Dilatancy controlled spatiotemporal slip evolution of a sealed fault with spatial variations of the pore pressure

A range of observations suggest the formation and maintenance of sealed and hence overpressured compartments in fluid-infiltrated fault zones. It is assumed that hydromechanical properties of regions with variable pore pressure states control the fault's stability and thus its characteristic response, that is, seismic or aseismic slip accumulation. We investigate in a systematic parameter space study the effects of spatial variations in pore pressure on spatiotemporal slip evolution along a hydraulically isolated fault plane. The 3-D continuum model is governed by rate-and-state friction and constitutive laws for porosity reduction. We show that the model response is sensitive to the degree of overpressurization and the efficiency of dilatant hardening mechanisms. Low pore pressures and small dilatancy effects result in unstable response types, whereas high pore pressures and large dilatant effects lead to stable and aseismic creep. Regions with an unstable response are shown to support most of the stresses accumulated during interseismic periods. Accelerated slip nucleates preferably in regions of low pore pressure. Statistical properties of model seismicity produce a wide range of event sizes for moderate and large earthquakes, in the case where dilatant mechanisms are inefficient. In case of efficient slip rate controlled porosity increase, less instabilities grow into large earthquakes. Final slip maps demonstrate the applicability of the chosen method to model seismicity controlled by frictional and hydraulic processes on a planar fault plane. The evolution of governing variables that depend on the pore pressure environment provide a conceptual basis for the interpretation of observed response characteristic

In situ observations of velocity changes in response to tidal deformation from analysis of the high-frequency ambient wavefield

We report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Piñon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ. Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity S_θ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low-velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of S_θ≈5 × 10^3 and a stress sensitivity of 0.5 MPa^(−1). The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here

A new species of Phrynobatrachus (Amphibia: Anura: Phrynobatrachidae) from north-western Guinea, West Africa

A new small Phrynobatrachus species from a gallery forest in north-western Guinea is described. Phrynobatrachus pintoi sp. nov. exhibits a combination of unique morphological characters and a distinctive color pattern, including: compact, oval body, short snout, warty dorsum and eyelid (although no eyelid cornicle is present), three pairs of large symmetric black spots on throat and breast, black spots on belly, more than one black bar on thighs and lower leg, finger and toe tips not expanded, and rudimentary web on foot. Furthermore, analysis of mitochondrial DNA from 16S rRNA reveals that this new species differs from other West African species of the genus by a minimum distance of 7%. Genetically the new species is closest to Phrynobatrachus fraterculus, P. cornutus, and P. gutturosus

Near-surface structure of the North Anatolian Fault zone from Rayleigh and Love wave tomography using ambient seismic noise

We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending similar to 1200 km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10 km of a 70 x 30 km region around Lake Sapanca. We observe low S-wave velocities ( 3.2 km s(-1)) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4 s. At longer periods up to 10 s, the fast direction aligns with the direction of maximum extension for the region (similar to 45 degrees). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties.Peer reviewe

Near-surface structure of the North Anatolian Fault zone from Rayleigh and Love wave tomography using ambient seismic noise

We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending ∼1200&thinsp;km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10&thinsp;km of a 70&thinsp;×&thinsp;30&thinsp;km region around Lake Sapanca. We observe low S-wave velocities (&lt;2.5&thinsp;km&thinsp;s−1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we image higher velocities (&gt;3.2&thinsp;km&thinsp;s−1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4&thinsp;s. At longer periods up to 10&thinsp;s, the fast direction aligns with the direction of maximum extension for the region (∼45∘). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties.</p

The SMC-5/6 Complex and the HIM-6 (BLM) Helicase Synergistically Promote Meiotic Recombination Intermediate Processing and Chromosome Maturation during<i> Caenorhabditis elegans</i> Meiosis

Meiotic recombination is essential for the repair of programmed double strand breaks (DSBs) to generate crossovers (COs) during meiosis. The efficient processing of meiotic recombination intermediates not only needs various resolvases but also requires proper meiotic chromosome structure. The Smc5/6 complex belongs to the structural maintenance of chromosome (SMC) family and is closely related to cohesin and condensin. Although the Smc5/6 complex has been implicated in the processing of recombination intermediates during meiosis, it is not known how Smc5/6 controls meiotic DSB repair. Here, using Caenorhabditis elegans we show that the SMC-5/6 complex acts synergistically with HIM-6, an ortholog of the human Bloom syndrome helicase (BLM) during meiotic recombination. The concerted action of the SMC-5/6 complex and HIM-6 is important for processing recombination intermediates, CO regulation and bivalent maturation. Careful examination of meiotic chromosomal morphology reveals an accumulation of inter-chromosomal bridges in smc-5; him-6 double mutants, leading to compromised chromosome segregation during meiotic cell divisions. Interestingly, we found that the lethality of smc-5; him-6 can be rescued by loss of the conserved BRCA1 ortholog BRC-1. Furthermore, the combined deletion of smc-5 and him-6 leads to an irregular distribution of condensin and to chromosome decondensation defects reminiscent of condensin depletion. Lethality conferred by condensin depletion can also be rescued by BRC-1 depletion. Our results suggest that SMC-5/6 and HIM-6 can synergistically regulate recombination intermediate metabolism and suppress ectopic recombination by controlling chromosome architecture during meiosis