ALBERT

Description: Although the time-averaged shear-wave velocity down to 30 m depth ( V S 30 ) can be a proxy for estimating earthquake ground-motion amplification, significant controversy exists about its limitations when used as a single parameter for the prediction of amplification. To examine this question in absence of relevant strong-motion records, we use a range of different methods to measure the shear-wave velocity profiles and the resulting theoretical site amplification factors (AFs) for 30 sites in the Newcastle area, Australia, in a series of blind comparison studies. The multimethod approach used here combines past seismic cone penetrometer and spectral analysis of surface-wave data, with newly acquired horizontal-to-vertical spectral ratio, passive-source surface-wave spatial autocorrelation (SPAC), refraction microtremor (ReMi), and multichannel analysis of surface-wave data. The various measurement techniques predicted a range of different AFs. The SPAC and ReMi techniques have the smallest overall deviation from the median AF for the majority of sites. We show that V S 30 can be related to spectral response above a period T of 0.5 s but not necessarily with the maximum amplification according to the modeling done based on the measured shear-wave velocity profiles. Both V S 30 and AF values are influenced by the velocity ratio between bedrock and overlying sediments and the presence of surficial thin low-velocity layers (〈2 m thick and 〈150 m/s), but the velocity ratio is what mostly affects the AF. At 0.2〈 T 〈0.4 s, the AFs are largely controlled by the surficial geology of a particular site. AF maxima are the highest in the hard classes, which is the inverse of the findings used in the Australian Building Code. Only for T 〉0.5 s do the amplification curves consistently show higher values for soft site classes and lower for hard classes.

Description: In 2001, a rare swarm of small, shallow earthquakes beneath the city of Spokane, Washington, caused ground shaking as well as audible booms over a five-month period. Subsequent Interferometric Synthetic Aperture Radar (InSAR) data analysis revealed an area of surface uplift in the vicinity of the earthquake swarm. To investigate the potential faults that may have caused both the earthquakes and the topographic uplift, we collected ~3 km of high-resolution seismic-reflection profiles to image the upper-source region of the swarm. The two profiles reveal a complex deformational pattern within Quaternary alluvial, fluvial, and flood deposits, underlain by Tertiary basalts and basin sediments. At least 100 m of arching on a basalt surface in the upper 500 m is interpreted from both the seismic profiles and magnetic modeling. Two west-dipping faults deform Quaternary sediments and project to the surface near the location of the Spokane fault defined from modeling of the InSAR data.

Description: Shear-wave velocity ( V S ) and time-averaged shear-wave velocity to 30 m depth ( V S 30 ) are the key parameters used in seismic site response modeling and earthquake engineering design. Where V S data are limited, available data are often used to develop and refine map-based proxy models of V S 30 for predicting ground-motion intensities. In this paper, we present shallow V S data from 27 sites in Puerto Rico. These data were acquired using a multimethod acquisition approach consisting of noninvasive, collocated, active-source body-wave (refraction/reflection), active-source surface wave at nine sites, and passive-source surface-wave refraction microtremor (ReMi) techniques. V S -versus-depth models are constructed and used to calculate spectral response plots for each site. Factors affecting method reliability are analyzed with respect to site-specific differences in bedrock V S and spectral response. At many but not all sites, body- and surface-wave methods generally determine similar depths to bedrock, and it is the difference in bedrock V S that influences site amplification. The predicted resonant frequencies for the majority of the sites are observed to be within a relatively narrow bandwidth of 1–3.5 Hz. For a first-order comparison of peak frequency position, predictive spectral response plots from eight sites are plotted along with seismograph instrument spectra derived from the time series of the 16 May 2010 Puerto Rico earthquake. We show how a multimethod acquisition approach using collocated arrays compliments and corroborates V S results, thus adding confidence that reliable site characterization information has been obtained.

Description: We characterize shallow subsurface faulting and basin structure along a transect through heavily urbanized Reno, Nevada, with high-resolution seismic reflection imaging. The 6.8 km of P -wave data image the subsurface to approximately 800 m depth and delineate two subbasins and basin uplift that are consistent with structure previously inferred from gravity modeling in this region of the northern Walker Lane. We interpret two primary faults that bound the uplift and deform Quaternary deposits. The dip of Quaternary and Tertiary strata in the western subbasin increases with greater depth to the east, suggesting recurrent fault motion across the westernmost of these faults. Deformation in the Quaternary section of the western subbasin is likely evidence of extensional growth folding at the edge of the Truckee River through Reno. This deformation is north of, and on trend with, previously mapped Quaternary fault strands of the Mt. Rose fault zone. In addition to corroborating the existence of previously inferred intrabasin structure, these data provide evidence for an active extensional Quaternary fault at a previously unknown location within the Truckee Meadows basin that furthers our understanding of both the seismotectonic framework and earthquake hazards in this urbanized region.

