Small Seismic Events Research Articles

Magnetotelluric monitoring of coal-seam gas and shale-gas resource development in Australia

Extraction of unconventional energy has become a major global industry in the last decade and is driven by changes in technology and increasing demand. One of the key factors for the success of gas extraction is establishing sufficient permeability in otherwise low-porosity and low-permeability formations. Permeability can be established through hydraulic stimulation of deep formations, either through existing fracture networks or by creating new pathways for fluids to flow, and through depressurization of coalbeds by extracting existing subsurface fluids. Geophysical monitoring of hydraulic stimulation and depressurization can be used to determine lateral and vertical constraints on fluid movements in the target lithologies. Such constraints help to optimize production and well placement. In addition, independent verification is critical for social and environmental regulation, to ensure that hydraulic stimulations and depressurization do not interact with overlying aquifers. To date, the primary and most successful geophysical technique has been microseismic, which measures small seismic events associated with rock fractures from arrays of surface and downhole geophones. The microseismic approach has been used widely for many types of unconventional energy-resource development. The magnetotelluric (MT) method is an alternative approach to monitoring hydraulic stimulations and depressurization. In contrast to microseismic, which delineates the locations of rock fractures, MT is sensitive directly to the presence of fluid as measured by the earth's bulk electrical resistivity, which is dependent on permeability. MT is sensitive to the direction of fluid connection, so it might yield important information on how fluids migrate with time. Because subsurface fluids conduct electrical current dependent on the porosity, connectivity, and ionic saturation of the fluid, it follows that the introduction or removal of fluids will change the electrical resistivity of the formation. The physics of the approach is outlined, and the feasibility of the MT method for monitoring unconventional energy-resource development is demonstrated. Two case studies are conducted, one for a shallow (CSG) depressurization and the second for a deep hydraulic stimulation of a shale-gas reservoir.

Seismic collapse prevention system for steel-frame buildings

This paper presents an innovative “collapse prevention” system for seismic resistant design in new construction and existing buildings. The collapse prevention system consists of a collapse inhibiting mechanism, such as a pair of slack cables or loose linkages, working in tandem with the main lateral-force resisting system and engaging the gravity framing to avert collapse. In this holistic design approach, the main lateral-force resisting system and gravity framing are used to provide adequate performance under low to moderate level ground motions, and the collapse inhibiting mechanism is deployed as a back-up to provide life safety under extreme ground motions. The collapse inhibiting mechanism may be augmented with energy dissipation devices (small viscous fluid or visco-elastic solid dampers) to enhance performance under wind or small seismic events. Analytical performance of archetypical 1-story, 2-story, 4-story, and 8-story steel-frame buildings employing collapse prevention systems indicate that collapse prevention systems substantially reduce the probability of collapse during Maximum Considered Earthquake (MCE) ground motions, depending on the building and collapse inhibiting mechanism deployed. Seismic hazard data suggests that collapse prevention systems would provide a 1% or less risk of collapse in 50years in many locations, mostly in the central and eastern United States.

Long term continuous radon monitoring in a seismically active area

We present the results of a long term, continuous radon monitoring experiment started in April 2010 in a seismically active area, affected during the 2010-2013 data acquisition time window by an intense micro seismic activity and by several small seismic events. We employed both correlation and cross-correlation analyses in order to investigate possible relationship existing between the collected radon data, seismic events and meteorological parameters. Our results do not support the feasibility of a robust one-to-one association between the small magnitude earthquakes characterizing the local seismic activity and single radon measurement anomalies, but evidence significant correlation patterns between the spatio-temporal variations of seismic moment release and soil radon emanations, the latter being anyway dominantly modulated by meteorological parameters variations.

