Ionospheric influence on the seismo-telluric current related to electromagnetic signals observed before the Wenchuan <i>M</i>_S = 8.0 earthquake

A three-layer (Earth-air-ionosphere) physical model, as well as a two-layer (Earth-air) model, is employed in this paper to investigate the ionospheric effect on the wave fields for a finite length dipole current source co-located with the main fault of an earthquake when the transmitter-receiver distance is up to one thousand kilometers or even more. The results show that all electrical fields are free of the ionospheric effect for different frequencies in a relative short range, e.g., ~ 300 km for f = 1 Hz, implying the ionospheric influence on electromagnetic fields can be neglected within this range that becomes smaller as the frequency increases. However, the ionosphere can give a constructive interference to the waves passed through and make them decay slowly when an observation is out of this range and the ionosperic effect can be up to 1–2 magnitudes of the electrical fields. For an observed 1.3 mV/m signal at 1,440 km away for the Wenchuan MS = 8.0 earthquake, the expected seismo-telluric current magnitude for the Earth-air-ionosphere model is of 5.0 × 104 kA , which is of one magnitude smaller than the current value of 3.7 × 105 kA obtained by the Earth-air model free of ionospheric effect. This indicates that the ionosphere facilitates the electromagnetic wave propagation, as if the detectability of the system is improved effectively and it is easier to record a signal even for stations located at distances beyond their detectability threshold.

Estimating the seismotelluric current required for observable electromagnetic ground signals

We use a relatively simple model of an underground current source co-located with the earthquake hypocenter to estimate the magnitude of the seismotelluric current required to produce observable ground signatures. The Alum Rock earthquake of 31 October 2007, is used as an archetype of a typical California earthquake, and the effects of varying the ground conductivity and length of the current element are examined. Results show that for an observed 30 nT pulse at 1 Hz, the expected seismotelluric current magnitudes fall in the range ~10–100 kA. By setting the detectability threshold to 1 pT, we show that even when large values of ground conductivity are assumed, magnetic signals are readily detectable within a range of 30 km from the epicenter. When typical values of ground conductivity are assumed, the minimum current required to produce an observable signal within a 30 km range was found to be ~1 kA, which is a surprisingly low value. Furthermore, we show that deep nulls in the signal power develop in the non-cardinal directions relative to the orientation of the source current, indicating that a magnetometer station located in those regions may not observe a signal even though it is well within the detectable range. This result underscores the importance of using a network of magnetometers when searching for preseismic electromagnetic signals.