Multiple Observations of the Prompt Elastogravity Signals Heralding Direct Seismic Waves

The recent first observations of the prompt elastogravity signals (PEGS) induced by the 2011 M-w=9.1 Tohoku megathrust earthquake generated interest in how these signals might best be observed, especially for lower-magnitude events. Simulations of these signals preceding the direct P wave, for different depths and focal mechanisms, first reveal that shallow strike-slip earthquakes offer a better detection potential than subduction megathrust earthquakes. Consistently, clear PEGS are observed at several broadband seismometers during the 2012 M-w=8.6 Wharton Basin earthquake. Due to their short source durations, large deep earthquakes are then shown to have an even larger detection potential, confirmed by the successful seismological observations for the 2018 M-w=8.2 Fiji and 1994 M-w=8.2 Bolivia earthquakes. Detection is even improved when an earthquake is recorded by a number of good-quality stations, allowing for stacking techniques. Thanks to the deployment of the USArray network across Alaska, the recent 2018 M-w=7.9 off-Alaska earthquake (strike slip) is thus observed with an excellent signal-to-noise ratio. Array stacking is also shown to reveal the PEGS induced by the large 2010 M-w=8.8 Maule megathrust earthquake, for which individual observations are impeded by the long-lasting radiation generated by a distant large earthquake. As a whole, we show new observations and successful modeling of the PEGS for five earthquakes in the 7.9-8.8 magnitude range. These findings demonstrate that, even without considering promising future instruments, the PEGS detection is not restricted to exceptional events, confirming their potential for magnitude and focal mechanism determination within the few minutes following a large earthquake.