Time-Frequency-Wavenumber Analysis of Surface Waves Using the Continuous Wavelet Transform

A modified approach to surface wave dispersion analysis using active sources is proposed. The method is based on continuous recordings, and uses the continuous wavelet transform to analyze the phase velocity dispersion of surface waves. This gives the possibility to accurately localize the phase information in time, and to isolate the most significant contribution of the surface waves. To extract the dispersion information, then, a hybrid technique is applied to the narrowband filtered seismic recordings. The technique combines the flexibility of the slant stack method in identifying waves that propagate in space and time, with the resolution of f-k approaches. This is particularly beneficial for higher mode identification in cases of high noise levels. To process the continuous wavelet transform, a new mother wavelet is presented and compared to the classical and widely used Morlet type. The proposed wavelet is obtained from a raised-cosine envelope function (Hanning type). The proposed approach is particularly suitable when using continuous recordings (e.g., from seismological-like equipment) since it does not require any hardware-based source triggering. This can be subsequently done with the proposed method. Estimation of the surface wave phase delay is performed in the frequency domain by means of a covariance matrix averaging procedure over successive wave field excitations. Thus, no record stacking is necessary in the time domain and a large number of consecutive shots can be used. This leads to a certain simplification of the field procedures. To demonstrate the effectiveness of the method, we tested it on synthetics as well on real field data. For the real case we also combine dispersion curves from ambient vibrations and active measurement

Photometric compliance of tablet screens and retro-illuminated acuity charts as visual acuity measurement devices

Mobile technology is increasingly used to measure visual acuity. Standards for chart-based acuity tests specify photometric requirements for luminance, optotype contrast and luminance uniformity. Manufacturers provide some photometric data but little is known about tablet performance for visual acuity testing. This study photometrically characterised seven tablet computers (iPad, Apple inc.) and three ETDRS (Early Treatment Diabetic Retinopathy Study) visual acuity charts with room lights on and off, and compared findings with visual acuity measurement standards. Tablet screen luminance and contrast were measured using nine points across a black and white checkerboard test screen at five arbitrary brightness levels. ETDRS optotypes and adjacent white background luminance and contrast were measured. All seven tablets (room lights off) exceeded the most stringent requirement for mean luminance (≥ 120 cd/m2) providing the nominal brightness setting was above 50%. All exceeded contrast requirement (Weber ≥ 90%) regardless of brightness setting, and five were marginally below the required luminance uniformity threshold (Lmin/Lmax ≥ 80%). Re-assessing three tablets with room lights on made little difference to mean luminance or contrast, and improved luminance uniformity to exceed the threshold. The three EDTRS charts (room lights off) had adequate mean luminance (≥ 120 cd/m2) and Weber contrast (≥ 90%), but all three charts failed to meet the luminance uniformity standard (Lmin/Lmax ≥ 80%). Two charts were operating beyond manufacturer’s recommended lamp replacement schedule. With room lights on, chart mean luminance and Weber contrast increased, but two charts still had inadequate luminance uniformity. Tablet computers showed less inter-device variability, higher contrast, and better luminance uniformity than charts in both lights-on and lights-off environments, providing brightness setting was >50%. Overall, iPad tablets matched or marginally out-performed ETDRS charts in terms of photometric compliance with high contrast acuity standards

Two-dimensional resonances in Alpine valleys identified from ambient vibration wavefields

Although numerical simulations have for long shown the importance of 2-D resonances in site effect estimations of sediment-filled valleys, this phenomenon is usually not taken into account by current hazard assessment techniques. We present an approach to identify the resonance behaviour of a typical Alpine valley by analysis of ambient noise recorded simultaneously on a dense array. The applicability of the method is evaluated further using synthetic ambient noise acquired with current 3-D numerical simulation techniques. Resonance frequencies of the fundamental mode SV and the fundamental and first higher mode of SH are identified from measured data with the reference station method, verifying results of previous studies. Patterns of spectral amplitude and phase behaviour obtained from observed and synthetic noise correlate well with properties expected for 2-D resonance. Application of a frequency-wavenumber technique shows that the noise wavefield is dominated by standing waves at low frequencies (0.25 to 0.50 Hz). The different 2-D resonance modes are creating prominent peaks in horizontal-to-vertical spectral ratios, which can not be interpreted in terms of 1-D resonance. We conclude that ambient noise records measured simultaneously on a linear array perpendicular to the valley axis may be used for identification of resonance modes in sediment-filled valley

