In tectonically active mountain belts, landslides contribute significantly to erosion. Statistical analysis of regional inventories of earthquake-triggered-landslides after large earthquakes (Mw > ...5.5) reveal a complex interaction between seismic shaking, landslide material, and rainfall. However, the contributions of each component have never been quantified due to a lack of in-situ data for active landslides. We exploited a 3-year geodetic and seismic dataset for a slow-moving landslide in Peru affected by local earthquakes and seasonal rainfalls. Here we show that in combination, they cause greater landslide motion than either force alone. We also show the rigidity of the landslide's bulk clearly decreasing during Ml ≥ 5 earthquakes. The recovery is affected by rainfall and small earthquakes (Ml < 3.6), which prevent the soil from healing, highlighting the importance of the timing between forcings. These new quantitative insights into the mechanics of landslides open new perspectives for the study of the mass balance of earthquakes.
Monitoring landslides is essential to understand their dynamics and to reduce the risk of human losses by raising warnings before a failure. A decade ago, a decrease of apparent seismic velocity was ...detected several days before the failure of a clayey landslide, that was monitored with the ambient noise correlation method. It revealed its potential to detect precursor signals before a landslide failure, which could improve early warning systems. To date, nine landslides have been monitored with this method, and its ability to reveal precursors before failure seems confirmed on clayey landslides. However three challenges remain for operational early-warning applications: to detect velocity changes both rapidly and with confidence, to account for seasonal and daily environmental influences, and to check for potential instabilities in measurements. The ability to detect a precursory velocity change requires to adapt the processing workflow to each landslide: the key factors are the filtering frequency, the correlation time window, and the choice of temporal resolution. Other optional processing steps are described, to better measure rapid velocity changes, improve signal-to-noise ratio, or estimate the measurement uncertainty. The velocity also fluctuates seasonally, by 1 to 6% on the reviewed landslide studies, due to environmental influences. This review reveals a linear trend between the amplitude of seasonal fluctuations and the filtering frequency over the 0.1–20 Hz range, encompassing both landslide and non-landslide studies. The environmental velocity fluctuations are caused mostly by groundwater levels and soil freezing/thawing, but could also be affected by snow height, air temperature and tide depending on the site. Daily fluctuations should also occur on landslides, and can be an issue when seeking to obtain a sub-daily resolution useful for early-warning systems. Finally, spurious fluctuations of apparent velocity—unrelated to the material dynamics—should be verified for. They can be caused by changes in noise sources (location or spectral content), in site response (change of scatterers, attenuation, or resonance frequency due to geometrical factors), or in inter-sensor distance. As a perspective, the observation of seismic velocity changes could contribute in assessing a landslide stability across time, both during the different creeping stages occurring before a potential failure, and during its reconsolidation after a failure.
Given that clay‐rich landslides may become mobilized, leading to rapid mass movements (earthflows and debris flows), they pose critical problems in risk management worldwide. The most widely proposed ...mechanism leading to such flow‐like movements is the increase in water pore pressure in the sliding mass, generating partial or complete liquefaction. This solid‐to‐liquid transition results in a dramatic reduction of mechanical rigidity in the liquefied zones, which could be detected by monitoring shear wave velocity variations. With this purpose in mind, the ambient seismic noise correlation technique has been applied to measure the variation in the seismic surface wave velocity in the Pont Bourquin landslide (Swiss Alps). This small but active composite earthslide‐earthflow was equipped with continuously recording seismic sensors during spring and summer 2010. An earthslide of a few thousand cubic meters was triggered in mid‐August 2010, after a rainy period. This article shows that the seismic velocity of the sliding material, measured from daily noise correlograms, decreased continuously and rapidly for several days prior to the catastrophic event. From a spectral analysis of the velocity decrease, it was possible to determine the location of the change at the base of the sliding layer. These results demonstrate that ambient seismic noise can be used to detect rigidity variations before failure and could potentially be used to predict landslides.
Key Points
Rigidity drop observed from ambient seismic noise correlation monitoring
Rigidity drop several days before the slope failure
Frequency analysis demonstrates the change to be at a depth of 9 to 11 m
Billions of Radio-Frequency Identification (RFID) passive tags are produced yearly to identify goods remotely. New research and business applications are continuously arising, including recently ...localization and sensing to monitor earth surface processes. Indeed, passive tags can cost 10 to 100 times less than wireless sensors networks and require little maintenance, facilitating years-long monitoring with ten's to thousands of tags. This study reviews the existing and potential applications of RFID in geosciences. The most mature application today is the study of coarse sediment transport in rivers or coastal environments, using tags placed into pebbles. More recently, tag localization was used to monitor landslide displacement, with a centimetric accuracy. Sensing tags were used to detect a displacement threshold on unstable rocks, to monitor the soil moisture or temperature, and to monitor the snowpack temperature and snow water equivalent. RFID sensors, available today, could monitor other parameters, such as the vibration of structures, the tilt of unstable boulders, the strain of a material, or the salinity of water. Key challenges for using RFID monitoring more broadly in geosciences include the use of ground and aerial vehicles to collect data or localize tags, the increase in reading range and duration, the ability to use tags placed under ground, snow, water or vegetation, and the optimization of economical and environmental cost. As a pattern, passive RFID could fill a gap between wireless sensor networks and manual measurements, to collect data efficiently over large areas, during several years, at high spatial density and moderate cost.
