No. 916

Title:

Preliminary Analysis of Observations on the Ultra-Low Frequency Electric Field in a Region around Beijing

Author(s):

Zhuang, Jiancang (Institute of Statistical Mathematics, 106-8569, Tokyo, Japan);
Vere-Jones, David (School of Mathematical and Computing Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand);
Guan, Huaping (Center for Analysis and Prediction, China Seismological Bureau, 100036, Beijing, China);
Ogata, Yosihiko (Institute of Statistical Mathematics, 106-8569, Tokyo, Japan);
Ma, Li (Center for Analysis and Prediction, China Seismological Bureau, 100036, Beijing, China)

Key words:

ultra-low frequency electric signal; earthquake; precursor; self-exciting and mutually exciting model

Abstract:

    This paper presents a preliminary analysis of observations on ultra-low frequency ground electric signals from stations operated by the China Seismological Bureau over the last 20 years. A brief description of the instrumentation, operating procedures and data processing procedures is given. The data analysed consists of estimates of the total strengths (cumulated amplitudes) of the electric signals during 24 hour periods. The thresholds are set low enough so that on most days a zero observation is returned. Non-zero observations are related to electric and magnetic storms, occasional man-made electrical effects, and, apparently, some pre-, co-, or post-seismic signals. The main purpose of the analysis is to investigate the extent that the electric signals can be considered as pre-seismic in character. For this purpose the electric signals from each of five stations are jointly analyzed with the catalogue of local earthquakes within circular regions around the selected stations. A version of Ogata's Lin-Lin algorithm is used to estimate and test the existence of a pre-seismic signal. This model allows the effect of the electric signals to be tested, even after allowing for the effects of earthquake clustering. It is found that, although the largest single effect influencing earthquake occurrence is the clustering tendency, there remains a significant preseismic component from the electrical signals. Additional tests show that the apparent effect is not post-seismic in character, and persists even under variations of the model and the time periods used in the analysis. Samples of the data are presented, and the full data sets have been made available on local websites.