Measuring waves from far away using microseisms

The paper by Meschede et al. has just appeared in the Journal of Geophysical Research . It is a new step in the project "MIcroseism Modelling for Oceanography and Seismology Applications" funded by  Agence Nationale de la Recherche, a partnership between IPGP and Ifremer. This work combines a detailed numerical modeling of ocean waves, and the analysis of networks of seismometers. This is the first study that analyses so much data over several years, and it shows that the theory is right: ocean waves in opposing direction excite seismic waves... this was well known for seismic Rayleigh waves. This time we also verified that the amplitude is a function of the water depths where the ocean waves are.  Thanks to body waves we can map the sources of microseisms, which we could not do with Rayleigh waves, and thus map the ocean wave properties. The seismic data will also be used to study the structure of the solid Earth, which is usually done with Earthquakes. Also, seismic waves recorded since the 1970s can be used to improve our knowledge of the wave climate and variability.

These body waves were first detected in the 1960s. A first analysis of source locations was done at Ifremer by Mathias Obrebski in 2013, following the theory of body wave generation by Ardhuin et Herbers (2013). A first "order of magnitude" validation was performed on one single event by  the IPGP team (Stutzmann et al. 2016), using a ocean wave to seismic wave conversion model by Lucia Gualtieri (Gualtieri et al. 2014). This time, several years have been analyzed using several networks.

Although Rayleigh waves have amplitudes of a few microns, the body waves only shake the ground by a few tens of nanometers. These are really very small motions.

The figure below shows the distribution (known as spectrum) of body wave energy as a function of their frequency. The double hump in  the measured data is caused by the water dephts. Indeed, the ocean rings like an organ pipe with frequencies and overtones that correspond to acoustic waves with  a wavelength that is 1/4, 3/4 ... of the water depth.

Thus the properties of waves and the seafloor topograhy lead to microseism amplitude and frequency that varies in space and time. This is very well illustrated in this animation by Mathias Meschede h:ttps://youtu.be/EnEgeGOT724