Observation of early signals of disruption of the Earth’s gravity field linked to multiple earthquakes
These new observations of gravity disturbances, preceding seismic waves, confirm the potential of these signals for earthquake early warning systems.
When major earthquakes occur, they displace huge quantities of mass. These movements create tiny disturbances in the Earth’s gravitational field, to which seismometers are immediately sensitive. These Prompt elastogravity signals (PEGS) therefore arrive before the seismic waves, and have the potential to improve early warning systems by reducing the time needed to estimate the size of strong earthquakes.
However, the low amplitude of these elastogravity signals has made them difficult to detect: the first observations of PEGS date back only to 2016 and 2017. Now, Martin Vallée and Kevin Juhel, researchers at the IPGP and authors of a study published in February in the Journal of Geophysical Research, are reporting numerous new observations of these signals from five earthquakes with a magnitude of between 7.9 and 8.8, much smaller than the Tohoku earthquake (magnitude 9.1) from which their existence was discovered.
The researchers identified these signals using a multi-stage approach. Firstly, to better understand the configurations in which PEGS are optimally observable, numerical simulations were carried out to assess the impact of the depth and type of earthquake on the expected amplitude of the signals. The results indicate that, at equal magnitudes, shallow stall earthquakes and deep earthquakes are more likely to be recorded than subduction earthquakes. This information guided the authors in their analysis of the recordings of major earthquakes over the last 25 years.
This analysis directly revealed the presence of PEGS in several large earthquakes: the 2012 Wharton Basin earthquake (magnitude 8.6, making it the largest unconducting earthquake ever recorded) and two deep earthquakes of magnitude 8.2 (Fiji, 2018 and Bolivia, 1994).
By combining the observations of several instruments, the researchers were also able to improve the signal-to-noise ratios sufficiently to detect the elastogravity waves of two additional events: the magnitude 7.9 unstuck earthquake off Alaska (2018) and the Maule subduction earthquake (Chile, 2010, magnitude 8.8).
These results show that PEGS observations are not limited to exceptional earthquakes (magnitude > 9). Moreover, in all the cases studied, the observations are very well reproduced by the numerical simulations, and the latter show that the PEGS are naturally sensitive to the overall characteristics of the seismic process. Elastogravity signals therefore have the potential to improve the speed and reliability of early warning systems in many contexts. The two most concrete applications are to speed up emergency response and to more accurately and quickly anticipate the scale of the coming tsunami.
Ref:
- Text translated and adapted from the “Research Spotlight” produced by the AGU on this study
- Vallée and Juhel, Journal Geophysical Research: Solid Earth, doi:10.1029/2018JB017130, 2019
- The article on the 2017 publication in Science: New signals preceding seismic waves: how early gravity perturbations quantify the magnitude of strong earthquakes
