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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.

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.

Geomagnetic field B = B x e x + B y e y + B z e z (where e_{x}, e_{y}, and e_{z} are unit vectors directed to North, East and downward correspondingly) continuously exists in and around the Earth and is recorded nowadays digitally with a time reading interval Δt. After the conventional processing using Fourier transform a B record of duration t2-t1 is transformed into a superposition of harmonic components with periods T 1 , T 2 , ⋯ , T n .

Response function (RF) is the term widely used in natural sciences and mathematics. In the geoelectromagnetic studies of electrical conductivity σ(x, y, z) of the Earth’s interior [

Induction vector C = A e x + B e y (A and B are determined from the linear equation: B z = A B x + B B y ). Real induction vectors C u = A u e x + B u e y possess an important property: in the notation of Wiese used here, they are directed away from a good conductor.

Anomalous horizontal magnetic variation tensor [M] is determined from the linear system of equations B x ( r i ) = M x x B x ( r 0 ) + M x y B y ( r 0 ) , B y ( r i ) = M y x B x ( r 0 ) + M y y B y ( r 0 ) , where r_{0} and r_{i} are coordinates of base (reference) and some other observation place. Tensor [M] reflects the change in geoelectric structures between two places.

The processing of observed geomagnetic field B for the monitoring of geodynamic and other environmental processes implies transformation of 3 component geomagnetic field time-series with time reading interval Δt (1 min or 1 s in our study) into a variety of time-series with temporal resolution Δτ ( Δ τ ≫ Δ t ) of different RF components: Re and Im, x and y at the set of periods T 1 , T 2 , ⋯ , T n of received harmonics ( Δ t ≪ T i ≪ Δ τ ).

The theory of geoelectromagnetic methods is developed for natural source field in the form of vertically incident plane wave (Tikhonov-Cagniard (T-C) model), which usually holds for an external source field of magnetosphere-ionosphere origin (named as magnetotelluric (MT) field) for the periods less than 10^{4} s. Ideally RF depends only on the Earth’s conductivity distribution which is sensitive to the stress variations and therefore to geodynamic processes including the earthquake (EQ) preparation.

Really the observed B(T) is composed of several sources: 1) MT-field, part of which meets the T-C model requirements, the rest does not meet them and makes an error, 2) ever-present noise, both man-made and natural and sometimes B_{LE}—internal fields of Lithosphere Emission.

RFs and their variations, especially in relation with the occurrence of EQs and EQ preparation, were studied in Japan for many years and were described in many works between which we cite only few [

It is recently agreed that different kinds of phenomena do take place prior to an EQ, including surface deformation, geochemical anomalies, EM radiation, atmospheric and ionospheric perturbations, etc. [

A huge Tohoku EQ (with magnitude of 9) happened offshore of the Tohoku area on 11 March, 2011, and many scientists have tried to find any EQ precursors to this EQ. Different kinds of precursors (middle- and short-term precursors) to this 2011 Tohoku EQ have been reported so far including EM, geochemical anomalies and others, though most were recognized only retrospectively [

We have obtained the geomagnetic data from 17 Japanese geomagnetic observatories (see _{u}, B_{u}, A_{v} and B_{v} for each day for 5 period intervals centered at 225, 450, 1800 and 3600 s. In order to reduce a great scatter of data, every daily values were monthly averaged.

Then for the conversion of geomagnetic field B time series into RF time series we used the advanced multi-windows rr (remote reference) robust programs with coherency control [

Code | Station name | Geom. lat. | Geom. long. | Geogr. lat. | Geogr. long. | Processed years |
---|---|---|---|---|---|---|

