[1] 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.

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[1] An important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting (SWS). In Sumatra, two temporary seismic networks were deployed between December 2007 and February 2009, covering the fore arc between the fore-arc islands to the back arc. We use SKS and local SWS measurements to determine the type, amount, and location of anisotropy. Local SWS measurements from the fore-arc islands exhibit trench-parallel fast directions which can be attributed to shape preferred orientation of cracks/fractures in the overriding sediments. In the Sumatran Fault region, the predominant fast direction is fault/ trench parallel, while in the back-arc region it is trench perpendicular. The trench-perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge, while the fault-parallel measurements are similar to the fault-parallel fast directions observed for two crustal events at the Sumatran Fault. This suggests that there are two layers of anisotropy: one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the Sumatran Fault. SKS splitting results show a NNW-SSE fast direction with delay times of 0.8-3.0 s. The fast directions are approximately parallel to the absolute plate motion of the subducting Indo-Australian Plate. The small delay times exhibited by the local SWS (0.05-0.45 s), in combination with the large SKS delay times, suggest that the anisotropy generating the teleseismic SWS is dominated by entrained flow in the asthenosphere below the slab.

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 measurements suggest transportation sources generate the majority of the noise wavefield across the entire wideband, except at the secondary microseismic peak (2< T <6 s). The effect of road traffic is greatest at short periods (T <0.4 s) where acceleration noise powers are ∼20 dB larger than the New High Noise Model; at T =0.1 s daytime root-mean-square acceleration amplitudes are 1000 times higher in central London than at an observatory station in Eskdalemuir, Scotland. Overground railways generate observable signals both at short periods (T <0.3 s), which are recorded in close proximity to the tracks, and at very long periods (T >20 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 metres 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 power spectral density values for all stations, to facilitate comparison with any future urban seismic deployments.