Analysis of Geomagnetic Secular Variation Anomalies in Japan to Deduce Existence of Earthquake-related Changes in the Geomagnetic Field

Abstract:

To find an evidence of earthquake-related geomagnetic changes is a challenging work. Some earlier studies have reported pre- or co-seismic changes in the geomagnetic field with sizable amplitudes (~10 nT). However the rarity of large earthquakes, combined with the sparsity of continuous geomagnetic stations, means that there are few opportunities to reach a reliable conclusion on the existence of earthquake-related geomagnetic changes.

As an alternative approach, I focus on geomagnetic secular variation (SV) anomalies that are NOT related to earthquakes or other prominent events. SV anomalies up to few nT are reported in several areas in Japan. On the other hand, anomalies in absolute values of the geomagnetic field in the same area are up to 100 nT. If the reported SV anomalies are persistent during several decades, a contradiction arises unless there are “counter changes” which possibly related to earthquakes. Therefore precise analysis of the SV anomalies may provide an evidence of the existence of earthquake-related changes in the geomagnetic field, although indirect.

To determine SV anomalies, the 1^st geomagnetic survey’s data at about 100 sites provided by the GeoSpatial Authority of Japan (GSI) during 1970–2000, together with observatories data provided by GSI and the Japan Meteorological Agency, are analized. Spatio-temporal variations in the geomagnetic field in the region are approximated by the Spherical Harmonic functions with low degrees, and the differences between the observed and approximated values are regarded as the SV anomalies.

In many sites, persistent SV anomalies of the order of 0.1 nT/yr are determined. Although they are smaller than those reported by an earlier study, it still suggest the SV cannot continue for several 1000 years unless “counter changes” in values exists, at least in each 100 years. To find generation mechanism(s) of the SV is a future challenge, which possibly leads to an insight on the relationship between the geomagnetic SV and tectonic processes.