Geology topics

Arthur F. McGarr

Publications and source records attributed to Arthur F. McGarr.

At least 19 recordsLinked to original sources

USGS2019

An investigation of seismicity induced by hydraulic fracturing in the Sichuan basin of China based on data from a temporary seismic network

Hydraulic fracturing has been inferred to trigger the majority of injection‐induced seismicity in the Zhaotong and Changning shale gas field, Sichuan basin of China, in contrast to the Midwestern United States, where massive wastewater disposal has been the dominant triggering mechanism. More than 15,000 earthquakes, with magnitudes ranging up to M w 4.7, were recorded by a temporary network in the Sichuan basin for three years, with a completeness magnitude of M L 1.1. A catalog of earthquakes relocated with code tomoDD, combined with comprehensive injection data during an eight day period, shows that the earthquakes were highly correlated spatiotemporally with hydraulic fracturing activities mostly from a single well pad. Three M L ≥4.0 events occurred during hydraulic fracturing operations from 12 to 19 January 2017, followed by the fourth and largest event, with moment magnitude (⁠M w⁠ ) 4.7, on 28 January. The hypocenters of the four largest events were located in dolomite of Cambrian age, between a shale gas reservoir and the top of the crystalline basement rocks. This was found to be similar to 60% of the smaller earthquakes in this cluster, at depths from 2.5 to 4.0 km.

Sichuan basin↗

USGS2017

Wastewater disposal and the earthquake sequences during 2016 near Fairview, Pawnee, and Cushing, Oklahoma

Each of the three earthquake sequences in Oklahoma in 2016—Fairview, Pawnee, and Cushing—appears to have been induced by high-volume wastewater disposal within 10 km. The Fairview M 5.1 main shock was part of a 2 year sequence of more than 150 events of M 3, or greater; the main shock accounted for about half of the total moment. The foreshocks and aftershocks of the M 5.8 Pawnee earthquake were too small and too few to contribute significantly to the cumulative moment; instead, nearly all of the moment induced by wastewater injection was focused on the main shock. The M 5.0 Cushing event is part of a sequence that includes 48 earthquakes of M 3, or greater, that are mostly foreshocks. The cumulative moment for each of the three sequences during 2016, as well as that for the 2011 Prague, Oklahoma, and nine other sequences representing a broad range of injected volume, are all limited by the total volumes of wastewater injected locally.

Oklahoma↗

USGS2017

Earthquake source properties from instrumented laboratory stick-slip

Stick-slip experiments were performed to determine the influence of the testing apparatus on source properties, develop methods to relate stick-slip to natural earthquakes and examine the hypothesis of McGarr [2012] that the product of stiffness, k , and slip duration, Δ t , is scale-independent and the same order as for earthquakes. The experiments use the double-direct shear geometry, Sierra White granite at 2 MPa normal stress and a remote slip rate of 0.2 µm/sec. To determine apparatus effects, disc springs were added to the loading column to vary k . Duration, slip, slip rate, and stress drop decrease with increasing k , consistent with a spring-block slider model. However, neither for the data nor model is k Δ t constant; this results from varying stiffness at fixed scale. In contrast, additional analysis of laboratory stick-slip studies from a range of standard testing apparatuses is consistent with McGarr's hypothesis. k Δ t is scale-independent, similar to that of earthquakes, equivalent to the ratio of static stress drop to average slip velocity, and similar to the ratio of shear modulus to wavespeed of rock. These properties result from conducting experiments over a range of sample sizes, using rock samples with the same elastic properties as the Earth, and scale-independent design practices.

