Moment tensors of double-couple microseismic sources in anisotropic formations

Geophysics ◽  
2019 ◽  
Vol 85 (1) ◽  
pp. KS1-KS11 ◽  
Author(s):  
Vladimir Grechka
Keyword(s):  
Hydraulic Fracturing ◽  
Transversely Isotropic ◽  
Focal Mechanisms ◽  
Weak Anisotropy ◽  
Moment Tensors ◽  
Numerical Examples ◽  
Double Couple ◽  
Microseismic Events ◽  
Anisotropy Coefficients

Moment tensors of kinematically double-couple microseismic events triggered in anisotropic formations are known to exhibit non-double-couple focal mechanisms. The weak anisotropy approximation of these mechanisms reveals the combinations of anisotropy coefficients of vertically transversely isotropic and orthorhombic focal regions responsible for the deviations of moment tensors from double couples. Numerical examples for models of typical unconventional shales indicate the non-double-couple components of moment tensors to be sufficiently large to cause misinterpretation of the nature of ruptures associated with hydraulic fracturing.

Dense surface seismic data confirm non-double-couple source mechanisms induced by hydraulic fracturing

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. KS207-KS217 ◽  
Author(s):  
Jeremy D. Pesicek ◽  
Konrad Cieślik ◽  
Marc-André Lambert ◽  
Pedro Carrillo ◽  
Brad Birkelo
Keyword(s):  
Hydraulic Fracturing ◽  
Seismic Data ◽  
P Wave ◽  
Source Modeling ◽  
Moment Tensors ◽  
Amplitude Data ◽  
Double Couple ◽  
Source Mechanisms ◽  
Source Models ◽  
Favorable Conditions

We have determined source mechanisms for nine high-quality microseismic events induced during hydraulic fracturing of the Montney Shale in Canada. Seismic data were recorded using a dense regularly spaced grid of sensors at the surface. The design and geometry of the survey are such that the recorded P-wave amplitudes essentially map the upper focal hemisphere, allowing the source mechanism to be interpreted directly from the data. Given the inherent difficulties of computing reliable moment tensors (MTs) from high-frequency microseismic data, the surface amplitude and polarity maps provide important additional confirmation of the source mechanisms. This is especially critical when interpreting non-shear source processes, which are notoriously susceptible to artifacts due to incomplete or inaccurate source modeling. We have found that most of the nine events contain significant non-double-couple (DC) components, as evident in the surface amplitude data and the resulting MT models. Furthermore, we found that source models that are constrained to be purely shear do not explain the data for most events. Thus, even though non-DC components of MTs can often be attributed to modeling artifacts, we argue that they are required by the data in some cases, and can be reliably computed and confidently interpreted under favorable conditions.

Focal mechanisms for small to intermediate earthquakes in the northern part of the Alps and their seismotectonic interpretation

2020 ◽  
Author(s):  
Thomas Plenefisch ◽  
Laura Barth ◽  
Keyword(s):  
Stress Field ◽  
Focal Mechanisms ◽  
Small Magnitude ◽  
Moment Tensors ◽  
Time Period ◽  
Double Couple ◽  
Geodynamic Processes ◽  
The Alps ◽  
Seismic Networks ◽  
Seismotectonic Interpretation

<p>In the framework of the AlpArray project more than 600 broadband stations have been installed and operated in the Alps and the surroundings. Together with the permanent stations in the area it is one of the most densely spaced seismic networks worldwide. Thereby, it offers an excellent opportunity to investigate the seismicity and seismotectonics of the Alpine chain. Due to the huge number of stations focal mechanisms can be calculated even for small magnitude earthquakes with high accuracy. The focal mechanisms are one important key to reveal the contemporary stress field and thus contribute to a better understanding of the geodynamic processes of the Alps.</p><p>In our study we focus on small to intermediate earthquakes in the Northern Alps, namely on four distinct sub-regions. These are from West to East the Lake Constance, the Arlberg region, the area of Garmisch-Partenkirchen and the broader region of Innsbruck. In order to calculate the focal mechanisms, we apply the FOCMEC program (Snoke, 2003), which inverts for a pure double-couple source. P-polarities as well as amplitude ratios of SH to P are used as input parameters for the inversion. Thanks to the dense network a good coverage of the focal sphere is achieved in most cases.</p><p>Altogether, we calculated focal mechanisms for 25 earthquakes in the magnitude range between 2.5 and 3.5 from the time period 2016 to 2019. Most of the focal mechanisms represent reverse or strike-slip faulting, normal faulting events are rather rare. The mechanisms are analysed with respect to lateral changes along the Northern Alpine. On one hand we compare the mechanisms with mechanisms of older studies as well as with moment tensors of events of slightly larger magnitudes. Those events are the scope of another subproject in the framework of the AlpArray (Petersen et al., 2019). On the other hand, we compare our mechanisms with geological indicators, namely orientation of faults. Finally, the focal mechanisms are used as input to invert for the stress field.</p>

