High-Frequency Waves in the Observed Seismograms of Large and Moderate Earthquakes

The origin of high-frequency waves superposed on low-frequency waves was investigated using observed seismograms at hard rock sites. Two sets of data were used in this paper. One is the seismograms of main shocks and foreshock or aftershock sequences (M2.0-5.2) which occurred in the Kinki district, Japan. The other is the seismograms of the aftershocks of the Central Japan Sea earthquake (M5.3-7.1). For the earthquakes in the Kinki district, P-wave velocitywaveforms of a couple of events which occurred almost at the same location, were compared in the time domain. It was found that the high-frequency waves superposed on low-frequency waves generated by main shocks are very similar to the waveforms of their foreshocks or aftershocks. For the aftershocks of the Central Japan Sea earthquakes, the high frequency waves of every event had almost the same shape, even if its hypocenter is different from each other. Furthermore, their three component waveforms indicated that the high frequency waves did not arrived from the direction of their sources. It was concluded that almost all the high frequency waves analyzed in this paper reflect the site effect rather than the source process.