The earthquake size distribution follows, in most instances, a power law, with the slope of this power law, the 'b value', commonly used to describe the relative occurrence of large and small events ...(a high b value indicates a larger proportion of small earthquakes, and vice versa). Statistically significant variations of b values have been measured in laboratory experiments, mines and various tectonic regimes such as subducting slabs, near magma chambers, along fault zones and in aftershock zones. However, it has remained uncertain whether these differences are due to differing stress regimes, as it was questionable that samples in small volumes (such as in laboratory specimens, mines and the shallow Earth's crust) are representative of earthquakes in general. Given the lack of physical understanding of these differences, the observation that b values approach the constant 1 if large volumes are sampled was interpreted to indicate that b = 1 is a universal constant for earthquakes in general. Here we show that the b value varies systematically for different styles of faulting. We find that normal faulting events have the highest b values, thrust events the lowest and strike-slip events intermediate values. Given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress.
Global review of human-induced earthquakes Foulger, Gillian R.; Wilson, Miles P.; Gluyas, Jon G. ...
Earth-science reviews,
March 2018, 2018-03-00, Volume:
178
Journal Article
Peer reviewed
Open access
The Human-induced Earthquake Database, HiQuake, is a comprehensive record of earthquake sequences postulated to be induced by anthropogenic activity. It contains over 700 cases spanning the period ...1868–2016. Activities that have been proposed to induce earthquakes include the impoundment of water reservoirs, erecting tall buildings, coastal engineering, quarrying, extraction of groundwater, coal, minerals, gas, oil and geothermal fluids, excavation of tunnels, and adding material to the subsurface by allowing abandoned mines to flood and injecting fluid for waste disposal, enhanced oil recovery, hydrofracturing, gas storage and carbon sequestration. Nuclear explosions induce earthquakes but evidence for chemical explosions doing so is weak. Because it is currently impossible to determine with 100% certainty which earthquakes are induced and which not, HiQuake includes all earthquake sequences proposed on scientific grounds to have been human-induced regardless of credibility. Challenges to constructing HiQuake include under-reporting which is ~30% of M ~4 events, ~60% of M ~3 events and ~90% of M ~2 events. The amount of stress released in an induced earthquake is not necessarily the same as the anthropogenic stress added because pre-existing tectonic stress may also be released. Thus earthquakes disproportionately large compared with the associated industrial activity may be induced. Knowledge of the magnitude of the largest earthquake that might be induced by a project, MMAX, is important for hazard reduction. Observed MMAX correlates positively with the scale of associated industrial projects, fluid injection pressure and rate, and the yield of nuclear devices. It correlates negatively with calculated inducing stress change, likely because the latter correlates inversely with project scale. The largest earthquake reported to date to be induced by fluid injection is the 2016 M 5.8 Pawnee, Oklahoma earthquake, by water-reservoir impoundment the 2008 M ~8 Wenchuan, People's Republic of China, earthquake, and by mass removal the 1976 M 7.3 Gazli, Uzbekistan earthquake. The minimum amount of anthropogenic stress needed to induce an earthquake is an unsound concept since earthquakes occur in the absence of industrial activity. The minimum amount of stress observed to modulate earthquake activity is a few hundredths of a megapascal and possibly as little as a few thousandths, equivalent to a few tens of centimeters of water-table depth. Faults near to failure are pervasive in the continental crust and induced earthquakes may thus occur essentially anywhere. In intraplate regions neither infrastructure nor populations may be prepared for earthquakes. Human-induced earthquakes that cause nuisance are rare, but in some cases may be a significant problem, e.g., in the hydrocarbon-producing areas of Oklahoma, USA. As the size of projects and density of populations increase, the potential nuisance of induced earthquakes is also increasing and effective management strategies are needed.
The occurrence of subduction earthquakes usually leads to considerable localized mass migration changes. To improve the detection of earthquake subductions as well as the constraint of fault ...parameters, we derive expressions that describe changes of the gravitational curvatures (GC), i.e., the third-order derivatives of the Earth's gravitational potential, caused by a point dislocation while adopting a spherical symmetric Earth model. As a 3-D tensor matrix, the GC have twenty-seven components of those seven are independent. First, we investigate the dislocation Love numbers of the Earth's gravitational potential and derive the Green's functions of GC caused by four independent point sources in a spherical inhomogeneous Earth model. We then present the GC changes in a half-space Earth model. Furthermore, we conduct a sensitivity study by using three physical quantities that involve gravitation, gravitational gradients, and GC to compare their abilities in a seismic source depth detection. Our numerical results reveal that changes in the GC are more sensitive to a medium information about the field source compared to gravitation and gravitational gradients. This finding indicates that GC measurements could provide a more detailed information about slip fault parameters when considering a heterogeneous slip. Despite a widespread application of gravity gradients in Earth science, especially after launching the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite mission, measurements of the third-order derivatives of the gravitational potential have an enormous potential in the study of the solid Earth, although further work is needed in terms of instrument design and development.
