LTV - SourceMod4PSHA Construction of a seismic source model for the Lower Tagus Valley to use in future probabilistic seismic hazard assessment
When aiming for the probabilistic earthquake hazard assessment (PSHA) for a given region, one studies the sources of the seismic waves which might cause damage, their propagation through the earth, and their potential ampliﬁcation at the sites of interest.
This project focuses on the ﬁrst aspect (the construction and the evaluation of models for the seismic sources relevant for the
Lower Tagus Valley (LTV)), while carrying out the hazard analysis to the production of probabilistic hazard maps. Large earthquakes happened in the past on the SW margin of Portugal and were strongly felt in Lisbon and in its surrounding region (in 1755, moment magnitude Mw 8.5 and on Feb. 28 1969, Mw 7.3). However, large events can also initiate in the LTV itself (see the Mw 6 to 7 earthquakes in 1344, 1531, and 1909). Geophysically-determined fault offsets also point to the recurrence of Mw 6 earthquakes every 2 to 5 thousands of years. Because of the short distances between sites and sources within the LTV, information on the
lateral extension of faults or seismic source zones will be essential, to use with Ground Motion Prediction Models. The
challenges will be to collect, combine, and possibly complement information of various types: the historical and
instrumental earthquake catalogues, mapped active faults, geophysical subsurface data on fault geometry and offsets, and
paleoseismological information as they become available. A probabilistic modeling framework will be designed and used to 1)
study the correspondence between faults and earthquakes, and 2) to test the pertinence of different ways of characterizing the
sources (faults, diffuse zones, etc.) in terms of the robustness of the resulting hazard maps (i.e., maps of expected ground
shaking level for given return periods). Forward numerical fault modeling will also be instrumental in 1) studying the geometry of
the faults and the tectonic regimes with respect to the boundary conditions on the region (indeed, discrepancies in the numerical
models might indicate where probable, yet unknown faults might lie), and 2) propose physics-based renewal models for
characteristic strong earthquakes. The project will propose a seismicity model ready for use in a PSHA for the LVT, and will identify the inputs the estimation of PSHA in the LTV are most sensitive to. The results of this research will be useful for any region of the world displaying similar seismic activity, and similar tectonic patterns.
Objectives, activities and expected/achieved results
We want to construct and design ways to use a source model in probabilistic seismic hazard assessment (PSHA) for a slowly
deforming region with rather rare but damaging events. We aim for a composite object (catalogues, faults or seismic zones,
physics-based probability density functions (pdf) for recurrence of each magnitude) with its uncertainties as a possible alternative
to logic trees with branches weighted by "expert panels", and mathematically-based only renewal models, as found in the
PSHA is of great societal importance for a large city such as Lisbon and its surrounding region. The project will propose a seismicity model ready for use in aPSHA for the LVT, and will identify the inputs the estimation of PSHA in the LTV are most sensitive to. The results of this research will be useful for any region of the world displaying similar seismic activity, and similar tectonic patterns.
We will publish and present our work in International Journals and Conferences, in particular the World Conference in Earthquake Engineering to be held in Lisbon in 2012.
We also plan two types of outreach actions in the framework of this project, and more particularly during
Task 1 on the historical seismicity and the gathering of the seismicity catalogue. They have to do with the transmission of individual and collective memory and experience to the younger generation, and with raising the awareness of the population towards seismic hazard.
1- with schools of the LTV region.
2- with the “Lars dos Idosos” in the LTV as well