Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

[1] We perform systematic simulations of slip using a quasi-dynamic continuum model of a two-dimensional (2-D) strike-slip fault governed by rate-and state-dependent friction. The depth dependence of the a À b and L frictional parameters are treated in an innovative way that is consistent with available laboratory data and multidisciplinary field observations. Various realizations of heterogeneous L distributions are used to study effects of structural variations of fault zones on spatiotemporal evolution of slip. We demonstrate that such realizations can produce within the continuum class of models realistic features of seismicity and slip distributions on a fault. We explore effects of three types of variable L distributions: (1) a depth-dependent L profile accounting for the variable width of fault zones with depth, (2) uncorrelated 2-D random distributions of L with different degrees of heterogeneity, and (3) a hybrid distribution combining the depth-dependent L profile with the 2-D random L distributions. The first type of L distribution, with relatively small L over the depth range corresponding to the seismogenic zone and larger L elsewhere, generates stick-slip events in the seismogenic zone and ongoing creep above and below that region. The 2-D heterogeneous parameterizations generate frequency-size statistics with event sizes spanning 4 orders of magnitude. Our results indicate that different degrees of heterogeneity of L distributions control (1) the number of simulated events and (2) the overall stress level and fluctuations. Other observable trends are (3) the dependency of hypocenter location on L and (4) different nucleation phases for small and large events in heterogeneous distributions.Citation: Hillers, G., Y. Ben-Zion, and P. M. Mai (2006), Seismicity on a fault controlled by rate-and state-dependent friction with spatial variations of the critical slip distance,

We observe seasonal seismic wave speed changes (dv/v) in the San Jacinto fault area and investigate several likely source mechanisms. Velocity variations are obtained from analysis of 6 yr data of vertical component seismic noise recorded by 10 surface and six borehole stations. We study the interrelation between dv/v records, frequency-dependent seismic noise properties, and nearby environmental data of wind speed, rain, ground water level, barometric pressure and atmospheric temperature. The results indicate peak-to-peak seasonal velocity variations of ∼0.2 per cent in the 0.5-2 Hz frequency range, likely associated with genuine changes of rock properties rather than changes in the noise field. Phase measurements between dv/v and the various environmental data imply that the dominant source mechanism in the arid study area is thermoelastic strain induced by atmospheric temperature variations. The other considered environmental effects produce secondary variations that are superimposed on the thermal-based changes. More detailed work with longer data on the response of rocks to various known external loadings can help tracking the evolving stress and effective rheology at depth.

[1] 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 Landès 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.

A seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault-plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse-faulting style, and the associated SH radiation patterns appear to control the first-order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient-noise cross-correlation functions reconstruct regional surface-wave propagation and path-dependent body-wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear-wave velocities around 3.3  km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.

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 Piñon 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 Sθ 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 Sθ≈5 × 103 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.

We have analyzed the time-dependent properties of the ambient seismic wavefield between 0.1 and 8 Hz to detect, resolve, monitor, and image the deformation induced by the water injection associated with the stimulation of the 2006 Deep Heat Mining Project in the city of Basel, Switzerland. The application of passive methods allowed the detection of an aseismic transient of approximately 35 days' duration that began with the onset of the reservoir stimulation. Peak deformation was reached some 15 days after the bleed-off and after the induced seismicity ceased. We resolved a significant increase in seismic velocities and a simultaneous decorrelation of the noise correlation coda waveforms. The wavefield properties implied that the material response was monitored mainly in the sedimentary layer (<2.5 km) above the stimulated volume that was approximately 4.5 km deep. We inverted the velocity-change and decorrelation data to estimate the spatial distribution of the medium changes. The resulting images showed that the strong velocity variations and medium perturbations were generally colocated with the lateral distribution of the induced seismicity. Positive velocity changes and damage around the injection site indicated subsidence, settling, and compaction of the material overlying the stimulated volume. Our results demonstrate that noise-based analysis tools can provide important observables that are complementary to results obtained with standard microseismicity tools. Passive monitoring and imaging have the potential to mature into routinely applied observation techniques that support reservoir management in a variety of geotechnical contexts, such as for mining, fluid injection, hydraulic fracturing, nuclear waste management, and CO 2 storage.

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