THE EFFECTS OF LOCAL SOIL CONDITIONS ON THE CHARACTERISTICS OF NEAR-FAULT DIRECTIVITY PULSE

Pulse like near-fault ground motions resulting from directivity effects are a special class of ground motions that are challenging characterize for seismic performance assessment. These motions contain a pulse in the velocity and sometimes in the acceleration time histories often occurring in the direction perpendicular to the fault rupture at locations near the fault where the earthquake rupture has propagated toward the site. Near-fault recordings from recent earthquakes indicate that this pulse is a narrow band pulse whose period increases with magnitude as expected from theory. This magnitude dependence of the pulse period causes the response spectrum to have a peak whose period and amplitude increase with magnitude or size of asperity(s) on a fault. Meanwhile, the effects of local soil conditions on the near-fault directivity pulse have not yet been investigated and need to be further examined. In this study, the relations among directivity pulse amplitude and period with local soil conditions are investigated. The strong ground motions from Chi-Chi (1999) earthquake are used here due to the large rupture length and significant number of stations in near-fault area with different soil conditions. This makes a good opportunity to investigate on the variation of forward directivity pulse parameters with the local soil properties at the sites. For this purpose, 44 accelerograms that contain forward directivity pulse are selected among 300 accelerograms recorded in near-fault area. The soil condition of recording stations is categorized into four soil types (I, II, III and IV; from very hard to loose) according to Iranian design code (standard 2800). Then, the acceleration response spectra for the accelerograms at each soil category are calculated and averaged. The amplification characteristics of directivity pulse are discussed by the mean spectral ratio of soil types II, III and IV with respect to soil type I (very hard or rock). The results show the spectral amplification up to 2.5 in the period range of 0.8 to 8 sec. Furthermore, the amplification value and period increase as the soil getting loose. Finally, the outcomes provide the field evidence on the directivity pulse amplification due to local soil conditions and their importance in design.

Keywords:

Near-fault Ground Motions , Directivity Pulse , Local Soil Condition , Soil Classification , Acceleration and Velocity Response Spectra