S24B-01 INVITED
Seismic ambient noise imaging and monitoring: a new tool for the investigation of the continuously deforming Earth
The interaction of the ocean and atmosphere with the solid Earth creates an ambient noise field throughout the Earth. Modern seismological networks with continuous recordings document this persistent motion of the surface of the Earth. We can use the continuous noise records to complement the earthquake data that are the traditional/classical basis of seismology. The principle underlying this new approach is that the recordings of the noise are correlated between distant stations. Under some conditions, it is theoretically established that the correlation function reveals the actual response of the Earth between two stations, that is, the Green's function that would be produced by a source at one of the stations. The efficiency and the accuracy of this approach are illustrated by direct comparison of correlation functions with the observed Green's function. A good agreement is obtained for wave shapes, arrival times and amplitude decays. Therefore, we can use noise records to build new extended data sets of dispersion curves that, in turn, can be used for surface wave 3D imaging. Not only do noise based measurements complement the earthquake data in regions where seismicity is lacking, but also they allow for an improved resolution by providing dense coverage of short paths making it possible to consider short period information. Because of the recent developments in geodesy, the Earth appears more and more as a continually deforming body with various time scales. While earthquakes provide snapshots of the Earth at the time of the earthquake, noise-based measurements allow for continuous monitoring. We present examples of monitoring the temporal changes in material properties associated with volcanoes and fault zones. We detect relative fluctuations of velocity smaller than 0.05 %. In the case of the San Andreas Fault zone at Parkfield, California, correlations of the ambient seismic noise show that the seismic velocities were strongly decreased by the 2003 San Simeon and the 2004 Parkfield earthquakes. Comparisons with geodetic observations and occurrence of non-volcanic tremors suggest that the measured seismic velocity changes are related to the co-seismic damage in the shallow layers and to deep co-seismic stress change and post-seismic stress relaxation within the fault zone and its surroundings. Analysis of ambient-noise offers the opportunity to produce new measurements of seismic velocities for the present-day refined tomography techniques, in the perspective of the high resolution imaging required to address fundamental questions of the dynamics of the Earth's interior. Ambient noise monitoring is also a seismological tool to investigate the temporal variations of the Earth's material properties at various time- scales, including tectonic transient deformations, response to underground industrial activities, hydro- meteorological effects, etc. The continuous recordings of the ambient noise open new opportunities for seismology to contribute to other fields of research.