Book chapter↗

USGS2017

2017 One‐year seismic‐hazard forecast for the central and eastern United States from induced and natural earthquakes

We produce a one‐year 2017 seismic‐hazard forecast for the central and eastern United States from induced and natural earthquakes that updates the 2016 one‐year forecast; this map is intended to provide information to the public and to facilitate the development of induced seismicity forecasting models, methods, and data. The 2017 hazard model applies the same methodology and input logic tree as the 2016 forecast, but with an updated earthquake catalog. We also evaluate the 2016 seismic‐hazard forecast to improve future assessments. The 2016 forecast indicated high seismic hazard (greater than 1% probability of potentially damaging ground shaking in one year) in five focus areas: Oklahoma–Kansas, the Raton basin (Colorado/New Mexico border), north Texas, north Arkansas, and the New Madrid Seismic Zone. During 2016, several damaging induced earthquakes occurred in Oklahoma within the highest hazard region of the 2016 forecast; all of the 21 moment magnitude ( M ) ≥4 and 3 M ≥5 earthquakes occurred within the highest hazard area in the 2016 forecast. Outside the Oklahoma–Kansas focus area, two earthquakes with M ≥4 occurred near Trinidad, Colorado (in the Raton basin focus area), but no earthquakes with M ≥2.7 were observed in the north Texas or north Arkansas focus areas. Several observations of damaging ground‐shaking levels were also recorded in the highest hazard region of Oklahoma. The 2017 forecasted seismic rates are lower in regions of induced activity due to lower rates of earthquakes in 2016 compared with 2015, which may be related to decreased wastewater injection caused by regulatory actions or by a decrease in unconventional oil and gas production. Nevertheless, the 2017 forecasted hazard is still significantly elevated in Oklahoma compared to the hazard calculated from seismicity before 2009.

Seismological Research Letters↗

USGS2016

Seismic‐hazard forecast for 2016 including induced and natural earthquakes in the central and eastern United States

The U.S. Geological Survey (USGS) has produced a one‐year (2016) probabilistic seismic‐hazard assessment for the central and eastern United States (CEUS) that includes contributions from both induced and natural earthquakes that are constructed with probabilistic methods using alternative data and inputs. This hazard assessment builds on our 2016 final model ( Petersen et al. , 2016 ) by adding sensitivity studies, illustrating hazard in new ways, incorporating new population data, and discussing potential improvements. The model considers short‐term seismic activity rates (primarily 2014–2015) and assumes that the activity rates will remain stationary over short time intervals. The final model considers different ways of categorizing induced and natural earthquakes by incorporating two equally weighted earthquake rate submodels that are composed of alternative earthquake inputs for catalog duration, smoothing parameters, maximum magnitudes, and ground‐motion models. These alternatives represent uncertainties on how we calculate earthquake occurrence and the diversity of opinion within the science community. In this article, we also test sensitivity to the minimum moment magnitude between M 4 and M 4.7 and the choice of applying a declustered catalog with b =1.0 rather than the full catalog with b =1.3. We incorporate two earthquake rate submodels: in the informed submodel we classify earthquakes as induced or natural, and in the adaptive submodel we do not differentiate. The alternative submodel hazard maps both depict high hazard and these are combined in the final model. Results depict several ground‐shaking measures as well as intensity and include maps showing a high‐hazard level (1% probability of exceedance in 1 year or greater). Ground motions reach 0.6 g horizontal peak ground acceleration (PGA) in north‐central Oklahoma and southern Kansas, and about 0.2 g PGA in the Raton basin of Colorado and New Mexico, in central Arkansas, and in north‐central Texas near Dallas–Fort Worth. The chance of having levels of ground motions corresponding to modified Mercalli intensity (MMI) VI or greater earthquake shaking is 2%–12% per year in north‐central Oklahoma and southern Kansas and New Madrid similar to the chance of damage at sites in high‐hazard portions of California caused by natural earthquakes. Hazard is also significant in the Raton basin of Colorado/New Mexico; north‐central Arkansas; Dallas–Fort Worth, Texas; and in a few other areas. Hazard probabilities are much lower (by about half or more) for exceeding MMI VII or VIII. Hazard is 3‐ to 10‐fold higher near some areas of active‐induced earthquakes than in the 2014 USGS National Seismic Hazard Model (NSHM), which did not consider induced earthquakes. This study in conjunction with the LandScan TM Database (2013) indicates that about 8 million people live in areas of active injection wells that have a greater than 1% chance of experiencing damaging ground shaking (MMI≥VI) in 2016. The final model has high uncertainty, and engineers, regulators, and industry should use these assessments cautiously to make informed decisions on mitigating the potential effects of induced and natural earthquakes.

