Frequency‐Dependent Moment Tensors of Induced Microearthquakes (original) (raw)
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Pure and Applied Geophysics, 2014
The geothermal reservoir at Soultz-sous-Forêts is a valuable natural laboratory for understanding the mechanisms of micro-earthquakes generated during stimulations and circulation tests. An ongoing effort currently exists regarding the retrieval of mechanisms aimed to indicate the type of fracturing of the rock massif. As a default, a moment tensor description has been applied. Nevertheless, the retrieval of the mode of fracturing still remains ambiguous. Recent studies indicate a prevailingly shear slip but, rarely, a non-shear pattern has also been observed. The moment tensor, used today as a universal tool for descriptions of the mechanism, captures general balanced dipole sources. However, in the case of small scale earthquakes, the moment tensor need not always be reliably determined.
Volumetric components in the earthquake source related to fluid injection and stress state
Geophysical Research Letters, 2017
We investigate source processes of fluid-induced seismicity from The Geysers geothermal reservoir in California to determine their relation with hydraulic operations and improve the corresponding seismic hazard estimates. Analysis of 869 well-constrained full moment tensors (M w 0.8-3.5) reveals significant non-double-couple components (>25%) for about 65% of the events. Volumetric deformation is governed by cumulative injection rates with larger non-double-couple components observed near the wells and during high injection periods. Source mechanisms are magnitude dependent and vary significantly between faulting regimes. Normal faulting events (M w < 2) reveal substantial volumetric components indicating dilatancy in contrast to strike-slip events that have a dominant double-couple source. Volumetric components indicating closure of cracks in the source region are mostly found for reverse faulting events with M w > 2.5. Our results imply that source processes and magnitudes of fluid-induced seismic events are strongly affected by the hydraulic operations, the reservoir stress state, and the faulting regime.
Preface to the special issue “Triggered and induced seismicity: probabilities and discrimination”
Journal of Seismology, 2013
Triggered and induced seismicity are the earth response to transient non-tectonic phenomena. In a common definition, a triggered earthquake is assumed as an event whose occurrence is anticipated in view of the background seismicity rate. The triggering process, caused by a transient phenomena, only concerns the nucleation of a small region of the rupture area, whereas the entire rupture is controlled by the background stress. An induced event, in change, is entirely (e.g. in terms of rupture size and energy released) controlled by its causative origin and would not occur without it. A complementary, stress-based approach to define the boundary among triggered and induced seismicity was discussed by , in the specific framework of anthropogenic seismicity. According to their classification, a broader term of "stimulated" seismicity could be used to describe both triggered and induced seismicity.
Rupture Characteristics of —M3 to M1 Hydraulically Induced Seismicity
SEG Technical Program Expanded Abstracts 2015, 2015
Besides the typical negative magnitude fractures induced in the treated formations by hydraulic fracture stimulation programs, small positive magnitude events associated to small faults located underneath or cross-cutting the reservoir also frequently occur. In this study we investigate discriminant source and rupture characteristics to distinguish between the two event types and also between reservoir fracture types. To achieve this goal we estimate source and failure properties of -M3 to M1 seismic events recorded during a hydraulic fracturing stimulation of a shale reservoir in Horn River Basin, Canada, for which the >M0 events are associated with slip on a pre-existing fault underneath the reservoir. Comparison between static and dynamic source parameters suggests distinct signatures of the two event types associated to two distinct failure processes. Positive magnitude events occurring beneath the reservoir have slightly higher static and dynamic stress drops, higher apparent stress and energy release, and rupture faster than shallower reservoir events. These differences reflect fracturing of harder rocks at higher confining stresses for the deep events, but also a possible release of a larger quantity of strain energy stored within the fault zone. The lower stress and energy release and slower rupturing fractures observed in the reservoir fractures, as well as overshoot type failure (slip weakening failure) indicates fluid lubrication by pore pressure increase and frictional resistance reduction. Some trends are also observed when looking only at reservoir fractures, where variations in average rupture velocities correlate with variations in formation depth and fault azimuth reflecting a dominance of the local stress field over other factors. Average rupture velocities also correlates with elapsed time showing an observable imprint of the changed local conditions during treatment over the regional conditions. Identification of more spatial and temporal families of events with similar rupture behaviors and source characteristics can be used as a proxy for specific fracture network development and hydrocarbon production and included in geo-mechanical models and fracture treatment designs. Reservoir and fault related induced events release less stress and radiates less energy than natural occurring tectonic earthquakes of comparable size at similar depths indicating a potential fluid influence in these failures. Considering the ongoing debate regarding the cause-effect relationship between fluid injection programs and nearby deeper earthquakes this study suggests that source parameters can be used as a discriminant factor between the two types of earthquakes.
