Gravimetry, a complementary tool to seismologyfor the study of the Earth’s deep interior:long-period seismic and Slichter modes

Speaker: Severine Rosat
“Knowledge of the Earth’s structure has considerably progressed however, some unsolved questions still remain. For instance, the presence or not of an excess density layer in the lower-most mantle has not been confirmed yet, we have poor information on the stratification of the outer core and the density jump at the inner-core boundary (ICB) is still poorly constrained.

Normal modes are a useful tool to investigate the Earth’s interior as they give us a direct image of the 3D-density structure. Particularly, below 1 mHz, the splitting of modes is strongly sensitive to the 3D-density structure inside the mantle and inside the core and is directly linked to the 1D-density, giving the possibility to estimate the density profile without any tradeoff on the seismic velocities Vp and Vs.

Below 1 mHz is indeed the frequency range where Superconducting Gravimeters (SGs) perform better than seismometers. In that way, SGs have contributed to the first detection of the degree-one 2S1 spheroidal mode after the 2001 Mw8.4 Peru earthquake and have led to high-resolution splitting analysis of the lowest seismic modes.

Besides the frequency splitting of modes, the coupling of modes below 1 mHz is also very sensitive to the 3D-density structure of the Earth. The example of 0S0 amplitude perturbation due to lateral heterogeneities is shown. This radial mode, so-called “breathing mode of the Earth”, represents a global dilatation and compression of the Earth. In a spherically non- rotating homogeneous Earth, the amplitude of 0S0 is the same everywhere on the surface. When we introduce rotation and ellipticity, an apparent latitude dependency of 0S0 amplitude appears because of the Earth’s flattening. In a more realistic 3D-laterally heterogeneous Earth, because of the coupling between multiplets, 0S0 amplitude depends also on longitude. We try to highlight this geographical perturbation of 0S0 amplitude thanks to SG data as, contrary to seismometers, SGs are precisely calibrated (better than 0.3%). We show the results of 0S0 amplitude analysis after the 2004 Mw9.3 Andaman-Sumatra, 2010 Mw8.8 Maule-Chile and 2011 Mw9 Off-Tohoku earthquakes.

In addition to the long-period seismic band, SGs are also the less noisy instruments in the sub- seismic frequency band, in which the Slichter mode lies. This mode, noted 1S1, corresponds to the translational oscillation of the inner core inside the fluid outer core. Apart from seismic modes for which the main restoring force is elastic, its feedback is Archimedean and hence its period is directly linked to the density jump at ICB. The detection of the Slichter mode would then constrain the density jump but also the viscosity at ICB through its damping. Despite many searches, this mode has never been undoubtedly observed. Theoretical predictions for various excitation mechanisms (large earthquakes, meteoroid impact, pressure flow in the core, surface atmospheric load) have shown that its surface amplitude would merely reach the nanogal level (1 nGal ~ 10-12 g) which is at the limit of detection of SGs. The quest for the Slichter mode is still open.”