Seismological Research Letters↗

USGS2016

2016 one-year seismic hazard forecast for the Central and Eastern United States from induced and natural earthquakes

The U.S. Geological Survey (USGS) has produced a 1-year seismic hazard forecast for 2016 for the Central and Eastern United States (CEUS) that includes contributions from both induced and natural earthquakes. The model assumes that earthquake rates calculated from several different time windows will remain relatively stationary and can be used to forecast earthquake hazard and damage intensity for the year 2016. This assessment is the first step in developing an operational earthquake forecast for the CEUS, and the analysis could be revised with updated seismicity and model parameters. Consensus input models consider alternative earthquake catalog durations, smoothing parameters, maximum magnitudes, and ground motion estimates, and represent uncertainties in earthquake occurrence and diversity of opinion in the science community. Ground shaking seismic hazard for 1-percent probability of exceedance in 1 year reaches 0.6 g (as a fraction of standard gravity [g]) in northern Oklahoma and southern Kansas, and about 0.2 g in the Raton Basin of Colorado and New Mexico, in central Arkansas, and in north-central Texas near Dallas. Near some areas of active induced earthquakes, hazard is higher than in the 2014 USGS National Seismic Hazard Model (NHSM) by more than a factor of 3; the 2014 NHSM did not consider induced earthquakes. In some areas, previously observed induced earthquakes have stopped, so the seismic hazard reverts back to the 2014 NSHM. Increased seismic activity, whether defined as induced or natural, produces high hazard. Conversion of ground shaking to seismic intensity indicates that some places in Oklahoma, Kansas, Colorado, New Mexico, Texas, and Arkansas may experience damage if the induced seismicity continues unabated. The chance of having Modified Mercalli Intensity (MMI) VI or greater (damaging earthquake shaking) is 5–12 percent per year in north-central Oklahoma and southern Kansas, similar to the chance of damage caused by natural earthquakes at sites in parts of California.

Open-File Report↗

USGS2015

Surface monitoring of microseismicity at the Decatur, Illinois, CO2 sequestration demonstration site

