Observations and modeling of the elastogravity signals preceding direct seismic waves

Abstract

Gravity gets into the earthquake game Earthquakes generate large movements of mass, which slightly change the gravitational field. Unlike the elastic waves that propagate from the earthquake, the gravity perturbations travel at the speed of light. Vallée et al. have finally observed these gravity perturbations in seismometer records from the great Tohoku earthquake in Japan in 2011. The signal would have allowed an accurate magnitude estimation in minutes, rather than hours, for this catastrophic earthquake. Science , this issue p. 1164

Multiple Observations of the Prompt Elastogravity Signals Heralding Direct Seismic Waves

“…For all these detected earthquakes, observed PEGS waveforms are in good agreement with modeled PEGS provided the appropriate source parameters (location, focal mechanism, and moment rate function) are used. This simultaneously supports the observations, confirms the accuracy of the modeling approaches described in Vallée et al () and Juhel, Montagner, et al (), and shows the PEGS sensitivity to earthquake characteristics.…”

“…In all these examples, we will show that waveform modeling always supports the observations, confirming that the approach taking into account both the direct gravity effect and the elastic gravity‐induced effect (Vallée et al, ; see also Juhel, Montagner, et al, ) is an accurate formalism. The global scope of this study is therefore twofold: showing new observations of PEGS not restricted to an exceptional event and demonstrating the potential of retrieving earthquake source parameters from these early signals.…”

“…Vallée et al () showed that the PEGS at frequencies higher than ≃ 0.002 Hz can be computed as the difference of two terms, a direct gravity perturbation Δg and a gravity‐induced elastic acceleration . Such formalism enabled us to accurately model the perturbation induced by the Tohoku earthquake at single station locations, using the discrete wavenumber AXITRA code (Cotton & Coutant, ).…”

“…In order to simulate the PEGS waveforms, the 2012 Wharton Basin earthquake is thus modeled by a point source (located at the hypocentral coordinates) whose focal mechanism and STF are determined by the SCARDEC method (Table and Figure a). We then follow the procedure described in Vallée et al (): The direct elastic displacement wavefield u is computed in the Mantle PREM model (Dziewonski & Anderson, ) using the AXITRA method (Cotton & Coutant, ) and Earth flattening corrections (Muller, ). This provides access to the direct gravity perturbation Δg through integration over the time‐evolving volume affected by u (Dahlen & Tromp, ).…”

“…This provides access to the direct gravity perturbation Δg through integration over the time‐evolving volume affected by u (Dahlen & Tromp, ). As shown by Vallée et al (), the PEGS are the difference between Δg and the gravity‐induced elastic ground acceleration , the latter term being calculated with the AXITRA method by considering that Δg also acts as a body force in the medium surrounding the observation point. The resulting synthetic vertical PEGS are finally processed with the same filtering as the observed waveforms and shown with the black line in Figure b.…”