Estimating the seismotelluric current required for observable electromagnetic ground signals

Abstract

Abstract. 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.

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

“…The results present that, the seismo-telluric currents with and without ionospheric effects must be about 5.0 × 10 7 A and 3.7 × 10 8 A, respectively. Compared with the expected seismo-telluric current ∼ 10-100 kA of the "Alum Rock" M W = 5.6 earthquake for an observed 30 nT pulse at 1 Hz and D = 2 km (Bortnik et al, 2010), this result is probably in a reasonable range.…”

“…For a finite length dipole source of the Wenchuan earthquake, its 2-D distributions of electrical field components |E x | and |E y |, which are orthogonal each other, on the Earth's surface show there are strong field power areas and weak field power areas around the source as illustrated by Bortnik et al (2010). The radiating pattern of the total |E| in this investigation is symmetric to the center circle outside of the source, which indicates a signal is always registered to anyone direction if a system is designed to measure the total field |E| or both |E x | and |E y | components instead of only one.…”

Fractal analysis of ULF electromagnetic emissions in possible association with earthquakes in China

“…As mentioned before, the epicentral distances are over 100 km, which is too far for the detection of seismogenic ULF emissions by means of simple analysis such as amplitude analysis. The most probable reason for this would be the radiation directivity of seismogenic ULF emissions depending on the relative direction between the generated current source and the receiving station, as already suggested by , Tian and Hata (1996), Dong et al (2005), and Bortnik et al (2010). If the epicentral distance is not so large, the directivity issue does not have such a significant effect.…”

Correlation of pre-earthquake electromagnetic signals with laboratory and field rock experiments

“…A combination of the larger distance (19 km) between the epicenter and magnetometer instrument, and the high conductivity of the area all contributed to attenuation of the signals. A new paper (Bortnik, 2010) investigated the effect of conductivity on ULF wave propagation, and confirms this attenuation effect when conductivity is high.…”