Sequestration of CO2 into subsurface reservoirs can play an important role in limiting future emission of CO2 into the atmosphere (e.g., Benson and Cole, 2008). For geologic sequestration to become a viable option to reduce greenhouse gas emissions, large-volume injection of supercritical CO2 into deep sedimentary formations is required. These formations offer large pore volumes and good pore connectivity and are abundant (Bachu, 2003; U.S. Geological Survey Geologic Carbon Dioxide Storage Resources Assessment Team, 2013). However, hazards associated with injection of CO2 into deep formations require evaluation before widespread sequestration can be adopted safely (Zoback and Gorelick, 2012). One of these hazards is the potential to induce seismicity on pre-existing faults or fractures. If these faults or fractures are large and critically stressed, seismic events can occur with magnitudes large enough to pose a hazard to surface installations and, possibly more critical, the seal integrity of the cap rock. The Decatur, Illinois, carbon capture and storage (CCS) demonstration site is the first, and to date, only CCS project in the United States that injects a large volume of supercritical CO2 into a regionally extensive, undisturbed saline formation. The first phase of the Decatur CCS project was completed in November 2014 after injecting a million metric tons of supercritical CO2 over three years. This phase was led by the Illinois State Geological Survey (ISGS) and included seismic monitoring using deep borehole sensors, with a few sensors installed within the injection horizon. Although the deep borehole network provides a more comprehensive seismic catalog than is presented in this paper, these deep data are not publicly available. We contend that for monitoring induced microseismicity as a possible seismic hazard and to elucidate the general patterns of microseismicity, the U.S. Geological Survey (USGS) surface and shallow borehole network described below provides an adequate event detection threshold. The formation targeted for injection is the Mount Simon Sandstone, which is laterally extensive, has high porosity and permeability and has the potential to host future CCS projects due to its favorable hydrologic characteristics and proximity to industrial sources of CO2 (Birkholzer and Zhou, 2009). At Decatur, CO2, a byproduct of ethanol production at the Archer Daniels Midland (ADM) facility, is compressed to supercritical state and injected at 2.1 km depth into the 460 m thick Mount Simon Sandstone. This sandstone has varying properties, ranging from the lower, fine- to coarse-grained sandstone with high permeability and porosity, to the middle and upper Mount Simon, which consist of planar, cross-bedded layers of varied permeability and porosity (Leetaru and Freiburg, 2014). The changes in permeability and porosity within the Mount Simon Sandstone, due to depositional and diagenetic differences, create horizontal baffles, which inhibit vertical flow and restrict the injected CO2 to remain near the injection horizon (Bowen et al., 2011). The lowest portion of the Mount Simon Sandstone overlying the Precambrian rhyolite basement is the Pre-Mount Simon interval, generally < 15 m in thickness and composed of fine- to medium-grain size sandstone that is highly deformed (Leetaru and Freiburg, 2014). The basement rhyolite has a clayrich matrix and is fractured, with significant alterations within the fractures. The primary sealing cap rock is the Eau Claire Formation, a 100–150 m thick unit at a depth of roughly 1.69 km (Leetaru and Freiburg, 2014). The Maquoketa Shale Group and the New Albany Shale serve as secondary and tertiary seals at shallower depths of ∼820 and ∼650 m, respectively. The ISGS managed the Illinois Basin–Decatur Project (IBDP), a three-year project beginning in November 2011, during which carbon dioxide was injected at a rate of ∼1000 metric tons per day until November 2014 (Finley et al., 2011, 2013). ADM manages the Illinois Industrial CCS (ICCS) project, which will inject ∼3000 metric tons/day into a second injection well starting in the summer of 2015. The USGS began monitoring microseismicity with a 13- station seismic network at Decatur in July 2013 (Fig. 1). This network provides good detection capabilities and azimuthal (focal sphere) coverage for microseismicity with moment magnitudes (Mw) above about −0:5. Here, we report on 19 months of microseismicity monitoring at the Decatur CO2 sequestration site, which permits a detailed look at the evolution and character of injection-induced seismicity.

Illinois↗

USGS2015

Moment tensors and other source parameters of mining‐induced earthquakes in TauTona Mine, South Africa

Induced seismicity exhibits diverse source mechanisms that are often difficult to constrain for small events. Here, we use data from the in‐mine seismic network, the Natural Earthquake Laboratory in South African Mines network, and a temporary Program for the Array Seismic Studies of the Continental Lithosphere deployment in TauTona Mine, South Africa, to determine full moment tensors of 100 mining‐induced earthquakes in the magnitude range −2.7< M w <2.5. Ground displacement derived from velocity and acceleration data show clear near‐field effects, indicating that the lowest frequencies are well resolved. Phase amplitudes of between 11 and 77 picks per event were inverted to obtain the six independent moment tensor components. The quality of each moment tensor solution is quantified using (1) the misfit between observed and synthetic waveforms, (2) bootstrap resampling to estimate uncertainties, and (3) the F ‐test to determine the need for including an isotropic component with an extra degree of freedom in the solution. The results indicate 82% of the events have well‐constrained solutions, and 45% of the well‐constrained events require an isotropic source term. Throughout the magnitude range, both deviatoric and implosive mechanisms are observed, with implosive ratios of volume change to shear deformation (ΔV/Σ τ ) of −1.03 to −0.15. Two explosive events are observed at M w −0.5 and −0.2, withΔV/Σ τ =0.15 and 0.51, respectively. For the largest events, we determine maximum slip and apparent stress ( τ a ) and find values consistent with those of natural tectonic earthquakes, with 0.1≤ τ a ≤9.2 MPa. Our results support previous speculation on the nature of isotropic components of mining‐induced earthquakes, in which events of all sizes begin as shear failure that may intersect a void (tunnel or stope) and cause collapse, whereas only small events result in explosive sources.

