IIIT Hyderabad Publications
Numerical Modeling of Near Fault Seismic Ground Motions for Strike-slip and Dip-slip faults
Author: Chenna Rajaram
Report no: IIIT/TH/2016/6
Advisor:Pradeep Kumar Ramancharla
Strong ground motion plays a vital role in engineering for design of important structures such as dams and nuclear power plants. The site of construction seldom contains past strong motion records, which is a major constraint in earthquake resistant design. Simulated strong motion records at such sites serve the purpose of safe design criteria. Researchers have been producing ground motions using analytical models, for the earthquakes that have occurred in the past. In the area of ground motion analysis, numerical studies on actual eld conditions are quite few. Proposed research is to generate ground motions in near fault regions for both strike-slip and dip-slip faults. For this purpose, these faults are modeled to simualte the ground motion records using numerical approach. Semi-empirical approach is used to validate the characteristics of ground motions of both the earthquakes. It is found that the PGA values follow attenuation with respect to epicentral distance. It is observed that the simulated ground motions are in good agreement with the observed ground motions along FN and FP directions. A ground motion record of PGA 0.62 g is estimated at maximum horizontal slip (8.8 m) of Denali fault during 2002 Denali earthquake. Further the analysis has been carried out to derive and propose geophysical parameters such as ground motion prediction equation (GMPE), stress drop, kappa parameter and frequency dependent quality parameter for Alaskan region. These parameters are important and play vital role in generating synthetic ground motions. The variation in magnitude of these parameters is significantly changed the peak ground acceleration and spectral acceleration. For this purpose, a study has been carried out on effect of geophysical parameters on ground motion. It is observed that there is no effect of quality factor on spectral acceleration in high period range, but slight effect is seen in low period range. Significant increase in spectral acceleration and PGA values is observed with increase in stress drop. As shear wave velocity increases, the rupture velocity increases as well and consequently the fault will be fractured more rapidly. Generally, the rupture velocity is assumed as 80-90% of shear wave velocity, which is also important parameter in accessing the ground motion. A simplied approach has been proposed to estimate Vr=Vs through numerical approach. It is observed that the ratios of rupture and shear wave velocities at 20 s, 25 s, 30 s, 35 s and 50 s are 0.75, 0.63, 0.84, 0.76 and 0.53, respectively. The maximum rupture velocity is equal to 0.76 times the shear wave velocity, whereas, v the assumed value was 0.8 times the shear wave velocity. Further, a uniform hazard spectrum (UHS) has been developed for Alaska region. The seismicity parameters a and b are generated for Alaska region and those are 5.76 and 0.776, respectively. A 10 % probability in 50 years corresponding to a 475 year return period for uniform hazard spectrum has been assumed. A maximum of 0.51 g spectral acceleration has been observed at the bed rock level.
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Centre for Earthquake Engineering
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