Michael Bostock

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Seismic velocity structure under Vancouver Island from travel time inversion : insight from Low Frequency Earthquakes (2018)

The Cascadia subduction zone is a warm subduction zone where Episodic Tremor and Slow Slip(ETS), a phenomenon with debated governing physical mechanisms, is observed. The eventualdevelopment of an accurate physical model of ETS relies on a comprehensive set of seismic andgeodetic observations. Low Frequency Earthquakes (LFE) are one of the seismic signaturesthat accompany slow slip and possess discernible phase arrivals that can be used for traveltimeinversion methods. I first address the issue of detecting LFEs by developing an automatedalgorithm that exploits cross-station waveform similarity. The algorithm detects thousands ofevents per ETS episode in southern Vancouver Island that reveal pronounced spatiotemporalclustering and complex propagation patterns that compare favourably with independent ob-servations in Cascadia. This method allows us to build a significantly improved catalogue ofLFE templates that we next employed for local double-difference earthquake tomography withthe aim of resolving structures in the vicinity of ETS and metamorphic controls on its generation. In southern Vancouver Island, tomographic images reveal high Poisson’s ratios associatedwith a dipping low‐velocity zone (LVZ) inferred to be overpressured, upper oceanic crust ofthe Juan de Fuca plate where LFEs and other slow‐slip phenomena occur. We also observe alow Poisson’s ratio anomaly ( ∼0.225) in the forearc continental crust above the mantle wedgecoinciding with high Vp and high levels of clustered microseismicity. We develop a conceptualmodel where quartz concentration is produced by metasomatism in the forearc crust and catalyzed by a focussed ingress of slab-derived fluids at high pore pressure. In northern VancouverIsland, we also observe the LVZ and a low Poisson’s ratio anomaly in a similar position, but noassociated microseismic activity. Distinct, parallel lineaments that trend slightly oblique to thestrike of the megathrust are resolved within seismicity concentrations of the Nootka Fault Zoneimmediately offshore Nootka Island. We speculate that these features may represent faultingdeveloped as a result of high strains associated with slab bending, amplified in the vicinity ofthe newly formed plate boundary. We also find possible evidence for a deep-seated, seismogenicfault near the Explorer Plate edge. Supplementary materials available at: http://hdl.handle.net/2429/68898

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Seismic detection of rockfalls on railway lines (2016)

Railway operators mitigate the risk of derailments caused by hazardous rocks falling onto the track by installing slide detector fences (SDF). These consist of electrical sensing wires strung on poles located uphill of the track; falling rocks snap these wires and trigger an alarm. Rocks of non-threatening size and migrating animals frequently break the wires causing prolonged false alarms and delaying rail traffic until the SDF is manually repaired, often in a hazardous environment. This thesis is concerned with the development of a prototype of the autonomous Seismic Rockfall Detection System (SRFDS) as a potential replacement for the SDF. Analysis and classification of natural and anthropogenic seismic signals which have been observed at the SRFDS field installations, is presented. A method for identification of hazardous rocks (>0.028 m³) using an empirical peak ground velocity attenuation model is outlined. Pattern recognition techniques which are based on cross-correlation and on variations in the short-term / long term averages of the ground vibrations are introduced for rail traffic identification and rockfall detection. The techniques allow the SRFDS to eliminate false activations by rail traffic, report hazardous rocks with minimum (
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Microseismic source inversion in anisotropic media (2014)

