ALBERT

Description: We used 2D finite-difference modeling and azimuthally binned receiver functions (RFs) to explore whether abrupt offsets in Moho depth can be detected by one or a few closely spaced P -wave RFs. Our results show that 2D synthetic RFs generated in the immediate vicinity above a Moho depth change can provide important clues to the abruptness of the offset. In particular, diffraction of the waves impinging onto the Moho offset may generate a split P S arrival, causing systematic variation of peak-to-peak P - P S delay times with increasing ray parameter, depending on the location relative to the Moho offset and the incidence direction of the RFs. We outline an approach using a slant-stack method to constrain the location of a relatively abrupt depth change of Moho ( ) using separate RF stacks incident from opposite directions. For a station located on the western border of the Caspian Sea in Azerbaijan (LKR), our 2D models with an ~8 km transition from a shallower Moho to the east and deeper Moho to the west generate synthetic RFs with features in general agreement with observations. These models, which include step- and ramp-like offsets of Moho, are in general agreement with estimates of crustal thickness from seismic data. Thus, our results suggest that characteristics in one or a few azimuthally binned radial P -wave RFs can be used in concert with a slant-stack analysis to pinpoint a relatively abrupt change in underlying Moho depth. Online Material: Discussion and figures of a verification study of receiver functions (RFs) computed by different methods, crustal phases generated in our 2D model using animations of the simulated wave propagations, as well as estimated crustal structures, and RFs for Moho offset models, including realistic levels of noise.

Description: Field studies of historic rupture traces show that fault stepovers commonly serve as endpoints to earthquake ruptures. This is an effect that is corroborated by past dynamic modeling studies. However, field studies also show a great deal of complexity in fault-zone structure within a stepover, which is often simplified out of modeling studies. In the present study, we use the 3D finite-element method to investigate the effect of one type of smaller-scale complexity on the rupture process: a smaller fault segment positioned between the two primary strands of a strike-slip fault stepover. We find that such small faults can have a controlling effect on whether or not a rupture is able to jump the stepover and on the resulting ground motions from these ruptures. However, this effect is neither straightforward nor linear: the length of the intermediate segment and its basal depth, as well as whether the stepover is extensional or compressional, all contribute to the rupture behavior and ground-motion distribution. These results have important implications for assessing the probability of a rupture propagating through small- and large-scale discontinuities in faults, as well as for evaluating ground-motion intensities near fault stepovers. Because of the sensitivity of results to so many parameters, these results also suggest that modeling studies on idealized fault geometries may not be sufficient to describe the rupture behaviors of specific complex fault systems. Site-specific modeling studies, where possible, will provide better inputs and constraints for probabilistic rupture length assessments as well as for ground-motion estimates.

Description: We predict broadband (BB, 0–10 Hz) ground motions for M  7 earthquakes on the Salt Lake City segment of the Wasatch fault (WFSLC), Utah, which include the effects of nonlinear site response. The predictions are based on low-frequency (LF, 0–1 Hz) finite-difference (FD) simulations for six different rupture models generated during a previous study ( Roten et al. , 2011 ), which we combine with high-frequency (HF, 1–10 Hz) shear-to-shear (S-to-S) back-scattering operators to generate BB synthetics. Average horizontal spectral accelerations at 5 and 10 Hz (0.2-s SAs and 0.1-s SAs, respectively) calculated from the linear BB synthetics exceed estimates from four recent ground-motion prediction equations (GMPEs) at near-fault (〈5 km) locations on the sediment by more than one standard deviation, but agree with the GMPEs at larger rupture distances. The overprediction of the near-fault GMPE values is largely eliminated after corrections of the BB synthetics for nonlinear soil effects are applied, reducing the SAs from the simulations by up to 70%. These corrections are based on amplitude-, frequency-, and site-dependent correction functions from 1D nonlinear simulations at ~450 locations in the Salt Lake basin, using a simple soil model based in part on published laboratory experiments on Bonneville clay samples. We obtain geometric mean 1-s SAs from from the six scenarios of more than 0.75 g on the hanging-wall side of the fault. Geometric mean 0.2-s SAs exceed 1 g on the hanging-wall and on the footwall sediments in the central Salt Lake basin, and peak horizontal ground accelerations range from 0.45 to 〉0.60 g in the same general locations. Online Material: Table of coefficients and amplitude-dependent correction functions for nonlinear soil effects, and figures showing maps of SAs at various frequencies, PGA and PGV, with and without correction for nonlinear soil effects, results of 1D nonlinear simulations, and comparison to ground motion prediction equations.

