Evaluation of seismo-electric anomalies using magnetic data in Taiwan

C H Chen,P Han,Y Y Sun,C H Wang,J Y Liu,S Wen,F Y Chang,H L Hsu,K Hattori,H Y Yen,T K Yeh

doi:10.5194/nhess-13-597-2013

Abstract

Abstract. The Parkinson vectors derived from 3-component geomagnetic data via the magnetic transfer function are discussed with respect to epicentre locations and hypocentre depths of 16 earthquakes (M ≥ 5.5) in Taiwan during a period of 2002–2005. To find out whether electric conductivity changes would happen particularly in the seismoactive depth ranges, i.e. in the vicinity of the earthquake foci, the frequency dependent penetration depth of the electromagnetic waves (skin effect) is taken into account. The background distributions involving the general conductivity structure and the coast effect at 20 particular depths are constructed using the Parkinson vectors during the entire study period. The background distributions are subtracted from the time-varying monitor distributions, which are computed using the Parkinson vectors within the 15-day moving window, to remove responses of the coast effect and underlying conductivity structure. Anomalous depth sections are identified by deviating distributions and agree with the hypocentre depths of 15 thrust and/or strike-slip earthquakes with only one exception of a normal fault event.

Highlights

Recognition of anomalous pre-earthquake phenomena has been reported for major earthquakes since 1990 (Fraser-Smith et al, 1990; Bernardi et al, 1991; Molchanov ductivity (Chiang et al, 2010) and its susceptibility (Yen et aall.i,n2e0a0r 9re) loaftioansspheicpifiexcOiastrsecabe.ePtawanrekeinSnsscohnoier(t1-np9ec5r9eio) dprvoaprioasteiodntshiant
The Parkinson vectors are utilised to expose the momentary existence of earthquake-related high conductivity materials associated with 16 earthquakes (M ≥ 5.5, Table 1; focal mechanisms in Fig. 1) that occurred in Taiwan during the period of 2002–2005
When the relationship between f and Dep is employed by using the magnetic transfer function, Dep with respect to the Parkinson vectors can be deduced from the frequencydependent coefficients A and B at f

Summary

Methodology

The skin depth describes the relationship between the penetration depth and the frequency of electromagnetic waves (Cheng, 1989; Simpson and Bahr, 2005). Influences of the coast effect and underlying conductivity structure on Parkinson vectors are assumed to be persistent within the 4-yr study period (i.e. from 2002 through 2005). To evaluate these persistent responses, the background distributions in azimuth and magnitude at Dep are constructed by Pa(Dep) and Pm(Dep) during the entire period (2002–2005). Anomalous responses at different azimuths are obtained by subtracting the normalised background azimuth distribution from the normalised time-varying one at each studied depth. Derived differences from the normalised background magnitude distribution are summed individually for each of the 20 depths These 20 summed values versus the related depths are utilised to construct anomalous depth sections. The computed depth of anomalous conductivity is defined by using the largest summed value from these depth sections

Observation and interpretation

Discussion and conclusions