Focal mechanisms produced by shear faulting in weakly transversely isotropic crustal rocks

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. D145-D151 ◽  
Author(s):  
Václav Vavryčuk
Keyword(s):  
Transverse Isotropy ◽  
Transversely Isotropic ◽  
Focal Mechanisms ◽  
Crustal Rocks ◽  
Linear Vector ◽  
Double Couple ◽  
Anisotropic Rocks ◽  
Anisotropy Parameters ◽  
Anisotropy Strength

Shear faulting in anisotropic rocks produces non-double-couple (non-DC) mechanisms. The non-DC mechanisms can comprise the isotropic (ISO) and compensated linear vector dipole (CLVD) components. The formulas for percentages of the ISO and CLVD are simplified under the assumption of weak transverse isotropy and can be expressed advantageously in terms of Thomsen’s anisotropy parameters. Shear faulting in crustal rocks with anisotropy strength of 10% can produce an ISO of up to 10% and a CLVD of up to 30%. Such values are significant and detectable in carefully determined focal mechanisms.

Seismic Source Processes of 25 Earthquakes (Mw>5) in the Gulf of California

2022 ◽  
Author(s):  
Eduardo Huesca-Pérez ◽  
Edahí Gutierrez-Reyes ◽  
Luis Quintanar
Keyword(s):  
Gulf Of California ◽  
Moment Tensor ◽  
Source Parameters ◽  
Seismic Source ◽  
Focal Mechanisms ◽  
Process Models ◽  
Moment Tensors ◽  
Time Space ◽  
Double Couple ◽  
Broadband Seismograms

ABSTRACT The Gulf of California (GoC) is a complex tectonic boundary that has been instrumented in the past several decades to record broadband seismograms. This volume of data has allowed us to study several source parameters systematically. Before, only a few source parameters of earthquakes greater than magnitude five had been studied in the GoC area. We re-examined the focal mechanisms of several earthquakes in the southern GoC that occurred over the last 20 yr using local–regional distance broadband seismograms. These focal mechanisms were then used as input data to retrieve the time–space history of the rupture for each earthquake. This work contributes to the study of 25 rupture-process models computed with the method proposed by Yagi et al. (1999). To investigate more about the nature of the seismicity in the GoC, we also calculated the non-double-couple component of moment tensors for 45 earthquakes. Previous studies (e.g., Ortega et al., 2013, 2016) have shown that non-double-couple components from moment tensors in this region are associated with complex faulting, suggesting that oblique faults or several parallel faults are interacting simultaneously. Our results show that, at least for moderate earthquakes (5 < M < 6), rupture processes in the GoC show a complex interaction between fault systems. It is revealed on the important contribution of non-double-couple component obtained in the full moment tensor analysis.