Description: Memory-variable methods have been widely applied to approximate frequency-independent quality factor Q in numerical simulation of wave propagation. The frequency-independent model is often appropriate for frequencies up to about 1 Hz but at higher frequencies is inconsistent with some regional studies of seismic attenuation. We apply the memory-variable approach to frequency-dependent Q models that are constant below, and follow a power-law above, a chosen transition frequency. We present numerical results for the corresponding memory-variable relaxation times and weights, obtained by nonnegative least-squares fitting of the Q ( f ) function, for a range of exponent values; these times and weights can be scaled to an arbitrary transition frequency and a power-law prefactor, respectively. The resulting memory-variable formulation can be used with numerical wave-propagation solvers based on methods such as finite differences (FDs) or spectral elements and may be implemented in either conventional or coarse-grained form. In the coarse-grained approach, we fit effective Q for low- Q values (〈200) using a nonlinear inversion technique and use an interpolation formula to find the corresponding weighting coefficients for arbitrary Q . A 3D staggered-grid FD implementation closely approximates the frequency–wavenumber solution to both a half-space and a layered model with a shallow dislocation source for Q as low as 20 over a bandwidth of two decades. We compare the effects of different power-law exponents using a finite-fault source model of the 2008 M w  5.4 Chino Hills, California, earthquake and find that Q ( f ) models generally better fit the strong-motion data than the constant Q models for frequencies above 1 Hz. Online Material: Figures comparing finite difference and frequency–wavenumber seismograms for an elastic layered-model point source simulation. Median spectral acceleration centered at 1 s and Fourier amplitude centered at 0.25 and 2.25 Hz for strong ground motion recordings and synthetics from the Chino Hills earthquake.

Description: The Colima-Jalisco (CJ) region in northwestern Mexico has generated large-magnitude earthquakes at least since 1800. For example, during the last century, three large, destructive, shallow-thrust subduction earthquakes occurred on 3 and 18 June 1932 with MS of 8.2 and 8, respectively, and on 9 October 1995 (Mw 8, MS 7.4). This historical seismicity and the lack of seismic recordings in the CJ region pose important constraints for the computation of reliable seismic-hazard studies for sites in this region of Mexico. Towards this aim, we have used a hybrid method to generate broadband (BB) synthetics for the Mw 8 CJ 1995 earthquake for the recording sites of the near (MZ) intermediate (CG), and far (COL) fields. The low-frequency (LF, [≤]0.5 Hz) synthetics were simulated by applying a 3D finite-difference method, and the high frequencies (HF, 〉0.5 Hz) were generated by the empirical Green's function technique. Finally, matched filters were applied to the LF and HF synthetics to obtain the BB time series. The LF synthetics were computed from a finite-fault description of the source with four asperities in a 2.5D model constrained by gravity and seismological data. Our preferred model includes an approximation of a thin accretionary prism. For the HF modeling, we also used the four-asperity source model as well as the recordings of the foreshock and aftershock of the Mw 8 1995 mainshock. Based on the comparisons of the BB synthetics with the observed strong ground motions for the 1995 CJ earthquake at the three stations, we believe that our hybrid method is a first step toward the generation of more reliable estimates of the seismic hazards in CJ region. Further improvement in the hazard estimates depends on the urgent deployment of seismological and strong ground motion infrastructure in the CJ region.

