cell biology, genetics, disease, neurobiology, behaviour, physiology and evolution. Despite remarkable similarities among these Drosophila species, we identified many putatively non-neutral changes in protein-coding genes, non-coding RNA genes, and cis-regulatory regions. These may prove to underlie differences in the ecology and behaviour of these diverse species

Evolution of genes and genomes on the Drosophila phylogeny

Affiliations des auteurs : cf page 216 de l'articleInternational audienceComparative analysis of multiple genomes in a phylogenetic framework dramatically improves the precision and sensitivity of evolutionary inference, producing more robust results than single-genome analyses can provide. The genomes of 12 Drosophila species, ten of which are presented here for the first time (sechellia, simulans, yakuba, erecta, ananassae, persimilis, willistoni, mojavensis, virilis and grimshawi), illustrate how rates and patterns of sequence divergence across taxa can illuminate evolutionary processes on a genomic scale. These genome sequences augment the formidable genetic tools that have made Drosophila melanogaster a pre-eminent model for animal genetics, and will further catalyse fundamental research on mechanisms of development, cell biology, genetics, disease, neurobiology, behaviour, physiology and evolution. Despite remarkable similarities among these Drosophila species, we identified many putatively non-neutral changes in protein-coding genes, non-coding RNA genes, and cis-regulatory regions. These may prove to underlie differences in the ecology and behaviour of these diverse species