Modelling seismic ground motion including source and site complexity for better knowledge of past earthquakes

​

Start in : As soon as possible

Workplace : Paris, France

Duration : 18 months

Historical earthquake catalogues are one of the building blocks for the assessment of seismic hazard. In spite of many years of research in the archives, many earthquakes remain poorly known. New sources of information are hence required. Among these, historical buildings are witnesses of natural catastrophes recorded in their walls as structural disorders, repairs, restorations. The ambition of the “ArChaeology, inventory of RecOnstruction, Seismology and Structural engineering” (ACROSS) ANR Project (PI Maria Lancieri, IRSN) is to study past earthquakes using buildings as “stone seismometers”, analysing the seismic ground motions required to explain building repairs/disorders, or their absence. The ACROSS project aims at developing a methodology to demonstrate that archaeological characterization of post-seismic repairs on buildings can be successfully used to infer key ground motion and earthquake source characteristics of historical earthquakes.

The site under study is located in Tuscany, Italy. Mugello is an intramontane basin bordered by two large antithetic normal fault systems (“Ronta” and “Sieve”) characterized by the same extensional regime causing the recent strong Italian earthquakes (i.e. Norcia - Amatrice 2016 Mw 6,5 ~200 km south along the Apennine chain), but with a lower seismicity rate. The strongest known events in Mugello occurred in 1542 (Mw~6) and in 1919 (Mw~6.3). Both earthquakes induced extensive damages over the whole region. On the basis of historical macroseismic intensities, the 1542 event is located in the north-western part of the basin, while the 1919 one is in the south-west. However, the fault system on which these earthquakes occurred is still a matter of debate. These two events led to disorders, repairs, restorations to five bell-towers located in the area, that will be studied by historians, archeologists, petrologists and civil engineers in the framework of the project.

The candidate PostDoc will be involved in the numerical modelling of ground motion generated by these two past earthquakes, especially at the location of the five bell-towers. A representative model (geometry and mechanical properties) of the Mugello area will be defined on the basis of data on faults and basin properties coming from scientific publications, reports, and national/regional databases. Eventually, additional data will be acquired in the field to complement basin characterization. Uncertainties related to this model will be defined as well.

The main challenges and tasks of the candidate PostDoc will be the following:

This first step aims at determining the capacity of the basin model to reproduce observed seismic ground motion, by comparing available seismic ground motions recorded in the area (e.g., 2008-2009 and 2019 local sequences, teleseismic events) with seismic ground motions computed in the model, especially inside the basin. Depending on these comparisons, the model of the basin could be updated.

This task is focused on the exploration of the physical parameters of the fault model that could have led to the 1542 and 1919 earthquakes, such as 1) the fault geometry (based on different geometries at depth for the two fault systems), 2) the fault kinematic model parameters (spatio-temporal slip evolution, stress drop & rupture velocity). At the end of this phase, a set of plausible scenarios of the 1542 and 1919 earthquakes will be chosen in accordance to the seismotectonic context of the area and other information, such as macroseismic intensity of past earthquakes.

The last task is the computation broadband 3D ground motions emitted by the chosen fault rupture scenarios and propagating inside the area including the basin, especially at the 5 bell-towers studied within the project. If needed, the high frequency part of the signal will be generated using a different technique and combined with the low frequency part coming from the wave propagation 3D numerical model.

Throughout this study, the post-doc will discuss with the ACROSS partners. In IRSN group, the candidate will benefit from strong interactions with a postdoc working on earthquake physics modeling, belonging to a neighboring undergoing project focused on central Italy faults, and with another postdoc involved in simulations of 3D wave propagation in a French basin.

MissionsOpportunity to model the seismic ground motion taking into account source rupture complexity and wave propagation in a 3D sedimentary basin. True integration of the ground motion models in a knowledge chain spanning between geology (build 3D model from inputs) and the damaged structures (outputs will be used to shake numerical models of historical buildings). Collaborate with a multidisciplinary team (historians, archeologists, petrologists, civil engineers, geologists, seismologists, mathematicians) from Italy and France. Learn to solve complex problems using high performance computing facilities

Profile

PhD - experience in numerical simulations of wave propagation or rupture dynamics in 3D models