Journal of Geophysical Research: Solid Earth, 2018
Full moment tensors of 1,421 microearthquakes in The Geysers geothermal field were calculated using waveform data from a field‐wide broadband network and the approach based on the principal component analysis. Spatial characteristics of faulting regime, stress tensor and the isotropic component (ISO) of moment tensors were investigated. The studied events form different clusters dominated by normal faults (NF) and strike‐slip (SS) faults, respectively. The SS‐dominated clusters are related to the SS stress state observed in the southwestern side of the field where two NW‐SE trending fault zones exist. Increasing proportions of SS faults were observed near the shallow and deep parts of the NF‐dominated clusters. Temperature differences between the upper and lower parts of the reservoir do not change the overall stress states of clusters. The stress ratios in the NW part are much smaller than in the SE part of the field. The retrieved ISOs range between −5% and 25% for 96% of events. ...
Geophysical Journal International, 2000
We have inverted the peak amplitudes of direct P waves of 45 micro-earthquakes with magnitudes between M = 1.4 and 2.9, which occurred during and after the 2003 massive fluid injection in the GPK3 borehole of the Soultz-sous-Forêts Hot Dry Rock facility. These events were recorded by a surface seismic network of 15 stations operated by the Ecole et Observatoire des Sciences de la Terre, University of Strasbourg. The unconstrained moment tensor (MT) expression of the mechanism was applied, allowing the description of a general system of dipoles, that is, both double-couple (DC) and non-DC sources, as tensile fractures. The mechanisms of all but one event are dominantly DCs with a few per cent additional components at the most. We have checked carefully the reliability of the MT retrieval in bootstrap trials eliminating some data, by simulating the mislocation of the hypocentre and by applying simplified velocity models of the area in constructing Green's functions. In some of the trials non-DC components amounting to several tens of per cent appear, but the F-test classifies them as insignificant. Even the only micro-earthquake with an exceptionally high non-DC mechanism cannot be classified unambiguously-the F-test assigns similar significance to the pure DC solution. The massive dominance of the DC indicates the shear-slip as the mechanism of the micro-earthquakes investigated. The mechanisms display large variability and are of normal dip-slip, oblique normal to strike-slip types. The T-axes are fairly stable, being concentrated subhorizontally roughly in the E-W direction. On the contrary, the P-axes are ill constrained varying in the N-S direction from nearly vertical to nearly horizontal, which points to heterogeneous stress in the Soultz injected volume. This is in agreement with the stress pattern from in situ measurements: the minimum stress axis is well constrained to E-W, whereas the maximum and intermediate stress values are close to one another, enabling the ambiguity of the P-axis direction. We found no significant dependence of source mechanisms either on magnitudes or depths. The time-space distribution of the events analysed suggests that the injection activated two segments of the natural faults existing in the area (I and II in our notation) showing different source mechanism patterns. The dip-slip regime is characteristic of fault segment I where the seismicity occurred during and also after injection, whereas the strike-slip regime prevails in segment II where the seismicity was triggered only after the injection shut in. This indicates that the tensile fractures, which are assumed to be created during injection, may have occurred on a smaller scale than the pure shear micro-earthquakes investigated.
We have inverted the peak amplitudes of direct P waves of 45 micro-earthquakes with magnitudes between M = 1.4 and 2.9, which occurred during and after the 2003 massive fluid injection in the GPK3 borehole of the Soultz-sous-Forêts Hot Dry Rock facility. These events were recorded by a surface seismic network of 15 stations operated by the Ecole et Observatoire des Sciences de la Terre, University of Strasbourg. The unconstrained moment tensor (MT) expression of the mechanism was applied, allowing the description of a general system of dipoles, that is, both double-couple (DC) and non-DC sources, as tensile fractures. The mechanisms of all but one event are dominantly DCs with a few per cent additional components at the most. We have checked carefully the reliability of the MT retrieval in bootstrap trials eliminating some data, by simulating the mislocation of the hypocentre and by applying simplified velocity models of the area in constructing Green's functions. In some of the trials non-DC components amounting to several tens of per cent appear, but the F-test classifies them as insignificant. Even the only micro-earthquake with an exceptionally high non-DC mechanism cannot be classified unambiguously-the F-test assigns similar significance to the pure DC solution. The massive dominance of the DC indicates the shear-slip as the mechanism of the micro-earthquakes investigated. The mechanisms display large variability and are of normal dip-slip, oblique normal to strike-slip types. The T-axes are fairly stable, being concentrated subhorizontally roughly in the E-W direction. On the contrary, the P-axes are ill constrained varying in the N-S direction from nearly vertical to nearly horizontal, which points to heterogeneous stress in the Soultz injected volume. This is in agreement with the stress pattern from in situ measurements: the minimum stress axis is well constrained to E-W, whereas the maximum and intermediate stress values are close to one another, enabling the ambiguity of the P-axis direction. We found no significant dependence of source mechanisms either on magnitudes or depths. The time-space distribution of the events analysed suggests that the injection activated two segments of the natural faults existing in the area (I and II in our notation) showing different source mechanism patterns. The dip-slip regime is characteristic of fault segment I where the seismicity occurred during and also after injection, whereas the strike-slip regime prevails in segment II where the seismicity was triggered only after the injection shut in. This indicates that the tensile fractures, which are assumed to be created during injection, may have occurred on a smaller scale than the pure shear micro-earthquakes investigated.