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•Expressions describing changes of the third-order derivatives of the Earth’s gravitational potential caused by point dislocation are derived.•Changes in third-order derivatives of the Earth’s gravitational potential provide a more detailed information on slip fault parameters.
SUMMARY
Seismic interrogation of the upper mantle from the base of the crust to the top of the mantle transition zone has revealed discontinuities that are variable in space, depth, lateral extent, ...amplitude and lack a unified explanation for their origin. Improved constraints on the detectability and properties of mantle discontinuities can be obtained with P-to-S receiver function (Ps-RF) where energy scatters from P to S as seismic waves propagate across discontinuities of interest. However, due to the interference of crustal multiples, uppermost mantle discontinuities are more commonly imaged with lower resolution S-to-P receiver function (Sp-RF). In this study, a new method called CRISP-RF (Clean Receiver-function Imaging using SParse Radon Filters) is proposed, which incorporates ideas from compressive sensing and model-based image reconstruction. The central idea involves applying a sparse Radon transform to effectively decompose the Ps-RF into its underlying wavefield contributions, that is direct conversions, multiples, and noise, based on the phase moveout and coherence. A masking filter is then designed and applied to create a multiple-free and denoised Ps-RF. We demonstrate, using synthetic experiment, that our implementation of the Radon transform using a sparsity-promoting regularization outperforms the conventional least-squares methods and can effectively isolate direct Ps conversions. We further apply the CRISP-RF workflow on real data, including single station data on cratons, common-conversion-point stack at continental margins and seismic data from ocean islands. The application of CRISP-RF to global data sets will advance our understanding of the enigmatic origins of the upper mantle discontinuities like the ubiquitous mid-lithospheric discontinuity and the elusive X-discontinuity.
On the Broadband Instrument Response of Fiber‐Optic DAS Arrays Lindsey, Nathaniel J.; Rademacher, Horst; Ajo‐Franklin, Jonathan B.
Journal of geophysical research. Solid earth,
February 2020, 2020-02-00, 20200201, Volume:
125, Issue:
2
Journal Article
Peer reviewed
Distributed Acoustic Sensing (DAS) is a novel tool in array seismology that measures the phase of backscattered laser pulses traveling in a fiber‐optic cable, and relates this measurement to the ...axial strain induced on the cable by a propagating seismic wavefield. Combining DAS with telecommunications optical fiber networks has begun to address a range of earth science questions where cost and field logistics have historically hindered observations. Unlike classic inertial seismometers, DAS instrument response is presently unquantified. This topic includes a variable sensing element—the fiber, including packaging and installation—which changes between experiments. Ignoring this element, one DAS record should approximate a fixed‐length strain gauge, which exactly measures Earth's motion down to quasi‐static frequencies relevant to geodesy. In this paper, we test this hypothesis using seismological observations of teleseismic earthquakes and microseism noise spanning the 1 to 120 s period range. We use a commercial DAS interrogator unit connected to an optical fiber previously used for telecommunication and a colocated broadband seismometer to estimate the DAS transfer function. We find a 1:1 correspondence with actual ground motion from 10–120 s. At shorter periods (1–10 s), DAS amplitude response is enhanced by 3–11 dB. Phase response is flat over this range of periods. We interpret the recovered DAS response function in terms of hypothesized fiber coupling and photonic effects. We propose this calibration methodology for future DAS experiments where seismic amplitude information is desired.