Sedimentary rocks and shales in particular are known to be anisotropic, sometimes strongly so, and hydraulic fracturing is now common practice in shale plays to enhance the extraction of hydrocarbons. One of the ways to understand the hydraulic fracturing process is through the micro--earthquakes that it generates; it is therefore of interest to study the impact that anisotropy may have on hydraulically induced seismicity. This thesis is concerned with the inclusion of anisotropy into the geophysical forward and inverse problems of microseismic sources.Ray theory is used for the forward problem --- dynamic ray tracing in a medium composed of homogeneous layers with vertical transverse isotropy (VTI) is used and the possibility of qSv triplication is considered. Novel approaches to the inverse problem are introduced, including waveform fitting in the frequency domain to recover the source function as well as moment tensor. Uncertainties in estimated moment tensor components are quantified with multi--variate normal sampling utilizing the full covariance matrix from the linear inversion. A new decomposition of the moment tensor is described that removes the distortions due to anisotropy local to the source and a new way to visualize an earthquake source is also introduced. The impact of anisotropy on moment tensor inversion and decomposition is shown to be significant.Field data recorded by two downhole arrays of multicomponent receivers are analyzed using the new techniques. The event collection suggests a source mechanism dominated by cylindrical dilatation. This is unexpected but is supported by results from another code. Future directions include extensions to lower symmetry forms of anisotropy and application to surface arrays. Preliminary analysis of downhole recordings of aftershocks of the 2008 Mw=7.9 Wenchuan earthquake is shown.

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Studies of seismic deconvolution and low-frequency earthquakes (2014)

Deconvolution of seismic data is an important component of signal processing that aims to remove the seismic source from seismograms, thereby isolating the Green’s function. By considering seismograms of multiple earthquakes from similar locations recorded at a given station and that therefore share the same Green’s function, we investigate a system of equations where the unknowns are the sources and source durations. Our solution is derived using direct linear inversion to recover the sources and Newton’s method to recover source durations. For the short seismogram durations considered, we are able to recover source time functions for noise levels at 1% of the direct P -wave amplitude. However, the nonlinearity of the problem renders the system expensive to solve and sensitive to noise; therefore consideration is limited to short seismograms with high signal-to-noise ratio (SNR). When SNR levels are low, but a large multiplicity of seismograms representing a common source-receiver path are available, we can apply a different deconvolution approach to recover the Green’s function. In an application to tectonic tremor in northern Cascadia, we implement an iterative blind deconvolution method that involves correlation, threshold detection and stacking of 1000’s of low frequency earthquakes (LFEs) that form part of tremor to generate templates that can be considered as empirical Green’s functions. We exploit this identification to compute hypocentres and moment tensors. LFE hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth. The majority of mechanisms are consistent with shallow thrusting in the direction of plate motion. We analyze the influence of ocean tides on the triggering of LFEs and find a spatially variable sensitivity to tidally induced up-dip shear stress (UDSS), suggesting that tidal sensitivity must partially depend on laterally heterogeneous physical properties. The majority of LFEs fail during positive and increasing UDSS, consistent with combined contributions from background slow slip and from tides acting directly on LFEs. We identified rapid tremor reversals in southern Vancouver Island with higher sensitivity to UDSS than the main front and which at least partially explains an observed increase in LFE sensitivity to UDSS with time.

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Improved teleseismic Green's functions and western Canada mantle structure and evolution (2009)

The present thesis is divided into three distinct parts and focuses both on the improvement of deconvolution technique in a teleseismic context for crustal and upper-mantle studies and on the understanding of western Canada structure and evolution through seismic imaging. The first part presents estimates of the P-component of the teleseismic-P Green's functions for three stations of the Canadian National Seismic Network (CNSN) obtained using a new deconvolution technique. Our results show evidence of the principal, first-order scattered Moho phases and, in particular, the Pp_Mp. The second part presents teleseismic receiver functions from 20 broadband three-component seismometers deployed along the MacKenzie-Liard Highway in Canada's Northwest Territories as part of the joint Lithoprobe-IRIS CAnadian NOrthwest Experiment (CANOE). These stations traverse a Paleoproterozoic suture and subduction zone that has been previously documented in detail to mantle depths using seismic reflection profiling. Our results reveal the response of the ~1.8 Ga subduction zone on both the radial and transverse component. The identification of this structure and its comparison with fine-scale mantle layering below the adjacent Slave province and from a range of Precambrian terranes provides an unambiguous connection between fossil subduction and fine-scale, anisotropic mantle layering found beneath cratons. Previous documentation of similar layering below the adjacent Slave province provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction. The last part presents P- and S wave velocity models for western Canada. In this part, we focus our attention on two distinct features: 1) the transition from Phanerozoic to cratonic mantle in northwestern Canada and 2) the complex tectonic environment at the northern terminus of the Cascadia subduction zone where the plate boundary changes from convergent to transform. We find that the main transition from Phanerozoic to cratonic mantle in northwestern Canada occurs at the Cordilleran deformation front. In northern Cascadia, we have imaged and characterized the signature of the subducting Juan de Fuca plate and observed evidence of subduction beyond the northern edge of the slab. Our result show that the Anahim hotspot track is underlain by a -2% low-velocity zone.

