On the initiation of sustained slip-weakening ruptures by localized stresses

Numerical simulations of dynamic earthquake rupture require an artificial initiation procedure, if they are not integrated in long-term earthquake cycle simulations. A widely applied procedure involves an ‘overstressed asperity’, a localized region stressed beyond the static frictional strength. The physical properties of the asperity (size, shape and overstress) may significantly impact rupture propagation. In particular, to induce a sustained rupture the asperity size needs to exceed a critical value. Although criteria for estimating the critical nucleation size under linear slip-weakening friction have been proposed for 2-D and 3-D problems based on simplifying assumptions, they do not provide general rules for designing 3-D numerical simulations. We conduct a parametric study to estimate parameters of the asperity that minimize numerical artefacts (e.g. changes of rupture shape and speed, artificial supershear transition, higher slip-rate amplitudes). We examine the critical size of square, circular and elliptical asperities as a function of asperity overstress and background (off-asperity) stress. For a given overstress, we find that asperity area controls rupture initiation while asperity shape is of lesser importance. The critical area obtained from our numerical results contrasts with published theoretical estimates when background stress is low. Therefore, we derive two new theoretical estimates of the critical size under low background stress while also accounting for overstress. Our numerical results suggest that setting the asperity overstress and area close to their critical values eliminates strong numerical artefacts even when the overstress is large. We also find that properly chosen asperity size or overstress may significantly shorten the duration of the initiation. Overall, our results provide guidelines for determining the size of the asperity and overstress to minimize the effects of the forced initiation on the subsequent spontaneous rupture propagation

Chronic vascular effects of oat phenolic acids and avenanthramides in pre- or stage 1 hypertensive adults

Wholegrain consumption is linked to a lower risk of cardiovascular disease. Evidence from randomized controlled trials have established that the consumption of wholegrain oats lowers blood cholesterol, via a mechanism partly mediated by β-glucan soluble fiber. However, oats contain an arrayof phenolic acids, including ferulic acid and also structurally related avenanthramides, which mayalso contribute to the cardiovascular health benefits of oat intake. We investigated whether 4 weeks, daily consumption of oat phenolics leads to improvement in markers of CVD risk men and women.In a 3 arm crossover single-blind, placebo-controlled trial, 28 volunteers consumed either: 1) oatmeal/oatcake intervention (-containing 48.9mg of phenolic acids and 19.2mg of avenanthramides); 2) oatbran concentrate+rice porridge/wheat cracker intervention (-containing 38.4mg of phenolic acidsand 0.5mg of avenanthramides) or 3) rice porridge/wheat cracker intervention (containing 13.8mg of phenolic acids). All treatments were matched in soluble fiber (4.8g) and energy (500kcal). The primary endpoint was FMD and other cardiovascular endpoints were blood pressure, LDI, LDL/HDL cholesterol, platelets and endothelial cell-derived extracellular vesicles (EVs). All measures were taken at baseline and after three, 4 week long intervention periods and two washout periods.Our data indicates an increase by 1.09 %±0.41 %(Mean± SEM) in FMD response following high phenolic oat intake with a significant difference (P=0.007) between baseline and postintervention. Consumption of high phenolic oats also led to a significant improvement in 24-hour SBP, day time SBP and night time SBP (P<0.01, P<0.01 and P<0.05) and day time and night time DBP (p<0.05). There was also a significant decrease with total and LDL cholesterol after the consumption of moderate and high phenolic oat interventions (P<0.05) and a small improvement in LDI (both Ach and SNP) but not significant. The number of resting endothelial EVs were also found to be increasing after the consumption of high phenolic oats.The findings of this study may provide evidence about the role of oat phenolic acids and avenanthramides in cardiovascular health and contribute to more effective public health advice about the consumption of oats and healthy cardiovascular aging