•Multiply scattered waves in sand and concrete are affected by fluid injection.•Fluid injection impacts more waveforms than the phase of the recordings.•Coda Wave Decorrelation can be applied to ...monitor fluid migration in such medium.
Seismic and ultrasonic waves are sometimes used to track fluid injections, propagation, infiltrations in complex material, including geological and civil engineered ones. In most cases, one use the acoustic velocity changes as a proxy for water content evolution. Here we propose to test an alternative seismic or acoustic observable: the waveform decorrelation.
We use a sample of compacted millimetric sand as a model medium of highly porous multiple scattering materials. We fill iteratively the sample with water, and track changes in ultrasonic waveforms acquired for each water level. We take advantage of the high sensitivity of diffuse coda waves (late arrivals) to track small water elevation in the material. We demonstrate that in the mesoscopic regime where the wavelength, the grain size and the porosity are in the same order of magnitude, Coda Wave Decorrelation (waveform change) is more sensitive to fluid injection than Coda Wave Interferometry (apparent velocity change). This observation is crucial to interpret fluid infiltration in concrete with ultrasonic record changes, as well as fluid injection in volcanoes or snow melt infiltration in rocky glaciers. In these applications, Coda Wave Decorrelation might be an extremely interesting tool for damage assessment and alert systems.
In the American College of Cardiology/American Heart Association guidelines, patients are considered to have true-severe stenosis when the mean gradient (MG) is ≥40 mm Hg with an aortic valve area ...(AVA) ≤1 cm2 during dobutamine stress echocardiography (DSE). However, these criteria have not been previously validated.
The aim of this study was to assess the value of these criteria to predict the presence of true-severe AS and the occurrence of death in patients with low-flow, low-gradient aortic stenosis (LF-LG AS).
One hundred eighty-six patients with low left ventricular ejection fraction (LVEF) LF-LG AS were prospectively recruited and underwent DSE, with measurement of the MG, AVA, and the projected AVA (AVAProj), which is an estimate of the AVA at a standardized normal flow rate. Severity of AS was independently corroborated by macroscopic evaluation of the valve at the time of valve replacement in 54 patients, by measurement of the aortic valve calcium by computed tomography in 25 patients, and by both methods in 8 patients. According to these assessments, 50 of 87 (57%) patients in the study cohort had true-severe stenosis.
Peak stress MG ≥40 mm Hg, peak stress AVA ≤1 cm2, and the combination of peak stress MG ≥40 mm Hg and peak stress AVA ≤1 cm2 correctly classified AS severity in 48%, 60%, and 47% of patients, respectively, whereas AVAProj ≤1 cm2 was better than all the previous markers (p < 0.007), with 70% correct classification. Among the subset of 88 patients managed conservatively (47% of the cohort), 52 died during a follow-up of 2.8 ± 2.5 years. After adjustment for age, sex, functional capacity, chronic kidney failure, and peak stress LVEF, peak stress MG and AVA were not predictors of mortality in this subset. In contrast, AVAProj ≤1 cm2 was a strong predictor of mortality under medical management (hazard ratio: 3.65; p = 0.0003).
In patients with low LVEF LF-LG AS, the DSE criteria of a peak stress MG ≥40 mm Hg, or the composite of a peak stress MG ≥40 mm Hg and a peak stress AVA ≤1 cm2 proposed in the guidelines to identify true-severe AS and recommend valve replacement, have limited value to predict actual stenosis severity and outcomes. In contrast, AVAProj better distinguishes true-severe AS from pseudo-severe AS and is strongly associated with mortality in patients under conservative management. (Multicenter Prospective Study of Low-Flow Low-Gradient Aortic Stenosis TOPAS; NCT01835028)
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Objectives The objective of this study was to compare the incidence of cerebral embolism (CE) as evaluated by diffusion-weighted magnetic resonance imaging (DW-MRI) following transapical (TA) ...transcatheter aortic valve implantation (TAVI) versus transfemoral (TF) TAVI. Background The TA-TAVI approach avoids both the manipulation of large catheters in the aortic arch/ascending aorta and the retrograde crossing of the aortic valve, and this avoidance might lead to a lower rate of CE. Methods This was a prospective multicenter study including 60 patients who underwent cerebral DW-MRI the day before and within the 6 days following TAVI (TF approach: 29 patients; TA approach: 31 patients). Neurologic and cognitive function assessments were performed at DW-MRI time points. Results The TAVI procedure was performed with the Edwards valve and was successful in all cases but one (98%). A total of 41 patients (68%) had 251 new cerebral ischemic lesions at the DW-MRI performed 4 ± 1 days after the procedure, 19 patients in the TF group (66%) and 22 patients in the TA group (71%; p = 0.78). Most patients (76%) with new ischemic lesions had multiple lesions (median number of lesions per patient: 3, range 1 to 31). There were no differences in lesion number and size between the TF and TA groups. No baseline or procedural factors were found to be predictors of new ischemic lesions. The occurrence of CE was not associated with a measurable impairment in cognitive function, but 2 patients (3.3%) had a clinically apparent stroke within the 24 h following the procedure (1 patient in each group). Conclusions TAVI is associated with a high rate of silent cerebral ischemic lesions as evaluated by DW-MRI, with no differences between the TF and TA approaches. These results provide important novel insight into the mechanisms of CE associated with TAVI and support the need for further research to both reduce the incidence of CE during these procedures and better determine their clinical relevance.
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