Abstract
We made a close investigation about the initial rising slopes of the envelope waveforms of the P-wave ground motion by using data for recent earthquakes. We have found that the P-wave initial slopes decrease almost linearly with increasing epicentral distance over the wide range of distance. This linear relation looks to be independent of the earthquake magnitudes and focal depths though the dispersion of data is somewhat large. We have found that the P-wave initial slopes of the foreshock M7.1 of the 2004 off the Kii peninsula earthquakes are very sharp. To the contrary, the slope of the main shock M7.4 is quite gentle. The main shock was preceded by a small event that occurred about 6.7 seconds before the main shock. This pre-event was generated near the initiation point of the main fracture. We are speculating that the difference in the initial slope between the foreshock and main shock may reflect the difference in the time variation of the stress drop at the early stage of the fracture and that the small pre-shock may be responsible for the slow rising of the initial P-wave motion of the main shock.
