| Popis: |
The main purpose of this study is to investigate the temporal changes in the attenuation of coda waves before and after the 7.3 magnitude Kumamoto earthquake in the Kumamoto region of Japan. In this study, the single back scattering method has been used to estimate the attenuation of coda waves. The attenuation was estimated using 474 local earthquakes recorded at 306 seismic stations with a magnitude between 2.5 and 5.5 and the focal depth of less than 200 km. 18,810 records were processed in seven frequency ranges of 1.5, 3, 4.5, 6, 9, 12 and 18 Hz and in three time intervals of 20, 30 and 40 seconds. The frequency ratio of quality coefficient for three lapsed time 20, 30 and 40 seconds as Qc = (53 ± 3) f (0.86 ± 0.06), Qc = (65 ± 4) f (0.87 ± 0.06) and Qc = (79 ± 4) f (0.88 ± 0.07). The results show that the study area is tectonically highly active and has a high seismicity rate. Increasing the quality coefficient along with increasing the lapse time shows that shallow structures have more heterogeneity than the deeper ones. Investigating the temporal attenuation changes in this region shows that by approaching the main Kumamoto earthquake, the number of earthquakes increases while the quality factor of coda waves decreases. This negative correlation can be justified by the increased stress in the region that will cause to earthquakes increases. Having more earthquakes in the region have caused fractures to increase and thus led to decrease the quality coefficient. After the main Kumamoto earthquake, there were results being observed in opposite to the foreshock results. The average quality coefficient before and after the main earthquake is respectively 68 and 60, which shows that the heterogeneity in the region has increased. The increase in heterogeneity in the region can be due to the increase in the aftershocks and consequently an increase in the cracks and fractures. |
