Determination of the Earth's Structure Based on Long-Period Surface Wave Recordings of Tidal Gravimeters

Abstract:

The use of seismic methods for gravimetric recordings of earthquakes can allow improving studies about the structure of the Earth's mantle. Tidal gravimeters are capable to detect surface waves of periods even up to 500-600 s, while a typical broad-band seismic sensor can detect them up to the periods of 200-300 s due to its mechanical limitation. The capability of using tidal gravimeters as long period seismometers has already been analyzed since the late '90s. These researches were mainly focused on the examination of the power spectral density of noise level of instruments (both gravimeter and seismometer) and the analysis of normal modes of the largest earthquakes recorded by gravimeters. The first attempt to apply seismic methods to gravimetric recordings, namely to calculate dispersion curves of Rayleigh surface waves was carried out lately.

Three sites located in Western and Central Europe (Black Forest, Membach and Rochfort), where broadband seismometer and tidal gravimeter are collocated have been selected. All data has been downloaded from the Incorporated Research Institutions for Seismology (IRIS) database. The consistent database of seismic (only vertical component) and gravimetric recordings of earthquakes has been created. Next, the group-velocity dispersion curves of fundamental-mode Rayleigh waves were calculated for each sensor by applying the multiple filter technique. The averaged dispersion curves for particular regions were inverted by weighted linear inversion and Monte Carlo inversion methods, which resulted in distributions of shear-wave seismic velocities with the depth in the Earth’s crust and mantle.

Resolution tests (resolution matrices, sensitive kernels, checkerboard tests and probability distribution) show that tidal gravimeters, because of their capabilities to record long-period Rayleigh waves (up to 500-600 s), can give us the direct information about the Earth’s structure even down to the depth of 1500 km. So, tidal gravimetric recordings of earthquakes could be complementary to the vertical component of seismic recordings, as in the case of high-rate GPS to horizontal ones.

This work was done in the research project No. 2017/27/B/ST10/01600 financed from the funds of the Polish National Science Centre.