Big Velocity Pulses and Their Influence on Near-fault Ground Motion Records

Based on the strong earthquake acceleration records in different fault distances from the Chi-Chi earthquake in Taiwan(1999) and Northridge Earthquake(1994) in the United States,the statistical analysis and parameter fitting are performed.It is obvious that the occurrence of frequency of the speed pulses appearing in ground motion records decreases with increasing fault distances.When the fault distance is more than 40 km,the occurrence of frequency of the velocity impulse is very small.Beyond this range,the effects of speed pulses can be neglected.The attenuation law of pulse peak with increasing fault distances is also observed.Result show that either for Chi-Chi earthquake or Northridge earthquake,the general attenuation of pulse peak(v_g and v_p) is very fast.