LOW FREQUENCY, DETERMINISTIC SIMULATION OF STRONG GROUND MOTION

Eser Durukal

(Thesis Supervisor: Mustafa Erdik)

ABSTRACT

In this study low frequency strong ground motions are simulated for the İzmit Bay area in Turkey, for the San Fransisco Bay area in California, USA and for the Dinar region in Turkey. Simulation is based on the evaluation of the triple integral of the analytically constructed frequency-wave number spectra of the displacement field in to space-time domain. The extended seismic source is represented by discretized double couples describing point sources. The earth medium is layered.

The effect of near source factors, namely the fault geometry, directivity and accordingly arising spatial variability have been discussed. The effect of source functions (Haskell and barrier types) and of source properties like rupture pattern, rise time, rupture velocity, effect of ground layering on the resulting ground motions are investigated.

The low frequency (0-2 Hz), deterministic modeling of strong ground motion is an important and useful tool for obtaining realistic ground displacements and velocities in the near field of causative faults. The recently reevaluated problems of effect of rupture directivity and fault geometry can be successfully modeled using deterministic models. The incoherent part of strong ground motion (~˃ 2 Hz), that depends on the details of rupture process and on the earth medium heterogeneities on the travel path of seismic waves between the fault and the station which can not be a-priorily assessed, is difficult to model deterministically due to its random nature. Therefore, as it seems today, the broadband simulation of strong ground motion in the coming years will be exercised as a rational combination of the deterministically found, coherent, low-frequency part with the stochastically determined incoherent, high frequency portion.