Aftershocks include those events that are triggered by other earthquakes while background events are those that occur independent of other earthquakes.
We choose a wide region so that we can include nearby earthquakes as background seismicity.
In doing so, we distinguished tremors from normal earthquakes and background noise using the criteria of source duration and the spectrum ratio between low and high frequencies.
Various declustering algorithms classifying earthquakes into background or triggered seismicity (aftershocks) have been investigated (e.g., Reasenberg, 1985; Zhuang et al., 2002).
Preceding the Kumamoto earthquakes, shallow background seismicity in and around Kumamoto Prefecture had long been present inland in the Japanese Archipelago (Ogata 2017c).
Therefore, the seismicity rate at time t is given by summing the effects of all prior earthquakes and background seismicity rate ⩽μas, (1) where c and p are parameters in the Omori-Utsu law, K and α control the aftershock productivity by a mainshock and its magnitude sensitivity, respectively.
Despite its prevalence in earthquake catalogs, the background physics still remains unclear (e.g., Vidale and Shearer, 2006).
The aftershocks are triggered because of internal stress adjustments in the seismogenic system initiated by the occurrence of an earthquake, while the background earthquakes are generally caused by forces related to plate tectonics.
In this study, background earthquake values are defined as MS = 6 for Alborz-Azarbayejan and Kopedagh provinces and MS = 5.5 for Central-East Iran province.
In their tomographic inversions, they used many data from aftershocks in addition to the background earthquake data, which enabled precise imaging of the source area within the slab.