TauTona Mine↗

USGS2015

Coping with earthquakes induced by fluid injection

Large areas of the United States long considered geologically stable with little or no detected seismicity have recently become seismically active. The increase in earthquake activity began in the mid-continent starting in 2001 ( 1 ) and has continued to rise. In 2014, the rate of occurrence of earthquakes with magnitudes ( M ) of 3 and greater in Oklahoma exceeded that in California (see the figure). This elevated activity includes larger earthquakes, several with M > 5, that have caused significant damage ( 2 , 3 ). To a large extent, the increasing rate of earthquakes in the mid-continent is due to fluid-injection activities used in modern energy production ( 1 , 4 , 5 ). We explore potential avenues for mitigating effects of induced seismicity. Although the United States is our focus here, Canada, China, the UK, and others confront similar problems associated with oil and gas production, whereas quakes induced by geothermal activities affect Switzerland, Germany, and others.

Science↗

USGS2015

Increasing seismicity in the U. S. midcontinent: Implications for earthquake hazard

Earthquake activity in parts of the central United States has increased dramatically in recent years. The space-time distribution of the increased seismicity, as well as numerous published case studies, indicates that the increase is of anthropogenic origin, principally driven by injection of wastewater coproduced with oil and gas from tight formations. Enhanced oil recovery and long-term production also contribute to seismicity at a few locations. Preliminary hazard models indicate that areas experiencing the highest rate of earthquakes in 2014 have a short-term (one-year) hazard comparable to or higher than the hazard in the source region of tectonic earthquakes in the New Madrid and Charleston seismic zones.

The Leading Edge↗

USGS2015

Incorporating induced seismicity in the 2014 United States National Seismic Hazard Model: results of the 2014 workshop and sensitivity studies

The U.S. Geological Survey National Seismic Hazard Model for the conterminous United States was updated in 2014 to account for new methods, input models, and data necessary for assessing the seismic ground shaking hazard from natural (tectonic) earthquakes. The U.S. Geological Survey National Seismic Hazard Model project uses probabilistic seismic hazard analysis to quantify the rate of exceedance for earthquake ground shaking (ground motion). For the 2014 National Seismic Hazard Model assessment, the seismic hazard from potentially induced earthquakes was intentionally not considered because we had not determined how to properly treat these earthquakes for the seismic hazard analysis. The phrases “potentially induced” and “induced” are used interchangeably in this report, however it is acknowledged that this classification is based on circumstantial evidence and scientific judgment. For the 2014 National Seismic Hazard Model update, the potentially induced earthquakes were removed from the NSHM’s earthquake catalog, and the documentation states that we would consider alternative models for including induced seismicity in a future version of the National Seismic Hazard Model. As part of the process of incorporating induced seismicity into the seismic hazard model, we evaluate the sensitivity of the seismic hazard from induced seismicity to five parts of the hazard model: (1) the earthquake catalog, (2) earthquake rates, (3) earthquake locations, (4) earthquake Mmax (maximum magnitude), and (5) earthquake ground motions. We describe alternative input models for each of the five parts that represent differences in scientific opinions on induced seismicity characteristics. In this report, however, we do not weight these input models to come up with a preferred final model. Instead, we present a sensitivity study showing uniform seismic hazard maps obtained by applying the alternative input models for induced seismicity. The final model will be released after further consideration of the reliability and scientific acceptability of each alternative input model. Forecasting the seismic hazard from induced earthquakes is fundamentally different from forecasting the seismic hazard for natural, tectonic earthquakes. This is because the spatio-temporal patterns of induced earthquakes are reliant on economic forces and public policy decisions regarding extraction and injection of fluids. As such, the rates of induced earthquakes are inherently variable and nonstationary. Therefore, we only make maps based on an annual rate of exceedance rather than the 50-year rates calculated for previous U.S. Geological Survey hazard maps.

