Characterizing broadband seismic noise in Central London

Recordings made at five broadband seismometers, deployed in central London during the summer of 2015, reveal the wideband nature (periods T of between 0.01 and 100 s) of anthropogenic noise in a busy urban environment. Temporal variations of power spectral density (PSD) measurements suggest that transportation sources generate the majority of the noise wavefield across the entire wideband, except at the secondary microseismic peak (220  s), which are recorded across the city. We record a unique set of signals 30 m above a subway (London Underground) tunnel interpreted as a short‐period dynamic component, a quasi‐static response to the train moving underneath the instrument and a very long period (T>30  s) response to air movement around the tunnel network. A low‐velocity clay and sand overburden tens of meters thick is shown to amplify the horizontal‐component wavefield at T∼1  s, consistent with properties of the London subsurface derived from engineering investigations. We provide tabulated median PSD values for all stations to facilitate comparison with any future urban seismic deployments

Damage and seismic velocity structure of pulverized rocks near the San Andreas Fault

A combination of seismic refraction tomography, laboratory ultrasonic velocity measurements, and microstructural observations was used to study the shallow velocity structure of a strand of the San Andreas fault (SAF) just south of Littlerock, California. The examined site has a strongly asymmetric damage structure with respect to the SAF core. The conglomerates to the southwest show little to no damage, whereas a ~100 m wide damage zone exists to the northeast with a ~50 m wide zone of pulverized granite adjacent to the fault core. Seismic P‐wave velocities of the damaged and pulverized granite were investigated over a range of scales. In situ seismic velocity imaging was performed on three overlapping profiles normal to the SAF with lengths of 350 m, 50 m, and 25 m. In the laboratory, ultrasonic velocities were measured on centimeter‐ to decimeter‐sized samples taken along the in situ profiles. The samples were also investigated microstructurally. Micro‐scale fracture damage intensifies with increasing proximity to the fault core, allowing a subdivision of the damage zone into several sections. Laboratory‐derived velocities in each section display varying degrees of anisotropy, and combined with microfracture analysis suggest an evolving damage fabric. Pulverized rocks close to the fault exhibit a preferred fault‐parallel orientation of microfractures, resulting in the lowest P‐wave velocity orientated in fault‐perpendicular direction. Closest to the fault, pulverized rocks exhibit a gouge‐like fabric that is transitional to the fault core. Comparison of absolute velocities shows a scaling effect from field to laboratory for the intact rocks. A similar scaling effect is absent for the pulverized rocks, suggesting that they are dominated by micro‐scale damage. Fault‐parallel damage fabrics are consistent with existing models for pulverized‐rock generation that predict strong dynamic reductions in fault‐normal stress. Our observations provide important constraints for theoretical models and imaging fault damage properties at depth using remote methods

An initial assessment of the environmental impact of grocery products

This report presents a series of analyses with the common purpose of establishing which grocery products are likely to contribute most to the environmental impacts (carbon footprint or embodied carbon, embodied energy, water, materials use and waste) associated with UK household consumption. Understanding and prioritising these has enabled reduction actions, interventions and further research to be directed more effectively at those products with the greatest potential to influence overall consumption impacts.The report includes a systematic review of 1,900 grocery carbon footprint data points for 191 products; believed to be the largest assessment of its kind at the time of publication

Mantle flow in regions of complex tectonics: insights from Indonesia

Indonesia is arguably one of the tectonically most complex regions on Earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion. It is thus an ideal location to study the interaction between subducting plates and mantle convection. Seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes, such as mantle flow, for example. Here, we present novel shear wave splitting results across the Indonesian region. Using three different shear phases (local S, SKS, and downgoing S) to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features. For example, the block rotation history of Borneo is reflected in coast-parallel fast directions, which we attribute to fossil anisotropy. Furthermore, we are able to unravel the mantle flow pattern in the Sulawesi and Banda region: We detect toroidal flow around the Celebes Sea slab, oblique corner flow in the Banda wedge, and sub-slab mantle flow around the arcuate Banda slab. We present evidence for deep, sub-520 km anisotropy at the Java subduction zone. In the Sumatran backarc, we measure trench-perpendicular fast orientations, which we assume to be due to mantle flow beneath the overriding Eurasian plate. These observations will allow to test ideas of, for example, slab–mantle coupling in subduction regions