Direct magnetic signals from earthquake rupturing: Iwate-Miyagi earthquake of M 7.2, Japan

The key point of this paper is that for the first time we successfully observed “co-rupturing” Earthʹs magnetic field changes due to the piezomagnetic effect caused by earthquake rupturing. Electromagnetic changes associated with earthquakes have been investigated previously. Most of them are “coseismic” step-like offsets in the geomagnetic field, which are regarded as the piezomagnetic effect caused by the total static stress changes resulting from an earthquake. Proton precession magnetometers have so far been mainly used to detect seismomagnetic effects, for which the measurement interval was usually 1 or 10 min and the measurement accuracy was 0.1 nT. For seismomagnetic observations we employed flux-gate magnetometers whose specifications are the measurement with accuracy of 0.01 nT and with the sampling interval of 0.5 or 1 s. By virtue of such highly sensitive magnetometers, we show observations of magnetic signals accompanying fault movement. Our observation site happened to be situated at an epicentral distance of 26 km from the 2008 Iwate-Miyagi Nairiku earthquake of M 7.2, in NE Japan. Magnetic field components began to change almost simultaneously with the onset of the earthquake rupture and grew non-linearly until the first P wave arrival. Such magnetic signals are most probably generated by the changing stress field due to rupturing, i.e. the piezomagnetic effect, rather than the seismic dynamo effect or electromagnetic induction within the conducting crust, because they lack oscillatory features like seismic waves. We attempt to interpret the observations with the aid of quasi-static piezomagnetic-model calculation which strongly supported our observation results.