Nonlinear time series analysis of geomagnetic pulsations

A detailed nonlinear time series analysis has been made of two daytime geomagnetic pulsation events being recorded at L'Aquila (Italy, L ≈ 1.6) and Niemegk (Germany, L ≈ 2.3). Grassberger and Procaccia algorithm has been used to investigate the dimensionality of physical processes. Surrogate data test and self affinity (fractal) test have been used to exclude coloured noise with power law spectra. Largest Lyapunow exponents have been estimated using the methods of Wolf et al. The problems of embedding, stability of estimations, spurious correlations and nonlinear noise reduction have also been discussed. The main conclusions of this work, which include some new results on the geomagnetic pulsations, are (1) that the April 26, 1991 event, represented by two observatory time series LAQ1 and NGK1 is probably due to incoherent waves; no finite correlation dimension was found in this case, and (2) that the June 18, 1991 event represented by observatory time series LAQ2 and NGK2, is due to low dimensional nonlinear dynamics, which include deterministic chaos with correlation dimension D2(NGK2) = 2.25 &#x00b1; 0.05 and D2(NDK2) = 2.02 &#x00b1; 0.03, and with positive Lyapunov exponents λ_max (LAQ2) = 0.055 &#x00b1; 0.003 bits/s and λ_max (NGK2) = 0.052 &#x00b1; 0.003 bits/s; the predictability time in both cases is ≈ 13 s

Quantification of frequency-dependent lengthening of seismic ground-motion duration due to local geology: Applications to the Volvi area (Greece).

International audienc

Initiation and arrest of earthquake ruptures due to elongated overstressed regions

The initiation of natural and induced earthquakes is promoted in fault areas where shear stress is close to fault strength. In many real-world situations, these overstressed fault areas (or 'asperities') are very elongated; for example, in the case of a fault intersecting a reservoir subject to fluid-injection, or the stress concentration along the bottom of a seismogenic zone induced by deep fault creep. Theoretical estimates of the minimum overstressed asperity size leading to runaway rupture and of the final size of self-arrested ruptures are only available for 2-D problems and for 3-D problems with an asperity aspect ratio close to one. In this study, we determine how the nucleation of ruptures on elongated asperities, and their ensuing arrest, depends on the size and aspect ratio of the asperity and on the background stress. Based on a systematic set of 3-D dynamic rupture simulations assuming linear slip-weakening friction, we find that if the shortest asperity side is smaller than the 2-D critical length, the problem effectively reduces to a 2-D problem in which rupture nucleation and arrest are controlled by the shortest length of the asperity. Otherwise, nucleation and rupture arrest are controlled by the asperity area, with a minor exception: for asperities with shortest side slightly larger than the 2-D critical length, arrested ruptures are smaller than predicted by the asperity area. The fact that rupture arrest is dominantly controlled by area, even for elongated asperities, corroborates the finding that observed maximum magnitudes of earthquakes induced by fluid injection are consistent with the theoretical relation between the magnitude of the largest self-arrested rupture and the injected volume. In the context of induced seismicity, our simulations provide plausible scenarios that could be either favourable or challenging for traffic light systems and provide mechanical insights into the conditions leading to these situations

Strengthening Effectiveness of Ancient Masonry Bridges

A study aimed to investigate the effectiveness of two new strengthening techniques for ancient masonry bridges is herein presented. In particular the spandrel walls are subjected to both in-plane and out-of- plane actions: the former mainly due to shear and the latter due to the filling thrust or earthquake excitation. The proposed techniques concern the application on the masonry surface of a GFRP reinforced lime and cement mortar coating or a reinforced repointing with a net of stainless strands. Some experimental diagonal compression tests evidenced the significant effectiveness of the first technique to shear. A numerical simula-tion of the experimental tests allowed determining the mechanical parameters to be used in the material model. A second numerical study was aimed to investigate the effectiveness of both strengthening techniques for out-of-plane actions (flexure). The results obtained in the study show good performances of the strengthening techniques both for in-plane and out-of-plane actions of two types of masonry walls: rubble stone and solid brick