The shallow Nice submarine slope is notorious for the 1979 tsunamigenic landslide that caused eig... more The shallow Nice submarine slope is notorious for the 1979 tsunamigenic landslide that caused eight casualties and severe infrastructural damage. Many previous studies have tackled the question whether earthquake shaking would lead to slope failure and a repetition of the deadly scenario in the region. The answers are controversial. In this study, we assess for the first time the factor of safety using peak ground accelerations (PGAs) from synthetic accelerograms from a simulated offshore Mw 6.3 earthquake at a distance of 25km from the slope. Based on cone penetration tests (CPTu) and multichannel seismic reflection data, a coarser grained sediment layer was identified. In an innovative geotechnical approach based on uniform cyclic and arbitrary triaxial loading tests, we show that the sandy silt on the Nice submarine slope will fail under certain ground motion conditions. The uniform cyclic triaxial tests indicate that liquefaction failure is likely to occur in Nice slope sediments in the case of a Mw 6.3 earthquake 25km away. A potential future submarine landslide could have a slide volume (7.7 × 10 6 m 3) similar to the 1979 event. Arbitrary loading tests reveal post-loading pore water pressure rise, which might explain post-earthquake slope failures observed in the field. This study shows that some of the earlier studies offshore Nice may have overestimated the slope stability because they underestimated potential PGAs on the shallow marine slope deposits.
Soil Dynamics and Earthquake Engineering, Feb 1, 2021
In this study we investigate the effect of prior seismic shaking on the monotonic shear strength ... more In this study we investigate the effect of prior seismic shaking on the monotonic shear strength of saturated Ottawa Sand 20/30. We perform a series of stress-controlled undrained cyclic triaxial tests with different seismic intensities intentionally without causing failure, followed by drainage of the excess pore pressure and an undrained monotonic loading test to determine the undrained shear strength. The experimental data show that small to moderate seismic events that do not fail the specimen can significantly increase undrained shear strength without much change in relative density. One prior seismic event with peak ground acceleration ~ 1.3 m/s 2 may increase the undrained shear strength of a specimen at ~ 10 m depth by around 30%. The results also show that as the intensity of the shaking increases, the increase in the monotonic shear strength increases. However, the strengthening effect does not increase with the number of seismic events although a small degree of global densification in the sample is observed. The results of this paper will help assess the change in static slope stability after a single or multiple small to moderate events occurred without causing initial instability.
This paper aims to understand the hydrothermal sites near the Yonaguni Knoll IV in the Okinawa Tr... more This paper aims to understand the hydrothermal sites near the Yonaguni Knoll IV in the Okinawa Trough, and to develop new techniques to study fluid flow patterns for hydrothermal systems and their impact on ore deposits on the seafloor. Hydraulic parameters are important for hydrothermal system studies, but in-situ measurements of fluid migration rates are difficult. Hydrothermal fluids can reach several hundred degrees Celsius, temperatures high enough to perturb hydrothermal fields and pore water migration patterns. Using in-situ temperature data as constraints, we model and synthesize 1-D and 3-D cylindrical hydrothermal models to fit the spatial variations of observed temperature fields. The 1-D modeling uses Péclet number analysis along the conduit. We also construct a 3-D cylindrical model to estimate the temperature and fluid velocity fields using a finite element software. All domains are set to be porous to allow the fluid to flow. The simulation is run until it reaches a semi steadystate solution, allowing both the temperature and velocity fields to stabilize. Results show the dimension of the thermal anomaly zone is likely controlled by advective heat transfer along the vent due to upward fluid flow. We estimate a Péclet number of-1.6, and the vertical fluid flow velocities at these sites are high, approximately 10-6 m s-1 , that is, about 100 m yr-1. This is a spatially averaged estimate over tens to hundreds of meters and does not take into account finer-scale venting, which may be very heterogeneous. The results of this work may help estimate the quantity of metal elements transported through pore fluid migration at modern hydrothermal sites.
