Lithosphere – Ionosphere – Magnetosphere Coupling

Analysis of the 28 September 2018 Palu (Indonesia) Mw=7.5  earthquake

On 28 September 2018 Indonesia was hit by a strong earthquake (Mw=7.5) that caused more than 2000 victims. Retrospective investigation of the data acquired by some payloads on-board CSES-01 satellite permitted to detect several ionospheric anomalous signals that could be possibly related to the preparation phase of such a powerful earthquake. It is fundamental to note that the reported anomalies are only potentially associated with this earthquake because, despite all the precautions, we cannot rule out that similar perturbations can be caused by other sources as well. Therefore, they cannot be yet used to make a prediction of a future event for which more research is necessary. On the other hand, this kind of study allows us to make significant steps forward to understand the physics underlying the preparation phase of a large earthquake.

In particular, CSES-01 was able to detect two types of anomalies possibly related to the impending earthquake: absolute and relative anomalies, both taken during quiet magnetic times, i.e. not disturbed by any geomagnetic storm. The first type of anomalies is a change of the absolute value of the parameter under study: in this case, we found an increase of the electron density (the amount of free electrons for a cubic centimetre) above the epicentre as part of an Equatorial Ionospheric Anomaly (EIA) that normally occurs only during the local daytime. The second type is a relative variation of the parameter of interest, detected as rapid oscillations of magnetic field and electron density above the epicentre at the CSES-01 altitude (~ 505 km). Regarding the absolute type of anomalies, the following picture shows a reconstruction of the electron density on the night of 19 August 2018, i.e. more than one month before the earthquake. It is possible to note a higher level of its value (not expected for the night-time standard behaviour of the ionosphere) that is contained inside the preparation area (defined by the Dobrovolsky radius that scales with magnitude) used for the investigation and represented by a yellow circle.

Figure 1 Electron density increase detected by CSES-01 together with ESA Swarm constellation (A and C) satellites on 19 August 2018, i.e., more than one month before the Mw=7.5 Indonesia 2018 earthquake. Full track profiles and further details are available in Marchetti et al., JAES, 2020.

Searching for the second type of anomalies, i.e. rapid variations of some ionospheric quantities, it was possible to extract several anomalous tracks of electron density, electron temperature and magnetic field. The following figures show two interesting examples.

Figure 2. From Left to right: CSES-01 electron density and electron temperature tracks, together with their corresponding first differences, acquired on 8 August 2018 at 18:12 UTC (02:01 Local Time), orbit number: 2850.1, and the corresponding geographical map where a yellow circle denotes the Dobrovolsky area, i.e. the theoretical representation of the preparation area of the earthquake, with its centre as the earthquake epicentre; the red line represents the satellite orbit as projected at the Earth’s surface. It is worth noting a rapid variation of electron density and electron temperature above the future epicentre. Furthermore, the absolute value of electron density increased close to the epicentre, while the one of electron temperature decreased. We note that an earthquake of magnitude 5.9 occurred on the following day inside the investigated area and, at present, it is not possible to distinguish if this track was caused by which earthquake or even confirm the lithospheric origin of the detected disturbance, but a some statistics presented in the paper would support the seismic origin of such detected perturbations. Reproduced from Marchetti et al., JAES, 2020.
Figure 3 CSES-01 High Precision Magnetometer (HPM) passing on August 14th, 2018 above the future epicentre (the red star in the map) of Mw7.5 Indonesia. The red circle underlines an anomaly in the East-Y component of the magnetic field at about the same latitude of the future epicentre (the tracks show also a few spikes due to some instrumental malfunctions). Reproduced from Marchetti et al., JAES, 2019, where also more details about the investigation of possible lithosphere atmosphere and ionosphere coupling are available.