Microseismic Monitoring of Hydraulic Fracturing – Data Interpretation Methodology with an Example from Pomerania

2017 ◽  
Author(s):  
Michał Antoszkiewicz ◽  
Mateusz Kmieć ◽  
Paweł Szewczuk ◽  
Marek Szkodo ◽  
Robert Jankowski
Keyword(s):  
Hydraulic Fracturing ◽  
Signal To Noise Ratio ◽  
Velocity Model ◽  
Data Interpretation ◽  
Microseismic Monitoring ◽  
Grid Search ◽  
Moment Tensors ◽  
Spatial Image ◽  
Microseismic Events ◽  
Breakage Mechanism

Microseismic monitoring is a method for localizing fractures induced by hydraulic fracturing in search for shale gas. The aim of this paper is to conduct the data interpretation of the microseismic monitoring based on the results from Pom-erania region of Poland. The data has been collected from an array of geophones deployed on the surface. Ground vibrations have been recorded and analyzed for fracture location, magnitude and breakage mechanism. A velocity model of underlying formations has been used for successful microseismic monitoring. The model has been further tuned with signal from perfora-tion shots of known location. Imaging of events has been done using software MicSeis, which utilizes diffraction stacking of waveforms from multiple stations to image microseismic events with low signal-to-noise ratio. The imaging of microseismic events in MicSeis uses a grid search over all possible origin times and locations in the selected rock volume. The seismic moment tensors are automatically determined from the amplitudes from the grid search procedure and are used to model po-larities of events which then enhance constructive interference. Function characterizing a maximum stack per time sample have been calculated over whole volume and analyzed using the STA/LTA algorithm. Once the event has been detected in time, location has been determined through analysis of the 3D spatial image function. The procedure has been used to detect five events during hydraulic fracturing in Pomerania.

Seismic Moment Tensors of Microseismic Events: General Solution vs Double-Couple Solutions

2015 ◽  
Author(s):  
Lindsay Smith-Boughner* ◽  
Adam Baig ◽  
Ted Urbancic ◽  
Eric Von Lunen
Keyword(s):  
General Solution ◽  
Seismic Moment ◽  
Moment Tensors ◽  
Double Couple ◽  
Microseismic Events

The Structure of Moment Tensors in Transversely Isotropic Focal Regions

2019 ◽  
Vol 109 (6) ◽  
pp. 2415-2426
Author(s):  
Çağrı Diner
Keyword(s):  
Moment Tensor ◽  
Transversely Isotropic ◽  
Geometric Interpretation ◽  
Elasticity Tensor ◽  
Point Of View ◽  
Focal Region ◽  
Moment Tensors ◽  
Double Couple ◽  
Isotropic Elasticity ◽  
The Moment

Abstract Full moment tensor inversion has become a standard method for understanding the mechanisms of earthquakes as the resolution of the inversion process increases. Thus, it is important to know the possible forms of non–double‐couple (non‐DC) moment tensors, which can be obtained because of either the different source mechanisms or the anisotropy of the focal regions. In this study, the form of the moment tensors of seismic sources occurring in transversely isotropic (TI) focal regions is obtained using the eigendecomposition of the elasticity tensor. More precisely, a moment tensor is obtained as a linear combination of the eigenspaces of TI elasticity tensor in which the coefficients of the terms are the corresponding eigenvalues multiplied with the projection of the potency tensor onto the corresponding eigenspaces. Moreover, the eigendecomposition method is also applied to obtain the three different forms of moment tensors in isotropic focal regions, in particular, for the shear source, tensile source, and for any type of potency tensor whose rank is three. This linear algebra point of view makes the structure of the moment tensors more apparent; for example, a shear source tensor is an eigenvector of isotropic elasticity tensor, and hence the resulting moment tensor is proportional to its shear source tensor. Moreover, a geometric interpretation for the scalar seismic moment, which is the norm of the moment tensor, for anisotropic focal regions is achieved through the eigendecomposition method. This method also gives a simple way to quantify the percentage of the isotropic component of the moment tensor of shear sources in TI focal regions. Hence, the complexities in the moment tensor introduced by the anisotropy of the focal region and by the source mechanism can be differentiated.

