New signals preceding seismic waves: how early gravity perturbations can quantify the magnitude of strong earthquakes
Earthquakes abruptly change the balance of forces in the Earth at rupture, generating seismic waves that propagate through the Earth's crust and mantle at speeds of between 3 and 10 km/sec. These potentially devastating waves are the main manifestations of earthquakes, and since the earliest days of seismology they have made it possible to locate and quantify them. However, when it comes to rapidly determining the magnitude of an earthquake - particularly important for anticipating the arrival of a possible tsunami - seismologists are faced with the incompressible delay associated with the propagation time of the waves before they reach the seismometers.
Elastogravitational signal
Publication date: 01/12/2017
Observatories, Press, Research
Related observatories : GEOSCOPE Observatory
Related themes : Natural Hazards
But these seismic waves are also the source of another physical phenomenon: as soon as they are generated by the earthquake – and then during their propagation – they cause dilations and compressions of the medium, the cumulative effect of which slightly but immediately (1) disturbs the gravity field throughout the Earth. And although present in the complete theoretical equations of seismic motion, this early signal linked to gravity has only very recently been considered. In an initial analysis in 2016 (2), such a signal was detected on a gravimeter during the Tohoku earthquake (Japan, March 11th 2011, magnitude 9.1).
In a study published in the journal Science on December 1st 2017, researchers from the IPGP, Paris Diderot University, CNRS and Caltech, have worked on this same earthquake again, and have enabled us to go much further in observing and understanding the phenomenon. Firstly, the signal was observed on a dozen broadband seismometers at distances of between 500 and 3000km from the earthquake. The signal is measured with a high signal-to-noise ratio. The quality of the signals then made it possible to examine their precise origin. The team of researchers thus understood that the shape of the signals is due to a combination of two effects: a direct effect due to the gravity disturbance that displaces the mass of the seismometer, and an induced effect due to the seismic waves generated by the gravity disturbances in the vicinity of the seismometer. Similar to an earthquake, but on a much smaller scale, a change in gravity disturbs the balance of forces in the earth’s environment, causing seismic waves.
Taking these two effects into account gives a very good explanation of the data and also shows that the early signal linked to gravity is very sensitive to the magnitude of the earthquake. In the case of the Tohoku earthquake, its magnitude of over 9 could thus have been determined within minutes of its occurrence, much earlier than can be done using conventional methods. The future challenge is to be able to use this information for much smaller earthquakes. For an earthquake of magnitude 8, the Earth’s seismic noise already blinds current instruments to the weak signal linked to gravity disturbances. Several technologies, including some inspired by instruments developed for the detection of gravitational waves, are being considered to take a new step forward in the detection of these precious signals.
Notes:
- 1 – More precisely, at the speed of light (300,000km/s).
- 2 – J.-P. Montagner et al, Nat. Commun. 7, 13349 (2016).
The map shows the location of seismometers (triangles) that detected an early signal following the Tohoku (star) earthquake on March 11th 2011 (Japan, magnitude 9.1). The focus here is on a station in north-eastern China (MDJ), located 1280km from the earthquake. At these distances, the direct seismic waves arrive around 165s after the original time, as shown in the inset. However, although of much lower amplitude, a clear signal was detected by the seismometer before these direct waves.
The origin of this signal can be understood by looking at the time between the time of origin and the time of arrival of the waves: for example, around 55s after the earthquake was triggered, the seismic waves propagated into the volume shown in grey, and are about to reach station MAJO. Within this volume, the waves cause compression and expansion of the medium, as shown in the cross-section below. The overall contribution of all these zones whose mass changes leads to an immediate change in the gravity detected by the seismometer (direct effect). The gravitational field is also disturbed throughout the Earth, and each of these disturbances is a force that causes secondary seismic waves (induced effect).
In the volume near the seismometer (shown in green), this secondary seismic field arrives before the direct waves. The seismometer thus records, before the direct seismic waves, an elasto-gravity signal resulting from the direct and induced effects of gravity disturbances.
Ref: M. Vallée, J. P. Ampuero, K. Juhel, P. Bernard, J.-P. Montagner & M. Barsuglia, Observations and modeling of the elastogravity signals preceding direct seismic waves, Science, doi : 10.1126/science.aao0746
