Arben Pitarka
Verified @llnl.gov
Seismologist, Atmospheric, Earth, and Energy Division
Lawrence Livermore National Laboratory
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Arsam Taslimi, Floriana Petrone, and Arben Pitarka
American Society of Civil Engineers (ASCE)
Mamun Miah, David McCallen, Arben Pitarka, and Floriana Petrone
Wiley
AbstractRecent advancements in high performance computing platforms and computational workflow for regional‐scale simulations are enabling unprecedented modeling of fault‐to‐structure earthquake processes. Regional simulations resolving ground motions at frequencies relevant to engineered systems are becoming computationally viable and provide a new capability to improve understanding of the geographical distribution and intensity of risk to buildings and critical infrastructure. As computational capabilities advance, it is essential to move beyond illustrative single rupture realizations for scenario earthquake events towards the development of a full suite of rupture realizations that appropriately characterize the range of risk to building systems. The work described in this article investigates the application of a suite of fault rupture realizations with the objective of assessing near‐fault, site‐specific seismic demand variability for building structures. A representative high‐performance regional‐scale computational model is utilized to execute ground motion and building response simulations based on 18 kinematic rupture realizations of an M7 strike‐slip scenario earthquake. The fault rupture models for the scenario earthquake are created by systematically perturbing the hypocenter location and stochastically generating rupture parameters (slip, rise time, rake angle) to represent a breadth of ground motion intensities resulting from the spatial and temporal variabilities of an earthquake rupture process. The resulting seismic demand variability for three‐story (short period) and forty‐story (long period) steel moment‐resisting frame buildings is characterized in terms of the median and distribution of peak inter‐story drift ratio for a range of near‐fault sites. The full suite of 18 fault rupture realizations and approximately 280,000 nonlinear dynamic building simulations indicate that the three‐story building undergoes higher median seismic demand and significantly greater variability of demand at a given site than the forty‐story building, which has important implications for the level of certainty in predicting building performance during an earthquake. The simulations performed provide deeper insight into the relationship between fault rupture parameterization and building response, which is essential information for developing a representative suite of rupture realizations for specific earthquake scenarios.
Maha Kenawy, David McCallen, and Arben Pitarka
SAGE Publications
Earthquake-induced ground shaking near rupturing faults is highly sensitive to the rupture characteristics, seismic wave propagation patterns and site conditions, and field recordings of near-fault shaking are relatively sparse. These challenges complicate the assessment of the seismic performance of near-fault structures. A common approach to representing near-fault ground motion in engineering analysis is to explicitly consider and select records with strong directivity pulses (pulse records). We use three-dimensional high-resolution physics-based earthquake simulations to test this approach in the context of scenario-based ground motion record selection, and to study the important characteristics of near-fault ground shaking. We highlight the deficiencies associated with classifying near-fault simulated records as “pulse” or “non-pulse,” based on the presence of a single dominating pulse in the velocity time history. We show that this approach is inadequate for characterizing near-fault shaking on soft soils which can be dominated by both forward rupture directivity and basin amplification effects. We conduct ground motion selection experiments for the analysis of near-fault structures with and without explicit classification of the pulse features in the records, and evaluate the bias in the predicted structural demands. We find that the maximum interstory drift demands on building structures imposed by unscaled site-specific simulated ground motion records selected based on relevant spectral shape features are not sensitive to the classification of records as pulse/non-pulse. Therefore, with regard to predicting the maximum interstory drifts in near-fault buildings, we do not find justification for the binary pulse classification of near-fault records.
