Observation and origin of the elastogravity signal preceding direct seismic waves. The map shows the location of the seismometers (triangles) detecting the prompt signals just after the beginning of the Tohoku earthquake (Japan, 11 March 2011, magnitude 9.1), indicated by the black star. We focus here on one of the stations (MDJ), located in north-east China, 1280km away from the Tohoku earthquake. At such distances, direct seismic waves arrive about 165 s after the earthquake starts, as shown in the inset reproducing the MDJ vertical seismogram. However, a clear, even if much weaker, acceleration signal is detected by the seismometer before the direct waves arrival. The origin of such signal can be understood by considering a time after the earthquake onset but before the arrival of the direct seismic waves. For example, about 55 s after origin time, direct waves have propagated inside the volume shown by the grey area, but are still far from arriving at MDJ station. However, inside this volume, seismic waves have caused compressions and dilations of the medium (as further indicated in the bottom cross-section), and the global contribution of all such elements whose mass has changed gives rise to a gravity perturbation, immediately detected by the seismometer (direct effect). The gravitational field is also modified everywhere in the Earth, and each of the elements affected by these perturbations is a secondary source of seismic waves (induced effect). In the green volume around the seismometer, this secondary seismic wavefield arrives before the direct waves. The seismometer therefore records a prompt elastogravity signal, due to the direct and induced effects of the gravity perturbations. This material relates to a paper that appeared in the 1 December 2017, issue of Science, published by AAAS. The paper, by M. Vallée at Institut de Physique du Globe de Paris in Paris, France, and colleagues was titled, "Observations and modeling of the elastogravity signals preceding direct seismic waves."
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