Abstract
As regards strike-slip faultings generated inland in Japan, we occasionally observe conjugate fault activity in association with main faultings. In most cases, conjugate faults appear on the side of main faults where frictional force on conjugate faults decreases in response to the main faulting. We present two examples which typically show mutually conjugate activities. These are the pair of the 1978 Izu-Oshima-kinkai earthquake (M7.0) and the 1990 earthquake near Izu-Oshima Island (M6.5) and the pair of the 1984 Western Nagano prefecture earthquake (M6.8) and its largest aftershock (M6.2). It seems rare that conjugate fractures were propagated in regions where frictional forces on conjugate faults were expected to increase. These facts indicate that changes in frictional force plays an important role in the generation of secondary fault activity at least in inland strike-slip faults in Japan. In other words, the frictional coefficient in the Coulomb failure function may not be as small as 0.1∼0.3 but as large as 0.5∼0.7 in its value in inland strike-slip faultings in Japan.
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