Arben Pitarka, Aybige Akinci, Pasquale De Gori, and Mauro Buttinelli
Seismological Society of America (SSA)
ABSTRACT The Mw 6.5 Norcia, Italy, earthquake occurred on 30 October 2016 and caused extensive damage to buildings in the epicentral area. The earthquake was recorded by a network of strong-motion stations, including 14 stations located within a 5 km distance from the two causative faults. We used a numerical approach for generating seismic waves from two hybrid deterministic and stochastic kinematic fault rupture models propagating through a 3D Earth model derived from seismic tomography and local geology. The broadband simulations were performed in the 0–5 Hz frequency range using a physics-based deterministic approach modeling the earthquake rupture and elastic wave propagation. We used SW4, a finite-difference code that uses a conforming curvilinear mesh, designed to model surface topography with high numerical accuracy. The simulations reproduce the amplitude and duration of observed near-fault ground motions. Our results also suggest that due to the local fault-slip pattern and upward rupture directivity, the spatial pattern of the horizontal near-fault ground motion generated during the earthquake was complex and characterized by several local minima and maxima. Some of these local ground-motion maxima in the near-fault region were not observed because of the sparse station coverage. The simulated peak ground velocity (PGV) is higher than both the recorded PGV and predicted PGV based on empirical models for several areas located above the fault planes. Ground motions calculated with and without surface topography indicate that, on average, the local topography amplifies the ground-motion velocity by 30%. There is correlation between the PGV and local topography, with the PGV being higher at hilltops. In contrast, spatial variations of simulated PGA do not correlate with the surface topography. Simulated ground motions are important for seismic hazard and engineering assessments for areas that lack seismic station coverage and historical recordings from large damaging earthquakes.
Arben Pitarka, Robert Graves, Kojiro Irikura, Ken Miyakoshi, Changjiang Wu, Hiroshi Kawase, Arthur Rodgers, and David McCallen
Seismological Society of America (SSA)
ABSTRACT The main objective of this study is to develop physics-based constraints on the spatiotemporal variation of the slip-rate function using a simplified dynamic rupture model. First, we performed dynamic rupture modeling of the 2019 Mw 7.1 Ridgecrest, California, earthquake, to analyze the effects of depth-dependent stress and material friction on slip rate. Then, we used our modeling results to guide refinements to the slip-rate function that were implemented in the Graves–Pitarka kinematic rupture generation technique. The dynamic ruptures were computed on a surface-rupturing, planar strike-slip fault that includes a weak (negative to low-stress-drop) zone in the upper 4 km of the crust. Below the weak zone, we placed high-stress-drop patches designed to mirror the large-slip areas seen in various rupture model inversions of the event. The locations of the high-stress-drop patches and the hypocenter were varied in multiple realizations to investigate how changing the dynamic conditions affected the resulting rupture kinematics, in particular, the slip rate. From these simulations, we observed a systematic change in the shape of the slip-rate function from Kostrov type below the weak zone to a predominantly symmetric shape within the weak zone, along with a depth-dependent reduction of peak slip rate. We generalized these shallow rupture features into a depth-dependent parametric variation of the slip-rate function and implemented it in the Graves–Pitarka kinematic rupture model generator. The performance of the updated kinematic approach was then verified in 0–4 Hz simulations of the Mw 7.1 Ridgecrest earthquake, which showed that incorporating the depth-dependent variation in the shape of the slip-rate function improves the fit to the observed near-fault ground motions in the 0.5–3 s period range.
