Yu Jeffrey Gu

Models of the mantle shear velocity and discontinuities in the pattern of lateral heterogeneities

Resolution of the pattern of large-scale shear velocity variations above and below the known and postulated mantle discontinuities could provide constraints on the nature of mineral phase transitions, changes in composition, and the scale of mantle convection. To achieve good resolution across a full range of depths, we use a diversified data set consisting of body and mantle wave waveforms, travel times, and surface wave phase velocities. Our main focus is on the 670-km discontinuity, long presumed to be an important barrier, or impediment, to whole mantle convection. Our data set has a relatively high radial resolution throughout the mantle; in the transition zone and some 200 km below it, the long period waveforms, dominated by multiple surface reflections, make a particularly important contribution. We use a local spline support to parameterize the model; this allows us to obtain a smooth model (twice differentiable) and simplifies calculation of the model and its derivatives in applications such as three-dimensional ray tracing. In one inversion we use a continuous radial representation throughout the mantle; in the other, a discontinuity is allowed across the 670-km boundary. Both models suggest that the long-wavelength anomalies of the transition zone and the mantle below 750 km are significantly different. Near 670 km these two models display notable differences in the peak amplitudes and lateral scales of major anomalies. The degree 2 spherical harmonic, which dominates the large-scale shear velocities in the transition zone, is strongly attenuated at the top of the lower mantle where the power spectrum is essentially white. Resolution tests show that these results are robust, which suggests a possible reorganization of the flow between the upper and lower mantle. At other depths the power spectra of our models as a function of depth indicate a modest change near 400 km, where the dominating effect of degree 5 (shields) is replaced by degree 2 (slabs). The power of the heterogeneity in at mid-mantle depths is low and nearly flat as a function of spherical harmonic degree up to 𝓁 = 12, with no detectable change near 1000 km. The increase of the power in the lowermost mantle is rather gradual, not characteristic of a discontinuous change. Cross sections of our models at major subduction zones indicate that major downwelling of cold slab material may occur at some locations. On the other hand, there are numerous examples of an abrupt change of the sign of the velocity anomalies across the 670-km discontinuity.

Preferential detection of the Lehmann discontinuity beneath continents

We perform a global survey for the presence of the Lehmann discontinuity using ∼20,000 long-period SS precursors. This data set is highly sensitive to upper mantle reflectors and the coverage is more complete than in previous studies. Our survey indicates that the Lehmann discontinuity is a local feature that is observed under continents more than twice as often as it is observed under oceans. We observe significant variations in travel times and waveforms associated with this shallow mantle reflector, indicating its complexity and lateral depth variations. Little signal is detected on the continent-scale stacks of the SS precursors, which provides further evidence for the intermittent and variable nature of the Lehmann discontinuity.

An investigation of seismicity clustered near the Cordel Field, west central Alberta, and its relation to a nearby disposal well

Historically, seismicity documented in the Western Canada Sedimentary Basin has been relatively quiescent and earthquakes are usually restricted to the foreland belt of the Rocky Mountains. However, exceptional clusters of events, which have remained active for decades, are recognized in Alberta. In this study we investigate the seismicity in this region using data obtained from recently established regional arrays, emphasizing the relationship between a disposal well in the Cordel Field and a nearby cluster of previously reported earthquakes. We explore temporal correlations of wastewater pumping rates and local seismic activity dating back to 1960. We find that the first statistically significant increase in seismicity lags the onset of wastewater injection (October 1991) by ~3.33 years. In particular, the waveform similarity of 32 events are analyzed from continuous data recorded at NOR, a nearby (~30 km) station operated by the University of Alberta starting in September of 2006. Results from this analysis suggest that many events are well correlated in the characteristics of the waveforms and thus are likely to share a similar origin and source mechanism. The most prolific of these multiplets repeats more than 10 times sporadically throughout the entire duration of recorded data from October 2006 to March 2012. Despite the limited availability of nearby stations, which adversely affects the resolution of our analysis, hypocenter depths could be relatively accurately determined from waveform synthesis and double difference methods. The results of our analysis provide first-order evidence that the seismicity is consistent with fluid injection-induced events.

