The b value of the frequency-magnitude relation, and thus the mean magnitude, in the off-Ito volcanic area is not uniform based on detailed b value tomography using about 10,000 events in an area of 10 km radius and in the upper 15 km of the crust. A high b value anomaly (b=1.5) below 7 km depth and with a radius of about 2 km, located below the coast southeast of Ito, contrasts with lower values of typically b=0.7 north of it and at shallower depths. On the basis of surface deformations, a tensile crack dislocation source was located above 7 km depth. Thus we surmise that the magma chamber in the off-Ito area was located below 7 km depth. The correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies. The magma body may be described as consisting of two parts: a lower, well-established chamber below 11 km depth and a shallower part (11--7 km depth). The b value in the volume surrounding the magma chamber increased from 0.7 during the 1980s to 1.5 during the earthquake swarms of the 1990s. This may reflect a trend of increasing crack density due to the intrusion activity.¿ 1997 American Geophysical Union