Abstract
Abstract We deployed 56 temporary seismic stations within approximately a month after the occurrence of the 2004 mid-Niigata prefecture earthquake. Using manually-picked arrival data obtained from the temporary and surrounding permanent seismic stations, 1056 aftershocks have been relocated. Based on the spatiotemporal variations in the relocated aftershocks, the cluster activities associated with the mainshock and some large aftershock events are identified. The aftershocks associated with the mainshock, the largest occurred on the two steep west-dipping planes at an angle of 60° and approximately 5 km away. In contrast, the aftershocks following the event on Oct. 27 are aligned on east-dipping plane at a low angle of 25°. It is further observed that the aftershock area extended in both northeastward and southwestward directions at a later stage. The triggered seismicity around the northeast edge was more significant than that around the southwest edge. This difference could be understood by the discrepancy in the shear stress level accumulated at the dynamic shear rupture due to the mainshock.
Highlights
A shallow Mw 6.6 inland earthquake occurred in northwestern Japan (Niigata prefecture) on Oct. 23, 2004 and resulted in serious seismic damages including landslides in the surrounding areas
It is remarkable that the number of large aftershocks (M>4) after this event is significantly greater than those after other large inland earthquakes [Japan Meteorological Agency (JMA) catalog] that have occurred recently in Japan
The aftershock distributions have changed with time due to the triggered aftershocks of large magnitudes and resulted in highly complex patterns comprising of many earthquake clusters
Summary
A shallow Mw 6.6 inland earthquake occurred in northwestern Japan (Niigata prefecture) on Oct. 23, 2004 and resulted in serious seismic damages including landslides in the surrounding areas. It is remarkable that the number of large aftershocks (M>4) after this event is significantly greater than those after other large inland earthquakes [Japan Meteorological Agency (JMA) catalog] that have occurred recently in Japan. This sequence of aftershocks with magnitudes greater than 4 continued for approximately 2 weeks after the mainshock. In order to investigate the accurate aftershock distributions and the development of the sequence of aftershocks, we deployed 56 temporary seismic stations for approximately one month in and around the source region of the mainshock. The details of the seismic activity within the first 5 days, which were determined by 14 temporary stations deployed immediately after the occurrence of the mainshock, are presented in another paper (Sakai et al, 2005)
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