Linking hematite (U-Th)/He dating with the microtextural record of seismicity in the Wasatch fault damage zone, Utah, USA

Techniques directly dating fault slip are few, limiting the ability to interpret the rock record of seismicity. Hematite is commonly found in fault zones, amenable to (U-Th)/He dating, and slip surface hematite may be reset by shear heating events and/or recrystallization. Glossy hematite-coated fault surfaces in the Wasatch fault footwall damage zone, Utah (USA), exhibit evidence of hematite cataclasis and preserve Pliocene hematite (U-Th)/He dates. Apatite (U-Th)/He and fission track data from the host gneiss indicate footwall unroofing through ∼2 km by ca. 4.5 Ma. Internally reproducible but disparate hematite (U-Th)/He dates 4.5 Ma and younger from isolated locations on a single fault surface do not reflect ambient cooling. We hypothesize that these dates, and associated iridescence and annealed crystal texture, document rapid cooling from friction-generated heat during small seismic slip events between ca. 4.5 and 2.5 Ma. Thus, hematite (U-Th)/He dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 10 5 –10 6 yr time scales.

A Robust Calibration Technique for Acoustic Emission Systems Based on Momentum Transfer from a Ball Drop

We describe a technique to estimate the seismic moment of acoustic emissions and other extremely small seismic events. Unlike previous calibration tech- niques, it does not require modeling of the wave propagation, sensor response, or signal conditioning. Rather, this technique calibrates the recording system as a whole and uses a ball impact as a reference source or empirical Green's function. To correctly apply this technique, we develop mathematical expressions that link the seismic mo- ment M0 of internal seismic sources (i.e., earthquakes and acoustic emissions) to the impulse, or change in momentum Δp, of externally applied seismic sources (i.e., me- teor impacts or, in this case, ball impact). We find that, at low frequencies, moment and impulse are linked by a constant, which we call the force-moment-rate scale factor C F _ MM0=Δp. This constant is equal to twice the speed of sound in the material from which the seismic sources were generated. Next, we demonstrate the calibration technique on two different experimental rock mechanics facilities. The first example is a saw-cut cylindrical granite sample that is loaded in a triaxial apparatus at 40 MPa confining pressure. The second example is a 2 m long fault cut in a granite sample and deformed in a large biaxial apparatus at lower stress levels. Using the empirical cali- bration technique, we are able to determine absolute source parameters including the seismic moment, corner frequency, stress drop, and radiated energy of these magnitude −2:5 to −7 seismic events.

Monitoring of hydraulic fracturing in Moomba, South Australia, using magnetotellurics

The microseismic method is the dominant geophysical tool for monitoring hydraulic fracturing. High-pressure injection of fluids creates fractures in the sub-surface, recorded as small seismic events. Inverting such events yields the temporal and spatial extent of deformation; however, there is no straightforward relationship between the location of micro-deformations and the migration of injected fluid. In this peer-reviewed paper, an alternative approach to deep-fluid monitoring using the magnetotelluric (MT) method is presented. As MT is primarily sensitive to the presence of fluids (through changes in the bulk electrical resistivity) and the orientation of fluid connection, this technique may yield important constraints on how fluids propagate from the well with time. This technique has been successfully used to monitor hydraulically induced fractures in two geothermal settings at depths of more than 3 km. A 10-stage horizontal-well hydraulic-fracture in the Cooper Basin for a shale-gas reservoir was monitored using MT. From the central injecting well, four lines of instruments were placed radially in a cross-pattern. These new electric and magnetic loggers were deployed for a continuous monitoring period of 65 days. As changes in the magnetic field are spatially uniform across the extent of the survey area, only two magnetic-field data recorders were placed in the array. A third logger was placed 10 km from the well, for redundancy and as a remote reference. Measurements were taken from mid-May to mid-July 2014, while the injection and drawback occurred for approximately one month. Processing regimes were created to convert field data into MT responses and resistivity models of the Earth. Theory, survey practice and outputs from this experiment are presented in this peer-reviewed paper.

