Variability of Near-Field Ground Motion from Dynamic Earthquake Rupture Simulations

Abstract

This study investigates near-field ground-motion variability due to dynamic rupture models with heterogeneity in the initial shear stress. Ground velocity seismograms are synthesized by convolving the time histories of slip velocity obtained from spontaneous dynamic rupture models with Green's functions of the medium calculated with a discrete wavenumber/finite-element method. Peak ground velocity (PGV) estimated on the synthetics generally matches well with an empirically derived attenuation relation, whereas spectral acceleration (SA) shows only an acceptable match at periods longer than 1 sec. Using the geometric mean to average the two orthogonal components leads to a systematic bias for the synthetics, in particular at the stations closest to the fault. This bias is avoided by using measures of ground motion that are independent of the sensor orientation.The contribution from stress heterogeneity to the overall ground-motion variability is found to be strongest close to the fault and in the backward directivity region of unilaterally propagating ruptures. In general, the intraevent variability originating from the radiation pattern and the effect of directivity is on the same order or larger than the interevent variability. The interevent ground-motion variability itself is dominated by the hypocenter-station configuration and is influenced only to a lesser extent by the differences in the dynamic rupture process due to the stress heterogeneity. In our modeling approach the hypocenter location is not picked arbitrarily but is determined to be mechanically consistent with the stress heterogeneity through a procedure emulating tectonic stress loading of the fault and nucleation. Compared to the peak ground motion recorded during the 2004 Parkfield, California, earthquake our simulated seismograms show enhanced spatial correlation that may be attributed to the simplicity of the assumed crustal model or to an incomplete representation of the spatial heterogeneity of dynamic rupture parameters. Nevertheless, the intraevent PGV variability in the near-fault region determined for the Parkfield dataset is of the same order of magnitude as for our simulations.

The role of topography and lateral velocity heterogeneities on near-source scattering and ground-motion variability

“…At such short distance, the ground shaking was generally thought to be dominated by source rupture complexity (e.g. Ripperger et al 2008). Intraevent standard deviation estimates obtained in this study are in agreement with the work of and indicate that the PGV variability and the apparent attenuation induced by topography is always lower than those associated with velocity heterogeneity, although the difference reduces sensibly at very short distance (e.g.…”

“…However, we note that PGV variability values derived from combining topography and velocity perturbations are comparable with those derived from dynamic rupture simulations in the near-source region as obtained by Ripperger et al (2008) and from empirical ground-motion prediction equations (e.g. Abrahamson et al 2008).…”

“…Eventually, the scattering of elastic waves has become the subject of a series of studies applying deterministic and stochastic modelling (Frankel & Clayton 1986;Roth & Korn 1993;Saito et al 2002;Nielsen & Thybo 2003;Pitarka 2009), targeted towards a deeper understanding of the scattering process, and deciphering the characteristics and distribution of heterogeneities in the Earth crust and mantle. Recently, have demonstrated that scattering can sensibly increase ground-motion complexity already at few kilometres away from the source, where source effects were generally thought to dominate (Guatteri et al 2003;Aagaard & Heaton 2004;Ripperger et al 2008).…”

Is Ground‐Motion Variability Distance Dependent? Insight from Finite‐Source Rupture Simulations

“…Finally, PGV is taken as the median value of all the 90 geometric means. Ripperger et al (2008) compared different PGV approximations from the two horizontal components and observed that GMRotD50 is a stable measure of the PGV showing a low dependence on the orientation of the horizontal components. Figure 6 shows the mean (with a standard deviation error bar) of ground motion in terms of natural log of PGV averaged over the different azimuths and along the R JB distances for the fault models considered in this study.…”

Ground motions characterized by a multi-scale heterogeneous earthquake model

“…Therefore, the locally identified SMGAs may be different from the main rupture front or characterized by a larger stress drop. A more complicated 'inhomogeneous' fault model (e.g., Ripperger et al 2008) might be more appropriate in such cases. Stochastically heterogeneous models (e.g., Ide and Aochi 2005) should be used in future ground motion estimation studies.…”