Broadband analysis, modelling and applications of seismic noise: from microseisms to Earth normal modes
Seismic noise is recorded by broadband seismometers in the absence of earthquakes. It is generated by the atmosphere-ocean system with different mechanisms in the different frequency bands. Even though some mechanisms have been known for decades, an integrated understanding of the noise in the broadband period band 1-300 s is still missing. The causes of this poor knowledge involve a very limited quantitative knowledge on ocean wave properties at periods shorter than 5 s or longer than 25 s and difficulties in modelling the propagation of seismic waves in 3D earth models that include ocean-continent boundaries. Recent breakthroughs in numerical modelling of ocean waves and seismic propagation modelling are used to solve these problems. In particular we combine on-going developments in ocean wave modelling with extension towards both short and long periods, and advanced seismic modelling technique such as spectral element method to provide the first modelling of broadband seismic noise.
We investigate the different possible mechanisms for modelling the sources in the different period bands. The secondary microseisms (noise of 3-12s period) are generated by the interaction of ocean waves of similar frequencies and coming from opposite direction and we successfully modeled them. Since recently, the modelling of noise at longer period was still an open question. The primary mechanism is generated when waves reach the coast (Hasselman, 1963) but it had never been checked numerically. We have shown recently that this mechanism can explain both the primary microseisms (noise of 10-20s period) when ocean waves hit the coast and the hum (noise of 20-300s period) when infragravity waves interact with the continental shelf. This first modelling will now be improved both by taking into account more accurate source models and 3D seismic propagation.
In order to verify key aspects of wave model parameterizations and noise source theory, we will carry out two types of in-situ experiments. First, we will perform detailed measurements of the wave directional spectrum from a fixed platform in shallow water, which will allow us to refine today's parameterizations of breaking waves, relevant for short period noise. A second experiment, with the year-long deployment of a vertical acoustic antenna in the Indian Ocean, will provide an absolute calibration of modeled noise sources with periods around 5s. These efforts will lead to more robust parameterization for the shape of the short wave spectrum and its impact on satellite measurements of wind, sea level and salinitBesides this work on sources, we will also improve on the seismic propagation modelling by using the spectral element method. This approach will be particularly used to investigate the effect of the ocean-continent boundary on seismic noise amplitude. This combination of calibrated sources and quantitative propagation should remove the need for empirical factors used in today's seismic noise models. We will further investigate whether propagation in 3D media that include bathymetry variability enables to generate Love waves. These waves are observed but the current models do not explain them. We will also model noise body waves to provide the first quantitative comparison between observed and modeled noise body waves that will then be used for improving the tomographic models.
We will analyse seismic data and synthetic seismograms in order to better understand the noise recorded by station on continents, on islands, at the ocean bottom and in the sofar channel. We will provide a well-documented catalogue of the strong noise sources. This catalogue can be useful for all noise related seismic studies. We will use seismic noise for monitoring velocity changes in 2 contexts: magma intrusion related to volcanic activity and earthquakes related to ground water extraction.
The MIMOSA project will thus provide results and models that will serve all geosciences, promoting seismic noise related research on solid Earth structure, ocean and atmosphere dynamics.