Off-fault damage and acoustic emission distributions during the evolution of structurally complex faults over series of stick-slip events

Variations in fault structure, for example, surface roughness and deformation zone width, influence the location and dynamics of large earthquakes as well as the distribution of small seismic events. In nature, changes in fault roughness and seismicity characteristics can rarely be studied simultaneously, so that little is known about their interaction and evolution. Here, we investigate the connection between fault structure and near-fault distributions of seismic events over series of stick-slip cycles in the laboratory. We conducted a set of experiments on rough faults that developed from incipient fracture surfaces. We monitored stress and seismic activity which occurred in the form of acoustic emissions (AEs). We determined AE density distributions as a function of fault normal distance based on high-accuracy hypocentre locations during subsequent interslip periods. The characteristics of these distributions were closely connected to different structural units of the faults, that is, the fault core, off-fault and background damage zone. The core deformation zone was characterized by consistently high seismic activity, whereas the off-fault damage zone displayed a power-law decay of seismic activity with increasing distance from the fault core. The exponents of the power-law-distributed off-fault activity increased with successive stick-slip events so that later interslip periods showed a more rapid spatial decay of seismic activity from the fault. The increase in exponents was strongest during the first one to three interslip periods and reached approximately constant values thereafter. The relatively rapid spatial decay of AE events during later interslip periods is likely an expression of decreasing fault zone complexity and roughness. Our results indicate a close relationship between fault structure, stress and seismic off-fault activity. A more extensive mapping of seismic off-fault activity-decay has the potential to significantly advance the understanding of fault zone properties including variations in fault roughness and stress.

The effects of lateral property variations on fault-zone reactivation by fluid pressurization: Application to CO2 pressurization effects within major and undetected fault zones

In this study, we performed in situ multidisciplinary analyses of two different fault zones in carbonate formations. One is a seismically active fault zone several kilometers long (the Roccasseira Fault Zone); the other is a small fault zone a few hundred meters long (the GAS Fault Zone). The smaller, “immature” fault zone displays a discontinuous damage zone, because tectonic deformations have been accommodated differently according to the initial properties of the host rock. The larger, “mature” fault zone displays a continuous damage zone caused by the presence of secondary fault cores embedded in a heavily fractured area inside the damage zone. These markedly different fault-zone architectures were reflected in two hydraulic and geomechanical fault models, both generated from a coupled fluid-flow and geomechanical simulator, to examine the impact of hydromechanical property distribution on fault stability when the faults are reactivated by CO2 injection. In the smaller fault zone, marked differences in hydromechanical properties (Young's modulus and permeability) favor fluid accumulation, inducing high pressurization in parts of the damage zone, potentially resulting in small seismic events. On the other hand in the mature fault zone, fluid flows more easily and thus fluid-induced earthquakes may not readily occur, because the fault-zone pressurization is much lower, insufficient for triggering a seismic event.

Lack of Spatiotemporal Localization of Foreshocks before the 1999 Mw 7.1 Duzce, Turkey, Earthquake

We use a matched‐filter technique to detect small seismic events before the M w 7.1 Duzce earthquake that are not included in the regular catalog. The study employs extensive waveform dataset recorded by a 10‐station fault zone array near the epicenter of the Duzce earthquake, deployed about three months before the event. We use 3134 earthquakes within 20 km of the Duzce epicenter listed in the local catalog as templates to scan through waveforms recorded within ∼65 hours before the Duzce earthquake. The analysis reveals 262 newly detected events in this time interval, which is ∼5 times more than the 55 events listed in the original catalog. Most of the events occur to the west of the Duzce epicenter, which was initiated between a more active western and relatively quite eastern fault segments. The results do not indicate a localizing foreshock process that accelerates in time and/or involves progressive concentration of activity around the mainshock hypocenter during the preceding 65 hour period. Instead, we find that the Duzce source region becomes less active during the ∼20 hours immediately before the mainshock. Our results, together with other recent studies, suggest that progressive acceleration and localization of foreshocks around the mainshock epicenter is not a general phenomenon. Online Material: Figures of timing of recorded waveforms, peak ground velocities, foreshock activity, seismicity rate, and temporal evolution of earthquake activity, vertical waveforms, and distribution of template events.

