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UCSB Method for Broadband Ground Motion for Kinematic Simulations of Earthquakes

Jorge G. Crempien, & Ralph J. Archuleta

Accepted 2014, SCEC Contribution #1980

The UCSB method simulates the earthquake source by using correlated probability density functions for the kinematic parameters. As such, the source is stochastic. The wave propagation is computed using Green’s functions for a specified 1D anelastic velocity structure. In the initial formulation of the UCSB method (Liu et al., 2006; Schmedes et al., 2010; 2013) a single velocity structure was used to compute the full frequency range for the broadband ground motion. While Green’s functions for layered structures may provide a template to compute arrival times, these layered structures, especially near surface layers, do not reproduce the amplitude of the signal. This is particularly true for waves arriving with a large incident angle, characteristic of near-source records from faults that have slip at shallow depth. Moreover, it is unrealistic to expect that near surface layers with constant thickness and constant velocity persist for 10’s or 100’s of kilometers. We modified the UCSB broadband (UCSB BB) method by using two different velocity structures. For frequencies greater than 1.0 Hz we compute 1D Green’s functions for a velocity structure that is homogeneous above the Moho. We amplify the amplitudes of these high-frequency waves with the quarter wavelength amplification method (QWAM) of Boore and Joyner (1997). For low frequencies, we use a detailed 1D or 3D velocity structure to compute Green’s functions. We ensure that the S-wave arrival matches for both structures. With the wavelet method of Liu et al. (2006), we stitch the low- and high-frequency Fourier spectra at the crossover frequency (nominally 1.0 Hz). Using the inverse Fourier transform we produce broadband times histories of ground motion from a kinematic source that generates both the low and high frequencies.

Crempien, J. G., & Archuleta, R. J. (2014). UCSB Method for Broadband Ground Motion for Kinematic Simulations of Earthquakes. Seismological Research Letters, (accepted).