No abstract

The Spector and Grant method, which has been in use for 25 years, relates average depths to source to rate of decay of the magnetic power spectra. This method, which assumes a uniform distribution of parameters for an ensemble of magnetized blocks, leads to a depth‐dependent exponential rate of decay. We show that also inherent in this model is a power‐law rate of decay that is independent of depth. For most cases, except for extreme depths and small block sizes, the observed power spectrum should be corrected for a power law decay rate of β∼3. If the depth distribution of the magnetic blocks is Gaussian, then the observed power spectrum should be corrected for both a depth independent power law and exponential decay. This power‐law decay is very similar to the scaling behavior, supposed as a fractal character, of observed magnetic fields in North America. As a general rule, when β∼3, further information is needed to discriminate between a fractal or Spector and Grant model. However, it is becoming quite clear that magnetic power spectra should be corrected for a power law decay before applying the Spector and Grant method for depth determination.

The study of the preparation phase of large earthquakes is essential to understand the physical processes involved, and potentially useful also to develop a future reliable short-term warning system. Here we analyse electron density and magnetic field data measured by Swarm three-satellite constellation for 4.7 years, to look for possible in-situ ionospheric precursors of large earthquakes to study the interactions between the lithosphere and the above atmosphere and ionosphere, in what is called the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC). We define these anomalies statistically in the whole space-time interval of interest and use a Worldwide Statistical Correlation (WSC) analysis through a superposed epoch approach to study the possible relation with the earthquakes. We find some clear concentrations of electron density and magnetic anomalies from more than two months to some days before the earthquake occurrences. Such anomaly clustering is, in general, statistically significant with respect to homogeneous random simulations, supporting a LAIC during the preparation phase of earthquakes. By investigating different earthquake magnitude ranges, not only do we confirm the well-known Rikitake empirical law between ionospheric anomaly precursor time and earthquake magnitude, but we also give more reliability to the seismic source origin for many of the identified anomalies.

A large earthquake of 7.8 magnitude occurred on 25 April 2015, 06:26 UTC, with the epicenter in Nepal. Here, taking advantage of measurements provided by the Swarm magnetic satellites, we investigate the possibility to detect some series of pre-earthquake magnetic anomalous signals, likely due to a lithosphere-atmosphere-ionosphere coupling, that can be a potential earthquake precursory pattern. Different techniques have been applied to Swarm data available during two months around earthquake occurrence. From the detected magnetic anomalies series (during night and magnetically quiet times or with an automatic detection algorithm), we show that the cumulative number of anomalies follows the same typical power-law behavior of a critical system approaching its critical time, and hence recovers as the typical recovery phase after a large event. The similarity of this behavior with the one obtained from seismic data analysis and the application of the analyses also to another period without significant seismicity do support a lithospheric-linked origin of the observed magnetic anomalies. We suggest that they might be connected to the preparation phase of the Nepal earthquake. Confidential manuscript submitted to Earth and Planetary Science Letters • earthquake [e.g. Occhipinti et al., 2013, and references therein]. This offers the possibility to retrieve seismic information from ionospheric observations. An important and debated question arises about the possibility that, during the phase of EQ preparation, electromagnetic waves and/or particles could be transferred from the solid Earth (in particular the lithosphere) to the atmosphere, with a particular effect in the ionosphere, above around 50 km [e.g. Pulinets and Boyarchuk, 2004; Freund, 2011; Pulinets and Ouzounov, 2011; De Santis et al., 2015]. One of the most general models of coupling is based on the emission of a radioactive gas [Pulinets and Boyarchuk, 2004] or metallic ions [Freund, 2011] before a large earthquake, which may change the distribution of electric potential above the surface of the Earth and then up to the ionosphere [e.g., Pulinets and Boyarchuk, 2004; Sorokin et al., 2001]. Penetration of the electric field to the ionosphere could produce ionospheric plasma density and/or conductivity anomalies, which are observed above seismic zones [e.g., Liu et al., 2006; Kon et al., 2011]. An alternative explanation is that the radon emitted before an earthquake would increase the conductivity of air at ground level and that the ensuing increase of current in the fair weather global circuit would lower the ionosphere [Harrison et al. 2010]. Therefore, it is expected that low Earth orbiting (LEO) satellites could be the best possible dedicated platforms of sensors to detect any electromagnetic, acoustic or infrared seismic-linked precursors. Certainly, space observations have to be investigated together with ground (and near-surface) seismic and other geophysical observations, in order to have a more complete picture of the possible involved phenomena.