A Comparison of Seismicity Characteristics and Fault Structure Between Stick–Slip Experiments and Nature

Fault zones contain structural complexity on all scales. This complexity influences fault mechanics including the dynamics of large earthquakes as well as the spatial and temporal distribution of small seismic events. Incomplete earthquake records, unknown stresses, and unresolved fault structures within the crust complicate a quantitative assessment of the parameters that control factors affecting seismicity. To better understand the relationship between fault structure and seismicity, we examined dynamic faulting under controlled conditions in the laboratory by creating saw-cut-guided natural fractures in cylindrical granite samples. The resulting rough surfaces were triaxially loaded to produce a sequence of stick–slip events. During these experiments, we monitored stress, strain, and seismic activity. After the experiments, fault structures were imaged in thin sections and using computer tomography. The laboratory fault zones showed many structural characteristics observed in upper crustal faults, including zones of localized slip embedded in a layer of fault gouge. Laboratory faults also exhibited a several millimeter wide damage zone with decreasing micro-crack density at larger distances from the fault axis. In addition to the structural similarities, we also observed many similarities between our observed distribution of acoustic emissions (AEs) and natural seismicity. The AEs followed the Gutenberg–Richter and Omori–Utsu relationships commonly used to describe natural seismicity. Moreover, we observed a connection between along-strike fault heterogeneity and variations of the Gutenberg–Richter b value. As suggested by natural seismicity studies, areas of low b value marked the nucleation points of large slip events and were located at large asperities within the fault zone that were revealed by post-experimental tomography scans. Our results emphasize the importance of stick–slip experiments for the study of fault mechanics. The direct correlation of acoustic activity with fault zone structure is a unique characteristic of our laboratory studies that has been impossible to observe in nature.

Application of real time recurrent neural network for detection of small natural earthquakes in Poland

This study is an application of a Real Time Recurrent Neural Network (RTRN) in the detection of small natural seismic events in Poland. Most of the events studied are from the Podhale region with a magnitude of 0.4 to 2.5. The population distribution of the region required that seismic signals be recorded using temporary stations deployed in populated areas. As a consequence, the high level of seismic noise that cannot be removed by filtration made it impossible to detect small events by STA/LTA based algorithms. The presence of high noise requires an alternate method of seismic detection capable of recognizing small seismic events. We applied the RTRN, which potentially can detect seismic signals in the frequency domain as well as in the phase arrival times. Data results of small local seismic events showed that the RTRN has the ability to correctly detect most of the events with fewer false detections than STA/LTA methods.

Spatio-temporal occurrence patterns among the foreshocks preceding the 2007 Noto Hanto earthquake

The 2007 Noto Hanto earthquake was a magnitude 6.8 crustal earthquake (Japan Metrological Agency) that occurred in central Japan. By analyzing continuous seismograms that recorded activity during the 25 days before the earthquake, we detected 36 small seismic events around the mainshock hypocenter that classified as either isolated events or members of three clusters. Three of the detected events are listed in official seismic records. Two clusters included more than ten events and occurred some distance from the mainshock fault, indicating that the clusters did not relate to the mainshock event. Only the third cluster was located along the mainshock fault. This cluster occurred in the vicinity of the mainshock hypocenter and consisted of four foreshocks with identical seismograms. This cluster began twelve minutes before the mainshock. The occurrence pattern of this clustered foreshock sequence is similar to that observed in association with the 2008 Iwate-Miyagi Inland earthquake.

Influence of continental margin on regional seismic wavefield

Regional seismic analysis is an only way to investigate the source properties of small or moderate-sized seismic events that occur in physically unaccessible regions such as oceanic regions and politically-prohibited regions. The ground motions by earthquakes need to be understood for better mitigation of seismic hazards particularly in continental margins that often incorporate high seismicity. It has not been fully understood how seismic waves interact with complex crustal structures in continental margins. Responses of regional waves to complex crustal structures need to be quantified. Analysis of regional waveforms from controlled sources is desirable for investigation of influence of crustal structures. The influence of continental margin around the Korean Peninsula is investigated in terms of spectral contents, horizontal-to-vertical (H/V) spectral ratios and quality factors using regional waveforms for the 2009 North Korean underground nuclear explosion test that was well recorded by stations in the southern Korean Peninsula. Regional waveforms and spectral amplitudes vary significantly by path. Spectral contents of regional phases are different among stations in common great-circle directions. All regional phases modulate highly in continental margins. Path-dependent seismic attenuation is strong in low frequencies (≤ 3 Hz), and weak in high frequencies (> 3 Hz). Continental margins cause directional energy partition of regional waves depending on the path. Regional waves attenuate highly in passage across continental margin, and then regrow on continental paths. The growth rate is stronger than inherent attenuation rate, causing seismic amplification. The shapes of H/V ratios are similar among various regional phases at common stations. On the other hand, the H/V ratios for common phases vary by station in the same azimuths. Characteristic differences in H/V ratios between two horizontal components suggest directional partition of seismic energy by crustal structures in the paths. • Influence of complex crustal structures in continental margin on regional waveforms • Quantification of regional-phase regrowth rates on oceanic-to-continental paths • Characteristic directional energy partition of regional waves by crustal structures

