Alan Poulos

Selected publications [full list]

1. 

   Orientation dependence of probabilistic seismic hazard estimates from CyberShake physics‐based simulations

   Poulos, A., Hirakawa, E., Parker, G. A., and Baltay, A.

   Bulletin of the Seismological Society of America

   Abstract HTML

   Earthquake ground‐motion intensities, such as pseudospectral accelerations (SAs), can vary significantly with horizontal orientation. However, conducting probabilistic seismic hazard analysis (PSHA) for each horizontal orientation is challenging because current ground‐motion models used in PSHA consider only a single horizontal intensity value, usually the median across all orientations, known as RotD50. To address this limitation, we employ physics‐based simulations for PSHA, which contain full waveforms from which ground‐motion intensities can be computed for all horizontal orientations to study directional seismic hazard. We apply our approach to the latest CyberShake study of the Greater Los Angeles metropolitan area, developed by the Statewide California Earthquake Center, finding that seismic hazard at a 2475‐yr return period, a common value used for earthquake‐resistant design, varies significantly with horizontal orientation. For instance, for SAs at 3 s, the maximum seismic hazard across all horizontal orientations is, on average, 15% higher than the median RotD50 hazard, with these differences becoming more pronounced at longer periods. These observed variations can generally be attributed to physical mechanisms that polarize seismic waves, such as the radiation pattern of the earthquake source and the influence of the subsurface structure. These results may have important implications for earthquake engineering applications, particularly for long‐period structures in areas with substantial horizontal variations in seismic hazard.

2. 

   Incorporating ground motion directionality into probabilistic seismic hazard analysis

   Poulos, A., and Miranda, E.

   Earthquake Spectra

   Abstract HTML Preprint Supp

   Probabilistic seismic hazard analysis (PSHA) is typically used to estimate a ground motion intensity measure representing a central tendency within the horizontal plane, such as the median intensity across all horizontal orientations. However, in many cases, ground motion intensity is needed in specific horizontal orientations, particularly along the principal response axes of structures. Depending on the earthquake source, path effects, and site conditions, a site may exhibit significant levels of ground motion directionality, that is, a variation in ground motion intensity within the horizontal plane. If such variations are systematic across the earthquakes that affect the site, they can lead to significant differences in ground motion intensity between horizontal orientations. This paper introduces directional PSHA, an extension of conventional PSHA that quantifies seismic hazard for a specified horizontal orientation. The method primarily involves adapting the outputs of existing ground motion models to be orientation specific. The approach is applied to strike-slip earthquakes, which tend to produce above-average ground motion intensity in the transverse orientation. Two simple case studies are used to illustrate the method: a line-segment fault and the San Francisco Bay Area, limited to the San Andreas and Hayward Faults. Results show strong orientation dependence in seismic hazard at sites located very close to a strike-slip fault. Similar levels of orientation dependence are found at sites located farther from the fault relative to its length. The proposed method offers a more detailed description of the seismic hazard at a site and can improve seismic risk assessment methods for both new and existing structures.

3. 

   Accounting for ground motion directionality and building orientations in urban seismic risk analysis

   Poulos, A., and Miranda, E.

   Earthquake Spectra

   Abstract HTML PDF Supp

   Regional seismic risk analysis methods are used to estimate the loss of a group of buildings subjected to earthquakes. Buildings usually have two perpendicular horizontal principal orientations, which tend to be similar among buildings within the same city due to the grid-like nature of street layouts. Current regional seismic risk analysis methods do not consider these building orientations to estimate losses because they usually consider orientation-independent measures of horizontal ground motion intensity. However, horizontal ground motions present directionality, meaning that their intensities vary with horizontal orientation. Thus, the response of a building depends on its orientation relative to the orientation of the ground motion. This work studies the effect of building orientations and ground motion directionality on groups of buildings within a city. A testbed group of buildings subjected to an example earthquake is used to estimate losses in terms of the aggregate repair cost of the buildings. The variance of these output losses is shown to be maximized when all buildings share the same principal orientations and minimized when these orientations are fully randomized. A variance-based sensitivity analysis is then performed to compare the contribution to the output variance of ground motion directionality with that of some other sources of uncertainty usually considered in regional seismic risk analyses. When buildings share the same principal orientations and the median ground motion intensities from all orientations are fixed, ground motion directionality is found to be an important source of uncertainty, and its importance increases as more buildings are considered in the analysis. These results suggest that considering ground motion directionality and building orientations could be important for future urban seismic risk analyses and that, under the same seismic hazard, the seismic risk of cities with regular orthogonal street arrangements is larger than that of cities with more irregular street layouts.

4. 

   Modification of ground-motion models to estimate orientation-dependent horizontal response spectra in strike-slip earthquakes

   Poulos, A., and Miranda, E.

   Bulletin of the Seismological Society of America

   Abstract HTML PDF Supp

   A model to estimate the 5% damped response spectra of horizontal components at specific orientations is presented. The model, which explicitly accounts for directionality, is based on prior research by the authors that identified that the orientation of maximum horizontal spectral response at a site in strike-slip earthquakes tends to occur at or close to the transverse orientation with respect to the epicenter. Using a database of 1962 ground motions recorded in shallow crustal earthquakes with strike-slip faulting, it is shown that there is a significantly larger probability of exceeding orientation-independent RotD50 intensities in the transverse orientation than in the radial orientation. Furthermore, the results indicate that, on average, spectral responses in the transverse orientation are significantly larger than those in the radial orientation and that these differences become more significant as the period of the oscillator increases. For example, spectral responses in the transverse orientation are, on average, 12% larger than those in the radial orientation for 1 s oscillators and 78% larger for 10 s oscillators. A period- and orientation-dependent model is developed and calibrated to estimate 5% damped response spectral ordinates at specific orientations by modifying orientation-independent RotD50 intensities. The proposed orientation-dependent model can explicitly account for directionality by modifying the means and standard deviations of any ground-motion model that estimates RotD50 response spectral ordinates for strike-slip earthquakes to obtain probability distributions of response spectral ordinates but now at specific horizontal orientations.