MMB | Memambetsu | 35.44 | −148.24 | 43.910 | 144.189 | 1993-2012 |

AKA | Akaigawa | 34.31 | −151.09 | 43.072 | 140.815 | 2001-2012 |

YOK | Yokohama | 32.28 | −150.43 | 40.993 | 141.240 | 2001-2012 |

ESA | Esashi | 30.55 | −150.09 | 39.237 | 141.355 | 1997-2012 |

MIZ | Mizusawa | 30.41 | −150.21 | 39.112 | 141.204 | 1997-2015 |

HAR | Haramachi | 28.90 | −150.25 | 37.615 | 140.953 | 2001-2015 |

SIK | Shika | 28.04 | −153.96 | 37.082 | 136.773 | 2001-2012 |

KAK | Kakioka | 27.47 | −150.78 | 36.232 | 140.186 | 1956-2015 |

HAG | Hagiwara | 26.98 | −153.47 | 35.985 | 137.186 | 2001-2012 |

OTA | Otaki | 26.54 | −150.63 | 35.292 | 140.230 | 2001-2015 |

KNZ | Kanozan | 26.48 | −150.87 | 35.256 | 139.956 | 1996-2016 |

YOS | Yoshiwa | 25.12 | −157.87 | 34.476 | 132.176 | 2001-2012 |

TTK | Totsukawa | 24.83 | −154.52 | 33.932 | 135.802 | 2001-2015 |

HTY | Hatizyo | 24.30 | −150.75 | 33.073 | 139.825 | 1991-2008 |

MUR | Muroto | 24.10 | −155.99 | 33.319 | 134.122 | 2004-2012 |

KUJ | Kuju | 23.65 | −158.58 | 33.061 | 131.260 | 2001-2015 |

KNY | Kanoya | 22.00 | −158.80 | 31.424 | 130.88 | 1991-2016 |

Analyzing large material of processed data for 15 years from 2001 to 2015, we found that aperiodic variations (or enhancement of annual variation) of induction vectors were observed at periods of 225, 450 and 900 s during 3 - 5 years before the Tohoku EQ at the following stations of HAR, KAK, OTA, KNZ and TTK, most clearly at period 450 s presented in

well-known phenomenon of spatial selectivity of EQ precursors known during the centuries for hydrological precursors and recently proven for B_{LE} registered in the form of Seismic Electric Signal (SES) [

Having 1 min time series we can analyze only geomagnetic variations with period T > 3 min and the most interesting shorter part of ULF spectra (0.01 - 10 Hz) where the strongest emissions (B_{LE}) have been observed [

Processing of records from 18 observatories (16 of them are shown in _{xx} and M_{yy} equals to ≈40% and ≈30% correspondingly at periods T < 500 s with decreasing at longer periods. This result was supported by direct visual measurements described below. At closely located observatories KNZ and OTA, the latitudinal (E-W) component of induction vector at period 450 s and shorter increased (in 2011 comparatively to 2001) in opposite directions: westward in KNZ and eastward in OTA (see

Considering the time series of geomagnetic field synchronous records ( [

Direct measurements of the strong MT variation amplitudes in each component provide a check (not precise but very reliable) of the results obtained by our processing. So, the enhancement of B_{x} at KNZ and OTA at approximately 30% - 40% exists and it can be interpreted only by the electrical conductivity anomaly under the observatories, i.e. under the central part of the Boso peninsula.

The relation between M_{xx} and M_{yy} anomalies in KNZ defines the WNW-ESE strike of the Boso conductivity anomaly, and geological data [_{xx} and M_{yy} in UCU, OTA and KYS are different as seen in

On the other hand, the plate tectonic evidences that the Boso anomaly is located over the Sagami trough, structure at the depth 15 - 20 km in the complex junction of three lithosphere plates (

The eastern part of both conductors (shallow sediments and deep trough) has a contact with sea water, while the western one suppositionally contacts with a magma reservoir. In such a circuit it can be some unstable area(s) with conductivity strongly dependent on stress and sensitive to stress changes related with EQs.

Results of the processing of single station and nighttime records at KNZ are given in _{u} and partly B_{v} components, because a railway is located to the west of KNZ and brings the distortions mainly in the eastern component. The northern component is less affected by noise at periods 100 s and more that opens the possibility to use it for the separation of EQ precursors.