Robert J. Mellors, Robert Abbott, David Steedman, David Podrasky, and Arben Pitarka
American Geophysical Union (AGU)
Floriana Petrone, Norman Abrahamson, David McCallen, Arben Pitarka, and Arthur Rodgers
Wiley
Arben Pitarka and Robert Mellors
Seismological Society of America (SSA)
ABSTRACT In an ongoing effort to improve 3D seismic-wave propagation modeling for frequencies up to 10 Hz, we used cross correlations between vertical-component waveforms from an underground chemical explosion to estimate the statistical properties of small-scale velocity heterogeneities. The waveforms were recorded by a dense 2D seismic array deployed during the Source Physics Experiments for event number 5 (SPE-5) in a series of six underground chemical explosions, conducted at the Nevada National Security Site. The array consisted of 996 geophones with a 50–100 m grid spacing, deployed at the SPE site at the north end of the Yucca Flat basin. The SPE were conducted to investigate the generation and propagation of seismic and acoustic waves from underground explosions. Comparisons of decay rates of waveform cross correlations as function of interstation distance, computed for observed and synthetic seismograms from the SPE-5 chemical explosion, were used to constrain statistical properties of correlated stochastic velocity perturbations representing small-scale heterogeneities added to a geology-based velocity model of the Yucca Flat basin. Using comparisons between recorded and simulated waveform cross correlations, we were able to recover sets of statistical properties of small-scale velocity perturbations in the velocity model that produce the best-fit between the recorded and simulated ground motion. The stochastic velocity fluctuations in the velocity model that produced the smallest misfits have a horizontal correlation distance of between 400 and 800 m, a vertical correlation distance between 100 and 200 m, and a standard deviation of 10% from the nominal model velocity in the alluvium basin layers. They also have a horizontal correlation distance of 1000 m, a vertical correlation distance of 250 m, and a standard deviation of 6% in the underlying and consolidated sedimentary layers, up to a depth of 4 km. Comparisons between observed and simulated wavefields were used to assess the proposed small-scale heterogeneity enhancements to the Yucca Flat basin model. We found that adding a depth-resolved stochastic variability to the geology-based velocity model improves the overall performance of ground-motion simulations of an SPE-5 explosion in the modeled frequency range up to 10 Hz. The results may be applicable to other similar basins.
G. A. Ichinose, S. R. Ford, K. Kroll, D. Dodge, M. Pyle, A. Pitarka, and W. R. Walter
American Geophysical Union (AGU)
Maha Kenawy, David McCallen, and Arben Pitarka
Wiley
David McCallen, Floriana Petrone, Mamun Miah, Arben Pitarka, Arthur Rodgers, and Norman Abrahamson
SAGE Publications
The existing observational database of the regional-scale distribution of strong ground motions and measured building response for major earthquakes continues to be quite sparse. As a result, details of the regional variability and spatial distribution of ground motions, and the corresponding distribution of risk to buildings and other infrastructure, are not comprehensively understood. Utilizing high-performance computing platforms, emerging high-resolution, physics-based ground motion simulations can now resolve frequencies of engineering interest and provide detailed synthetic ground motions at high spatial density. This provides an opportunity for new insight into the distribution of infrastructure seismic demands and risk. In the work presented herein, the EQSIM fault-to-structure computational framework described in a companion paper, McCallen et al., is employed to investigate the regional-scale response of buildings to large earthquakes. A representative M = 7.0 strike-slip event is used to explore the distribution and amplitude of building demand, and comparisons are made between building response computed with fault-to-structure simulations and building response computed with existing measured near-fault earthquake records. New information on the distribution and variability of building response from high-performance parallel simulations is described and analyzed, and favorable first comparisons between building response predicted with both fault-to-structure simulations and real ground motions records are presented.
David McCallen, Anders Petersson, Arthur Rodgers, Arben Pitarka, Mamun Miah, Floriana Petrone, Bjorn Sjogreen, Norman Abrahamson, and Houjun Tang
SAGE Publications
Computational simulations have become central to the seismic analysis and design of major infrastructure over the past several decades. Most major structures are now “proof tested” virtually through representative simulations of earthquake-induced response. More recently, with the advancement of high-performance computing (HPC) platforms and the associated massively parallel computational ecosystems, simulation is beginning to play a role in increased understanding and prediction of ground motions for earthquake hazard assessments. However, the computational requirements for regional-scale geophysics-based ground motion simulations are extreme, which has restricted the frequency resolution of direct simulations and limited the ability to perform the large number of simulations required to numerically explore the problem parametric space. In this article, recent developments toward an integrated, multidisciplinary earth science-engineering computational framework for the regional-scale simulation of both ground motions and resulting structural response are described with a particular emphasis on advancing simulations to frequencies relevant to engineered systems. This multidisciplinary computational development is being carried out as part of the US Department of Energy (DOE) Exascale Computing Project with the goal of achieving a computational framework poised to exploit emerging DOE exaflop computer platforms scheduled for the 2022–2023 timeframe.