5. 

   Effect of style of faulting on the orientation of maximum horizontal earthquake response spectra

   Poulos, A., and Miranda, E.

   Bulletin of the Seismological Society of America

   Abstract HTML PDF Supp

   Horizontal earthquake ground motions have intensities that vary with changes in orientation, that is, they depend on the azimuth. Previous studies have found that the maximum horizontal spectral acceleration at mid-to-long periods is more likely to be closer to the strike-normal orientation than to the strike-parallel orientation for ground motions recorded very close to earthquake ruptures. However, this trend disappears relatively quickly as the distance to the rupture increases and becomes almost nonexistent for rupture distances longer than 5 km. Using a database of ground motions from shallow crustal earthquakes in active tectonic regimes, this work studies the orientation of maximum horizontal spectral acceleration of ground motions from earthquakes with strike-slip and reverse faulting. For strike-slip earthquakes, these orientations of maximum intensity are found to be close to the transverse orientation, that is, an orientation that is perpendicular to the line segment between the recording station and the earthquake epicenter. Contrary to what occurs with respect to the previously studied strike-normal orientation, the orientation of maximum spectral response remains close to the transverse orientation regardless of source-to-site distance. Moreover, on average, the orientations of maximum spectral response tend to become closer to the transverse orientation as the period increases. On the other hand, ground motions from reverse earthquakes do not show any trend in the orientation of maximum spectral response relative to the transverse orientation. Probability distributions of the angular difference between the orientation of maximum spectral response and the transverse orientation for strike-slip earthquakes were fitted at several periods between 0.01 and 10 s. These directionality effects from strike-slip earthquakes could be considered in future seismic hazard analyses.

6. 

   Damping-dependent correlations between response spectral ordinates

   Poulos, A., and Miranda, E.

   Earthquake Engineering & Structural Dynamics

   Abstract HTML PDF Supp Code

   Correlations between response spectral ordinates at different periods are used in several seismic hazard computations, such as for the construction of conditional mean spectra and conditional spectra. Conventionally, these correlations have been computed and reported only for a damping ratio of 5%; however, structures may have damping ratios substantially lower or higher than 5%. Therefore, in those cases, one requires correlations of spectral ordinates at different periods but having the same damping ratios that are different than 5%, correlations of spectral ordinates at the same period but having different damping ratios, or the general case of correlation between spectral ordinates of two oscillators having different damping ratios and different periods. This work computes such damping-dependent correlations by using the NGA-West2 ground motion database. In general, it is found that correlations increase as the damping ratio of any of the two spectral ordinates increases and as the ratio of periods of vibration of the two oscillators departs from one. A nonlinear regression model is fitted to the resulting damping-dependent correlations to simplify future computations. Finally, the use of the new damping-dependent correlations is illustrated by computing example conditional spectra for damping ratios differing from 5%. The results show that using 5%-damped correlations for the construction of condition mean spectra, overestimates spectral ordinates for damping ratios lower than 5% and underestimates spectral ordinates for damping ratios higher than 5%.

7. 

   Probabilistic characterization of the directionality of horizontal earthquake response spectra

   Poulos, A., and Miranda, E.

   Earthquake Engineering & Structural Dynamics

   Abstract HTML PDF Supp

   Earthquake response spectral ordinates vary significantly with changes in orientation within the horizontal plane. This variation is characterized probabilistically in this study using a large database of recorded earthquake ground motions. For each ground motion record, response spectral ordinates are computed in all horizontal orientations as a function of the rotation angle with respect to the azimuth of maximum response and then normalized by (1) the maximum and (2) the median spectral ordinate from all these orientations. Nonlinear regression models are then fitted to the means, standard deviations, and correlations of both ratios, as a function of rotation angle. To achieve a more complete probabilistic description, probability distributions are fitted to both ratios at each rotation angle. These results can be used for several probabilistic seismic hazard computations, such as the sampling of response spectral ordinates at specific orientations within the same site.

8. 

   Evaluation of earthquake response spectra directionality using stochastic simulations

   Poulos, A., Miranda, E., and Baker, J. W.

   Bulletin of the Seismological Society of America

   Abstract HTML PDF

   For earthquake-resistant design purposes, ground-motion intensity is usually characterized using response spectra. The amplitude of response spectral ordinates of horizontal components varies significantly with changes in orientation. This change in intensity with orientation is commonly known as ground-motion directionality. Although this directionality has been attributed to several factors, such as topographic irregularities, near-fault effects, and local geologic heterogeneities, the mechanism behind this phenomenon is still not well understood. This work studies the directionality characteristics of earthquake ground-motion intensity using synthetic ground motions and compares their directionality to that of recorded ground motions. The two principal components of horizontal acceleration are sampled independently using a stochastic model based on finite-duration time-modulated filtered Gaussian white-noise processes. By using the same stochastic process to sample both horizontal components of motion, the variance of horizontal ground acceleration has negligible orientation dependence. However, these simulations’ response spectral ordinates present directionality levels comparable to those found in real ground motions. It is shown that the directionality of the simulated ground motions changes for each realization of the stochastic process and is a consequence of the duration being finite. Simulated ground motions also present similar directionality trends to recorded earthquake ground motions, such as the increase of average directionality with increasing period of vibration and decrease with increasing significant duration. These results suggest that most of the orientation dependence of horizontal response spectra is primarily explained by the finite significant duration of earthquake ground motion causing inherent randomness in response spectra, rather than by some physical mechanism causing polarization of shaking.