Discriminating induced seismicity from natural earthquakes using moment tensors and source spectra
Journal of Geophysical Research: Solid Earth, 2016
Earthquake source mechanisms and spectra can provide important clues to aid in discriminating between natural and induced events. In this study, we calculate moment tensors and stress drop values for eight recent induced earthquakes in the Western Canadian Sedimentary Basin with magnitudes between 3.2 and 4.4, as well as a nearby magnitude 5.3 event that is interpreted as a natural earthquake. We calculate full moment tensor solutions by performing a waveform-fitting procedure based on a 1-D transversely isotropic velocity model. In addition to a dominant double-couple (DC) signature that is common to nearly all events, most induced events exhibit significant non-double-couple components. A parameter sensitivity analysis indicates that spurious non-DC components are negligible if the signal to noise ratio (SNR) exceeds 10 and if the 1-D model differs from the true velocity structure by less than 5%. Estimated focal depths of induced events are significantly shallower than the typical range of focal depths for intraplate earthquakes in the Canadian Shield. Stress drops of the eight induced events were estimated using a generalized spectral-fitting method and fall within the typical range of 2 to 90 MPa for tectonic earthquakes. Elastic moduli changes due to the brittle damage production at the source, presence of multiple intersecting fractures, dilatant jogs created at the overlapping areas of multiple fractures, or non-planar pre-existing faults may explain the non-DC components for induced events. This study focuses on the Western Canadian Sedimentary Basin (WCSB), a region characterized by induced seismicity and relatively infrequent natural earthquakes. As elaborated below, a marked increase in seismicity in the WCSB since 2009 appears to be associated with subsurface fluid injection. For example, in the Horn River Basin, northeastern British Columbia, 38 seismic events were observed from 2009 to 2011 ranging in ZHANG ET AL.
Mechanical effect of fluid migration on the complexity of seismicity
Journal of Geophysical Research, 1997
Spatio-temporal variation of earthquake activity is modeled assuming fluid migration in a narrow porous fault zone whose boundaries are impermeable. The duration of earthquake sequence is assumed to be much shorter than the recurrence period of characteristic events on the fault. Principle of the effective stress coupled to the Coulomb failure criterion introduces mechanical coupling between fault slip and pore fluid pressure. A linear relation is assumed in our simulations between the accumulated slip and fault zone width on the basis of laboratory and field observations. High complexity is observed in the rupture activity so long as an inhomogeneity is introduced in the spatial distribution of initial strength, which is defined as the fracture threshold stress before the intrusion of the fluid. Frequency-magnitude statistics of intermediate-size events obeys the Gutenberg-Richter relation for all the models in which spatial heterogeneity is introduced for the initial strength. The behavior of larger-size events seems to be rather model dependent. It is also observed that the rupture occurrence tends to be inactivated immediately before the occurrence of the largest event in a sequence. This never happens if a brittle rupture is assumed in an elastic medium with no mechanical effect of fluid. This inactivation will occur because it takes much time to build up fluid pressure to break a fault segment having high initial strength, whose rupture triggers the largest event in a sequence. Our calculations also show that a single predominant principal event cannot be observed in a sequence when both the variance and average value of the distributed initial strengths are large. This may explain a feature observed for earthquake swarm. tive stress coupled to the Coulomb failure criterion [Raleigh et al., 1976], which is thought to be applicable at least to the top few kilometers of the crust [Raleigh et al., 1976; Zoback and Hickman, 1982; Zoback and Healy, 1984; Hickman et al., 1995]. The role of pore fluid in reducing the effective value of the confining stress in bulk samples and the normal stress across frictional surfaces has been demonstrated in laboratory experiments [e.g., Brace and Martin, 1968; Byeflee and Brace, 1972]. Field evidence comes from earthquakes induced either through direct injection of fluids down boreholes or from the filling of large reservoirs with subsequent infiltration of water into the underlying rock mass [e.g., Healy et al., 1968; Raleigh et al., 1976]. Additional evidence of mechanical involvement of fluids in earthquake faulting comes from the substantial change in groundwater level, and surface discharge before and after some earthquakes in the shallow crust. For example, the variation in groundwater flow is observed at many locations before and after the 1995 Hyogoken-Nanbu (Kobe) earthquake; Tsunogai and Wakita [1995] report a steady increase in C1-and SO•-concentrations with time from