Corine Frischknecht

Seismic Soil Effect in an Embanked Deep Alpine Valley: A Numerical Investigation of Two-Dimensional Resonance

This paper presents a 2D numerical study of the seismic response of a deep embanked sediment-filled valley (Rhone valley, Switzerland) to incident SH and SV waves. The 2D modeling is performed with a program developed by Pedersen et al. (1995), based on the IBEM technique. This valley exhibits variation in subsurface geometry. Therefore, three representative cross-sections are considered in order to investigate the influence of symmetric and asymmetric morphology on the seismic response. Comparison between 1D and 2D modeling shows that both resonant frequencies and amplification values differ to a large extent. Looking in the central part of the valley, the 2D predicted amplification value is about twice higher than in the 1D case, whereas the fundamental resonant frequency value is higher by a factor of 1.7. Moreover 2D modeling results show that this valley presents a twodimensional resonance due to its shape ratio and shear-wave velocity contrast. The main characteristic is the unchanged position of the fundamental resonance frequency, regardless of the surface site along the cross-section under consideration and the incidence angle. However, the asymmetric cross-section induces a specific spectral amplification pattern at the surface site and along the cross-section, which is also a function of the direction of incidence of the seismic wave. Such results show the importance of considering the morphology of a deep and narrow valley when investigating the local seismic response.

A fast GIS-based risk assessment for tephra fallout: the example of Cotopaxi volcano, Ecuador-Part II: vulnerability and risk assessment

In order to develop efficient strategies for risk mitigation and emergency management, planners require the assessment of both the expected hazard (frequency and magnitude) and the vulnerability of exposed elements. This paper presents a GIS-based methodology to produce qualitative to semi-qualitative thematic risk assessments for tephra fallout around explosive volcanoes, designed to operate with datasets of variable precision and resolution depending on data availability. Due to the constant increase in population density around volcanoes and to the wide dispersal of tephra from volcanic plumes, a large range of threats, such as roof collapses, damage to crops, blockage of vital lifelines and health problems, concern even remote communities. To address these issues, we have assessed the vulnerability and the risk levels for five themes relevant to tephra fallout: (1) social, (2) economic, (3) environmental, (4) physical and (5) territorial. Risk and vulnerability indices for each theme are averaged to the fourth level of administrative unit (parroquia, parish). In a companion paper, Biass and Bonadonna (this volume) present a probabilistic hazard assessment for tephra fallout at Cotopaxi volcano (Ecuador) using the advection-diffusion model TEPHRA2, which is based on field investigations and a global eruption database (Global Volcanism Program, GVP). The scope of this paper is to present a new approach to risk assessment specifically designed for tephra fallout, based on a comprehensive hazard assessment of Cotopaxi volcano. Our results show that an eruption of moderate magnitude (i.e. VEI 4) would result in the possible collapse of ∼9,000 houses in the two parishes located close to the volcano. Our study also reveals a high risk on agriculture, closely linked to the economic sector, and a possible accessibility problem in case of an eruption of any size, as tephra is likely to affect the only major road running from Quito to Latacunga (Panamerican Highway). As a result, this method fits into the ongoing effort to better characterize and evaluate volcanic risk, and more specifically the risk associated with tephra fallout. Although this methodology relies on some assumptions, it can serve as a rapid and efficient starting point for further investigations of the risk level around explosive volcanoes.

Toward Seismic Microzonation—2-D Modeling and Ambient Seismic Noise Measurements: The Case of an Embanked, Deep Alpine Valley

In the Sion area of Switzerland, part of a deep, embanked sediment-filled valley, investigations on soil site effects have been conducted using two independent methods. Two-dimensional (2-D) modeling was performed with a program based on the Indirect Boundary Element Method (Pedersen et. al. 1994). Numerical simulations allow taking into account the subsurface geometry of the valley and its peculiar characteristics, such as a variable shape ratio and a high shear-wave velocity contrast. The H/V method has been applied on ambient seismic noise measurements recorded on sites as close as possible to the 2-D modeling. This technique allows capturing the fundamental resonant frequency of the deepest sediments as well as identifying the existence of a surficial deposit. Both approaches agree on the fact that the fundamental resonant frequency of the valley is below 1 Hz. The amplification level of the predominant frequency obtained with numerical simulation is up to two times higher than the one given by the H/V ratio. These results provide the basis for further investigations in order to resolve differences.

Vulnerability Assessment and Risk Mitigation: The Case of Vulcano Island, Italy

This paper reports on a comprehensive vulnerability analysis based on a research work developed within the EC ENSURE Project (7FP) dealing with the assessment of different volcanic phenomena and induced mass-movements on Vulcano Island (S Italy) as a key tool for proactive efforts for multi-risk mitigation. The work is mainly focused on tephra sedimentation and lahar hazards and related physical, systemic and mitigation capacities.

Participatory Approach For Flood Risk Assessment: The Case Of Yeumbeul Nord (YN), Dakar, Senegal

Flooding has emerged lately as a major threat for poor people in the suburban area of Dakar (Senegal), a densely populated area. In this region, flood events are mainly controlled by rainfall intensity and groundwater level fluctuations. To assess flood risk at a neighbourhood level, accurate data on flood extent, exposure and vulnerability is required. The objective of the present study is to obtain these data combining remote sensing data and local knowledge. Field work data were collected through interviews with inhabitants. About 500 respondents were surveyed with the support of a local association and very detailed mapping was carried out to clearly identify elements at risk. Information collected at the household level concerns: 1) socio-economic data, 2) information on the property, 3) flooded houses and 4) strategies of risk reduction. This research demonstrates that local knowledge is an important tool to obtain accurate data useful for understanding flood hazard and vulnerability patterns. It provides quantitative data at the household level that can be used to complement conventional GIS and remote sensing data.