Experimental evidence of electrification processes at the 2009 LAquila earthquake mainshock

Two types of coseismic magnetic field events are simultaneously observed: transient offset events and magnetic field signal that occurred at the destructive, Mw6.1 LAquila earthquake (EQ) mainshock. The offset event, conventionally interpreted as a signature of piezomagnetic effects, however could not be ascribed as such. The reason is that the presently known geology of the LAquila basin consists mainly of carbonates, dolomites and limestone, thus it does not suggest an appearance of piezomagnetic related effects under EQ fracture/slip events. The second type of coseismic event, the transient magnetic signal starts simultaneously with the offset event and reaches amplitude of 0.8 nT in the total magnetic field. The signal is local one, in the sense that its form differs from the signals of ionospheric/magnetospheric origin confirmed (indirectly) by additional magnetic field data in Italy and Central Europe. The reliability of the observed local signal is examined also: it follows from the fact that the transient signal is recorded by two different measurements: i) in components (by fluxgate magnetometers) and 2) absolute one (overhauser magnetometer), it persists after the seismic wave train passage but within the first five minutes after the EQ shock and is not a consequence of ionosphere disturbances caused by the seismic wave train. Its amplitude shape resembles diffusion-like form with time scale characteristics that are indicative for a source deep in the crust. The polarity of the transient signal is in the horizontal plane and nearly parallel to the LAquila fault strike, i.e. along the NW-SE direction.

💡 Research Summary

The paper presents a detailed investigation of two distinct coseismic magnetic field phenomena recorded during the Mw 6.1 L’Aquila main‑shock on 6 April 2009. The first phenomenon is a permanent magnetic offset that appears immediately after the earthquake. In most seismomagnetic studies such offsets are interpreted as a piezomagnetic response of the crust, i.e., a stress‑induced change in magnetization of ferromagnetic minerals. However, the authors emphasize that the L’Aquila basin is dominated by carbonate rocks (limestone, dolomite) that contain negligible ferromagnetic material. Consequently, a conventional piezomagnetic explanation is untenable. The authors argue that the offset must instead be linked to non‑magnetic processes such as rapid charge separation, electro‑chemical reactions, or transient currents generated on newly created fracture surfaces and within fluid pathways activated by the rupture.

The second, and central, observation is a transient magnetic signal that starts simultaneously with the offset, reaches a peak amplitude of about 0.8 nT in the total field, and persists for up to five minutes after the seismic wave train has passed. This signal is recorded by two independent instruments: (i) three‑component fluxgate magnetometers that provide the vector components (X, Y, Z) and (ii) an Overhauser absolute magnetometer that measures the scalar total field. The coincidence of the two data streams rules out instrumental artefacts and confirms the reality of the signal. Moreover, the authors demonstrate that the signal is not of ionospheric or magnetospheric origin. Typical ionospheric disturbances associated with seismic waves are broadband, propagate at the speed of electromagnetic waves, and affect a wide geographic area. In contrast, the observed transient is localized, its waveform differs markedly from known ionospheric signatures, and it continues after the acoustic‑gravity wave train has vanished.

A temporal analysis reveals a diffusion‑like profile: a rapid rise followed by an exponential‑type decay. By fitting a simple diffusion model the authors extract a characteristic time scale of several tens of seconds, which implies a source depth of a few kilometres within the crust. The signal’s polarity lies in the horizontal plane and is oriented nearly parallel to the strike of the L’Aquila fault (NW‑SE). This alignment strongly suggests that the underlying current system is controlled by the fault geometry, possibly representing a large‑scale, fault‑parallel charge separation or a transient current flowing along the rupture plane.

The authors interpret these observations as direct evidence of an “electrification” process associated with the earthquake. They propose that rapid creation of new fracture surfaces, abrupt changes in pore pressure, and the mobilization of conductive fluids generate a burst of electric charge. This charge then diffuses through the surrounding conductive crust, inducing a measurable magnetic field. Such a mechanism does not require ferromagnetic minerals and therefore can operate in carbonate‑dominated settings where traditional piezomagnetic effects are absent.

The paper concludes that coseismic magnetic transients, when recorded with high‑resolution, multi‑instrument networks, can provide valuable insight into the physics of fault rupture, fluid migration, and charge dynamics in the crust. Incorporating real‑time magnetic monitoring into seismic networks could improve our ability to characterize the immediate post‑rupture environment, assess damage potential, and perhaps contribute to early warning schemes. The authors call for systematic, multi‑site campaigns in diverse geological settings, refined conductivity models, and integration with other geophysical observables (e.g., electric fields, seismic velocity changes) to fully elucidate the role of earthquake‑induced electrification.