Description: We conducted active and passive seismic imaging investigations along a 5.6-km-long, east–west transect ending at the mapped trace of the Wasatch fault in southern Utah Valley. Using two-dimensional (2D) P-wave seismic reflection data, we imaged basin deformation and faulting to a depth of 1.4 km and developed a detailed interval velocity model for prestack depth migration and 2D ground-motion simulations. Passive-source microtremor data acquired at two sites along the seismic reflection transect resolve S-wave velocities of approximately 200??m/s at the surface to about 900??m/s at 160 m depth and confirm a substantial thickening of low-velocity material westward into the valley. From the P-wave reflection profile, we interpret shallow (100–600 m) bedrock deformation extending from the surface trace of the Wasatch fault to roughly 1.5 km west into the valley. The bedrock deformation is caused by multiple interpreted fault splays displacing fault blocks downward to the west of the range front. Further west in the valley, the P-wave data reveal subhorizontal horizons from approximately 90 to 900 m depth that vary in thickness and whose dip increases with depth eastward toward the Wasatch fault. Another inferred fault about 4 km west of the mapped Wasatch fault displaces horizons within the valley to as shallow as 100 m depth. The overall deformational pattern imaged in our data is consistent with the Wasatch fault migrating eastward through time and with the abandonment of earlier synextensional faults, as part of the evolution of an inferred 20-km-wide half-graben structure within Utah Valley. Finite-difference 2D modeling suggests the imaged subsurface basin geometry can cause fourfold variation in peak ground velocity over distances of 300 m.

Description: A BSTRACT :  Deep-water mudstones from ancient epicontinental settings are significant repositories for organic matter, but the detailed temporal variations of, and controls on, the abundance and type of organic matter (OM) is little studied. Using micro-petrographic and geochemical data from late Mississippian mudstones of the Widmerpool Gulf, UK, the processes that delivered fine-grained sediment to this basin during a glacioeustatic sea-level cycle are interpreted from detailed lithofacies analysis. Seven primary lithofacies are identified from core, which show specific and systematic variations in total organic carbon (TOC) content and bulk carbon isotope composition of organic material ( 13 C org ). During sea-level highstands, thin-bedded carbonate-bearing mudstones are the dominant facies deposited, contain up to 6.6% TOC (average 4.6 ± 1.3%), and have mean 13 C org of –28.5 ± 0.9. During phases of lower sea level, thin-bedded silt-bearing clay-rich mudstones with up to 4.1% TOC (average 2.3 ± 0.8%; mean 13 C org : –28.2 ± 1.0) were interbedded with more organic-lean graded silt-bearing mudstones and sand-bearing silt-rich mudstones (average TOC: 1.7 ± 0.6%) derived from turbidity currents. The latter (mean 13 C org : –26.2 ± 0.7) are closely linked to significant proportions of terrestrial plant material, while some rare plant debris- and sand-bearing mudstones produced from debris flows have more than 7.0% TOC and 13 C org ≥ –26.0. The 13 C values of wood fragments ranged from –27.1 to –24.0 and therefore the 13 C org is interpreted as a function of the ratio of marine and terrestrial organic matter. More negative values in the carbonate-bearing and the clay-rich mudstones indicate marine planktonic algae whereas the least negative values reflect greater contribution of terrestrial plant material. The data suggest that the marine conditions prevailed and supported marine planktonic algae throughout different sea-level stages. Marine OM was delivered to the sea floor by continuous hemipelagic settling whereas terrestrial OM was delivered by sediment density flows. Variations in bioproductivity and dilution by siliciclastics influenced the burial rate of marine OM. Organic-rich mudstones preserved in these marine basins are potential hydrocarbon source rocks, especially as unconventional (shale gas) reservoirs. Detailed microtextural and compositional analysis coupled with rigorous geochemical parameters as used in this study are important for the understanding of the source-rock potential of basinal mudstones, and of fine-grained organic-rich sediments in general.

Description: We ran finite-difference earthquake simulations for great subduction zone earthquakes in Cascadia to model the effects of source and path heterogeneity for the purpose of improving strong-motion predictions. We developed a rupture model for large subduction zone earthquakes based on a k –2 slip spectrum and scale-dependent rise times by representing the slip distribution as the sum of normal modes of a vibrating membrane. Finite source and path effects were important in determining the distribution of strong motions through the locations of the hypocenter, subevents, and crustal structures like sedimentary basins. Some regions in Cascadia appear to be at greater risk than others during an event due to the geometry of the Cascadia fault zone relative to the coast and populated regions. The southern Oregon coast appears to have increased risk because it is closer to the locked zone of the Cascadia fault than other coastal areas and is also in the path of directivity amplification from any rupture propagating north to south in that part of the subduction zone, and the basins in the Puget Sound area are efficiently amplified by both north and south propagating ruptures off the coast of western Washington. We find that the median spectral accelerations at 5 s period from the simulations are similar to that of the Zhao et al. (2006) ground-motion prediction equation, although our simulations predict higher amplitudes near the region of greatest slip and in the sedimentary basins, such as the Seattle basin.

Description: Previous investigators have argued that the northwest-striking Reelfoot fault of northwest Tennessee and southeastern Missouri is segmented. One segment boundary is at the intersection of the northeast-striking Cottonwood Grove and Ridgely strike-slip faults with the Reelfoot fault. We use seismic reflection and geologic mapping to locate and determine the history of the Reelfoot South fault across this boundary zone. One reflection profile revealed a southwest-dipping (81°) Reelfoot South reverse fault that displaces the top of the Paleozoic 65 m, Cretaceous 40 m, Paleocene 31 m, Eocene Wilcox Group 20 m, and Eocene Memphis Sand 16 m. A second reflection profile reveals a north-dipping (84°) reverse fault 4.3 km south of the Reelfoot South fault, which defines the southwest margin of the Tiptonville dome. A geologic profile of the base of the ~3.1 Ma Upland complex (Mississippi River terrace alluvium) within the Mississippi River bluffs reveals ~6 m of displacement across the Reelfoot South fault. Similarly, Quaternary stream terrace distribution suggests ~6 m of Reelfoot South hanging-wall (Tiptonville dome) uplift that is probably Holocene. Fault strike trends show the Reelfoot fault and its hanging-wall Tiptonville dome are not laterally offset across the Cottonwood Grove and Ridgely faults. The Reelfoot South fault northwest and southeast of the Cottonwood Grove and Ridgely faults has very similar vertical displacement on common stratigraphic marker horizons in the upper 900 m. These data indicate the Reelfoot fault/Tiptonville dome has acted as one continuous fault zone across the Cottonwood Grove and Ridgely faults since Late Cretaceous.

Description: We examine the variability of long‐period (⁠T≥1  s⁠) earthquake ground motions from 3D simulations of Mw 7 earthquakes on the Salt Lake City segment of the Wasatch fault zone, Utah, from a set of 96 rupture models with varying slip distributions, rupture speeds, slip velocities, and hypocenter locations. Earthquake ruptures were prescribed on a 3D fault representation that satisfies geologic constraints and maintained distinct strands for the Warm Springs and for the East Bench and Cottonwood faults. Response spectral accelerations (SA; 1.5–10 s; 5% damping) were measured, and average distance scaling was well fit by a simple functional form that depends on the near‐source intensity level SA0(T) and a corner distance Rc:SA(R,T)=SA0(T)(1+(R/Rc))−1⁠. Period‐dependent hanging‐wall effects manifested and increased the ground motions by factors of about 2–3, though the effects appeared partially attributable to differences in shallow site response for sites on the hanging wall and footwall of the fault. Comparisons with modern ground‐motion prediction equations (GMPEs) found that the simulated ground motions were generally consistent, except within deep sedimentary basins, where simulated ground motions were greatly underpredicted. Ground‐motion variability exhibited strong lateral variations and, at some sites, exceeded the ground‐motion variability indicated by GMPEs. The effects on the ground motions of changing the values of the five kinematic rupture parameters can largely be explained by three predominant factors: distance to high‐slip subevents, dynamic stress drop, and changes in the contributions from directivity. These results emphasize the need for further characterization of the underlying distributions and covariances of the kinematic rupture parameters used in 3D ground‐motion simulations employed in probabilistic seismic‐hazard analyses.

Description: Significant uncertainty remains concerning how and where crustal shortening occurs throughout the eastern Cascade Range in Washington State. Using light detection and ranging (lidar) imagery, we identified an ∼5‐km‐long lineament in Swakane canyon near Wenatchee, roughly coincident with a strand of the Entiat fault. Topographic profiles across the lineament reveal a southwest‐side‐up break in slope, with an average of 2–3 m of vertical separation of the hillslope surface. We consider a range of possible origins for this feature, including differential erosion across a fault‐line scarp, slope failure (sackung or landslide), and surface deformation across an active fault strand. Based on trenching, radiocarbon and luminescence dating, and ground‐penetrating radar (GPR) across the lineament, we conclude that warped saprolite observed in the shallow subsurface is most consistent with southwest‐side‐up folding caused by blind reverse faulting at depth. Following this reasoning, dating of overlying colluvial deposits suggests that at least one Holocene earthquake occurred on this strand of the southern Entiat fault, with an approximate vertical separation of ≥1  m. GPR reveals up to 4 m of cumulative vertical separation of the saprolite, suggesting a history of multiple earthquakes on the structure. Taken in context with other potential fault‐related lineaments along the Entiat fault, our interpretation of Holocene earthquakes in Swakane canyon could suggest reactivation of longer sections of the Entiat fault, as well as of other bedrock faults in the eastern Cascades. Although active erosion and slow strain rates lead to a subdued geomorphic expression of recent deformation, we conclude that the reactivated Entiat fault represents a seismogenic structure that should be considered in regional seismic hazard analyses. The difficulty of recognizing low‐slip‐rate structures in forested and mountainous terrain underscores the importance of additional lidar surveys and geological and geophysical studies for fully understanding seismic hazard in regions with infrequent but potentially large earthquakes.