Description: Finite-difference modeling of 3D long-period (〉2 s) ground motions for large ( M w  6.8) scenario earthquakes is conducted to investigate effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. Scenario earthquakes include deep (〉40 km) subducting Juan de Fuca (JdF) plate earthquakes, simulated in locations congruent with known seismicity. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The chosen peak motion metric is the geometric mean of the two orthogonal horizontal components of motion. The ratio between predicted peak ground velocity (PGV) for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. A total of 10 deep subducting JdF plate earthquakes are simulated within 100 km of Greater Vancouver. Simulations are calibrated using records from the 2001 M w  6.8 Nisqually earthquake. On average, the predicted level of average PGV at stiff soil sites across Greater Vancouver for an M w  6.8 JdF plate earthquake is 3.2 cm/s (modified Mercalli intensity IV–V). The average increase in PGV due to basin structure across Greater Vancouver is 3.1. Focusing of north-northeast-propagating surface waves by shallow (〈1 km) basin structure increases ground motion in a localized region of south Greater Vancouver; hence, scenario JdF plate earthquakes located ≥80 km south-southwest of Vancouver are potentially the most hazardous. Online Material: Depth slices of 3D velocity model, peak ground velocity maps, and snapshots and videos of wave propagation.

Description: We predict ground motions in the Salt Lake basin (SLB) during M 7 earthquakes on the Salt Lake City segment of the Wasatch fault (WFSLC). First we generate a suite of realistic source representations by simulating the spontaneous rupture process on a planar, vertical fault with the staggered-grid split-node finite-difference (FD) method. The initial distribution of shear stress is the sum of both a regional depth-dependent shear stress appropriate for a dipping, normal fault and a stochastically generated residual shear stress field associated with previous ruptures. The slip-rate histories from the spontaneous rupture scenarios are projected onto a detailed 3D model geometry of the WFSLC that we developed based on geological observations. Next, we simulate 0- to 1-Hz wave propagation from six source models with a 3D FD code, using the most recent version of the Wasatch Front Community Velocity model. Horizontal spectral accelerations at two seconds (2-s SAs) reveal strong along-strike rupture direction effects for unilateral ruptures, as well as significant amplifications by the low-velocity sediments on the hanging-wall side of the fault. For ruptures nucleating near the southern end of the segment, we obtain 2-s SAs of up to 1.4g near downtown SLC, caused by a combination of rupture-direction and basin-edge effects. Average 3-s SAs and 2-s SAs from the six scenarios are generally consistent with values predicted by four next-generation attenuation models.

