Immediately following Japan's 2011 Tohoku earthquake, while seismic waves still traveled to seismic stations to offer insight into the event's magnitude, seismographs recorded a gravity change reflective of this value, researchers report. Going forward, they say, such prompt "elasto-gravity" signals could be used to make earlier estimates of large earthquake magnitudes - an idea that represents a new approach for examining tremblors that massively rupture the Earth. The elastic waves generated by earthquakes are a well-known phenomenon, carrying valuable information about earthquake characteristics. P waves, in particular, have been thought to be the fastest way to transmit seismic information, though some seismologists have speculated otherwise. Here, knowing that, like any major disruption to mass, earthquakes and their elastic wave fields cause perturbations to Earth's gravitational field, Martin Vallée and colleagues sought to observe these gravity-induced signals, which travel at the speed of light (far faster than P waves). By closely evaluating seismic data recorded during the Tohoku earthquake, they finally did so, finding these signals to consistently arrive to seismic monitoring stations before their seismic wave counterparts. If this approach had been available in 2011, say the authors, the fact that the moment magnitude 9.1 Tohoku quake had a magnitude larger than 9 could have been detected within minutes. As it were, the near-real-time magnitude provided by the Japan Meteorological Agency was 7.9, corrected three hours later to 8.8 (again, an underestimation).
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Science