Hydraulic fracturing and the Crooked Lake Sequences: Insights gleaned from regional seismic networks

Within central Alberta, Canada, a new sequence of earthquakes has been recognized as of 1 December 2013 in a region of previous seismic quiescence near Crooked Lake, ~30 km west of the town of Fox Creek. We utilize a cross-correlation detection algorithm to detect more than 160 events to the end of 2014, which is temporally distinguished into five subsequences. This observation is corroborated by the uniqueness of waveforms clustered by subsequence. The Crooked Lake Sequences have come under scrutiny due to its strong temporal correlation (>99.99%) to the timing of hydraulic fracturing operations in the Duvernay Formation. We assert that individual subsequences are related to fracturing stimulation and, despite adverse initial station geometry, double-difference techniques allow us to spatially relate each cluster back to a unique horizontal well. Overall, we find that seismicity in the Crooked Lake Sequences is consistent with first-order observations of hydraulic fracturing induced seismicity.

Probing long bones with ultrasonic body waves

This study uses seismological principles and techniques including waveform simulations and travel time calculations to reveal the nature of ultrasoundwave propagation in long bones at small offsets (source-to-receiver distances) and times. Our experiments confirm the existence of reflected and converted body waves originating from wave interaction at the internal interfaces of a bone structure. We demonstrate that the bone ultrasound responses can be successfully simulated and properly interpreted based on a simple, horizontally layered approximation.

A seismological overview of the induced earthquakes in the Duvernay play near Fox Creek, Alberta

This paper summarizes the current state of understanding regarding the induced seismicity in connection with hydraulic fracturing operations targeting the Duvernay Formation in central Alberta, near the town of Fox Creek. We demonstrate that earthquakes in this region cluster into distinct sequences in time, space, and focal mechanism using (i) cross-correlation detection methods to delineate transient temporal relationships, (ii) double-difference relocations to confirm spatial clustering, and (iii) moment tensor solutions to assess fault motion consistency. The spatiotemporal clustering of the earthquake sequences is strongly related to the nearby hydraulic fracturing operations. In addition, we identify a preference for strike-slip motions on subvertical faults with an approximate 45° P axis orientation, consistent with expectation from the ambient stress field. The hypocentral geometries for two of the largest-magnitude (M ~4) sequences that are robustly constrained by local array data provide compelling evidence for planar features starting at Duvernay Formation depths and extending into the shallow Precambrian basement. We interpret these lineaments as subvertical faults orientated approximately north-south, consistent with the regional moment tensor solutions. Finally, we conclude that the sequences were triggered by pore pressure increases in response to hydraulic fracturing stimulations along previously existing faults.

Crustal imprints of Precambrian orogenesis in western Laurentia

Crustal low-velocity zones (LVZs) have been reported in active orogens such as the Himalayas and the Andes but rarely in stable cratonic regions. In this study, we provide compelling evidence for a significant midcrustal LVZ beneath eastern-central Alberta, an integral part of the Precambrian Canadian Shield covered by thick Phanerozoic sedimentary deposits. This 200 km wide, over 10 km thick midcrustal LVZ is well resolved by shear velocity inversions using P-to-S receiver functions from more than 4600 earthquakes. It is generally overlain by a high-velocity upper crust in the depth range of 8–15 km, especially in western-central Alberta, which coincides with the previously documented Winagami reflection sequence. We interpret the LVZ to be of granitic composition, potentially in connection with the crystallization of partially molten crust during the Paleoproterozoic eon. In addition to the Precambrian tectonic history of western Laurentia, which featured plate convergence conducive to crustal melting, our crustal model is further supported by (1) a moderate spatial correlation between the LVZ and heat flow, and (2) shear velocities consistent with that of granite. The well preserved Winagami reflection sequence and the LVZ are potential evidence of distinct episodes of magmatism and crust modification in the Precambrian basement of the Western Canada Sedimentary Basin. The existence of a broad crustal LVZ suggests extensive subduction, orogenesis, and crustal melting during the Precambrian assembly of the North American craton.

