The 1998-1999 volcanic unrest of Iwate volcano, northeastern Japan, was marked by 350 deep low-frequency earthquakes (DLFs) and 120 intermediate-depth low-frequency earthquakes (ILFs), as well as an intense swarm of shallow volcanic earthquakes that began during 1998 April. The rate of occurrence of the DLFs increased approximately 5 d before that of the shallow volcanic earthquakes increased, and the number of ILFs gradually increased from the middle of 1998. To investigate the relationship between the shallow volcanic activity and the activities of the DLFs and ILFs, we determined their precise hypocentres and source mechanisms by analysing waveform data recorded by a dense seismic network of 47 three-component seismometers located on and around the volcano. The hypocentres of DLFs are concentrated within three regions: the first region is at depths from 31 to 34 km, approximately 10 km south of the summit, the second is at depths from 32 to 36 km approximately 10 km northeast of the summit and the third is at a 37 km depth, approximately 7 km northeast of the summit. In contrast, ILFs are located within a vertical pipe-like region just beneath the summit and sometimes show a vertical migration of the focal depth. Our moment tensor inversion using spectral ratios of body waves indicates that the source mechanisms of the DLFs and ILFs have a significant double couple and a compensated linear-vector dipole component. It is also found that a significant volumetric change is included in the source mechanisms for some DLFs. Such source mechanisms of DLFs and ILFs can be explained by the motion of a tensile crack coupled either with a shear crack or with an oblate spheroid magma chamber. However, the orientations and polarities of the crack motions are not the same in each region of DLFs and ILFs. These results suggest that a complex magma system is present at the source regions of the DLFs and ILFs.
- Source mechanism
- Spectral ratio