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Incorporation of Forward Directivity in Near-Fault Ground Motions into Probabilistic Seismic Hazard Analysis

Zarnousheh Farahani, Gholamreza | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41834 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Golafshani, Ali Akbar
  7. Abstract:
  8. Ground motions in close proximity to the causative fault of an earthquake can be significantly affected by the propagation of rupture. In particular, when the rupture and slip direction relative to a site coincide and a significant portion of the fault ruptures towards the site, the ground motion can exhibit the effects of Forward-Directivity (FD). Pulse-like near-fault ground motions resulting from directivity effects are a special class of ground motions that are challenging to characterize for seismic performance assessment. These motions contain a pulse in the velocity time history of the motion, often occurring in the direction perpendicular to the fault rupture at locations near the fault where the earthquake rupture has propagated towards the site. A recently proposed Moving Average (MA)-based signal processing approach was employed on a large ground motion library to empirically identify these pulses in ground motions. By using MA filter one can decompose near-fault records into two components, i.e., Pulse Type Record (PTR) and Back Ground Record (BGR). The first component is defined as the low-frequency smoothed part of each record, which is extracted by applying a middle-assigned MA filter, while the latter one is considered as the difference between the original record and the PTR. On the other hand current attenuation laws are not able to capture FD effects, and therefore current Probabilistic Seismic Hazard Analysis (PSHA) is not able to predict this peculiar spectral shape. This failure may possibly lead to an underestimation of, in particular, the nonlinear demands. Accounting for pulse-type records should be reflected both in the PSHA and in the record selection strategy for seismic assessment of structures. These applications require a model for the probability of occurrence of pulse-like records. Herein such a model is proposed on an empirical basis. A set of pulse-like fault-normal ground motions is selected from the Next Generation Attenuation of Ground Motions (NGA) Project dataset. The independent variables studied are chosen from those considered by seismologists to affect the amplitude of directivity pulses. Issues related to the dataset and the explanatory powers of the proposed models are also discussed.
    The response spectra of these records are then studied using this approach, and it is seen that their spectra can be described using an existing ground motion prediction (attenuation) model coupled with a narrow-band amplification function in the region of the pulse period. This modified prediction can be incorporated in probabilistic seismic hazard analysis, providing a direct and transparent method of accounting for directivity effects.
    After modification of the conventional PSHA to incorporate FD effects in a hazard analysis, the proposed PSHA methodology will be applied for seismic hazard analysis of a specific site in Tehran
  9. Keywords:
  10. Forward Directivity ; Moving Average (MA) ; Near-Field Earthquake ; Probability Analysis ; Probabilistic Seismic Hazard Analysis (PSHA) ; Pulse Probability

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