Synthetic seismograms and the optimal period measured from Rayleigh wave amplitude spectra. (a) Vertical displacement waveforms at depth of 5 km, which are aligned to P onset. Numbers indicate the epicentral distance. The source time function is a triangle function with a duration of 0.3 s, which is estimated from the scaling law for an M_w 4 event (Somerville, 1999). (b) For depth of 10 km. (c) Rayleigh wave amplitude spectra (dots) calculated for waveforms at depth of 5 km. Grey regions indicate the optimal periods calculated based on eigenfunctions for distance 30–100 km. (d) Same as (c), but for depth of 10 km.