Acoustically-induced slip in sheared granular layers: application to dynamic earthquake triggering

A fundamental mystery in earthquake physics is ``how can an earthquake be triggered by distant seismic sources?'' Here, we use discrete element method simulations of a granular layer, during stick-slip, that is subject to transient vibrational excitation to gain further insight into the physics of dynamic earthquake triggering. Using Coulomb friction law for grains interaction, we observe delayed triggering of slip in the granular gouge. We find that at a critical vibrational amplitude (strain) there is an abrupt transition from negligible time-advanced slip (clock advance) to full clock advance, {\it i.e.}, transient vibration and triggered slip are simultaneous. The critical strain is order of $10^{-6}$, similar to observations in the laboratory and in Earth. The transition is related to frictional weakening of the granular layer due to a dramatic decrease in coordination number and the weakening of the contact force network. Associated with this frictional weakening is a pronounced decrease in the elastic modulus of the layer. The study has important implications for mechanisms of triggered earthquakes and induced seismic events and points out the underlying processes in response of the fault gouge to dynamic transient stresses

Onset of strain localization in sheared glacial till

see Abstract Volum

Healing of simulated fault gouges aided by pressure solution: results from rock analogue experiments

see Abstract Volum

Childhood sexual experiences with an older partner among men who have sex with men in Buenos Aires, Argentina

This study sought to describe childhood sexual experiences with older partners (CSEOP) among men who have sex with men (MSM) in Buenos Aires, Argentina. MSM were recruited through respondent driven sampling. They responded to a computer administered self-interview with questions on CSEOP, operationalized as manual, oral, genital, or anal contact prior to age 13 with a partner at least 4 years older. Of the 500 respondents, only 25% identified as gay. Eighteen percent of the respondents reported CSEOP, the majority of whom did not feel they were hurt by the experience and did not consider it to be childhood sexual abuse (CSA). Over two-thirds of MSM who reported CSEOP said that their older partner was a female. Only 4% of those with a female partner felt their experience was CSA compared to 44% of those who had a male partner. Among all men reporting CSEOP, those who felt sexually abused were more likely to have been physically forced or threatened, physically hurt, and emotionally hurt than those who did not feel sexually abused. Having CSEOP, being hurt by the experiences, and perceiving the experiences as sexual abuse were not associated with current HIV sexual risk or substance use behavior. In this sample of MSM in Argentina, a substantial minority reported CSEOP. Those who felt they had been sexually abused were much more likely to have had an older male partner than an older female partner, and were more likely to report having been physically forced and threatened by their older partner.Fil: Dolezal, Curtis. Columbia University; Estados UnidosFil: Carballo Diéguez, A.. Columbia University; Estados UnidosFil: Balán, Iván C.. Columbia University; Estados UnidosFil: Pando, María de los Ángeles. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas en Retrovirus y Sida. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas en Retrovirus y Sida; ArgentinaFil: Mabragaña, Marina. Columbia University; Estados UnidosFil: Marone, Ruben. Nexo Asociación Civil; ArgentinaFil: Barreda, Victoria. Nexo Asociación Civil; ArgentinaFil: Ávila, María Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas en Retrovirus y Sida. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas en Retrovirus y Sida; Argentin

The mechanisms of spatial and temporal earthquake clustering

The number of earthquakes as a function of magnitude decays as a power law. This trend is usually justified using spring-block models, where slips with the appropriate global statistics have been numerically observed. However, prominent spatial and temporal clustering features of earthquakes are not reproduced by this kind of modeling. We show that when a spring-block model is complemented with a mechanism allowing for structural relaxation, realistic earthquake patterns are obtained. The proposed model does not need to include a phenomenological velocity weakening friction law, as traditional spring-block models do, since this behavior is effectively induced by the relaxational mechanism as well. In this way, the model provides also a simple microscopic basis for the widely used phenomenological rate-and-state equations of rock friction.Comment: 7 pages, 10 figures, comments welcom

Post-seismic and interseismic fault creep II: transient creep and interseismic stress shadows on megathrusts

We use idealized subduction megathrust models to examine aseismic, frictional fault creep throughout the interseismic period. We consider rate-dependent and rate- and state-dependent friction. When there is significant post-seismic creep surrounding locations of coseismic slip, the creep rates surrounding an asperity may be lower than the plate convergence rate late in the seismic cycle. This lowering of the creep rates is due to stress shadows forming around the interseismically locked asperities. The size of the stress shadows increases as there is more transient post-seismic creep. Larger asperities produce larger interseismic stress shadows, and multiple asperities can act together to produce a stress shadow larger than the sum of the effects of the individual asperities. For rate-state frictional megathrusts, there is a wide range of transient post-seismic creep that occurs: from pulses of post-seismic creep with decreasing creep rates through time, to delayed post-seismic creep. Delayed post-seismic creep may occur well into the interseismic period, with transient creep lasting over a significant portion of the seismic cycle. Delayed post-seismic creep is generally favoured in velocity strengthening regions with either a larger magnitude of the frictional direct effect or a larger effective normal stress. In addition, regions of the fault undergoing delayed post-seismic creep must be above steady state following coseismic slip.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78703/1/j.1365-246X.2009.04482.x.pd

Evolution of elastic and mechanical properties during fault shear. The roles of clay content, fabric development, and porosity

Phyllosilicates weaken faults due to the formation of shear fabrics. Although the impacts of clay abundance and fabric on frictional strength, sliding stability, and porosity of faults are well studied, their influence on elastic properties is less known, though they are key factors for fault stiffness. We document the role that fabric and consolidation play in elastic properties and show that smectite content is the most important factor determining whether fabric or porosity controls the elastic response of faults. We conducted a suite of shear experiments on synthetic smectite-quartz fault gouges (10–100 wt% smectite) and sediment incoming to the Sumatra subduction zone. We monitored Vp, Vs, friction, porosity, shear and bulk moduli. We find that mechanical and elastic properties for gouges with abundant smectite are almost entirely controlled by fabric formation (decreasing mechanical and elastic properties with shear). Though fabrics control the elastic response of smectite-poor gouges over intermediate shear strains, porosity is the primary control throughout the majority of shearing. Elastic properties vary systematically with smectite content: High smectite gouges have values of Vp ~ 1,300–1,800 m/s, Vs ~ 900–1,100 m/s, K ~ 1–4 GPa, and G ~ 1–2 GPa, and low smectite gouges have values of Vp ~ 2,300–2,500 m/s, Vs ~ 1,200–1,300 m/s, K ~ 5–8 GPa, and G ~ 2.5–3 GPa. We find that, even in smectite-poor gouges, shear fabric also affects stiffness and elastic moduli, implying that while smectite abundance plays a clear role in controlling gouge properties, other fine-grained and platy clay minerals may produce similar behavior through their control on the development of fabrics and thin shear surfaces