[MUSIC] Hello in this sequence will discover how the seismic motion may be observed at the surface of the earth crust from now on we'll name it the seismic ground motion since it represents the motion of the ground surface. Later on in this Mooc, this seismic ground motion will become the seismic excitation applied on civil engineering structures. The seismic ground motion may be characterized by the use of sensors located at the surface. Depending on the type of sensors, we may measure the ground acceleration or the ground velocity with respect to time. The first quake detector in history is probably this chinese jar from the middle age. As you can see, each dragon keeps the ball in his mouth. In case of shaking, one of these balls may fall in one of the frog's mouth. We met us know the direction of the shaking, but very little information about the shaking amplitude. In the 20th century, the first seismometers were developed. A mass, spring, dashpot system allows to record the motion of the mass, which may be easily related to that of the ground. Originally, the recording consisted in burning a paper. Afterwards, the seismic ground motion was written on the rotating cylinder. At the end of the 20th century electronic systems allowed to record the seismic motion in a digital way and then to store it in a large database. Where are these sensors located around the world? Permanent observation networks exist at the regional, or national level. Here is the European network showing stations recording either particle velocities or accelerations of the ground. Here is now one of the Japanese networks named KiK-net. At each of these numerous stations, the surface ground motion is recorded. At depth, in a borehole, a seismological station also records the seismic motion. It is thus called the borehole motion. Periodic networks may also be installed for a detailed study of a specific area for a given time period. It is useful when a seismic crisis occurs. Finally, we may combine various observations as it is the case for the European Plate Observing System. This observing system stores such various data as seismic ground motions, geological and geodetic data and volcanic observations as well. The recording data are gathered in large databases such as the Orfeus database in Europe, the Cosmos database in the US, or the KiK-net database in Japan. The recorded seismic motions maybe then sorted by the end users through dedicated platforms. For example, this seismological station operated by INGV. The Italian Institute of geology and volcanology has been chosen in order to select and download data. The downloaded seismic ground motions allowed to analyze the wave features and the way it propagates. They can also be used as seismic excitations in structural dynamics for numerical computations. But even for experiments on shaking tables. In most of the seismological networks, some sensors are also located in buildings to assess their actual seismic response. For instance, the Millikan Library at Caltech has been continuously instrumented for several decades. It allows to compare the seismic response of a building for various events, but also to monitor the potential damage due to the repeated seismic loadings or even the influence of the aging of the structure. On this plot, the observed fundamental frequency of the Millikan Library progressively decreases with time due to the influence of the repeated seismic shaking. Each frequency drop corresponds to a strong seismic event. Once it has been recorded, the seismic ground motion may be characterized from a seismological point of view. We may consider accelerograms giving the variations of the ground acceleration with respect to time. The particle velocity or the displacement may also be investigated. Instead of the whole motion history, we may consider a single scalar value: the maximum acceleration of the ground. It is named PGA for Peak Ground Acceleration. PGV stands for Peak Ground Velocity and PGD for the maximum displacement. With this single value it is possible to compare one seismic event to the other in terms of maximum, seismic ground motion. The seismic loading fastly varies with time, and we don't know how fast. By computing the Fourier transform of the accelerogram, we may investigate the frequency content of the signal. If the amplitudes are small in the lower frequency range, here below 1 Hz, it will be beneficial for high rise buildings. Conversely, smaller amplitudes in the higher frequency range will be favorable for low rise buildings or industrial equipments. In this case, structures having a fundamental frequency close to 6 Hz will be shaken more strongly. The impact of seismic shaking depends on the number of loading cycles and thus on the shaking duration. Looking at recorded accelerograms, it is not so easy to point out the beginning and the end of the shaking because of the very small peaks. Otherwise, we may define the integral of the squared acceleration. From this monotonic function, we may choose two thresholds for the starting, let's say 5% of the maximum of the shaking, and ending times, let's say 95% of the maximum. The earthquake duration will be the time elapsed between the moment the 5% threshold is reached and that where the 95% is reached. For this accelerogram, the duration of the shaking is thus 22.5 seconds. In this sequence, we have discovered the various ways to observe and characterize the seismic ground motion through sensors, this old fashioned one or much modern electronic wireless devices, the networks, the databases and last but not least, the main features of the ground shaking. Knowing the seismic ground motion for some past earthquakes, can we predict the ground shaking for future events? To do so, we need to analyze seismic wave propagation in geological structures, between the fault and the ground surface.
