Yutaka Mamada

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

Strong attenuation of shear waves in the focal region of the 1997 northwestern Kagoshima earthquakes, Japan

In 1997, two moderate earthquakes ( M JMA 6.5 on March 26 and M JMA 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 ( \(Q_{\mathrm{S}}^{-1}\) ) 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 \(Q_{\mathrm{S}}^{-1}\) 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 \(Q_{\mathrm{S}}^{-1}\) 9s have significantly larger values compared to other previous studies in the crust. From our results, \(Q_{\mathrm{S}}^{-1}\) can be approximately represented as a function of frequency f by \(Q_{\mathrm{S}}^{-1}(f)=(9.93{\times}10^{-2})f^{-0.95}\) . In order to check the significance of the large values of \(Q_{\mathrm{S}}^{-1}\) 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 \(Q_{\mathrm{S}}^{-1}\) outside the focal region can be approximately represented as a function of frequency by \(Q_{\mathrm{S}}^{-1}(f)=(1.03{\times}10^{-2})f^{-0.59}\) . This is similar to the crustal \(Q_{\mathrm{S}}^{-1}\) in the surrounding area obtained by Kato (1999), suggesting that the values of \(Q_{\mathrm{S}}^{-1}\) in the focal region are correctly estimated. Comparison of the values of \(Q_{\mathrm{S}}^{-1}\) demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.