Michelle Scalise, Arben Pitarka, John N. Louie, and Kenneth D. Smith
Seismological Society of America (SSA)
ABSTRACT Explosions are traditionally discriminated from earthquakes, using the relative amplitude of compressional and shear waves at regional and teleseismic distances known as the P/S discriminant. Pyle and Walter (2019) showed this technique to be less robust at shorter distances, in detecting small-magnitude earthquakes and low-yield explosions. The disparity is largely due to ground motion from small, shallow sources being significantly impacted by near-surface structural complexities. To understand the implications of wave propagation effects in generation of shear motion and P/S ratio during underground chemical explosions, we performed simulations of the Source Physics Experiment (SPE) chemical explosions using 1D and 3D velocity models of the Yucca Flat basin. All simulations used isotropic point sources in the frequency range 0–5 Hz. We isolate the effect of large-scale geological structure and small-scale variability at shallow depth (<5 km), using a regional 3D geologic framework model (GFM) and the GFM-R model derived from the GFM, by adding correlated stochastic velocity perturbations. A parametric study of effects of small-scale velocity variations on wave propagation, computed using a reference 1D velocity model with stochastic perturbations, shows that the correlation length and depth of stochastic perturbations significantly impact wave scattering, near-surface wave conversions, and shear-wave generation. Comparisons of recorded and simulated waveforms for the SPE-5 explosion, using 3D velocity models, demonstrate that the shallow structure of the Yucca Flat basin contributes to generation of observed shear motion. The inclusion of 3D wave scattering, simulated by small-scale velocity perturbations in the 3D model, improves the fit between the simulated and recorded waveforms. In addition, a relatively low intrinsic attenuation, combined with small-scale velocity variations in our models, can confirm the observed wave trapping and its effect on duration of coda waves and the spatial variation of P/S ratio at basin sites.
Arthur J. Rodgers, Arben Pitarka, Ramesh Pankajakshan, Bjorn Sjögreen, and N. Anders Petersson
Seismological Society of America (SSA)
ABSTRACT Large earthquake ground-motion simulations in 3D Earth models provide constraints on site-specific shaking intensities but have suffered from limited frequency resolution and ignored site response in soft soils. We report new regional-scale 3D simulations for moment magnitude 7.0 scenario earthquakes on the Hayward Fault, northern California with SW4. Simulations resolved significantly broader band frequencies (0–10 Hz) than previous studies and represent the highest resolution simulations for any such earthquake to date. Seismic waves were excited by a kinematic rupture following Graves and Pitarka (2016) and obeyed wave propagation in a 3D Earth model with topography from the U.S. Geological Survey (USGS) assuming a minimum shear wavespeed, VSmin, of 500 m/s. We corrected motions for linear and nonlinear site response for the shear wavespeed, VS, from the USGS 3D model, using a recently developed ground-motion model (GMM) for Fourier amplitude spectra (Bayless and Abrahamson, 2018, 2019a). At soft soil locations subjected to strong shaking, the site-corrected intensities reflect the competing effects of linear amplification by low VS material, reduction of stiffness during nonlinear deformation, and damping of high frequencies. Sites with near-surface VS of 500 m/s or greater require no linear site correction but can experience amplitude reduction due to nonlinear response. Averaged over all sites, we obtained reasonable agreement with empirical ergodic median GMMs currently used for seismic hazard and design ground motions (epsilon less than 1), with marked improvement at soft sedimentary sites. At specific locations, the simulated shaking intensities show systematic differences from the GMMs that reveal path and site effects not captured in these ergodic models. Results suggest how next generation regional-scale earthquake simulations can provide higher spatial and frequency resolution while including effects of soft soils that are commonly ignored in scenario earthquake ground-motion simulations.