地震入力レベルを考慮した連結機構摩擦ダンパーの免震戸建て住宅への適用性

For the extremely large or the long period ground motions, it is feared that the base isolation layer collides with the side wall due to the extremely large displacement. In case dampers with large capacity are incorporated with the base isolation layer, the effect of the base isolation decreases due to the large reaction force of dampers in small seismic events. To realize the control of the displacement and the acceleration which is the important factor of the base isolation effect, the friction damper with coupling mechanism is proposed. Analytical studies are conducted using the base isolated detached houses with slide bearings and the proposed dampers. To decide a series of damper parameters, the optimum design method is used. Results indicate that the friction damper with coupling mechanism can control the maximum displacement and the maximum acceleration within the design criteria which is decided individually for the input ground motion level.

Geomechanical Modeling of Fault Responses and the Potential for Notable Seismic Events During Underground CO2 Injection

We summarize a number of recent modeling studies related to the potential for fault reactivations and induced seismicity during underground CO2 injection. The model simulations were conducted using coupled multiphase fluid flow and geomechanics, including fault-frictional weakening enabling analysis of sudden (seismic) fault rupture, with some of the numerical analyses extended to dynamic modeling of seismic source, wave propagation, and ground motion. These model simulations show that the critical factors in determining the likelihood and magnitude of such an event are the local in situ stress field, fault orientation and size, fault strength, and injection pressure. We analyzed the case of activation of a 1km long minor fault that might have gone undetected during the site investigation and show that the maximum seismic magnitudes would likely be less than about 3.6, even if the entire 1km fault would to be activated. We then include seismic wave propagation generated by the rupture and show how the acceleration and deceleration of the rupture generate waves and result in a peak ground acceleration of about 0.1g, except for a localized –0.6g of horizontal peak acceleration at the faults intersection with the ground surface. The modeling shows that these are high frequency events that would not cause any substantial damage but could certainly be felt by the local population. We may also considered that fault reactivation, even associated with relatively small seismic or aseismic events, could potentially increase CO2 seepage out of the intended storage complex and therefore reduce the effectiveness of a CO2 storage operation. Under these circumstances, we recommend a staged, learn-as-you-go approach, involving a gradual increase of injection rates combined with continuous monitoring of geomechanical changes, as well as siting beneath a multiple layered overburden for multiple flow barrier protection, should an unexpected deep fault activation occur.

Seismic anisotropy beneath the northeastern Caribbean: implications for the subducting North American lithosphere

Abstract Active plate boundaries in the Caribbean form a complex tectonic environment that includes transform and subduction zones. The Caribbean-North American plate boundary is one such active margin, where subduction transitions from arc- to oblique-type off the northeast coast of Puerto Rico. Understanding mantle flow in this region will not only help determine the nature of tectonic activity and mantle dynamics that control these margins, but will also aid our understanding of the fate of subducting lithosphere. The existence of tears, windows, and gaps in subducting slabs has been proposed at various locations around the world but few have been confirmed. Since mantle flow and crustal deformation are believed to produce seismic anisotropy in the asthenosphere and lithosphere, searching for changes in, for example, SKS splitting parameters can help identify locations at which subducting slabs have been disrupted. Several lines of evidence support the notion of a slab tear within the subducting North American plate at this transition zone, including the counter-clockwise rotation of the Puerto Rico microplate over the past ~10 Ma, clusters of small seismic events, and trench collapse initiating ~3.3 m.y. Here we present results from a detailed investigation of seismic anisotropy from 28 stations across six networks in the Northeast Caribbean that support the hypothesis of a significant slab gap in the vicinity of the U.S. and British Virgin islands. A regional synthesis of our results reveals fast shear wave polarizations that are generally oriented parallel to the plate boundary with intermediate to high SH-SV delay times. For example, polarization directions are oriented roughly NE-SW along the bulk of the Lesser Antilles, E-W along the Puerto Rico trench and the northern Lesser Antilles, and NW-SE beneath Hispaniola. Beneath the U.S. and British Virgin Islands, however, the fast polarization direction differs markedly from the regional pattern, becoming almost perpendicular to the plate boundary. Stations on Anegada, British Virgin islands and St. Croix, U.S. Virgin islands show a fast polarization direction that is oriented nearly NNE-SSW and smaller delay times than surrounding stations. These results suggest that mantle flow is redirected NE-SW at this location through a gap in the subducted lithosphere of the North American plate.

