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/

Serine Phosphoacceptor Sites within the Core Protein of Hepatitis B Virus Contribute to Genome Replication Pleiotropically

The core protein of hepatitis B virus can be phosphorylated at serines 155, 162, and 170. The contribution of these serine residues to DNA synthesis was investigated. Core protein mutants were generated in which each serine was replaced with either alanine or aspartate. Aspartates can mimic constitutively phosphorylated serines while alanines can mimic constitutively dephosphorylated serines. The ability of these mutants to carry out each step of DNA synthesis was determined. Alanine substitutions decreased the efficiency of minus-strand DNA elongation, primer translocation, circularization, and plus-strand DNA elongation. Aspartate substitutions also reduced the efficiency of these steps, but the magnitude of the reduction was less. Our findings suggest that phosphorylated serines are required for multiple steps during DNA synthesis. It has been proposed that generation of mature DNA requires serine dephosphorylation. Our results suggest that completion of rcDNA synthesis requires phosphorylated serines

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

Coupling long-term tectonic loading and short-term earthquake slip.

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Arial; -webkit-text-stroke: #000000} span.s1 {font-kerning: none} <p>Initiation, propagation, and termination of an earthquake rupture are highly sensitive to initial stress conditions. However, due to the difficulty of determining stresses, in-situ many studies have considered only uniform regional stresses without trying to estimate and include a realistic pre-stress constrained by a regional stress state. A geodynamic model can solve this issue of constraining initial stresses for a large region with great depth. A regional geodynamic model can include a fault zone, which can spontaneously form from a pre-defined one in the model. In this work, we show a rupture simulation on 2D large offset normal fault. </p

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

Seismic velocity changes, strain rate and non-volcanic tremors during the 2009–2010 slow slip event in Guerrero, Mexico

International audienceSeismic velocity changes, strain rate and non-volcanic tremors during the S U M M A R Y We use ambient noise cross-correlations to monitor small but reliable changes in seismic velocities and to analyse non-volcanic tremor (NVT) intensities during the slow slip event (SSE) that occurred in 2009 and 2010 in Guerrero. We test the sensitivity of the seismic velocity to strain variations in absence of strong motions. The 2009-2010 SSE presents a complex slip sequence with two subevents occurring in two different portions of the fault. From a seismic array of 59 seismometers, installed in small antennas, we detect a velocity drop with maximum amplitude at the time of the first subevent. We analyse the velocity change at different period bands and observe that the velocity perturbation associated with the SSE maximizes for periods longer than 12 s. Then a linearized inversion of the velocity change measured at different period bands is applied in order to determine the depth of the portion of the crust affected by this perturbation. No velocity change in the first 10 km is detected. Below, the velocity perturbation increases with depth, affecting the middle and lower crust. Finally, we compute the transient deformation produced by the SSE in an elastic model using the slip evolution recovered from the inversion of continuous GPS. The comparison between the velocity changes and the deformation suggests that the velocity change is correlated with the strain rate rather than with the strain. This result is similar to what was observed during the 2006 SSE in the same region and suggests a non-linear behaviour of the crust. The velocity changes can be interpreted together with other observables such as NVTs. During the 2009-2010 SSE we measure NVT activity using continuous seismic records filtered between 2 and 8 Hz. We observe a correlation between velocity changes (for period band greater than 14 s) and tremor activity whereas no correlation exists between velocity changes and seismic noise energy measured at long periods. These observations suggest that both seismic velocity change and NVT can be used as indication of transient deformation at depth

A Suite of Exercises for Verifying Dynamic Earthquake Rupture Codes

The article of record as published may be found at http://dx.doi.org/10.1785/0220170222We describe a set of benchmark exercises that are designed to test if computer codes that simulate dynamic earthquake rupture are working as intended. These types of computer codes are often used to understand how earthquakes operate, and they produce simulation results that include earthquake size, amounts of fault slip, and the patterns of ground shaking and crustal deformation. The benchmark exercises examine a range of features that scientists incorporate in their dynamic earthquake rupture simulations. These include implementations of simple or complex fault geometry, off-fault rock response to an earthquake, stress conditions, and a variety of formulations for fault friction. Many of the benchmarks were designed to investigate scientific problems at the forefronts of earthquake physics and strong ground motions research. The exercises are freely available on our website for use by the scientific community.Southern California Earthquake Center (SCEC