Shear wave reflectivity imaging of the Nazca‐South America subduction zone: Stagnant slab in the mantle transition zone?

[1] In this study we utilize over 5000 SS waveforms to investigate the high-resolution mantle reflectivity structure down to 1200 km beneath the South American convergent margin. Our results indicate that the dynamics of the Nazca subduction are more complex than previously suggested. The 410- and 660-km seismic discontinuities beneath the Pacific Ocean and Amazonian Shield exhibit limited lateral depth variations, but their depths vary substantially in the vicinity of the subducting Nazca plate. The reflection amplitude of the 410-km discontinuity is greatly diminished in a 1300-km wide region in the back-arc of the subducting plate, which is likely associated with a compositional heterogeneity on top of the upper mantle transition zone. The underlying 660-km discontinuity is strongly depressed, showing localized depth and amplitude variations both within and to the east of the Wadati-Benioff zone. The width of this anomalous zone (1000 km) far exceeds that of the high-velocity slab structure and suggesting significant slab deformation within the transition zone. The shape of the 660-km discontinuity and the presence of lower mantle reflectivity imply both stagnation and penetration are possible as the descending Nazca slab impinges upon the base of the upper mantle. Citation: Contenti, S., Y. J. Gu, A. Okeler, and M. D. Sacchi (2012), Shear wave reflectivity imaging of the Nazca-South America subduction zone: Stagnant slab in the mantle transition zone?, Geophys. Res. Lett., 39, L02310, doi:10.1029/ 2011GL050064.

Probing the sources of ambient seismic noise near the coasts of southern Italy

[1] In this study we probe the source of ambient noise in the southern Apennines and the Calabrian Arc by crosscorrelating two months of ambient seismic noise records collected during the Calabria-Apennine-Tyrrhenian/ Subduction-Collision-Acretion Network (CAT/SCAN) project. Significant Rayleigh wave energy is observed on the vertical component of the noise correlation stacks and reveals multiple sources of ambient noise in southern Italy. The most dominant noise sources are found along (1) the Tyrrhenian coast of northern Calabria-southern Campania and (2) the Adriatic Sea near the Gargano Promontory. Enhanced ocean currents evident from buoy records during the study period could be responsible for the observed microseisms. We validate the source locations using earthquake records and the consistency between noise and earthquake correlation functions supports the observed dominant directions of ambient seismic noise. Citation: Gu, Y. J., C. Dublanko, A. Lerner-Lam, K. Brzak, and M. Steckler (2007), Probing the sources of ambient seismic noise near the coasts of southern Italy, Geophys. Res. Lett., 34, L22315, doi:10.1029/2007GL031967.

Hydraulic fracturing volume is associated with induced earthquake productivity in the Duvernay play

Seismicity curbed by lowering volume Determining why hydraulic fracturing (also known as fracking) triggered earthquakes in the Duvernay Formation in Canada is important for future hazard mitigation. Schultz et al. found that injection volume was the key operational parameter correlated with induced earthquakes in the Duvernay. However, geological factors also played a considerable role in determining whether a large injection volume would trigger earthquakes. These findings provide a framework that may lead to better forecasting of induced seismicity. Science, this issue p. 304 Induced seismicity from hydrofracturing in Canada is related to the well fluid injection volumes. A sharp increase in the frequency of earthquakes near Fox Creek, Alberta, began in December 2013 in response to hydraulic fracturing. Using a hydraulic fracturing database, we explore relationships between injection parameters and seismicity response. We show that induced earthquakes are associated with completions that used larger injection volumes (104 to 105 cubic meters) and that seismic productivity scales linearly with injection volume. Injection pressure and rate have an insignificant association with seismic response. Further findings suggest that geological factors play a prominent role in seismic productivity, as evidenced by spatial correlations. Together, volume and geological factors account for ~96% of the variability in the induced earthquake rate near Fox Creek. This result is quantified by a seismogenic index–modified frequency-magnitude distribution, providing a framework to forecast induced seismicity.