Strong Attenuation of Shear Waves in the Focal Region of the 1997 Northwestern Kagoshima Earthquakes, Japan

In 1997, two moderate earthquakes (MJMA 6.5 on March 26 and MJMA 6.3 on May 13) occurred in northwestern Kagoshima prefecture, Japan. Using the records of aftershocks with focal depths from 5 to 10 km, recorded at a station located above the center of the aftershock zone, attenuation values for shear waves (Qs^-1) were determined for the focal region. This region consists of the aftershock zones, including the rupture zones of the two mainshocks. A coda normalization method was applied for the measurement of Qs^-1 and determined as a function of frequency in five frequency bands centered at 1.5, 3, 6, 12, and 24 Hz. Before applying the method, a correction for the effect of the radiation pattern of shear waves on the spectral amplitude was made for frequency bands centered at 3 Hz and lower. Estimated Qs^-1ʹs have significantly larger values compared to other previous studies in the crust. From our results,Qs^-1 can be approximately represented as a function of frequency f by Qs^-1(f)=(9.93*10^-2)f^-0.95. In order to check the significance of the large values of Qs^-1 in the focal region, we also determined the attenuation in the surrounding area, using shear waves that propagate through both the focal region and the surrounding region. Our results show that the estimated Qs^-1 outside the focal region can be approximately represented as a function of frequency by Qs^-1(f)=(1.03*10^-2)f^0.59 . This is similar to the crustal Qs^-1 in the surrounding area obtained by Kato (1999), suggesting that the values of Qs^-1 in the focal region are correctly estimated. Comparison of the values of Qs^-1 demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.