Anna L Stork

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the ∼60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of ∼2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975–84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia–Arabia plate boundary.

Comparison of geomechanical deformation induced by megatonne-scale CO2 storage at Sleipner, Weyburn, and In Salah

Significance The economic and political viability of carbon capture and sequestration (CCS) is dependent on the secure storage of CO2 in subsurface geologic reservoirs. A key leakage risk is that posed by geomechanical deformation generating fractures in otherwise sealing caprocks. This study examines this risk, comparing and contrasting deformation induced at three large-scale CCS sites—Sleipner (Norwegian North Sea), Weyburn (Canada), and In Salah (Algeria). These sites show very different geomechanical responses, highlighting the importance of systematic geomechanical appraisal prior to injection, and comprehensive, multifaceted monitoring during injection at any future large-scale CCS operations. Geological storage of CO2 that has been captured at large, point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions. However, this technology will only be viable if it can be guaranteed that injected CO2 will remain trapped in the subsurface for thousands of years or more. A significant issue for storage security is the geomechanical response of the reservoir. Concerns have been raised that geomechanical deformation induced by CO2 injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO2 leakage. In this paper, we examine three large-scale sites where CO2 is injected at rates of ∼1 megatonne/y or more: Sleipner, Weyburn, and In Salah. We compare and contrast the observed geomechanical behavior of each site, with particular focus on the risks to storage security posed by geomechanical deformation. At Sleipner, the large, high-permeability storage aquifer has experienced little pore pressure increase over 15 y of injection, implying little possibility of geomechanical deformation. At Weyburn, 45 y of oil production has depleted pore pressures before increases associated with CO2 injection. The long history of the field has led to complicated, sometimes nonintuitive geomechanical deformation. At In Salah, injection into the water leg of a gas reservoir has increased pore pressures, leading to uplift and substantial microseismic activity. The differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site.

Source Parameter Scaling for Small Earthquakes Observed at the Western Nagano 800-m-Deep Borehole, Central Japan

Source parameters of 68 small earthquakes (0.6 < M W < 3.0) in western Nagano, Japan, are investigated to determine the scaling of static stress drop (Δσs) and apparent stress (σa) with seismic moment ( M ). Data from the 800-m-deep borehole in the area provides clean recordings containing a wide range of frequencies. Source parameters are determined by manipulation of P and S spectra in the frequency domain using individually determined time window lengths for arrivals on each seismogram. Frequency-independent quality factors, Q , corner frequencies, f c and the amplitude spectra levels are estimated with the best-fitting Brune (1970) ω2 model. A frequency-dependent attenuation model, Q ( f ), is calculated by spectra normalization. Static stress drop Δσs is self-similar for 1010 1011 N m, and 0.002 < σa < 0.2 MPa for M < 1011 N m, a narrower range than Q analysis results. Limits in recorded frequencies, variations in time window length, and source complexity are not found to significantly affect the calculation of σa. Therefore, the constant scaling of Δσs with M and the nonsimilarity and breakdown in σa scaling could be true characteristics of small earthquakes ( M W <1.3). Manuscript received 22 October 2002.

Uppermost mantle velocity from Pn tomography in the Gulf of Aden

We determine the lateral variations in seismic velocity of the lithospheric mantle beneath the Gulf of Aden and its margins by inversion of Pn (upper mantle high-frequency compressional P wave) traveltimes. Data for this study were collected by several temporary seismic networks and from the global catalogue. A least-squares tomographic algorithm is used to solve for velocity variations in the mantle lithosphere. In order to separate shallow and deeper structures, we use separate inversions for shorter and longer ray path data. High Pn velocities (8.2–8.4 km/s) are observed in the uppermost mantle beneath Yemen that may be related to the presence of magmatic underplating of the volcanic margins of Aden and the Red Sea. Zones of low velocity (7.7 km/s) are present in the shallow upper mantle beneath Sana’a, Aden, Afar, and along the Gulf of Aden that are likely related to melt transport through the lithosphere feeding active volcanism. Deeper within the upper mantle, beneath the Oman margin, a low-velocity zone (7.8 km/s) suggests a deep zone of melt accumulation. Our results provide evidence that the asthenosphere undergoes channelized flow from the Afar hotspot toward the east along the Aden and Sheba Ridges.