Locally triggered seismicity in the central Swiss Alps following the large rainfall event of August 2005

In 2005 August, an unusual series of 47 earthquakes occurred over a 12-hr period in central Switzerland. The earthquakes occurred at the end of 3-d period of intensive rainfall, with over 300 mm of precipitation. The highest seismicity occurred as two distinct clusters in the region of Muotatal and Riemenstalden, Switzerland, a well-known Karst area that received a particularly large amount of rainfall. The large increase in seismicity, compared to the background, and the short time delay between the onset of the intense rainfall and the seismicity strongly suggest that earthquakes were triggered by rainfall. In our preferred model, an increase in fluid pressure at the surface due to a large amount of rain leads to a local increase in pore fluid pressure at depth. The increase in pore fluid pressure will reduce the shear strength of a porous medium by counteracting normal stress and, at the end, provoke failure. The series of triggered earthquakes in central Switzerland occurred in regions that have been seismically active in the past, showing similar hypocentre locations and magnitudes. This suggests that these earthquakes occurred on existing faults that were critically stressed. We modelled the intense rainfall as a step increase in fluid pressure at the surface that migrates to greater depths following the solution of the one-dimensional diffusion equation in a homogeneous half space. This allowed us to estimate the hydraulic diffusivity by plotting triggered seismicity in a time-depth plot. We found values of hydraulic diffusivity in the range from 0.01 to 0.5 m2 s−1 for our study area. These values are in good agreement with previous studies on earthquakes that were triggered by fluids, supporting the idea that the observed earthquake series was triggered by the large amount of rainfal

Partner selection supported by opaque reputation promotes cooperative behavior

Reputation plays a major role in human societies, and it has been proposed as an explanation for the evolution of cooperation. While the majority of previous studies equates reputation with a transparent and complete history of players' past decisions, in real life, reputations are often ambiguous and opaque. Using web-based experiments, we explore the extent to which opaque reputation works in isolating defectors, with and without partner selection opportunities. Our results show that low reputation works as a signal of untrustworthiness, whereas medium or high reputation are not taken into account by participants for orienting their choices. We also find that reputation without partner selection does not promote cooperative behavior; that is, defectors do not turn into cooperators only for the sake of getting a positive reputation. Finally, in a third study, we find that, when reputation is pivotal to selection, then a substantial proportion of would-be defectors turn into cooperators. Taken together, these results provide insights on the characteristics of reputation and on the way in which humans make use of it when selecting partners but also when knowing that they will be selected

A comparison of observed and simulated site response in the Rhône valley

Site effects in the city of Sion in the Rhône valley are analysed from weak motion signals recorded on a dense temporary array. We simulate the recorded events with a 3-D finite difference method for frequencies up to 4 Hz using a recently developed velocity model of the Sion basin. Site-to-reference Fourier spectral ratios are computed from 16 local and regional events. All sites exhibit amplification factors of up to 12 between 0.5 and 0.6 Hz, which can be reproduced by the numerical simulations. By rotating the weak motion to directions parallel and perpendicular to the valley axis, we show that this low-frequency amplification is caused by the SH00 and SV0 fundamental modes of 2-D resonance. Additional peaks of amplification can be observed at higher frequencies, with amplification factors of up to 20 at some sites. Application of the high-resolution frequency-wavenumber and the multiple signal characterization method to the vertical component of recorded and simulated signals show that edge-generated surface waves arriving from almost all directions dominate the wavefield at 1.25 and 2.50 Hz. Peak ground velocities computed from the simulated ground motion show interference patterns that depend strongly on the incidence direction, and the computed amplification of peak ground velocities are generally in agreement with the observations. We conclude that the complex 3-D geometry of the basin needs to be considered to evaluate site effects up to at least 2.5 H

Moho depth and Poisson's ratio in the Western-Central Alps from receiver functions