Geometrical spreading in a layered transversely isotropic medium with vertical symmetry axis

Geophysics ◽  
2003 ◽  
Vol 68 (6) ◽  
pp. 2082-2091 ◽  
Author(s):  
Bjørn Ursin ◽  
Ketil Hokstad
Keyword(s):  
Ray Tracing ◽  
Seismic Data ◽  
Symmetry Axis ◽  
Transversely Isotropic ◽  
Analytical Approximation ◽  
Geometrical Spreading ◽  
S Wave ◽  
Weak Anisotropy ◽  
P Waves ◽  
Amplitude Versus

Compensation for geometrical spreading is important in prestack Kirchhoff migration and in amplitude versus offset/amplitude versus angle (AVO/AVA) analysis of seismic data. We present equations for the relative geometrical spreading of reflected and transmitted P‐ and S‐wave in horizontally layered transversely isotropic media with vertical symmetry axis (VTI). We show that relatively simple expressions are obtained when the geometrical spreading is expressed in terms of group velocities. In weakly anisotropic media, we obtain simple expressions also in terms of phase velocities. Also, we derive analytical equations for geometrical spreading based on the nonhyperbolic traveltime formula of Tsvankin and Thomsen, such that the geometrical spreading can be expressed in terms of the parameters used in time processing of seismic data. Comparison with numerical ray tracing demonstrates that the weak anisotropy approximation to geometrical spreading is accurate for P‐waves. It is less accurate for SV‐waves, but has qualitatively the correct form. For P waves, the nonhyperbolic equation for geometrical spreading compares favorably with ray‐tracing results for offset‐depth ratios less than five. For SV‐waves, the analytical approximation is accurate only at small offsets, and breaks down at offset‐depth ratios less than unity. The numerical results are in agreement with the range of validity for the nonhyperbolic traveltime equations.

A teleseismic body-wave analysis of the May 1980 Mammoth Lakes, California, earthquakes

1983 ◽  
Vol 73 (2) ◽  
pp. 419-434
Author(s):  
Jeffery S. Barker ◽  
Charles A. Langston
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Normal Fault ◽  
Strike Slip ◽  
Body Waves ◽  
P Wave ◽  
Moment Tensors ◽  
Double Couple ◽  
The Moment ◽  
Mammoth Lakes

abstract Teleseismic P-wave first motions for the M ≧ 6 earthquakes near Mammoth Lakes, California, are inconsistent with the vertical strike-slip mechanisms determined from local and regional P-wave first motions. Combining these data sets allows three possible mechanisms: a north-striking, east-dipping strike-slip fault; a NE-striking oblique fault; and a NNW-striking normal fault. Inversion of long-period teleseismic P and SH waves for the events of 25 May 1980 (1633 UTC) and 27 May 1980 (1450 UTC) yields moment tensors with large non-double-couple components. The moment tensor for the first event may be decomposed into a major double couple with strike = 18°, dip = 61°, and rake = −15°, and a minor double couple with strike = 303°, dip = 43°, and rake = 224°. A similar decomposition for the last event yields strike = 25°, dip = 65°, rake = −6°, and strike = 312°, dip = 37°, and rake = 232°. Although the inversions were performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first motion polarities at local, regional, and teleseismic distances. The stress axes inferred from the moment tensors are consistent with N65°E extension determined by geodetic measurements by Savage et al. (1981). Seismic moments computed from the moment tensors are 1.87 × 1025 dyne-cm for the 25 May 1980 (1633 UTC) event and 1.03 × 1025 dyne-cm for the 27 May 1980 (1450 UTC) event. The non-double-couple aspect of the moment tensors and the inability to obtain a convergent solution for the 25 May 1980 (1944 UTC) event may indicate that the assumptions of a point source and plane-layered structure implicit in the moment tensor inversion are not entirely valid for the Mammoth Lakes earthquakes.

A least squares method for earthquake mechanism determination using S-wave data

1964 ◽  
Vol 54 (6A) ◽  
pp. 2037-2047
Author(s):  
Agustin Udias
Keyword(s):  
Least Squares ◽  
Statistical Evaluation ◽  
Least Squares Method ◽  
Numerical Approach ◽  
Focal Mechanisms ◽  
Least Square ◽  
S Wave ◽  
Double Couple ◽  
Earthquake Mechanism

abstract In this paper a numerical approach to the determination of focal mechanisms based on the observation of the polarization of the S wave at N stations is presented. Least-square methods are developed for the determination of the orientation of the single and double couple sources. The methods allow a statistical evaluation of the data and of the accuracy of the solutions.

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