Source processes of the 1978 and 2005 Miyagi‐oki, Japan, earthquakes: Repeated rupture of asperities over successive large earthquakes

Highly similar seismograms commonly observed along transform or convergent plate boundaries indicate that repeated small to moderate earthquakes occur within a single area. The results of source‐process inversions for seismic events within the northeastern Japan subduction zone suggest that large earthquakes and their component asperities also exhibit this repeating nature under certain conditions. However, sets of similar seismograms that might provide direct evidence of the repeated rupture of large‐earthquake asperities have not been observed. This paper analyzes two offshore earthquakes near Miyagi Prefecture, Japan, the most recent occurring in 2005 with a JMA magnitude (M) of 7.2 in the source region of the 1978 earthquake (M7.4). We demonstrate the similarity in waveforms from the seismograms recorded during the 1978 and 2005 earthquakes. The early portions of the 2005 seismograms resemble the 1978 seismograms, suggesting that the asperities located close to the 1978 hypocenter ruptured again during the 2005 event. The seismogram similarities provide the first direct evidence for the repeated rupture of asperities during successive large earthquakes. The results of our waveform inversions further indicate that the two asperities of the 2005 event largely coincide with the southern two asperities of the 1978 event. These repeating asperities are recorded in the early portions of the 2005 seismograms with 60% of the amplitude recorded in the 1978 seismograms. The characteristic behavior of the asperities supports the slip‐predictable recurrence model of earthquake rupture rather than the time‐predictable recurrence model.