Current knowledge about deep crustal structure of the Alpine orogen has mainly been derived from P-wave velocity models obtained from active and passive seismic experiments. A complementary S-wave model to provide lithological constraints necessary for unique structural interpretation has been missing to date. In this paper, we present important new information on S-wave velocity model in the Alps. We applied the receiver function method using 6 yr of high quality data from 61 permanent and temporary stations sampling the Western—Central Alps. We determined first-order crustal features Moho depth (H) and average Vp/Vs ratio (κ) with the H-κ stacking technique that uses timing of direct and multiple P-to-S converted phases from the Moho interface. Synthetic tests reveal a dipping Moho interface, expected beneath an orogen, causes a systematic bias of H and κ potentially leading to misinterpretation. We thus applied corrections determined from synthetic data to remove the bias, providing better fit of recovered Moho depths with active seismic estimates. For each site, we also obtained independent H and κ estimates based on the timing of the strong Ps-phase. Our results show a gently south—southeast dipping European Moho at a depth of ∼24-30 km beneath the Northern Alpine Foreland, steepening rapidly towards the Europe—Africa suture zone to reach a maximum depth of ∼55 km. South of the suture, the Moho of the Adriatic crust, promontory of the African plate, is at ∼35-45 km depth. In the previously ill-constrained Western Alps, we found the European Moho at ∼30 km depth beneath the more external units dipping east—northeast to reach ∼50-55 km in the inner core of the Alps. The Poisson's ratio clearly correlates with the tectonic units that comprise the Alps. Average crustal values in the European Alpine Foreland are close to 0.25. In the Alps, we observe low values (0.22) in the highly deformed nappes of the Mesozoic Helvetic and Southern Alps indicating a thickening of felsic upper-crustal material. In contrast, the Poisson's ratio is significantly higher (0.26) in the Penninic and Austroalpine units near the suture zone. This rapid and significant change marks a clear rupture between the Alpine forelands and the suture domain. We assign this high Poisson's ratio to doubling of mafic lower crust consistent with results from previous active seismic experiments. A continuation of the lower crustal wedge into the central part of the Western Alps, however, seems unlikely based on low observed Poisson's ratio

Bayesian imaging of the 2000 Western Tottori (Japan) earthquake through fitting of strong motion and GPS data

We image the rupture process of the 2000 Western Tottori earthquake (Mw = 6.6) through fitting of strong motion and GPS data. We consider an observational network consisting of 18 strong motion and 16 GPS stations, located within three fault lengths from the epicentre. We assume a planar fault and compute Green's functions for a 1-D velocity model. The earthquake rupture is described as a shear dislocation parameterized in terms of peak slip velocity, rake angle, rupture time and rise time, defined on a regular grid of nodes on the fault surface and derived at inner points through bilinear interpolation. Our inversion procedure is based on a Bayesian approach. The solution of the inverse problem is stated in terms of a posterior probability density function (pdf), representing the conjunction of prior information with information contained in the data and in the physical law relating model parameters with data. Inferences on model parameters are thus expressed in terms of posterior marginal pdfs. Due to the non-linearity of the problem, we use a Markov Chain Monte Carlo (MCMC) method, based on the Metropolis algorithm, to compute posterior marginals. Except for a few cases posterior marginals do not show a Gaussian-like distribution. This prevents us from providing a mean model and from characterizing uncertainties in terms of standard deviations only. Resolution on each single parameter is analysed by looking at the difference between prior and posterior marginal pdfs. Posterior marginals indicate that the best resolved feature is a major slip patch (peak value of 311 ± 140 cm), located between the hypocentre and the top edge of the fault, centered at a depth of 4.5 km. This shallow slip patch is triggered about 3 s after the earthquake nucleated and required about 4 s to reach its final slip value. The presence of this shallow slip patch is common to all previous studies. In contrast to some previous studies, we do not identify any significant slip (>1 m) at the bottom of the fault. We also compare inferences from both strong motion and GPS data with inferences derived from strong motion data only. In both cases the shallow slip patch is identified. At other locations, the main effect of the GPS data is in reducing the probability associated with high values of slip. GPS data reduce the presence of spurious fault slip and therefore strongly influence the resulting final seismic momen

Educación ambiental, vida en la naturaleza y ciencia escolar : Estado de situación y posibles estrategias de integración

La construcción de relaciones entre Vida en la Naturaleza, Educación Ambiental y Ciencia Escolar da lugar a un campo de trabajo educativo sobre el cual es posible trazar diversas configuraciones didácticas. Desde esta perspectiva, en el presente estudio se analiza el estado de situación de este campo relacional a partir de los resultados obtenidos de encuestas realizadas a estudiantes de Institutos Superiores de Formación Docente en Educación Física, para luego proponer posibles estrategias de intersección didáctica, siempre desde el marco referencial que brinda la tríada conceptual: motricidad, ambiente próximo y planetariedad. Las propuestas están dirigidas a enriquecer la formación inicial de futuros educadores así como también a contribuir en la formación permanente de docentes de diversos profesorados, pudiendo ser extendidas a facilitadores del ámbito educativo no forma