In this study we investigate the effect of prior seismic shaking on the monotonic shear strength ... more In this study we investigate the effect of prior seismic shaking on the monotonic shear strength of saturated Ottawa Sand 20/30. We perform a series of stress-controlled undrained cyclic triaxial tests with different seismic intensities intentionally without causing failure, followed by drainage of the excess pore pressure and an undrained monotonic loading test to determine the undrained shear strength. The experimental data show that small to moderate seismic events that do not fail the specimen can significantly increase undrained shear strength without much change in relative density. One prior seismic event with peak ground acceleration ~ 1.3 m/s 2 may increase the undrained shear strength of a specimen at ~ 10 m depth by around 30%. The results also show that as the intensity of the shaking increases, the increase in the monotonic shear strength increases. However, the strengthening effect does not increase with the number of seismic events although a small degree of global densification in the sample is observed. The results of this paper will help assess the change in static slope stability after a single or multiple small to moderate events occurred without causing initial instability.
Terrestrial, Atmospheric and Oceanic Sciences, 2019
This paper aims to understand the hydrothermal sites near the Yonaguni Knoll IV in the Okinawa Tr... more This paper aims to understand the hydrothermal sites near the Yonaguni Knoll IV in the Okinawa Trough, and to develop new techniques to study fluid flow patterns for hydrothermal systems and their impact on ore deposits on the seafloor. Hydraulic parameters are important for hydrothermal system studies, but in-situ measurements of fluid migration rates are difficult. Hydrothermal fluids can reach several hundred degrees Celsius, temperatures high enough to perturb hydrothermal fields and pore water migration patterns. Using in-situ temperature data as constraints, we model and synthesize 1-D and 3-D cylindrical hydrothermal models to fit the spatial variations of observed temperature fields. The 1-D modeling uses Péclet number analysis along the conduit. We also construct a 3-D cylindrical model to estimate the temperature and fluid velocity fields using a finite element software. All domains are set to be porous to allow the fluid to flow. The simulation is run until it reaches a semi steadystate solution, allowing both the temperature and velocity fields to stabilize. Results show the dimension of the thermal anomaly zone is likely controlled by advective heat transfer along the vent due to upward fluid flow. We estimate a Péclet number of-1.6, and the vertical fluid flow velocities at these sites are high, approximately 10-6 m s-1 , that is, about 100 m yr-1. This is a spatially averaged estimate over tens to hundreds of meters and does not take into account finer-scale venting, which may be very heterogeneous. The results of this work may help estimate the quantity of metal elements transported through pore fluid migration at modern hydrothermal sites.
This dissertation investigates the phenomenon of "seismic strengthening", which can be ... more This dissertation investigates the phenomenon of "seismic strengthening", which can be relevant to submarine landslide studies in active continental margins. Given that seismic strengthening is complex in nature and highly variable in different types of mixed sediments in the marine realm, this dissertation focuses on two types of siliceous soils only: natural diatomaceous mud offshore Japan, and a silica sand standard from the USA for generic laboratory testing. These materials underwent multi-methodological geotechnical testing, and the results are presented in three manuscripts in collaboration with partner institutions in Norway, France and Austria. The first manuscript of this study presents a comprehensive analysis of the sedimentology and geotechnical properties of four recently acquired sediment cores (5 meters long) on the continental slopes adjacent to the Japan Trench and Nankai Trough. We observe unexpectedly high undrained shear strength and apparent overconso...
In this study we investigate the effect of prior seismic shaking on the monotonic shear strength ... more In this study we investigate the effect of prior seismic shaking on the monotonic shear strength of saturated Ottawa Sand 20/30. We perform a series of stress-controlled undrained cyclic triaxial tests with different seismic intensities intentionally without causing failure, followed by drainage of the excess pore pressure and an undrained monotonic loading test to determine the undrained shear strength. The experimental data show that small to moderate seismic events that do not fail the specimen can significantly increase undrained shear strength without much change in relative density. One prior seismic event with peak ground acceleration ~ 1.3 m/s2 may increase the undrained shear strength of a specimen at ~ 10 m depth by around 30%. The results also show that as the intensity of the shaking increases, the increase in the monotonic shear strength increases. However, the strengthening effect does not increase with the number of seismic events although a small degree of global den...
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Papers by Ting-Wei Wu