Variations of Geomagnetic Response Functions before the 2011 Tohoku Earthquake

Igor I Rokityansky,Artem V Terehyns,Masashi Hayakawa,Valeriia I Babak

doi:10.4236/ojer.2019.82005

Abstract

Induction vectors have been extensively calculated using data from 19 Japanese observatories for a dozen years preceding the huge 2011 Tohoku earthquake (EQ). At 6 observatories anomalous variations of induction vectors were separated in the years of 2008-2010 that can be identified as middle-term precursors. These observatories are located not at the shortest distance from the EQ epicenter, that is in agreement with the widely known phenomenon of spatial selectivity of EQ precursors. The analysis of horizontal tensors reveals a conductivity anomaly under the central part of the Boso peninsula (at 30 km from Tokyo) with a WNW-ESE strike coinciding both with the Sagami trough strike and the strike of well conducting 3 km thick sediments. A joint analysis of geoelectric and tectonic data leads to a preliminary conclusion that the Boso conductivity anomaly connects two large scale conductors: Pacific sea water and a deep magma reservoir beneath a volcanic belt. Between two so different conductors an unstable transition zone can be expected which should be sensitive to changes of stress. Applying our original processing including two steps analysis and elimination of annual and monthly periods, a short-term two-month-long precursor of bay-like form was successfully separated at the observatory of Kanozan, KNZ (over the Boso anomaly) despite its strong noise. All the results were obtained with advanced multi-windows multi-rr (remote reference) robust programs with a coherency control. Dependence of the results of induction vector calculation on geomagnetic activity was carefully studied, and this dependence is relatively strong when the magnetotelluric field and noise have approximately the same magnitude. But even in this case we could identify the precursor field.

Highlights

First of all, we will explain our methodology in the analyses of geomagnetic field variations, and we will describe our research purpose on finding out any precursors to the 2011 Tohoku earthquake (EQ) from geomagnetic data with special reference to induction vectors.1.1
These observatories are located not at the shortest distance from the EQ, that is in agreement with well-known phenomenon of spatial selectivity of EQ precursors known during the centuries for hydrological precursors and recently proven for BLE registered in the form of Seismic Electric Signal (SES) [14] [15] [16] [17]
1) Thorough extensive analyses of very noisy geomagnetic data of Japanese observatories allow us to separate EQ anomalies which can be attributed to the medium-term EQ precursors to the disastrous Tohoku EQ on 11 March 2011

Summary

Introduction

We will explain our methodology in the analyses of geomagnetic field variations, and we will describe our research purpose on finding out any precursors to the 2011 Tohoku earthquake (EQ) from geomagnetic data with special reference to induction vectors.1.1. If some other magnetic field which is not so stable (let it be a precursor field), is superimposed on the field of noise, exactly the phase of induction vector will be the most sensitive component for such a precursor separation The nature of this “other” magnetic field can be BLE (data of [23] evidence that after February 22, 2011 at an observatory UCU (in the south of the Boso peninsula) BLE appeared with maximum at periods 30 - 100 s). We apply a new approach developed by V.I.Tregubenko [24], who used it for processing the data of seismo-prognosis monitoring network in Ukraine He separated precursors before few strongest (M ≈ 4) Crimean EQs occurred during 15 years, in particular before the Sudak, Crimea EQ M3.9 on 24.01.2005 [24]. We applied this approach to KNZ, KAK and ESA 1-s data, but the precursor was found only in KNZ

Results

Discussion

Conclusion