Rasool Anooshehpoor

Scale-Model and Numerical Simulations of Near-Fault Seismic Directivity

Foam rubber experiments simulating unilaterally propagating strike-slip earthquakes provide a means to explore the sensitivity of near-fault ground motions to rupture geometry. Subsurface accelerometers on the model fault plane show rupture propagation that approaches a limiting velocity close to the Rayleigh velocity. The slip-velocity waveform at depth is cracklike (slip duration of the order of narrower fault dimension W divided by S -wave speed β ). Surface accelerometers record near-fault ground motion enhanced along strike by rupture-induced directivity. Most experimental features (initiation time, shape, duration and absolute amplitude of acceleration pulses) are successfully reproduced by a 3D spontaneous-rupture numerical model of the experiments. Numerical- and experimental-model acceleration pulses show similar decay with distance away from the fault, and fault-normal components in both models show similar, large amplitude growth with distance along fault strike. This forward directivity effect is also evident in response spectra: the fault-normal spectral response peak (at period ∼ W /3 β ) increases approximately sixfold along strike, on average, in the experiments, with similar increase (about fivefold) in the corresponding numerical simulation. The experimental- and numerical-model response spectra agree with an empirical directivity model for natural earthquakes at long periods (near ∼ W / β ), and both overpredict shorter-period empirical directivity effects, with the amount of overprediction increasing systematically with diminishing period. We attribute this difference to rupture- and wavefront incoherence in natural earthquakes, due to fault-zone heterogeneities in stress, frictional resistance, and elastic properties present in the Earth but absent or minimal in the experimental and numerical models. Rupture-front incoherence is an important component of source models for ground-motion prediction, but finding an effective kinematic parameterization may be challenging.

The 1992 Little Skull Mountain Earthquake Sequence, Southern Nevada Test Site

The ML 5.6-5.8 Little Skull Mountain, Nevada, 29 June 1992 earth- quake occurred in the southwest portion of Nevada Test Site (NTS) approximately 20 km from Yucca Mountain, a potential site for a high-level radioactive waste repository. The earthquake involved predominantly down-to-the-southeast dip-slip motion with a small left-slip component on a steeply dipping, 70, northeast-striking fault. The mainshock nucleated near the base of the aftershock zone and ruptured up and to the northeast, and the mainshock rupture and the majority of the aftershock sequence were confined to depths between 6 and 12 km. All three ML 4 (largest ML 4.5) aftershocks occurred off the mainshock fault plane on secondary structures within the aftershock zone. The nearest strong-motion instrument located 11 km southwest of the epicenter recorded a peak acceleration of 0.206g. The earthquake occurred adjacent to the Rock Valley fault zone within an area of prior concentrated background seismicity and near the intersection of several northeast-striking faults in the southern NTS that have experienced Quaternary motion.

Constraints on Probabilistic Seismic-Hazard Models from Unstable Landform Features in New Zealand

We undertake the first New Zealand-based pilot study to investigate the use of ancient precariously balanced rocks (rocks that are unstably balanced on top of a pedestal) as a criteria for testing estimates of earthquake shaking from probabilistic seismic hazard models for long-return periods. To date, research to test seismic-hazard models in New Zealand has been restricted to the short historical record of earthquakes. Our survey of five sites in the South Island of New Zealand has yielded a total of 28 precariously balanced rocks which, on the basis of established methodology, are used to provide estimates of the maximum ground motions that could have occurred at the sites since the rocks became precarious. Age estimates for the precariously balanced rocks (40,400 to 55,300 years for central Otago schist rocks, and 77,300 years for northwest Nelson granitic rocks) are made from a limited number of cosmogenic dates obtained from bedrock removed from the pedestals of the rocks. Comparisons of the maximum peak ground accelerations and ages of the precariously balanced rocks with seismic-hazard curves derived from the New Zealand national seismic-hazard model show that the rocks indicate considerably lower hazard than the seismic-hazard model at sites located within 5 km of active faults, whereas the agreement is favorable for sites located away from active faults. The variability about the median estimates of peak ground acceleration for the fault sources and/or the median accelerations for the fault sources assumed in the seismic- hazard model may therefore be overestimated for the sites near active faults.