Alan Poulos

Selected publications [full list]

1. BSSA

   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 Preprint 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.

2. BSSA

   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 Preprint 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.

3. EE&SD

   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%.

4. EE&SD

   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.

5. BSSA

   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.

6. Earthq. Spec.

   Proposal of orientation-independent measure of intensity for earthquake-resistant design

   Poulos, A., and Miranda, E.

   Earthquake Spectra

   Abstract HTML PDF

   A new measure of ground motion intensity in the horizontal direction is proposed. Similarly to other recently proposed measures of intensity, the proposed intensity measure is also independent of the as-installed orientation of horizontal sensors at recording stations. This new measure of horizontal intensity, referred to as MaxRotD50, is defined using the maximum 5%-damped response spectral ordinate of two orthogonal horizontal directions and then computing the 50th percentile for all non-redundant rotation angles, that is, the median of the set of spectral ordinates in a range of 90º. This proposed measure of intensity is always between the median and maximum spectral ordinate for all non-redundant orientations, commonly referred to as RotD50 and RotD100, respectively. A set of 5065 ground motion records is used to show that MaxRotD50 is, on average, approximately 13-16% higher than Rot50 and 6% lower than RotD100. The new measure of intensity is particularly well suited for earthquake-resistant design where a major concern for structural engineers is the probability that the design ground motion intensity is exceeded in at least one of the two principal horizontal components of the structure, which for most structures are orthogonal to each other. Currently, design codes in the United States are based on RotD100, and hence using MaxRotD50 for structures with two orthogonal principal horizontal components would result in a reduction of the ground motion intensities used for design purposes.

7. BSSA

   Relations between MaxRotD50 and some horizontal components of ground-motion intensity used in practice

   Poulos, A., and Miranda, E.

   Bulletin of the Seismological Society of America

   Abstract HTML PDF Supp

   The most commonly used intensity measure of ground motion in earthquake engineering is the 5% damped spectral ordinate, which varies in different directions. Several different measures have been proposed over the years to combine the intensity of the two horizontal recorded ground motions to derive ground-motion models as well as for design purposes. This study provides the relation to seven previously used measures of horizontal ground motion with respect to a recently proposed orientation-independent measure of horizontal ground-motion intensity referred to as MaxRotD50. This new measure of horizontal intensity is defined as the median value of the maximum spectral ordinate of two orthogonal directions computed for all possible nonredundant orientations. The relations are computed using 5065 pairs of horizontal ground motions taken from the database of ground motions recorded in shallow crustal earthquakes in active tectonic regions developed as part of the Pacific Earthquake Engineering Research Center’s Next Generation Attenuation-West2 project. Empirically derived period-dependent relations are presented for three quantities that permit transforming any of the seven other definitions of horizontal ground-motion intensity to MaxRotD50, namely, (1) geometric mean of the ratio of MaxRotD50 to any of the seven other measures of intensities, (2) standard deviation of the natural logarithm of the ratio of MaxRotD50 to any of the seven other measures of intensities, and (3) the correlation between the natural logarithm of the ratio of MaxRotD50 to the other measures of intensities and the natural logarithm of the other measure of intensity. In addition, the influence of site class at the recording station, earthquake magnitude, and distance to the horizontal projection of the rupture is examined on the geometric mean of the ratio of MaxRotD50 to the median intensity of all nonredundant orientations (i.e., RotD50), showing negligible influence of site class and only a relatively small influence of magnitude and distance.