The challenge of earthquake ground motion estimation

A primary goal of any seismic hazard study is to estimate how the ground moves in response to an earthquake at specific locations of interest. The seismic cycle (recurrence period for a characteristic, damaging earthquake to occur at the same point on a plate boundary or along an active fault) is much longer than our lifetimes or that of an infrastructure. In addition, the earthquake phenomenon is too large compared to the physical experiments that we can undertake in controlled laboratory experiments. This is why it is important for current seismic hazard and risk evaluation to take advantage of numerical procedures and also to understand the observed phenomena and the recorded data during the recent large earthquakes in the world. Indeed, as seen in recent large earthquakes, like the 2004 Sumatra event, earthquake disasters are now a worldwide problem. Furthermore, when a building is subjected to ground shaking from an earthquake, elastic waves travel through the structure and the building begins to vibrate at various frequencies depending on the stiffness and shape of the structure. Earthquakes generate ground motions over a wide range of frequencies, from static displacements to tens of Hertz. Most structures have resonant vibration frequencies in the 0.1 Hz to 10 Hz range. A structure is most sensitive to ground motions with frequencies near its natural resonant frequency. Damage to a building thus depends on its properties and the character of the earthquake ground motions, such as peak acceleration and velocity, duration, frequency content, and spatial coherence, among other things. Realistic ground motion time histories are needed for nonlinear dynamic analysis of structures to engineer earthquake-resistant buildings and critical structures, such as dams, bridges, and lifelines. The estimation of earthquake ground motion relies on state-of-the-art studies concerning the source, path and site effects as a whole. However, due to the lack of knowledge of geological and geophysical information of these terms, one must take into account the uncertainties for the whole earthquake process when performing a reliable seismic hazard analysis. Their study constitutes a challenge for both seismology and earthquake engineering communities, and this is the topic of the present talk.