ALBERT

Description: The overturning fragility of a freestanding block such as a precariously balanced rock (PBR) has been parameterized as a function of a vector of ground-motion intensity measures. Methodologies are outlined to estimate the failure probabilities of such objects given their residence times. For deterministic seismic hazard analyses (DSHAs), a PBR is exposed to the scenario earthquakes that occur during its exposure time providing an estimate of the probability that the PBR survives the ensemble of events. For probabilistic seismic hazard analyses (PSHAs), the PBR overturning fragility is multiplied by the ground-motion occurrence rate from a vector-valued probabilistic seismic hazard analysis (VPSHA), yielding the marginal overturning rate for each ground-motion bin. Summing the marginal rates over all ground-motion bins produces the total overturning rate. For time-independent Poisson-based PSHA estimates, the probability of block failure can be easily calculated as a function of exposure time. This latter method is used to test VPSHA estimates similar to the 2002 U.S. Geological Survey (USGS) National Seismic Hazard Maps via PBR residence times. PBR overturning fragilities are estimated at sites in southern California near the San Andreas fault, between the San Jacinto and Elsinore faults, and near the White Wolf fault. The resulting failure probabilities for several of the PBRs are very high, suggesting that they are inconsistent with the 2002 USGS ground motions. An investigation of the hazard calculated with zero aleatory variability in the ground-motion prediction equations (GMPEs) suggests that the median ground motions or the earthquake rupture rates are too high at certain PBR sites.

Description: 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 beta ). 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 approximately W/3beta ) 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 approximately W/beta ), 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.