Open-File Report↗

USGS2014

The 2001-present induced earthquake sequence in the Raton Basin of northern New Mexico and southern Colorado

We investigate the ongoing seismicity in the Raton Basin and find that the deep injection of wastewater from the coal‐bed methane field is responsible for inducing the majority of the seismicity since 2001. Many lines of evidence indicate that this earthquake sequence was induced by wastewater injection. First, there was a marked increase in seismicity shortly after major fluid injection began in the Raton Basin in 1999. From 1972 through July 2001, there was one M≥4 earthquake in the Raton Basin, whereas 12 occurred between August 2001 and 2013. The statistical likelihood that such a rate change would occur if earthquakes behaved randomly in time is 3.0%. Moreover, this rate change is limited to the area of industrial activity. Earthquake rates remain low in the surrounding area. Second, the vast majority of the seismicity is within 5 km of active disposal wells and is shallow, ranging between 2 and 8 km depth. The two most carefully studied earthquake sequences in 2001 and 2011 have earthquakes within 2 km of high‐volume, high‐injection‐rate wells. Third, injection wells in the area are commonly very high volume and high rate. Two wells adjacent to the August 2011 M 5.3 earthquake injected about 4.9 million cubic meters of wastewater before the earthquake, more than seven times the amount injected at the Rocky Mountain Arsenal well that caused damaging earthquakes near Denver, Colorado, in the 1960s. The August 2011 M 5.3 event is the second‐largest earthquake to date for which there is clear evidence that the earthquake sequence was induced by fluid injection.

Colorado;New Mexico↗

USGS2014

Maximum magnitude earthquakes induced by fluid injection

Analysis of numerous case histories of earthquake sequences induced by fluid injection at depth reveals that the maximum magnitude appears to be limited according to the total volume of fluid injected. Similarly, the maximum seismic moment seems to have an upper bound proportional to the total volume of injected fluid. Activities involving fluid injection include (1) hydraulic fracturing of shale formations or coal seams to extract gas and oil, (2) disposal of wastewater from these gas and oil activities by injection into deep aquifers, and (3) the development of enhanced geothermal systems by injecting water into hot, low-permeability rock. Of these three operations, wastewater disposal is observed to be associated with the largest earthquakes, with maximum magnitudes sometimes exceeding 5. To estimate the maximum earthquake that could be induced by a given fluid injection project, the rock mass is assumed to be fully saturated, brittle, to respond to injection with a sequence of earthquakes localized to the region weakened by the pore pressure increase of the injection operation and to have a Gutenberg-Richter magnitude distribution with a b value of 1. If these assumptions correctly describe the circumstances of the largest earthquake, then the maximum seismic moment is limited to the volume of injected liquid times the modulus of rigidity. Observations from the available case histories of earthquakes induced by fluid injection are consistent with this bound on seismic moment. In view of the uncertainties in this analysis, however, this should not be regarded as an absolute physical limit.