A. Pitarka, R. Graves, K. Irikura, K. Miyakoshi, and A. Rodgers
Springer Science and Business Media LLC
We analyzed a kinematic earthquake rupture generator that combines the randomized spatial field approach of Graves and Pitarka (Bull Seismol Soc Am 106:2136–2153, 2016 ) (GP2016) with the multiple asperity characterization approach of Irikura and Miyake (Pure Appl Geophys 168:85–104, 2011 ) (IM2011, also known as Irikura recipe). The rupture generator uses a multi-scale hybrid approach that incorporates distinct features of both original approaches, such as small-scale stochastic rupture variability and depth-dependent scaling of rupture speed and slip rate, inherited from GP2016, and specification of discrete high slip rupture patches, inherited from IM2011. The performance of the proposed method is examined in simulations of broadband ground motion from the 2016 Kumamoto, Japan earthquake, as well as comparisons with ground motion prediction equations (GMPEs). We generated rupture models with multi-scale heterogeneity, including a hybrid one in which the slip is a combination of high- slip patches and stochastic small scale variations. We find that the ground motions simulated with these rupture models match the general characteristics of the recorded near-fault motion equally well, over a broad frequency range (0–10 Hz). Additionally, the simulated ground motion is in good agreement with the predictions from Ground Motion Prediction Equations (GMPEs). Nonetheless, due to sensitivity of the ground motion to the local fault rupture characteristics, the performance among the models at near-fault sites is slightly different, with the hybrid model producing a somewhat better fit to the recorded ground velocity waveforms. Sensitivity tests of simulated near-fault ground motion to variations in the prescribed kinematic rupture parameters show that average rupture speeds higher than the default value in GP2016 (average rupture speed = 80% of local shear wave speed), as well as slip rate durations shorter than the default value in GP2016 (rise time coefficient = 1.6), generate ground motions that are higher than the recorded ones at periods longer than 1 s. We found that these two parameters also affect the along strike and updip rupture directivity effects, as illustrated in comparisons with the Kumamoto observations.
Arthur J. Rodgers, Arben Pitarka, and David B. McCallen
Seismological Society of America (SSA)
Abstract We investigated the effects of fault geometry and assumed minimum shear wavespeed (VSmin) on 3D ground-motion simulations (0–2.5 Hz) in general, using a moment magnitude (Mw) 6.5 earthquake on the Hayward fault (HF). Simulations of large earthquakes on the northeast-dipping HF using the U.S. Geological Survey (USGS) 3D seismic model have shown intensity asymmetry with stronger shaking for the Great Valley Sequence east of the HF (hanging wall) relative to the Franciscan Complex to the west (footwall). We performed simulations with three fault geometries in both plane-layered (1D) and 3D models. Results show that the nonvertical fault geometries result in larger motions on the hanging wall relative to the vertical fault for the same Earth model with up to 50% amplifications in single-component peak ground velocity (PGV) within 10 km of the rupture. Near-fault motions on the footwall are reduced for the nonvertical faults, but less than they are increased on the hanging wall. Simulations assuming VSmin values of 500 and 250 m/s reveal that PGVs are on average 25% higher west of the HF when using the lower VSmin, with some locations amplified by a factor of 3. Increasing frequency content from 2.5 to 5 Hz increases PGV values. Spectral ratios of these two VSmin cases show average amplifications of 2–4 (0.5–1.5 Hz) for the lower VSmin west of the fault. Large differences (up to 2×) in PGV across the HF from previous studies persist even for the case with a vertical fault or VSmin of 250 m/s. We conclude that assuming a VSmin of 500 m/s underestimates intensities west of the HF for frequencies above 0.5 Hz, and that low upper crustal (depth <10 km) shear wavespeeds defined in the 3D model contribute most to higher intensities east of the HF.
Arthur J. Rodgers, N. Anders Petersson, Arben Pitarka, David B. McCallen, Bjorn Sjogreen, and Norman Abrahamson
Seismological Society of America (SSA)
Robert J. Mellors, Arben Pitarka, Eric Matzel, Steven Magana‐Zook, Douglas Knapp, William R. Walter, Ting Chen, Catherine M. Snelson, and Robert E. Abbott
Seismological Society of America (SSA)
Arthur J. Rodgers, Arben Pitarka, N. Anders Petersson, Björn Sjögreen, and David B. McCallen
American Geophysical Union (AGU)
Author(s): Rodgers, Arthur J; Pitarka, Arben; Petersson, N Anders; Sjogreen, Bjorn; McCallen, David B