Key Points
DAS instrument response for a telecom cable experiment is quantified empirically using a broadband seismometer
Amplitude response is flat to true ground motion from 10–120 s but enhanced at shorter periods; phase response is flat from 1–120 s
Our approach can be used to calibrate DAS arrays, enabling absolute ground motion amplitude measurements in the future
Application of the SCARDEC method provides the apparent source time functions together with seismic moment, depth, and focal mechanism, for most of the recent earthquakes with magnitude larger than ...5.6–6. Using this large dataset, we have developed a method to systematically invert for the rupture direction and average rupture velocity Vr, when unilateral rupture propagation dominates. The approach is applied to all the shallow (z < 120km) earthquakes of the catalog over the 1992–2015 time period. After a careful validation process, rupture properties for a catalog of 96 earthquakes are obtained. The subsequent analysis of this catalog provides several insights about the seismic rupture process. We first report that up-dip ruptures are more abundant than down-dip ruptures for shallow subduction interface earthquakes, which can be understood as a consequence of the material contrast between the slab and the overriding crust. Rupture velocities, which are searched without any a-priori up to the maximal P wave velocity (6000–8000m/s), are found between 1200m/s and 4500m/s. This observation indicates that no earthquakes propagate over long distances with rupture velocity approaching the P wave velocity. Among the 23 ruptures faster than 3100m/s, we observe both documented supershear ruptures (e.g. the 2001 Kunlun earthquake), and undocumented ruptures that very likely include a supershear phase. We also find that the correlation of Vr with the source duration scaled to the seismic moment (Ts) is very weak. This directly implies that both Ts and Vr are anticorrelated with the stress drop Δσ. This result has implications for the assessment of the peak ground acceleration (PGA) variability. As shown by Causse and Song (2015), an anticorrelation between Δσ and Vr significantly reduces the predicted PGA variability, and brings it closer to the observed variability.
•A new catalog of rupture velocities and directions for 96 shallow earthquakes•Subduction interplate earthquakes tend to propagate up-dip rather than down-dip.•Average rupture velocities slower than 3100m/s represent 75% of the catalog.•Several undocumented fast ruptures, very likely supershear, are reported.•The catalog reveals an anticorrelation between rupture velocity and stress drop.
SUMMARY
The use of the probabilistic approach to solve inverse problems is becoming more popular in the geophysical community, thanks to its ability to address nonlinear forward problems and to ...provide uncertainty quantification. However, such strategy is often tailored to specific applications and therefore there is a need for common platforms to solve different geophysical inverse problems and showing potential and pitfalls of the methodology. In this work, we demonstrate a common framework within which it is possible to solve such inverse problems ranging from, for example, earthquake source location to potential field data inversion and seismic tomography. This allows us to fully address nonlinear problems and to derive useful information about the subsurface, including uncertainty estimation. This approach can, in fact, provide probabilities related to certain properties or structures of the subsurface, such as histograms of the value of some physical property, the expected volume of buried geological bodies or the probability of having boundaries defining different layers. Thanks to its ability to address high-dimensional problems, the Hamiltonian Monte Carlo (HMC) algorithm has emerged as the state-of-the-art tool for solving geophysical inverse problems within the probabilistic framework. HMC requires the computation of gradients, which can be obtained by adjoint methods. This unique combination of HMC and adjoint methods is what makes the solution of tomographic problems ultimately feasible. These results can be obtained with ‘HMCLab’, a numerical laboratory for solving a range of different geophysical inverse problems using sampling methods, focusing in particular on the HMC algorithm. HMCLab consists of a set of samplers (HMC and others) and a set of geophysical forward problems. For each problem its misfit function and gradient computation are provided and, in addition, a set of prior models can be combined to inject additional information into the inverse problem. This allows users to experiment with probabilistic inverse problems and also address real-world studies. We show how to solve a selected set of problems within this framework using variants of the HMC algorithm and analyse the results. HMCLab is provided as an open source package written both in Python and Julia, welcoming contributions from the community.
SUMMARY
Although observation of gravity perturbations induced by earthquakes is possible, simulation of seismic wave propagation in a self-gravitating, rotating Earth model with 3-D heterogeneity is ...challenging due to the numerical complexities associated with the unbounded Poisson/Laplace equation that governs gravity perturbations. Therefore, gravity perturbations are generally omitted, and only the background gravity is taken into account using the so-called Cowling approximation. However, gravity perturbations may be significant for large earthquakes (Mw ≥ 6.0) and long-period responses.
In this study, we develop a time-domain solver based on the spectral-infinite-element approach, which combines the spectral element method inside the Earth domain with a mapped-infinite-element method in the infinite space outside. This combination allows us to solve the complete, coupled momentum-gravitational equations in a fully discretized domain while accommodating complex 3-D Earth models. We compute displacement and gravity perturbations considering various Earth models, including Preliminary Reference Earth Model and S40RTS and conduct comprehensive benchmarks of our method against the spherical harmonics normal-mode approach and the direct radial integration method. Our 3-D simulations accommodate topography, bathymetry, rotation, ellipticity and oceans. Results show that our technique is accurate and stable for long simulations. Our method provides a new scope for incorporating earthquake-induced gravity perturbations into source and adjoint tomographic inversions.
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