ALBERT

Description: We estimate the a rms -stress drop, , ( Hanks, 1979 ) using acceleration time records of 59 earthquakes from two earthquake sequences in eastern Honshu, Japan. These acceleration-based static stress drops compare well to stress drops calculated for the same events by Baltay et al. (2011) using an empirical Green’s function (eGf) approach. This agreement supports the assumption that earthquake acceleration time histories in the bandwidth between the corner frequency and a maximum observed frequency can be considered white, Gaussian, noise. Although the is computationally simpler than the eGf-based -stress drop, and is used as the "stress parameter" to describe the earthquake source in ground-motion prediction equations, we find that it only compares well to the at source-station distances of ~20 km or less because there is no consideration of whole-path anelastic attenuation or scattering. In these circumstances, the correlation between the and is strong. Events with high and low stress drops obtained through the eGf method have similarly high and low . We find that the inter-event standard deviation of stress drop, for the population of earthquakes considered, is similar for both methods, 0.40 for the method and 0.42 for the , in log 10 units, provided we apply the ~20 km distance restriction to . This indicates that the observed variability is inherent to the source, rather than attributable to uncertainties in stress-drop estimates. Online Material: Earthquake catalog including additional source parameters.

Description: The Next Generation Attenuation-West 2 (NGA-West 2) 2014 ground-motion prediction equations (GMPEs) model ground motions as a function of magnitude and distance, using empirically derived coefficients (e.g., Bozorgnia et al. , 2014 ); as such, these GMPEs do not clearly employ earthquake source parameters beyond moment magnitude ( M ) and focal mechanism. To better understand the magnitude-dependent trends in the GMPEs, we build a comprehensive earthquake source-based model to explain the magnitude dependence of peak ground acceleration and peak ground velocity in the NGA-West 2 ground-motion databases and GMPEs. Our model employs existing models ( Hanks and McGuire, 1981 ; Boore, 1983 , 1986 ; Anderson and Hough, 1984 ) that incorporate a point-source Brune model, including a constant stress drop and the high-frequency attenuation parameter 0 , random vibration theory, and a finite-fault assumption at the large magnitudes to describe the data from magnitudes 3 to 8. We partition this range into four different magnitude regions, each of which has different functional dependences on M . Use of the four magnitude partitions separately allows greater understanding of what happens in any one subrange, as well as the limiting conditions between the subranges. This model provides a remarkably good fit to the NGA data for magnitudes from 3〈 M 〈8 at close rupture distances ( R rup ≤20 km). We explore the trade-offs between and 0 in ground-motion models and data, which play an important role in understanding small-magnitude data, for which the corner frequency is masked by the attenuation of high frequencies. That this simple, source-based model matches the NGA-West 2 GMPEs and data so well suggests that considerable simplicity underlies the parametrically complex NGA GMPEs. Online Material: Figures providing detail on the V S 30 distribution in the subset of the Next Generation Attenuation-West 2 (NGA-West 2) data used, the quarter-wavelength amplifications used in the model, the statistical test for the large magnitude portion of the model, and the magnitude dependence.

Description: Tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground-motion prediction and hazard mapping. Traditionally, recorded earthquakes of magnitude ~3–8 are used to develop ground-motion prediction equations; however, typical earthquake records may be sparse in areas of high hazard. In this study, we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor, which is plentiful in some regions. Tectonic tremor occurs in the frequency band of interest for ground-motion prediction (i.e., ~2–8 Hz) and is located on the subducting plate interface, at the lower boundary of where future large earthquakes are expected. We empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones: Nankai, Japan; Cascadia, United States–Canada; Jalisco, Mexico; and southern Chile. With the large amount of data available from tremor, we show that in the upper plate, the lower crust is less attenuating than the upper crust. We apply the same analysis to intraslab events in Nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic crust. This suggests that high pore-fluid pressure is present in the tremor source region. These differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface.

Description: 〈span〉〈div〉Abstract〈/div〉Ground‐motion modeling requires accurate representation of the earthquake source, path, and site. Site amplification is often modeled by VS30, the time‐averaged shear‐wave velocity of the top 30 m of the Earth’s surface, though recent studies find that its ability to accurately predict site effects varies. Another measure of the site is κ0, the attenuation of high‐frequency energy near the site (〈a href="https://pubs.geoscienceworld.org/bssa#rf5"〉Anderson and Hough, 1984〈/a〉). We develop a novel application of the 〈a href="https://pubs.geoscienceworld.org/bssa#rf7"〉Andrews (1986)〈/a〉 method to simultaneously invert the spectra of 3357 earthquakes in Southern California into source and site components. These earthquakes have magnitudes 2.5–5.72 and were recorded on 16 stations for a total of 52,297 records. We constrain the inversion with an individual earthquake, demonstrating the most Brune‐like shape to preserve the site spectra. We then solve for κ0 site amplification at each station in three frequency bands: 1–6 Hz, 6–14 Hz, and 14–35 Hz. The resulting values of κ0 range from 0.017 s at ANZA station PFO to 0.059 s at ANZA station SND. We compare our results to values of site κ0 from other studies, as well as site residuals from ground‐motion prediction equations. We find good agreement between our site κ0 and previous studies in the region. We find that κ0 and high‐frequency site amplification (14–35 Hz band) correlates well with independent site residuals, making it a good first‐order approximation for the effects of site attenuation or amplification on ground motion.〈/span〉

Description: 〈span〉In the months before he passed away, Jack Boatwright asked us to review a manuscript on source properties (specifically, stress drop) of earthquakes in northeastern North America (NENA). This manuscript originated in research that was funded by the U.S. Nuclear Regulatory Commission (NRC), described in his final report to NRC and published as U.S. Geological Survey (USGS) Open‐File Report 2018–1073 (〈a href="https://pubs.geoscienceworld.org/srl#rf1"〉Boatwright, 2018〈/a〉). We wish to call attention to 〈a href="https://pubs.geoscienceworld.org/srl#rf1"〉Boatwright (2018)〈/a〉 for those who may not be aware of what Jack was unable to put into journal publications.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Ground‐motion modeling requires accurate representation of the earthquake source, path, and site. Site amplification is often modeled by VS30, the time‐averaged shear‐wave velocity of the top 30 m of the Earth’s surface, though recent studies find that its ability to accurately predict site effects varies. Another measure of the site is κ0, the attenuation of high‐frequency energy near the site (〈a href="https://pubs.geoscienceworld.org/bssa#rf5"〉Anderson and Hough, 1984〈/a〉). We develop a novel application of the 〈a href="https://pubs.geoscienceworld.org/bssa#rf7"〉Andrews (1986)〈/a〉 method to simultaneously invert the spectra of 3357 earthquakes in Southern California into source and site components. These earthquakes have magnitudes 2.5–5.72 and were recorded on 16 stations for a total of 52,297 records. We constrain the inversion with an individual earthquake, demonstrating the most Brune‐like shape to preserve the site spectra. We then solve for κ0 site amplification at each station in three frequency bands: 1–6 Hz, 6–14 Hz, and 14–35 Hz. The resulting values of κ0 range from 0.017 s at ANZA station PFO to 0.059 s at ANZA station SND. We compare our results to values of site κ0 from other studies, as well as site residuals from ground‐motion prediction equations. We find good agreement between our site κ0 and previous studies in the region. We find that κ0 and high‐frequency site amplification (14–35 Hz band) correlates well with independent site residuals, making it a good first‐order approximation for the effects of site attenuation or amplification on ground motion.〈/span〉