G. Hillers

Global oceanic microseism sources as seen by seismic arrays and predicted by wave action models.

We analyze global microseism excitation patterns between July 2000 and June 2001. Seismological observations are compared with modeling results to isolate robust activity features of relevant source processes. First, we use observations of microseism source locations estimated by Landes et al. (2010) based on array processing of ambient noise correlations. Second, we construct synthetic activity patterns by coupling sea state estimates derived from wave action models to the excitation theory for microseisms. The overall spatiotemporal evolution of both estimates is characterized by a seasonal character that is associated with strong activity during winter months. The distribution of landmass causes seasonal changes on the Northern Hemisphere (NH) to exceed the variability on the Southern Hemisphere (SH). Our systematic comparison of the two estimates reveals significant microseism excitation along coastlines and in the open ocean. Since coastal reflections are not accounted for in the modeling approach, the consistent mismatch between near-coastal observations and predictions suggests that relevant microseism energy arriving at the networks is generated in these areas. Simultaneously, systematic coincidence away from coastlines verifies the open ocean generation hypothesis. These conclusions are universal and robust with respect to the seismic network locations on the NH. The spatially homogeneous resolution of our synthetics provides a valuable resource for the assessment of the global microseism weather. Similar to previously identified hot spot areas in the North Atlantic, the modeled distributions hypothesize regions of strong localized activity on the SH, which are only partially confirmed by the analyzed data sets.

Seismic fault zone trapped noise

Systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics. Here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array. The frequency-dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field. A critical frequency fc ≈ 0.5 Hz defines a threshold above which the in-fault scattered wavefield has increased isotropy and coherency compared to the ambient noise. The increased randomization of in-fault propagation directions produces a wavefield that is trapped in a waveguide/cavity-like structure associated with the low-velocity damage zone. Dense spatial sampling allows the resolution of a near-field focal spot, which emerges from the superposition of a collapsing, time reversed wavefront. The shape of the focal spot depends on local medium properties, and a focal spot-based fault normal distribution of wave speeds indicates a ∼50% velocity reduction consistent with estimates from a far-field travel time inversion. The arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern, providing a noise-based imaging tool that can complement analyses of trapped ballistic waves. The results can have wide applicability for investigating the internal properties of fault damage zones, because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves.

Scaling Relations of Strike-Slip Earthquakes with Different Slip-Rate-Dependent Properties at Depth

Abstract Empirical observations suggest that earthquake stress drop is generally constant. To investigate the effect of rupture width on earthquake scaling relations, we analyze synthetic seismicity produced by a 3D vertical strike-slip fault model using two different profiles of frictional slip-rate behavior below the seismogenic zone. Within the rate-and-state framework, a relatively abrupt transition of the a – b profile from velocity weakening to strengthening at the base of the seismogenic crust produces increasing slip and stress drop with increasing event size. Choosing a smoother transition allows large earthquakes to propagate deeper, leading to similar slip-length scaling but constant stress-drop scaling. Our numerical experiments support the idea that the maintenance of constant stress drop across the entire range of observed earthquake magnitudes may be achieved by allowing coseismic slip to rupture to depths below the seismogenic layer.

In situ observations of velocity changes in response to tidal deformation from analysis of the high‐frequency ambient wavefield

We report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Pinon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ. Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity S_θ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low-velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of S_θ≈5 × 10^3 and a stress sensitivity of 0.5 MPa^(−1). The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here.

Focal spot imaging based on zero lag cross‐correlation amplitude fields: Application to dense array data at the San Jacinto fault zone

We image the subsurface below a dense seismic array straddling the Clark branch of the San Jacinto fault zone in Southern California. The analysis is based on focal spots of surface waves associated with the zero lag amplitudes of noise cross-correlations computed between all stations of the dense array. Local medium properties are inferred from the spatially variable focal spot size and shape based on the first zero crossing of amplitude versus distance distributions. The method provides simultaneous estimates of wave speed, apparent attenuation, and anisotropy without solving a tomographic inverse problem. The obtained images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far-field Rayleigh wave tomography. We observe an anticorrelation between our apparent attenuation coefficient and seismic velocity, and a fault-parallel alignment of fast propagation directions with greater structural complexity to the southwest of the fault. The results imply a complex fault zone structure including a waveguide to the northeast of the fault that is continuous across the observed depth range and a low-velocity structure to the southwest associated with a shallow sedimentary basin.

Toward Seismic Metamaterials: The METAFORET Project

We report on a seismic metamaterial experiment in a pine-tree forest environment where the dense collection of trees behaves as subwavelength coupled resonators for surface seismic waves. For the METAFORET experiment, more than 1000 seismic sensors were deployed over a 120 m × 120 m area to study the properties of the ambient and induced seismic wavefield that propagates in the ground and in trees. The goal of the experiment was to establish a link between seismic-relevant scales and microscale and mesoscale studies that pioneered the development of metamaterial physics in optics and acoustics. The first results of the METAFORET experiment show the presence of frequency band gaps for Rayleigh waves associated with compressional and flexural resonances of the trees, which confirms the strong influence that a dense collection of trees can have on the propagation of seismic waves.

Sources of Long‐Range Anthropogenic Noise in Southern California and Implications for Tectonic Tremor Detection

We study anthropogenic noise sources seen on seismic recordings along the central section of the San Jacinto fault near Anza, southern California. The strongest signals are caused by freight trains passing through the Coachella Valley north of Anza. Train-induced transients are observed at distances of up to 50 km from the railway, with durations of up to 20 min, and spectra that are peaked between 3 and 5 Hz. Additionally, truck traffic through the Coachella Valley generates a sustained hum with a similar spectral signature as the train transients but with lower amplitude. We also find that wind turbine activity in northern Baja California introduces a seasonal modulation of 1– to 5-Hz energy across the Anza network. We show that the observed train-generated transients can be used to constrain shallow attenuation structure at Anza. Using the results from this study as well as available borehole data, we further evaluate the performance of approaches that have been used to detect nonvolcanic tremor at Anza. We conclude that signals previously identified as spontaneous tremor (Hutchison and Ghosh, 2017) were probably generated by other nontectonic sources such as trains. Electronic Supplement: Seismograms during a transient, decomposition of the large amplitude signal into sequences of repeating patterns, detection statistics in the two stages of the tremor search procedure, velocity amplitude spectra of transients from two collocated borehole–surface station pairs, median weekday–night spectra for all stations in the Plate Boundary Observatory (PBO) Anza network, time variations of root mean square (rms) velocity for all stations in the PBO Anza network, cumulative population and kilometers of railroad on which the allowed speed for freight trains exceeds 50 mph, envelopes of vertical ground motions as a function of time, vertical ground motions recorded by the Anza borehole stations, and spectrograms of vertical ground motion and corresponding wind speeds.