The robustness of seismic moment and magnitudes estimated using spectral analysis

We present an assessment of how microseismic moment magnitude, MW ,e stimates vary with the method and parameters used to calculate seismic moment. This is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity. It is therefore imperative that these parties know and understand what errors exist in given magnitude values, something that is poorly reported. This study concentrates on spectral analysis techniques and compares MW computed in the time and frequency domains. Using recordings of MW > −1. 5e vents at Cotton Valley, east Texas, the maximum discrepancy between MW estimated using the different methods is 0.6 units, a significant variation. By adjusting parameters in the MW calculation we find that the radiation pattern correction term can have the most significant effect on M W , generally up to 0.8 units. Following this investigation we make a series of recommendations for estimating microseismic M W using spectral methods. Noise should be estimated and removed from recordings and an attenuation correction should be applied. The spectral level can be measured by spectral fitting or taken from the low frequency level. Significant factors in obtaining reliable microseismic MW estimates include using at least four receivers recording at ≥1000 Hz and making radiation pattern corrections based on focal mechanism solutions, not average values.

Study of Recorded Seismicity at the In Salah (Algeria) Carbon Capture and Storage Project

The In Salah Joint Venture carbon capture and storage project at Krechba, Algeria began injecting CO2 in 2004. This study makes use of microseismic event data recorded by a single geophone. Information from shear waves enables fracture directions and densities to be inferred and we find fractures striking N145°E, in agreement with the regional direction of maximum horizontal stress. There is no evidence for any temporal change in shear-wave splitting parameters in the time frame studied. This implies that CO2 injection is reactivating pre-existing fractures. We are able to obtain only very approximate event locations which suggest that events occur at or below the reservoir level, with no change over time.

Real Time Forecasting of Maximum Expected Induced Seismicity

Fluid injection into the subsurface is performed for a variety of reasons, such as hydraulic fracturing, and waste storage. It is well established that fluid injection can trigger seismic activity of sufficient magnitude to be felt by local populations. The industry wishes to avoid such events. We develop a statistical model to forecast the largest event that might be induced by a given injection operation. The model is continuously updated as microseismic data is processed in real time. We use this model in a prospective sense, updating our forecasts through the injection period. We apply our model to two case studies: a hydraulic fracture stimulation in a tight gas reservoir; and at the In Salah CCS project, Algeria. In both cases, our model is capable of forecasting the largest event prior to its occurrence, providing an early-warning for the operator.

The importance of accurate and consistent microseismic event magnitudes

Seismicity induced by fluid injection during hydraulic fracturing; carbon capture and storage (CCS); and geothermal developments is of concern to industry and the public. However, the magnitudes of such events are often reported without a clear understanding of the meaning or implications. Using microseismic events recorded at injection sites we show how moment magnitude estimates depend on the monitoring set-up and the calculation, in the time or frequency domain. Magnitude estimates, and in particular moment magnitude estimates, vary by tenths of a magnitude unit depending on the geophone array configuration and how the calculation is made. This raises important questions for the monitoring of industrial activities, such as hydraulic fracturing, if the regulation of such activities is to be based on the magnitude of seismic events induced by industrial projects.

Baseline Microseismic Monitoring for CO2 Injection Sites

This submission describes a baseline passive seismic monitoring experiment for the Field Research Station in Alberta. The analysis does not reveal any local seismicity and we estimate a minimum magnitude (M) of detection of -1.4 for a source-station distance of 500m during quiet periods. Injection tests indicate that the detection threshold will rise to M=-0.1 during injection. Using perforation shots in the injection well we find the array is able to locate seismic events at the planned injection point to within 20m.

Geophysical Investigations to Assess the Extent of Disturbance in Excavated Rock Faces at Hinkley Point C

Initial field trials to investigate the potential for seismic surveys to estimate the extent of damage caused during slope excavation and to estimate a disturbance factor show that a velocity contrast due weathering/disturbance effects in a cliff face is detectable with an active seismic survey with geophones attached to the face. Modelling results confirm the resolution of the method and the experimental set-up is being deployed to monitor excavated faces at Hinkley Point C construction site.