The first estimations of soil-radon activity near faults in Central Mongolia

Estimation of soil-radon activity, Q was first carried out for faults in Central Mongolia. Eight study sites were located in epicentral zones of Mogod (M = 7.8; 05 January 1967) and Avdar (M = 3.8; 22 March 2009) earthquakes, and in the vicinity of Ulaanbaatar, where small seismic events (M = 1.0–2.5) occurred in the past few years. Profile radon surveys were conducted at fifteen faults that differ in size and geodynamic activity, yet clearly topographically manifested as scarps or straightened segments of valleys of ephemeral streams. By applying the formalized method of processing of the survey results, it was possible to reveal radon anomalies and to establish that their shape, intensity and contrast are mainly determined by the structure of the fault zone. Due to heterogeneous permeability of fault zones, shapes and quantitative parameters of radon anomalies are variable at different faults and in individual cross-sections of one and the same fracture. Radon anomalies in Central Mongolia are diverse, yet the most frequent are the cases where (1) a radon anomaly is discontinuous in shape due to the presence of small domains with minimum values of Q; (2) a major part of the anomaly is located in one fault wall; and (3) a fault scarp is marked by a minimum value of Q. In Central Mongolia, intensities of radon anomalies, Qmax near neotectonic faults differ by more than an order of magnitude. The most intense anomaly (20,200 Bq/m3) is registered at Hustai fault in the vicinity of Ulaanbaatar, which indicates the importance of assessment of radon hazard for the capital city of Mongolia, where almost half the population of the country reside. The contrast of radon anomaly, KQ is determined as a ratio of a maximum value of Qmax to a minimum value of Qmin outside the fault zone; it varies from 1.4 to 17.3 for faults of Central Mongolia. Faults characterized by ultra-high (KQ > 10), high (10 ≥ KQ > 5), increased (5 ≥ KQ > 3), medium (3 ≥ KQ > 2) and low (KQ ≤ 2) radon activity are distinguished. A relative index, KQ can be effectively applied for assessment of geodynamic activity of faults in Central Mongolia. On the one hand, it correlates with sizes and seismic potential of the studied faults; on the other hand, it significantly reduces the complicating influence of regional factors, such as radioactivity of rocks, sediment thickness, meteorological conditions of measurements, etc. The application of KQ, the formalized method of detection of radon anomalies, long-distance base cross-sections, and reduction of the measurement interval near faults – these key features of the profile survey are recommendable for further more accurate estimations on the basis of the first measurements of soil-radon activity in Central Mongolia.

