Slow initial phase generated by microearthquakes occurring in the Western Nagano prefecture, Japan - The source effect

Near-source observations of microearthquakes occurring in the Western Nagan o prefecture show that durations of recorded slows initial phases were almo st constant independently of focal distance. Although a closer inspection r evealed that the durations of the slow initial phases slightly increased wi th focal distance, they were not proportional to focal distance. Thus, it i s concluded that the slow initial phases do not mainly reflect the whole pa th Q. If the slow initial phases were attributed to the strong anelastic at tenuation in the surface layer combined with that of the whole path Q, the rise time of the P-wave velocity pulse at the source for a M2.4 event would be estimated as about 3 ms and the source radius of the M2.4 event would b e estimated as about 10 m. This value of the source radius is too small for M2.4. This suggests that the slow initial phase does not arise From the st rong anelastic attenuation in the surface layer. it is likely that the slow initial phase does not mainly reflect a path effect but a source effect.