Continuous and repeated geomagnetic observations have been performed at 8 stations in the eastern part of
Hokkaido, NE Japan, to confirm a detailed picture of geomagnetic secular changes. The observation delineated
anomalously large secular changes of about 1 nT/year that have lasted at least for about 3 to 30 years (depending
on the period of observations) at 3 stations situated in the remarkable geomagnetic anomaly region. Contributions
from the earth’s core or ionospheric origin are ruled out as source mechanisms because of the local distribution
of the anomalous stations. Heat-triggered volcanomagnetic effect cannot be the origin of such secular changes
because the stations are quite far from the volcanoes. Instead, we propose the changes originate from stressinduced
tectonomagnetic effect (piezomagnetism). We performed piezomagnetic modeling under the condition
that observed regional tectonic stress has been applied to the highly magnetized rock bodies inferred from the
analysis of the observed geomagnetic anomalies. The modeling explained well the secular changes by assuming
the stress sensitivity of the order of 10−2 MPa−1, which is one order larger than the ordinarily used value.
Key words: Geomagnetic secular change, tectonomagnetism, stress sensitivity.