Abstract
One of the strongest earthquakes, with magnitude M 8.9, occurred at the sea bottom near to the east coast of Japan on March 11, 2011. This study is devoted to the investigation of anomaly disturbances in the main magnetic field of the Earth and in ultra-low frequency magnetic variations (F <10 Hz) observed before this earthquake. Secular variations of the main geomagnetic field were investigated using three-component 1-h data from three magnetic observatories over the 11-year period of January 1, 2000, to January 31, 2011. The Esashi and Mizusawa magnetic stations are situated northwest of the earthquake epicenter, at distances of around 170 km to 200 km, and the Kakioka observatory is situated southwest of the earthquake epicenter, at a distance of about 300 km. During this period, there were four local anomalies in the secular variations. The last anomaly was the biggest, which began around 3 years prior to the earthquake moment. All of the anomalies can be most distinctly recognized, in the form of differences in the corresponding magnetic components at these remote magnetic stations. For investigations of the ultra-low frequency magnetic field disturbances, three-component 1-s data at two magnetic stations (Kakioka and Uchiura) were used. The Uchiura station is situated 119 km south of Kakioka, at a distance of about 420 km from the earthquake epicenter. Data from the time interval of February 18, 2011 to March 10, 2011 (only at night-time: 01:00 to 04:00 local time) were investigated in a wide frequency range. In the frequency range of 0.033 Hz to 0.01 Hz, there was the clearest anomaly, seen as a decrease in the correlation coefficients of the corresponding magnetic components at these two stations, from February 22, 2011. Differences in the Z components showed an increase, and became positive after this date. This might suggest that the ultra-low frequency lithospheric source appeared north of the Kakioka station. Outside this specified frequency range, the anomalies were not well defined.
Highlights
One of the strongest earthquakes, with a magnitude Mw8.9 ( Japan Meteorological Agency classification), occurred at the sea bottom near the east coast of Japan on March 11, 2011
We have studied the precursors of this earthquake in the secular variations of the main geomagnetic field and in the ultra-low frequency (ULF) magnetic disturbances
Two geomagnetic poles in the secular variations closest to Japan are situated in eastern Siberia and in the Pacific Ocean, as based on the computations by Demina et al [2008] using a dynamic model of sources of the main geomagnetic field
Summary
One of the strongest earthquakes, with a magnitude Mw8.9 ( Japan Meteorological Agency classification), occurred at the sea bottom near the east coast of Japan on March 11, 2011. Three components (H, D, Z) of the secular variations of the constant magnetic field at the ESA and KAK magnetic stations over an 11-year period ( January 1, 2000, to January 31, 2011) are plotted, as the 1-h data.
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