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[SSS06-P05] Aftershock activity and its duration following inland earthquakes in Japan
Keywords:inland earthquakes, aftershock activity, duration of aftershock activity
A major earthquake (main shock) will be followed by many smaller earthquakes, which are called aftershocks, around its source area. Aftershock activity of a large earthquake is clearly detectable even in the number of earthquakes in all over Japan as a function of their size (magnitude). The analysis of aftershock activity is also important for disaster mitigation. In this study, we analyzed the number and duration of aftershocks for inland earthquakes in Japan and investigated their relationships to the size of major earthquakes.
In order to distinguish aftershocks from the steady seismic activity, it is necessary to define aftershocks in space and time. We first picked up earthquakes shallower than 30 km in the circular area from the epicenter of a large earthquake with a radius of 10^(0.5Mw-2) km as aftershock candidates, where Mw indicates the moment magnitude of the large main shock. We then define the seismic activity within 180 days before the main shock as the regular activity and investigated the period until the earthquake activity becomes lower than n times of the regular one, which is regarded as the aftershock duration. We analyzed three values of n=3, 5, 7 and confirmed that the conclusion is independent of the n value. Earthquakes with Mj equal to 3 and greater were investigated in this study for confirming the detection completeness, especially immediately after the 2011 Tohoku earthquake, where Mj stands for the magnitude value determined by Japan Meteorological Agency.
Although the temporal aftershock activity seems not to have a clear relationship to the size of main shocks, we detected a strong correlation between the size of main shocks and the number of aftershocks. We also found a weak correlation of the aftershock duration to the size of main shocks. We also investigated the effect of the regular earthquake rate to the results of aftershock duration. Earthquakes in a high seismic rate were likely to have a shorter aftershock duration, but we found no clear correlation between activities of regular earthquakes and aftershocks. These results suggest that the aftershock activity will be affected by other factors in addition to the main shock size, including the complexity of the rupture characteristics and the depth of the fault plane.
In order to distinguish aftershocks from the steady seismic activity, it is necessary to define aftershocks in space and time. We first picked up earthquakes shallower than 30 km in the circular area from the epicenter of a large earthquake with a radius of 10^(0.5Mw-2) km as aftershock candidates, where Mw indicates the moment magnitude of the large main shock. We then define the seismic activity within 180 days before the main shock as the regular activity and investigated the period until the earthquake activity becomes lower than n times of the regular one, which is regarded as the aftershock duration. We analyzed three values of n=3, 5, 7 and confirmed that the conclusion is independent of the n value. Earthquakes with Mj equal to 3 and greater were investigated in this study for confirming the detection completeness, especially immediately after the 2011 Tohoku earthquake, where Mj stands for the magnitude value determined by Japan Meteorological Agency.
Although the temporal aftershock activity seems not to have a clear relationship to the size of main shocks, we detected a strong correlation between the size of main shocks and the number of aftershocks. We also found a weak correlation of the aftershock duration to the size of main shocks. We also investigated the effect of the regular earthquake rate to the results of aftershock duration. Earthquakes in a high seismic rate were likely to have a shorter aftershock duration, but we found no clear correlation between activities of regular earthquakes and aftershocks. These results suggest that the aftershock activity will be affected by other factors in addition to the main shock size, including the complexity of the rupture characteristics and the depth of the fault plane.