Journal of Geophysical Research B: Solid Earth↗

USGS2014

Seismic monitoring at the Decatur, Ill., CO2 sequestration demonstration site

The viability of carbon capture and storage (CCS) to reduce emissions of greenhouse gases depends on the ability to safely sequester large quantities of CO2 over geologic time scales. One concern with CCS is the potential of induced seismicity. We report on ongoing seismic monitoring by the U.S. Geological Survey (USGS) at a CCS demonstration site in Decatur, IL, in an effort to understand the potential hazards posed by injection-induced seismicity associated with geologic CO2 sequestration. At Decatur, super-critical CO2 is injected at 2.1 km depth into the 550-m-thick Mt. Simon Sandstone, which directly overlies granitic basement. The primary sealing cap rock is the Eau Claire Shale, a 100- to 150-m-thick unit at a depth of roughly 1.5 km. The USGS seismic network consists of 12 stations, three of which have surface accelerometers and three-component borehole geophones. We derived a one-dimensional velocity models from a vertical seismic profile acquired by Archer-Daniels-Midland (ADM) and the Illinois State Geological Survey (ISGS) to a depth of 2.2 km, tied into shallow acoustic logs from our borehole stations and assuming a 6 km/sec P-wave velocity for granite below 2.2 km. We further assume a constant ratio of P- to S-wave velocities of 1.83, as derived from velocity model inversions. We use this velocity model to locate seismic events, all of which are within the footprint of our network. So far magnitudes of locatable events range from Mw = -1.52 to 1.07. We further improved the hypocentral precision of microseismic events when travel times and waveforms are sufficiently similar by employing double-difference relocation techniques, with relative location errors less than 80 m horizontally and 100 m vertically. We observe tend to group in three distinct clusters: ∼0.4 to 1.0 km NE, 1.6 to 2.4 km N, and ∼1.8 to 2.6 km WNW from the injection well. The first cluster of microseismicity forms a roughly linear trend, which may represent a pre-existing geologic structure. Most of these microearthquakes occur in the granitic basement at depths greater than 2.2 km, well below the caprock, and likely do not compromise the integrity of the seal. We conclude that because the observed microseismicity is occurring in the granitic basement, the integrity of the caprock seal has not been compromised by CCS activities.

Illinois↗

USGS2014

Analysis of induced seismicity in geothermal reservoirs – An overview

In this overview we report results of analysing induced seismicity in geothermal reservoirs in various tectonic settings within the framework of the European G eothermal E ngineering I ntegrating Mitigation of Induced Se ismicity in R eservoirs (GEISER) project. In the reconnaissance phase of a field, the subsurface fault mapping, in situ stress and the seismic network are of primary interest in order to help assess the geothermal resource. The hypocentres of the observed seismic events (seismic cloud) are dependent on the design of the installed network, the used velocity model and the applied location technique. During the stimulation phase, the attention is turned to reservoir hydraulics (e.g., fluid pressure, injection volume) and its relation to larger magnitude seismic events, their source characteristics and occurrence in space and time. A change in isotropic components of the full waveform moment tensor is observed for events close to the injection well (tensile character) as compared to events further away from the injection well (shear character). Tensile events coincide with high Gutenberg-Richter b -values and low Brune stress drop values. The stress regime in the reservoir controls the direction of the fracture growth at depth, as indicated by the extent of the seismic cloud detected. Stress magnitudes are important in multiple stimulation of wells, where little or no seismicity is observed until the previous maximum stress level is exceeded (Kaiser Effect). Prior to drilling, obtaining a 3D P -wave ( Vp ) and S -wave velocity ( Vs ) model down to reservoir depth is recommended. In the stimulation phase, we recommend to monitor and to locate seismicity with high precision (decametre) in real-time and to perform local 4D tomography for velocity ratio ( Vp / Vs ). During exploitation, one should use observed and model induced seismicity to forward estimate seismic hazard so that field operators are in a position to adjust well hydraulics (rate and volume of the fluid injected) when induced events start to occur far away from the boundary of the seismic cloud.

Geothermics↗

USGS2013

Constraints on behaviour of a mining‐induced earthquake inferred from laboratory rock mechanics experiments

