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: Eight years of ozone measurements retrieved from the Ozone Monitoring Instrument (OMI) and the Microwave Limb Sounder, both on the EOS Aura satellite, have been assimilated into the Goddard Earth Observing System version 5 (GEOS-5) data assimilation system. This study evaluates this assimilated product, highlighting its potential for science. The impact of observations on the GEOS-5 system is explored by examining the spatial distribution of the observation-minus-forecast statistics. Independent data are used for product validation. The correlation of the lower-stratospheric (the tropopause to 50 hPa) ozone column with ozonesondes is 0.99 and the (high) bias is 0.5%, indicating the success of the assimilation in reproducing the ozone variability in that layer. The upper-tropospheric (500 hPa to the tropopause) assimilated ozone column is about 10% lower than the ozonesonde column but the correlation is still high (0.87). The assimilation is shown to realistically capture the sharp cross-tropopause gradient in ozone mixing ratio. Occurrence of transport-driven low ozone laminae in the assimilation system is similar to that obtained from the High Resolution Dynamics Limb Sounder (HIRDLS) above the 400 K potential temperature surface but the assimilation produces fewer laminae than seen by HIRDLS below that surface. Although the assimilation produces about 25% fewer occurrences per day during the three years of HIRDLS data, the interannual variability is captured correctly.This data-driven assimilated product is complementary to ozone fields generated from chemistry and transport models. Applications include study of the radiative forcing by ozone and tracer transport near the tropopause.

Description: This study is our first step toward the generation of 6 hourly 3-D CO2 fields that can be used to validate CO2 forecast models by combining CO2 observations from multiple sources using ensemble Kalman filtering. We discuss a procedure to assimilate Atmospheric Infrared Sounder (AIRS) column-averaged dry-air mole fraction of CO2 (Xco2) in conjunction with meteorological observations with the coupled Local Ensemble Transform Kalman Filter (LETKF)-Community Atmospheric Model version 3.5. We examine the impact of assimilating AIRS Xco2 observations on CO2 fields by comparing the results from the AIRS-run, which assimilates both AIRS Xco2 and meteorological observations, to those from the meteor-run, which only assimilates meteorological observations. We find that assimilating AIRS Xco2 results in a surface CO2 seasonal cycle and the N-S surface gradient closer to the observations. When taking account of the CO2 uncertainty estimation from the LETKF, the CO2 analysis brackets the observed seasonal cycle. Verification against independent aircraft observations shows that assimilating AIRS Xco2 improves the accuracy of the CO2 vertical profiles by about 0.5–2 ppm depending on location and altitude. The results show that the CO2 analysis ensemble spread at AIRS Xco2 space is between 0.5 and 2 ppm, and the CO2 analysis ensemble spread around the peak level of the averaging kernels is between 1 and 2 ppm. This uncertainty estimation is consistent with the magnitude of the CO2 analysis error verified against AIRS Xco2 observations and the independent aircraft CO2 vertical profiles.

Description: The upper troposphere and lower stratosphere (UTLS) plays an important role in climate and atmospheric chemistry. Despite its importance on the point of causing deep intrusions of tropics originated air into the midlatitudes, the quasi-horizontal transport process in the UTLS, represented by global chemistry-transport models (CTMs) or chemistry-climate models (CCMs), cannot easily be diagnosed with conventional analyses on isobaric surfaces. We use improved diagnostic tools to better evaluate CTMs and CCMs relative to satellite observations in the region of UTLS. Using the Hellinger distance, vertical profiles of probability density functions (PDFs) of chemical tracers simulated by the Model for OZone And Related chemical Tracers 3.1 (MOZART-3.1) are quantitatively compared with satellite data from the Microwave Limb Sounder (MLS) instrument in the tropopause relative altitude coordinate to characterize features of tracer distributions near the tropopause. Overall, the comparison of PDFs between MLS and MOZART-3.1 did not satisfy the same population assumption. Conditional PDFs are used to understand the meteorological differences between global climate models and the real atmosphere and the conditional PDFs between MOZART-3.1 and MLS showed better agreement compared to the original PDFs. The low static stability during high tropopause heights at midlatitudes suggests that the variation of tropopause height is related to transport processes from the tropics to midlatitudes. MOZART-3.1 with the GEOS4 GCM winds reproduces episodes of the tropical air intrusions. However, our diagnostic analyses show that the GEOS4 GCM did not properly reproduce the high tropopause cases at midlatitudes especially in spring.

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: [1]  The extratropical stratosphere–troposphere exchange (STE) of ozone from 2005 to 2010 is estimated by combining Microwave Limb Sounder ozone observations and MERRA reanalysis meteorological fields in an established direct diagnostic framework. The multiyear mean ozone STE is 275 Tg yr −1 and 214 Tg yr −1 in the Northern and Southern Hemispheres, respectively. The year-to-year variability is greater in the Northern Hemisphere, where the difference between the highest and the lowest annual flux is 15% of the multiyear mean compared with 6% in the Southern Hemisphere. Variability of lower stratospheric ozone and variability of the net mass flux both contribute to interannual variability in the Northern Hemisphere ozone flux. The flux across the extratropical 380 K surface determines the amount of flux across the extratropical tropopause, and the greatest seasonal variability of the 380 K ozone flux occurs in the late winter/early spring, around the time of greatest flux. Both the mass flux and the ozone mixing ratios on the 380 K surface show recurring spatial patterns, but interannual variability of these quantities and their alignment contribute to the ozone flux variability. The spatial and temporal variability are not well represented when zonal and/or monthly mean fields are used to calculate the ozone STE, although this results in a small high bias of the seasonal amplitude and annual magnitude. If the climatological variability over these 6 years is representative, the estimated number of years required to detect a 2 − 3% decade −1 trend in ozone STE using this diagnostic is 35 − 39 years.

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: Observations from long-term ozonesonde measurements show robust variations and trends in the evolution of ozone in the middle and upper troposphere over Réunion Island (21.1°S, 55.5°E) in June-August. Here we examine possible causes of the observed ozone variation at Réunion Island using hindcast simulations by the stratosphere-troposphere Global Modeling Initiative chemical transport model (GMI-CTM) for 1992–2014, driven by assimilated Modern-Era Retrospective Analysis for Research and Applications (MERRA) meteorological fields. Réunion Island is at the edge of the subtropical jet, a region of strong stratospheric-tropospheric exchange (STE). Our analysis implies that the large interannual variation (IAV) of upper tropospheric ozone over Réunion is driven by the large IAV of the stratospheric influence. The IAV of the large-scale, quasi-horizontal wind patterns also contributes to the IAV of ozone in the upper troposphere. Comparison to a simulation with constant emissions indicates that increasing emissions do not lead to the maximum trend in the middle and upper troposphere over Réunion during austral winter implied by the sonde data. The effects of increasing emission over southern Africa are limited to the lower troposphere near the surface in August - September.

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.