As the first application of the method of Hashida and Shimazaki (J. Phys. Earth, 32, 299-316, 1984), a three-dimensional seismic attenuation structure and source strengths are estimated by inversion of seismic intensity data for earthquakes that occurred in the Tohoku district, Japan. We carefully selected 1, 630 intensity data from 101 earthquakes so that the intensities are consistent with accelerations converted from Kawasumi's relation and so that a well-resolved attenuation structure is obtained. A reasonable fit of the formula proposed in our preceding paper to the actual intensity data guarantees that a systematically biased attenuation structure is not obtained. A comparison of the obtained attenuation structure with velocity structures estimated by previous studies shows that high (low) Q nearly corresponds to high (low) V. The correlation of both structures indicates that the attenuation structure estimated by the proposed method is reliable. The resultant attenuation structure shows a remarkable contrast in the attenuation coefficient and two prominent features. The first feature is low-Q zones down to a depth of 90 km, which corresponds to the distribution of volcanoes. The second is high-Q zones that correspond to the subducting Pacific slab. The high-Q slab is in contact with the high-Q zone in a depth range of 30-60 km, which lies on the east side of the volcanic front. The presence of high-stress earthquakes in this depth range, such as the 1978 Miyagi-ken-oki earthquake, is explained by a model in which the contact of the underthrusting Pacific plate with the surface high-Q zone accumulates higher stress and thus causes stronger seismic coupling. The estimated source strength, which is expressed as a point source acceleration, correlates well with earthquake magnitude. Normalized source acceleration, which is an average acceleration over a source area, is estimated. The acceleration suggests that the stress drop of an earthquake becomes higher with magnitude and with depth. The relation between JMA magnitude (M_J) and seismic intensity at a hypocentral distance of 100 km (I₁₀₀) is found to be I₁₀₀=1.5 M_J-6.5 for crustal events. This I₁₀₀ is in agreement with the value reported by Utsu (Bull. Earthq. Res. Inst., 59, 219-233, 1984) which was determined from events excluding those which show anomalous distributions of intensity data. This agreement suggests that our method of estimating earthquake magnitude from intensity data is effective for removing the effect of structure.