On December 12, 2004, an earthquake of magnitude 2.2, located in the TauTona Gold Mine at a depth of about 3.65 km in the ancient Pretorius fault zone, was recorded by the in-mine borehole seismic network, yielding an excellent set of ground motion data recorded at hypocentral distances of several km. From these data, the seismic moment tensor, indicating mostly normal faulting with a small implosive component, and the radiated energy were measured; the deviatoric component of the moment tensor was estimated to be M0 = 2.3×1012 N·m and the radiated energy ER = 5.4×108 J. This event caused extensive damage along tunnels within the Pretorius fault zone. What rendered this earthquake of particular interest was the underground investigation of the complex pattern of exposed rupture surfaces combined with laboratory testing of rock samples retrieved from the ancient fault zone (Heesakkers et al.2011a, 2011b). Event 12/12 2004 was the result of fault slip across at least four nonparallel fault surfaces; 25 mm of slip was measured at one location on the rupture segment that is most parallel with a fault plane inferred from the seismic moment tensor, suggesting that this segment accounted for much of the total seismic deformation. By applying a recently developed technique based on biaxial stick-slip friction experiments (McGarr2012, 2013) to the seismic results, together with the 25 mm slip observed underground, we estimated a maximum slip rate of at least 6.6 m/s, which is consistent with the observed damage to tunnels in the rupture zone. Similarly, the stress drop and apparent stress were found to be correspondingly high at 21.9 MPa and 6.6 MPa, respectively. The ambient state of stress, measured at the approximate depth of the earthquake but away from the influence of mining, in conjunction with laboratory measurements of the strength of the fault zone cataclasites, indicates that during rupture of the M 2.2 event, the normal stress acting on the large-slip fault segment was about 260 MPa, the yield stress was 172 MPa and the seismic efficiency was 0.05. Thus, for event 12/12 2004, 5% of the energy released by the earthquake was radiated and the remaining 95% was consumed in overcoming fault friction and expanding the zone of rupture.

Conference Paper↗

USGS2012

Observed source parameters for dynamic rupture with non-uniform initial stressand relatively high fracture energy

We have conducted dynamic rupture propagation experiments to establish the relations between in-source stress drop, fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 MPa. To produce high fracture energy in the source we use a rough fault that has a large slip weakening distance. An artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture, making the initial stress non-uniform. Shear stress, particle velocity, fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault-normal distances. Stress drop increases weakly with normal stress. Average slip rate depends linearly on the fault strength loss and on static stress drop, both with a nonzero intercept. A minimum fracture energy of 1.8 J/m 2 and a linear slip weakening distance of 33 μm are inferred from the intercept. The large slip weakening distance also affects the average slip rate which is reduced by in-source energy dissipation from on-fault fracture energy. Because of the low normal stress and small per event slip (∼86 μm), no thermal weakening such as melting or pore fluid pressurization occurs in these experiments. Despite the relatively high fracture energy, and the very low heat production, energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties. The product of fracture energy and fault area is larger than the radiated energy. Seismic efficiency is low at ∼2%. The ratio of apparent stress to static stress drop is ∼27%, consistent with measured overshoot. The fracture efficiency is ∼33%. The static and dynamic stress drops when extrapolated to crustal stresses are 2–7.3 MPa and in the range of typical earthquake stress drops. As the relatively high fracture energy reduces the slip velocities in these experiments, the extrapolated average particle velocities for crustal stresses are 0.18–0.6 m/s. That these experiments are consistent with typical earthquake source properties suggests, albeit indirectly, that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops, dominate earthquake source properties only for exceptional events unless crustal stresses are low.

Journal of Structural Geology↗

USGS2009

Extension of Gutenberg‐Richter distribution to MW −1.3, no lower limit in sight

With twelve years of seismic data from TauTona Gold Mine, South Africa, we show that mining‐induced earthquakes follow the Gutenberg‐Richter relation with no scale break down to the completeness level of the catalog, at moment magnitude M W −1.3. Events recorded during relatively quiet hours in 2006 indicate that catalog detection limitations, not earthquake source physics, controlled the previously reported minimum magnitude in this mine. Within the Natural Earthquake Laboratory in South African Mines (NELSAM) experiment's dense seismic array, earthquakes that exhibit shear failure at magnitudes as small as M W −3.9 are observed, but we find no evidence that M W −3.9 represents the minimum magnitude. In contrast to previous work, our results imply small nucleation zones and that earthquake processes in the mine can readily be scaled to those in either laboratory experiments or natural faults.

TauTona Mine↗