HR: 15:10h
AN: S42F-06    [PDF]
TI: Simulation of Broadband Time Histories Combining Deterministic and Stochastic Methodologies
AU: * Graves, R W
EM: robert_graves@urscorp.com
AF: URS Corporation, 566 El Dorado Street, Pasadena, CA 91101 United States
AU: Pitarka, A
EM: arben_pitarka@urscorp.com
AF: URS Corporation, 566 El Dorado Street, Pasadena, CA 91101 United States
AB: We present a methodology for generating broadband (0 - 10 Hz) ground motion time histories using a hybrid technique that combines a stochastic approach at high frequencies with a deterministic approach at low frequencies. Currently, the methodology is being developed for moderate and larger crustal earthquakes, although the technique can theoretically be applied to other classes of events as well. The broadband response is obtained by summing the separate responses in the time domain using matched butterworth filters centered at 1 Hz. We use a kinematic description of fault rupture, incorporating spatial heterogeneity in slip, rupture velocity and rise time by discretizing an extended finite-fault into a number of smaller subfaults. The stochastic approach sums the response for each subfault assuming a random phase, an omega-squared source spectrum and simplified Green's functions (Boore, 1983). Gross impedance effects are incorporated using quarter wavelength theory (Boore and Joyner, 1997) to bring the response to a generic baserock level (e.g., $V_s = 1000$ m/s). The deterministic approach sums the response for many point sources distributed across each subfault. Wave propagation is modeled using a 3D viscoelastic finite difference algorithm with the minimum shear wave velocity set at 620 m/s. Short- and mid-period amplification factors provided by Borcherdt (1994) are used to develop frequency dependent site amplification functions. The amplification functions are applied to the stochastic and determinsitic responses separately since these may have different (computational) reference site velocities. The site velocity is taken as the measured or estimated value of ${V_s}^{30}$. The use of these amplification factors is attractive because they account for non-linear response by considering the input acceleration level. We note that although these design factors are strictly defined for response spectra, we have applied them to the Fourier amplitude spectra of our simulated time histories. This process appears to be justified since the amplification functions vary slowly with frequency and the method produces favorable comparisons with observed broadband motions. We have tested the methodology using ground motion recordings of the Northridge and Loma Prieta earthquakes.
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting