Seeing Through the Noise: Seismic Reflection Profiling in Urban Areas

Studies for urban hazard or resource assessment often take place in densely populated areas characterized by considerable cultural noise. These site conditions can severely compromise seismic reflection data quality. We have collected vibroseis and hammer (weight drop) seismic reflection data in a range of geologic conditions to image stratigraphy and structures in the upper one km along regional highways, city streets,and power line access roads. In addition to the challenges of safety and outreach, acquisition efforts along busy streets and highways often encounter poor receiver coupling and large-amplitude coherent noise from traffic and power lines. Although higher quality seismic reflection data may be obtained by simply choosing alternate sites with less cultural noise, modifications to the acquisition and processing step scan minimize the effects of cultural noise and poor coupling where profiling is most relevant. Flagging crews, flyers and public announcements assist with outreach and safety concerns, and the local news media are often enthusiastic about publicizing geologic studies. Recording long-record vibroseis data reduces the effects of noise by itself,but data quality can be further optimized by recording uncorrelated,unstacked data and applying precorrelation amplitude adjustments and filters. Recording individual hammer shots likewise allows gains or mutes to normalize or remove traffic noise prior to vertical stacking. Large numbers of receiver channels allow attenuation of random noise and velocity filtering to remove coherent noise. Because ground roll and normal moveout (NMO) corrections minimize near-surface coverage, asymmetric source-receiver geometry allows for additional near-surface fold while muting large amplitude ground rolland NMO stretch. Source and geophone coupling on road shoulder scan degrade signal quality due to variable materials and topography,but these problems are often addressed with static corrections. Our experience is that high-quality seismic data can be obtained in noisy urban areas, but many recorded channels and a careful attention to acquisition and processing procedures can significantly improve the results

米国ワシントン州ピュージェット低地におけるサイト応答特性と盆地効果と減衰

Site response, basin amplification, and attenuation within the Puget Lowland, in the Cascade forearc of Washington State, have been measured using large temporary deployments of seismometers during 3 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS) experiments, and the permanent strong-motion network. Horizontal-component recordings of shear waves from teleseisms and local earthquakes were used to compute spectral ratios with respect to the average of several bedrock sites. Results show amplifications of 0.3 to 0.8 Hz seismic waves by factors of 6 to 12 at sites over the deep Seattle sedimentary basin (＞2.5km of sedimentary strata), with the peak amplification being at about 0.3 Hz. All sites in the central Puget Lowland are amplified significantly, however, which indicates that the deposits in the upper 2.5km extending beyond the margins of the deep basin are responsible for much of the amplification. The deep basin itself appears to amplify waves by factors of up to 2 in the 0.05 to 0.2 Hz frequency range, which is consistent with resonance in the deep strata. The horizontal to vertical ratios and crosscorrelations of arrivals at adjacent stations on the basin are consistent with the presence of scattered body-wave energy and Rayleigh waves in the later portions of the long-period signal. Estimates of the shear-wave attenuation factors (Qs) between 2 to 20 Hz increase from 5 to 40 for shallow sedimentary deposits to about 250 for the deep sedimentary strata (7km depth)

Megathrust Splay Faults at the Focus of the Prince William Sound Asperity, Alaska

[1] High-resolution sparker and crustal-scale air gun seismic reflection data, coupled with repeat bathymetric surveys, document a region of repeated coseismic uplift on the portion of the Alaska subduction zone that ruptured in 1964. This area defines the western limit of Prince William Sound. Differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 Great Alaska earthquake was focused along a series of subparallel faults beneath Prince William Sound and the adjacent Gulf of Alaska shelf. Bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the seafloor as submarine terraces on the Cape Cleare bank southwest of Montague Island. Sparker seismic reflection data provide cumulative Holocene slip estimates as high as 9 mm/yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism. Crustal seismic data show that these megathrust splay faults root separately into the subduction zone décollement. Splay fault divergence from this megathrust correlates with changes in midcrustal seismic velocity and magnetic susceptibility values, best explained by duplexing of the subducted Yakutat terrane rocks above Pacific plate rocks along the trailing edge of the Yakutat terrane. Although each splay fault is capable of independent motion, we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the Holocene

'Reclaiming the criminal' : the role and training of prison officers in England, 1877-1914

This article examines the role and training of prison officers in England, between 1877 and 1914. It is concerned with the changing penal philosophies and practices of this period and how these were implemented in local prisons, and the duties of the prison officer. More broadly, this article argues that the role of the prison officer and their training (from 1896) reflect wider ambiguities in prison policy and practice during this period

Amplification of Earthquake Ground Motions in Washington, DC, and Implications for Hazard Assessments in Central and Eastern North America

The extent of damage in Washington, DC, from the 2011 MW 5.8 Mineral, VA, earthquake was surprising for an epicenter 130 km away; U.S. Geological Survey “Did-You-Feel-It” reports suggest that Atlantic Coastal Plain and other unconsolidated sediments amplified ground motions in the city. We measure this amplification relative to bedrock sites using earthquake signals recorded on a temporary seismometer array. The spectral ratios show strong amplification in the 0.7 to 4 Hz frequency range for sites on sediments. This range overlaps with resonant frequencies of buildings in the city as inferred from their heights, suggesting amplification at frequencies to which many buildings are vulnerable to damage. Our results emphasize that local amplification can raise moderate ground motions to damaging levels in stable continental regions, where low attenuation extends shaking levels over wide areas and unconsolidated deposits on crystalline metamorphic or igneous bedrock can result in strong contrasts in near-surface material properties

Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

The low energy muon (LEM) technique has been used to probe local changes in the dynamical spectrum of thin film polymer samples taking place as a function of the temperature and the implantation depth below the free surface. The studies have been made on samples of polydimethylsiloxane (PDMS) and polybutadiene (PB) using the transverse magnetic field (TF) configuration and diamagnetic probe muons. In PDMS evidence is found for suppression of the glass transition temperature near the surface, along with significantly modified dynamics in the near-surface region as well as at depths significantly below the surface. For PB the LEM technique reveals well-defined layers of dynamical and spatial inhomogeneity at depths of order 0.1–0.2 μm below the free surface. These inhomogeneous regions may be assigned to nanopores produced by solvent streaming during preparation of spin-cast films. A thermal annealing procedure is shown to significantly reduce the thickness of these inhomogeneous layers. These results demonstrate that using LEM in the TF configuration provides a promising new method for studying surface-modified local dynamics of polymers that is also able to reveal nanostructured buried layers in polymer films