Interpreting Microseismic Measurements: Practical Guide

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 144067, ’A Practical Guide To Interpreting Microseismic Measurements,’ by C. Cipolla, SPE, S. Maxwell, SPE, M. Mack, SPE, and R. Downie, Schlumberger, prepared for the 2011 SPE North American Unconventional Gas Conference and Exhibition, The Woodlands, Texas, 14-16 June. The paper has not been peer reviewed. Thousands of hydraulic-fracture treatments have been monitored in the past 10 years with microseismic mapping, providing measurements that show a surprising degree of diversity in event patterns. Interpretation of the microseismic data to determine the geometry and complexity of hydraulic fractures continues to focus on visualization of event patterns and on qualitative estimates of the stimulated volume, which has led to wide variations and inconsistencies. Comparing the energy input during a hydraulic-fracture treatment with the resultant energy released by microseismic events demonstrates that the seismic deformation is a very small portion of the total deformation. Introduction Hydraulic-fracture microseismic monitoring (MSM) has increased our understanding of fracture propagation, especially in unconventional reservoirs. MSM, which is a straightforward application of earthquake-seismology principles, consists of the detection, location, and further analysis of extremely small seismic events induced by the fracturing process. Typically, multiple-receiver arrays are positioned over a 300- to 1,000-ft vertical interval in a single nearby offset well. The microseismic events usually are not detectable as discrete events at the surface, but only from other wellbores within approximately 3,000 ft of the fracture-treatment well. Typically, microseisms are represented adequately by a double-couple source, which implies shear slippage that releases stress over some area. In competent rocks, shear slippages occur on pre-existing planes of weakness as a result of the changes in stress and pore pressure induced by the fracturing process. When fracture growth is relatively planar, these changes in stress and pore pressure usually result in a narrow band of microseisms that clearly defines the azimuth and geometry of the hydraulic fracture. However, complex fracture behavior is often interpreted from microseismic data, as evidenced by very intricate event patterns with width/length aspect ratios greater than 0.5. MSM data have revealed a large variation in hydraulic-fracture growth, from relatively simple planar fractures to very complex fracture networks. Understanding fracture growth and the degree and nature of hydraulic-fracture complexity is essential to selecting the appropriate stimulation design and completion strategy.

Low signal-to-noise event detection based on waveform stacking and cross-correlation: application to a stimulation experiment

We study the microseismicity (M L < 2) in the region of Landau, SW Germany. Here, due to thick sediments (~3 km) and high cultural seismic noise, the signal-to-noise ratio is in general very low for microearthquakes. To gain new insights into the occurrence of very small seismic events, we apply a three-step detection approach and are able to identify 207 microseismic events (−1 < M L < ~1) with signal-to-noise ratios smaller than 3. Recordings from a temporary broadband network are used with station distances of approximately 10 km. First, we apply a short-term to long-term average detection algorithm for data reduction. The detection algorithm is affected severely by transient noise signals. Therefore, the most promising detections, selected by coinciding triggers and high-amplitude measures, are reviewed manually. Thirteen seismic events are identified in this way. Finally, we conduct a cross-correlation analysis. As master template, we use the stacked waveforms of five manually detected seismic events with a repeating waveform. This search reveals additional 194 events with a cross-correlation coefficient exceeding 0.65 which ensures a stable identification. Our analysis shows that the repeating events occurred during the stimulation of a geothermal reservoir within a source region of only about 0.5 km3. Natural background seismicity exceeding our detection level of M L ~ 0.7 is not found in the region of Landau by our analysis.

Source Parameters of Picoseismicity Recorded at Mponeng Deep Gold Mine, South Africa: Implications for Scaling Relations

Abstract We investigate the source parameters of picoseismic and nanoseismic events ( M w >-4.1) recorded with a high-sensitivity seismic network at the Mponeng gold mine in South Africa to gain new insights into the scaling of small seismic events. The Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) network, composed of one three-component (3C) accelerometer (sensitivity 50 Hz to 25 kHz) and 8 acoustic emission (AE) sensors, (sensitivity 1 kHz to 180 kHz) is located at a depth of 3268 m and covers the limited volume of approximately 300×300×300 m. The AE sensors are calibrated with respect to the 3C accelerometer in the frequency band 400 Hz–17 kHz. The waveform data of two datasets are analyzed; (1) the aftershock sequence of an M w 1.9 event that occurred approximately 30 m from our network, and (2) the postblasting activity recorded during working days, located at a distance >90 m from the network at the exploitation level. For the analysis we applied spectral fitting and spectral ratio methods. The calculated values of M w range from -0.8 down to -4.1 with corner frequencies 0.8 kHz–13.6 kHz (source sizes from 8 cm to 1.3 m). We observe static stress drops ranging from 1 MPa to 10.0 MPa with apparent stresses of 0.01 MPa–1.00 MPa. Stress drops are independent of the moment, suggesting self-similar behavior in the analyzed magnitude range -4.1 M w