Abstract
Changes in the underlying conductivity around hypocenters are generally considered one of the promising mechanisms of seismo-electromagnetic anomaly generation. Parkinson vectors are indicators of high-conductivity materials and were utilized to remotely monitor conductivity changes during the MW 6.5 Jiuzhaigou earthquake (103.82°E, 33.20°N) on 8 August 2017. Three-component geomagnetic data recorded in 2017 at nine magnetic stations with epicenter distances of 63–770 km were utilized to compute the azimuths of the Parkinson vectors based on the magnetic transfer function. The monitoring and background distributions at each station were constructed by using the azimuths within a 15-day moving window and over the entire study period, respectively. The background distribution was subtracted from the monitoring distribution to mitigate the effects of underlying inhomogeneous electric conductivity structures. The differences obtained at nine stations were superimposed and the intersection of a seismo-conductivity anomaly was located about 70 km away from the epicenter about 17 days before the earthquake. The anomaly disappeared about 7 days before and remained insignificant after the earthquake. Analytical results suggested that the underlying conductivity close to the hypocenter changed before the Jiuzhaigou earthquake. These changes can be detected simultaneously by using multiple magnetometers located far from the epicenter. The disappearance of the seismo-conductivity anomaly after the earthquake sheds light on a promising candidate of the pre-earthquake anomalous phenomena.
Highlights
Seismo-electromagnetic anomalies have been observed in a wide frequency band in several previous studies [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]
There are some magnetic disturbances from the space, the durations of those disturbances are generally limited within 1–5 days that is significantly different with the time durations (10 days for the median frequency band, 25 days for the lowest frequency band, and 12 days for the highest frequency band) of the conductivity anomaly associated with the Juizhaigou earthquake
The seismo-conductivity anomalies before the Jiuzhaigou earthquake can be simultaneously observed by the geomagnetic stations with an epicentral distance from 63 km to 385 km
Summary
Seismo-electromagnetic anomalies have been observed in a wide frequency band in several previous studies [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Enhancements were observed in a relatively-low frequency band of 0.005–0.03 Hz about 1.5–1.0 months before the Biak earthquake [11]. With advantage of studies in a long time, the ultra-low-frequency (ULF) band mainly ranging between 0.005 Hz and 0.01 Hz is promising for detecting seismo-electromagnetic anomalies [2,5,7,11,15]. The causal mechanisms of seismo-electromagnetic anomalies remain unclear, stressed rocks [20,21], small conductivity fluctuations [22], piezomagnetism effects [23,24] and positive hole effect [25] are potential reasons for their occurrence
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