Probability of Occurrence of Velocity Pulses in Near-Source Ground Motions

Abstract

Near-source ground-motion records affected by directivity may show unusual features in the signal resulting in low-frequency cycle pulses in the velocity time history, especially in the fault-normal component. Such an effect causes the seismic demand for structures to deviate from that of so-called ordinary records. This circumstance may be particularly hazardous for structural engineering applications if it is not properly accounted for. In fact, current attenuation laws are not able to capture such effects well, if at all, and therefore current probabilistic seismic hazard analysis (PSHA) is not able to predict this peculiar spectral shape. This failure may possibly lead to an underestimation of, in particular, the nonlinear demand. Accounting for pulse-type records in earthquake engineering practice should be reflected both in the PSHA and in the record selection for seismic assessment of structures. These applications require a model for the probability of occurrence of pulselike records. Herein such a model is proposed on an empirical basis. A set of pulselike fault-normal ground motions from the Next Generation Attenuation of Ground Motions (NGA) Project dataset, as systematically identified by Baker (2007), is used. The independent variables studied are chosen from those considered by seismologists to affect the amplitude of directivity pulses. Issues related to the dataset and the explanatory power of the proposed models are also discussed.

Collapse Risk Evaluation of Self-Centering Steel MRFS With Viscous Dampers in Near-Fault Regions

“…Different IM are used in these equations (in this work S a (T 1 ) is adopted). PSHA has been recently modified to account for near source conditions, i.e., Near-source Probabilistic Seismic Hazard Analysis (NS-PSHA); more details, including implementation and applications, can be found in [2,[45][46][47][48][49]. According to this methodology, Equation (4) is adjusted to account for potential near-source directivity by an additional term, Z, which defines the site-to-source geometry: …”

“…The probability of a pulse occurring is a function of site-to-source geometry and decreases with distance from the fault and for shorter fault rupture lengths [2,46]. The pulse period distribution is a function of earthquake magnitude, with larger magnitude events usually causing longer pulse periods [46,50].…”

“…The location of earthquake epicenters is uniformly distributed along the fault while six sites with site-to-source distances equal to 5, 10 and 15 km at the end ("End-of-Fault" sites) and midpoint ("Midfault" sites) of the fault line are considered in this study (Figure 6(a)). The probability of a pulse occurring is computed by using the model in [2] and depends, in the case of a SS rupture, on rupture-to-site distance, R, the distance from the epicenter to the site measured along the rupture direction, s, and angle between the fault strike and the path from the epicenter to the site,  (a similar model is introduced in [46]). As noted in [49], a deterministic relationship between these parameters and rupture length, position of the rupture on the fault and epicenter location exist, allowing to easily simulate the uncertainty involved for the hazard computations.…”

“…Figure 7(b) shows the fragility curves obtained for the HP20D SC-MRF located at the midfault sites for site-to-source distances equal to 5, 10 and 15 km. It observed that the probability of collapse is reduced with distance from the fault, because the likelihood that a pulse occurs, P[Pulse|S a = x], decreases with distance and that affects the first part of Equation (2).…”

“…Such pulse is typically observed at the beginning of the fault-normal (FN) ground velocity time-history and has a probability of occurrence that depends on the site-tosource geometry, earthquake magnitude and other parameters [1][2]. Starting from the pioneer studies of Veletsos and Newmark [3] and Bertero et al [4], the response of yielding singledegree-of-freedom systems subjected to near-fault earthquake ground motions has been extensively studied by many researchers [5-6 and references therein].…”