Description: 〈span〉〈div〉Abstract〈/div〉We numerically model broadband ground motion (up to 5–7.5  Hz) from blind‐thrust scenario earthquakes matching the fault geometry of the 1994 Mw 6.7 Northridge earthquake. Several realizations are modeled (by varying the hypocenter location in the dynamic rupture simulation) in a 1D‐layered velocity profile. In addition, we include Q(f ), nonlinear effects from Drucker–Prager plasticity, and superimpose small‐scale medium complexity in both a 1D‐layered and 3D velocity model within the subsequent wave propagation. We investigate characteristics of the ground motion and its variability up to 50 km from the fault by comparing them with ground‐motion prediction equations (GMPEs), simple proxy metrics, as well as strong ground motion records from the Northridge event. We find that median ground motion closely follows the trend predicted by GMPEs and that the intraevent standard deviation, although varying with hypocenter location, lies near that of GMPE models. Plasticity affects ground‐motion amplitudes in regions near the source, reducing intraevent variability above ∼0.5  Hz. Heterogeneity in the velocity structure on both the regional and small scales is needed for the simulated data to match two proxy metrics: the period‐to‐period correlation of spectral acceleration (SA) and the ratio of maximum‐to‐median SA. Although small‐scale heterogeneity has a negligible effect on median SA for this style of rupture, it serves to significantly increase the cumulative absolute velocity, better agreeing with observations. When compared with strong‐motion data, we find that long‐wavelength velocity structure within our deterministic simulations reduces bias at both short and long periods. Finally, synthetic ground motion at both footwall and hanging‐wall sites has no clear dependence on the distance to rupture (at both short and long periods); directivity is likely overpowering any hanging‐wall effect.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We model deterministic broadband (0–7.5 Hz) ground motion from an Mw 7.1 bilateral strike‐slip earthquake scenario with dynamic rupture propagation along a rough‐fault topography embedded in a medium including small‐scale velocity and density perturbations. Spectral accelerations (SAs) at periods 0.2–3 s and Arias intensity durations show a similar distance decay (at the level of 1–2 interevent standard deviations above the median) when compared to Next Generation Attenuation‐West2 (NGA)‐West2 ground‐motion prediction equations (GMPEs) using a Q(f ) power‐law exponent of 0.6–0.8 above 1 Hz in models with a minimum VS of 750  m/s. With a trade‐off from Q(f ), the median ground motion is slightly increased by scattering from statistical models of small‐scale heterogeneity with standard deviation (σ) of the perturbations at the lower end of the observed range (5%) but reduced by scattering attenuation at the upper end (10%) when using a realistic 3D background velocity model. The ground‐motion variability is strongly affected by the addition of small‐scale media heterogeneity, reducing otherwise large values of intraevent standard deviation closer to those of empirical observations. These simulations generally have intraevent standard deviations for SAs lower than the GMPEs for the modeled bandwidth, with an increasing trend with distance (most pronounced in low‐to‐moderate scattering media) near the level of observations at distances greater than 35 km from the fault. Durations for the models follow the same increasing trend with distance, in which σ ∼ 5% produces the best match to GMPE values. We find that a 3D background‐velocity model reduces the pulse period into the expected range by breaking up coherent waves from directivity, generating a lognormal distribution of ground‐motion residuals. These results indicate that a strongly heterogeneous medium is needed to produce realistic deterministic broadband ground motions. Finally, the addition of a thin surficial layer with low, frequency‐independent Q in the model (with a minimum VS of 750  m/s) controls the high‐frequency decay in energy, as measured by the parameter κ, that may be necessary to include as simulations continue to extend to higher frequencies.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We model deterministic broadband (0–7.5 Hz) ground motion from an Mw 7.1 bilateral strike‐slip earthquake scenario with dynamic rupture propagation along a rough‐fault topography embedded in a medium including small‐scale velocity and density perturbations. Spectral accelerations (SAs) at periods 0.2–3 s and Arias intensity durations show a similar distance decay (at the level of 1–2 interevent standard deviations above the median) when compared to Next Generation Attenuation‐West2 (NGA)‐West2 ground‐motion prediction equations (GMPEs) using a Q(f ) power‐law exponent of 0.6–0.8 above 1 Hz in models with a minimum VS of 750  m/s. With a trade‐off from Q(f ), the median ground motion is slightly increased by scattering from statistical models of small‐scale heterogeneity with standard deviation (σ) of the perturbations at the lower end of the observed range (5%) but reduced by scattering attenuation at the upper end (10%) when using a realistic 3D background velocity model. The ground‐motion variability is strongly affected by the addition of small‐scale media heterogeneity, reducing otherwise large values of intraevent standard deviation closer to those of empirical observations. These simulations generally have intraevent standard deviations for SAs lower than the GMPEs for the modeled bandwidth, with an increasing trend with distance (most pronounced in low‐to‐moderate scattering media) near the level of observations at distances greater than 35 km from the fault. Durations for the models follow the same increasing trend with distance, in which σ ∼ 5% produces the best match to GMPE values. We find that a 3D background‐velocity model reduces the pulse period into the expected range by breaking up coherent waves from directivity, generating a lognormal distribution of ground‐motion residuals. These results indicate that a strongly heterogeneous medium is needed to produce realistic deterministic broadband ground motions. Finally, the addition of a thin surficial layer with low, frequency‐independent Q in the model (with a minimum VS of 750  m/s) controls the high‐frequency decay in energy, as measured by the parameter κ, that may be necessary to include as simulations continue to extend to higher frequencies.〈/span〉