In the time intervals with high MT field the RF corresponds to the conductivity distribution and can be used for the study of precursors arisen from changes of conductivity. As was discussed above the anomalous conductor under KNZ can be a part of the large scale electrical circuit (sea water—magma reservoir) strongly sensitive to stress changes. The shallow geoelectric structure of Kanto plain sediments is the part of this circuit. The changes of electrical currents in the circuit before an EQ can be recorded even with the use of strong dominant DC noise field. This idea is supported by the use of strong noise from DC railways for the study of geoelectrical structure [

From the analysis of magnetograms and frequency characteristics of induction vector and from the study of the correlation coefficient (CC) between induction vector variations and variations of K_{p} index, it has been proven [

The induction vector derived from very noisy records, practically from noise field, has a small stable phase. Therefore, 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 B_{LE} (data of [_{LE} appeared with maximum at periods 30 - 100 s). An alternative explanation of the precursor is the change of anomalous current in the large scale circuit including the Boso anomaly. In any case phase in such a noisy environment can be the most sensitive parameter for precursor separation.

Below we apply a new approach developed by V.I.Tregubenko [

The processing was made with the use of the program developed by Varentsov [

Step 1. 7 years long (2005-2011) geomagnetic field time series with every 1 s reading was processed for every month as a single unit. arg(A) time series with every month reading were received as in

Step 2. 2 years long (2010-2011) 1 s time series were processed for every day as single unit. A large scatter of every day results was reduced by averaging with moving window of 5 days long with 1 day shift. From these curves, i.e. arg(A) time series with every day reading, we extracted first, second and seventh (quasi-annual) order polynomials determined in Step 1. The result is shown by the grey rough curve in

The variations of arg(A) given by the bold solid line in

The time of beginning of a bay-like variation and its duration depends on the magnitude of an expected EQ. This time is equal to approximately 2 weeks for the processed Crimean EQs with magnitude approximately 4 [

We have calculated induction vectors using data from Japanese observatories for many years preceding the 2011 Tohoku huge EQ, and we will summarize the main results obtained in this paper as follows.

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.

2) Under the central part of Boso peninsula at a distance of 30 km from Tokyo, we could reliably detect a conductor anomaly. A change of anomalous field over the anomaly before the EQ was identified. Tectonic interpretation of the results contains a supposition that the Boso anomaly can be a fragment of large-scale anomaly between the Pacific water and the magma reservoir 50 - 100 m deep.

3) The EQ precursor can occur by perturbations of the field of strong noise from DC railway, which may be the case for the Boso anomaly. The Boso anomaly is a very attractive subject for our further detailed study, probably as a sensitive place for monitoring EQ precursors.

4) A short-term EQ precursor (two months long) just two months before the EQ was clearly identified in the phase data (less sensitive to noise) at the observatory, KNZ with the use of the method developed by Tregubenko and often used in Ukraine.

5) Finally the RF approach is found to be still a useful tool for presentation and storage of EM monitoring data even in any noisy environment.

Finally we try to compare our findings with the similar previous analyses for the same Tohoku EQ by other researchers. Kopytenko et al. [

The authors are grateful to Geospatial Information Authority of Japan, Japan Meteorological Agency and World Data Center for Geomagnetism (Kyoto) for providing us with good quality data. We are also thankful to V. I. Tregubenko for his valuable consultations on the application of his method, Drs. T. A. Klymkovych and I. M. Varentsov for their processing programs. Finally the authors would like to express their sincere thanks to Prof. K. Hattori of Chiba University for kindly providing the data in the Boso area.

The authors declare no conflicts of interest regarding the publication of this paper.

Rokityansky, I.I., Babak, V.I., Tereshyn, A.V. and Hayakawa, M. (2019) Variations of Geomagnetic Response Functions before the 2011 Tohoku Earthquake. Open Journal of Earthquake Research, 8, 70-84. https://doi.org/10.4236/ojer.2019.82005