Objectives and research hypothesis
Between 1998-2017, earthquakes caused nearly 750 000 deaths globally; more than half of all deaths related to natural disasters (World Health Organization). Such significant human and/or economic losses occurred by earthquakes near urban areas such as Haiti (2010), Christchurch (NZ, 2011), Nepal (2015), Kumamoto (Japan, 2016), Amatrice-Norcia (Italy, 2016). The general objective of this project is to respond to the needs of low-probability-high-consequences (LPHC) seismic risk in urban areas subjected to nearby earthquakes faults, by incorporating recent research progresses made in earthquake data observation and numerical modelling beyond the current seismic risk assessment framework.
Specific objectives, organized as work packages and related tasks in the project, are summarized in Figure 1 and explained as follows. Objective 1: near-fault simulation of ground motion (including spatial variability). Near-field ground motion is complex and spatially heterogeneous due to the fault geometry, near-surface geology, off-fault plasticity and secondary faults (special volume by Dalguer et al., 2020). Impulsive pulses observed near the fault from past earthquakes (e.g. 2003 Denali, 2016 Kumamoto), also known as “killer pulse” during the 1995 Kobe earthquake, reflect the mechanical properties of the causal earthquake fault. Therefore, we need to take into account its mechanical behavior in three-dimensional Physics-Based Simulations (PBSs) when predicting the near-source ground motion (e.g. Aochi and Ulrich, 2015). Objective 2: Soil/Structure time dependent behavior. Recent robust analysis of seismic data recorded in soils and buildings found that the media properties such as the frequency content and shear wave velocity vary with time (Astorga et al., 2018 for a building; Bonilla et al., 2019 for a sedimentary site, both during the Mw9 Tohoku 2011 earthquake). Often related to primary co-seismic nonlinear response of the uppermost soil layers or the structural elements of buildings, the damaging and healing time-dependent processes of the medium are not clear and must be deeply investigated in relation to the particular near-field ground motion features. Objective 3: Near-fault seismic risk analysis for town. The near-fault characteristics should be considered in seismic risk assessment, particularly related to the spatial variability of the induced ground motion including cross-coupling with the buildings. Seismic ground motion in urban environment will be analyzed, with a special focus on the rotational ground motion and the impulse wave impact on the building response (linear and nonlinear) and the contamination of the ground motion features due to the presence of neighboring buildings. Finally, rotational motion and spatial variability will be integrated to estimate their impact for future near-fault earthquake scenarios in Quito (Ecuador) as a target.
Our common research hypothesis is that the medium and materials are damaged and their properties vary with time during strong ground shaking or repeated seismic excitations. This can be detected by analysis of seismic records using advanced time-frequency signal processing tools. However, it is not clear where such nonlinear processes come from, other than the site effect in the near surface and the classical nonlinear response of buildings. We are particularly interested in two prevalent near-fault ground motion features (a) impulsive wave and (b) rotational motion and their impact on this nonlinear response. Both features will be considered commonly in this project beyond the current practice, and the abundant data sets of recent earthquakes will allow us to analyze more precisely these phenomena in space and time. The intensive numerical models including nonlinear behavior will help to assess the realistic near-fault seismic excitation levels and their impacts. Quito (Ecuador) is known as high-seismic risk city, and has often been targeted in previous ANR projects (Andes du Nord, 2008-2012; and REMAKE, 2015-2019). We can benefit from the past experiences and will provide a physics-based seismic risk assessment to the city.
