Physical Characteristics Controlling Radiation from Heterogeneous Ruptures—Finite Faults

Abstract

ABSTRACT Kinematic simulations of ground motion require representations of the earthquake source: the distribution of final slip, parameters of the source time function, and the velocity of rupture travel. There is a significant ambiguity in prescribing these physical characteristics, causing uncertainty in the resulting motions that needs to be quantified. The representation integral is an appropriate tool: it allows exact calculation of the source effect in both the near and far fields in the frequency band of practical interest. The commonly used distributions of slip have a k-square shape of their wavenumber spectra. Various k-square slips change the slope of the radiated spectra in the range of ∼−2.5 and −4.0 in both the far and near fields. The spectra generated by randomly disturbed constant slip are indistinguishable from those emitted by k-square faults. In both cases, variations in peak values of ground velocity and acceleration between realizations are relatively insignificant: under ∼15% for the same hypocenter position. The slopes of the Fourier spectra produced exclusively by the form of the slip function and the slip heterogeneity are equivalent to using a formal kappa filter with κ ranging from ∼0.025 to 0.045 s. No ad hoc high-frequency filtering (of kappa or fmax type) is required if fault finiteness is accounted for. Geometric irregularity of rupture fronts, at least for the way the front progression is randomized in our case, does not appreciably affect the slopes of the spectra. Its principal effect is in blurring the directivity, reducing the sharpness of radiated pulses. The most influential parameter affecting the peak ground motions for several commonly used slip functions is the maximum velocity of slip: scaling of vmax causes a proportional scaling in peak ground acceleration. This parameter is the most important to constrain to reduce ambiguities in predicted ground motions.