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Seismic absorption estimation and compensation (2009)

As seismic waves travel through the earth, the visco-elasticity of the earth's medium will cause energy dissipation and waveform distortion. This phenomenon is referred to as seismic absorption or attenuation. The absorptive property of a medium can be described by a quality factor Q, which determines the energy decay and a velocity dispersion relationship. Four new ideas have been developed in this thesis to deal with the estimation and application of seismic absorption. By assuming that the amplitude spectrum of a seismic wavelet may be modeled by that of a Ricker wavelet, an analytical relation has been derived to estimate a quality factor from the seismic data peak frequency variation with time. This relation plays a central role in quality factor estimation problems. To estimate interval Q for reservoir description, a method called reflectivity guided seismic attenuation analysis is proposed. This method first estimates peak frequencies at a common midpoint location, then correlates the peak frequency with sparsely-distributed reflectivities, and finally calculates Q values from the peak frequencies at the reflectivity locations. The peak frequency is estimated from the prestack CMP gather using peak frequency variation with offset analysis which is similar to amplitude variation with offset analysis in implementation. The estimated Q section has the same layer boundaries of the acoustic impedance or other layer properties. Therefore, the seismic attenuation property obtained with the guide of reflectivity is easy to interpret for the purpose of reservoir description. To overcome the instability problem of conventional inverse Q filtering, Q compensation is formulated as a least-squares (LS) inverse problem based on statistical theory. The matrix of forward modeling is composed of time-variant wavelets. The LS de-absorption is solved by an iterative non-parametric approach. To compensate for absorption in migrated seismic sections, a refocusing technique is developed using non-stationary multi-dimensional deconvolution. A numerical method is introduced to calculate the blurring function in layered media, and a least squares inverse scheme is used to remove the blurring effect in order to refocus the migrated image. This refocusing process can be used as an alternative to regular migration with absorption compensation.

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Seismic and mechanical attributes of lithospheric deformation and subduction in western Canada (2008)

Convergent continental margins are regions of intense deformation caused by the interaction of oceanic plates with continents. The spatial extent of deformation is broadly commensurate with the specific time scale of the causative phenomenon. For example, subduction-related short-term deformation is limited to
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Master's Student Supervision (2010 - 2018)
Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap (2018)

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutatplateau, but their origins are unclear due to lack of depth constraints. We haveprocessed tremor recordings to extract low-frequency earthquakes (LFEs), andgenerated a set of six LFE waveform templates via iterative network matched filteringand stacking. The timing of impulsive P (compressional) wave and S (shear) wavearrivals on template waveforms places LFEs at 40–58 km depth, near the upperenvelope of intraslab seismicity and immediately updip of increased levels ofintraslab seismicity. S waves at near-epicentral distances display polarities consistentwith shear slip on the plate boundary. We compare characteristics of LFEs, seismicity,and tectonic structures in central Alaska with those in warm subduction zones, andpropose a new model for the region’s unusual intraslab seismicity and the enigmaticDenali volcanic gap (i.e., an area of no volcanism where expected). We argue thatfluids in the Yakutat plate are confined to its upper crust, and that shallow subductionleads to hydromechanical conditions at the slab interface in central Alaska akin tothose in warm subduction zones where similar LFEs and tremor occur. Theseconditions lead to fluid expulsion at shallow depths, explaining strike-parallelalignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack ofdouble seismic zone and restriction of deep intraslab seismicity to a persistentlow-velocity zone are simple consequences of anhydrous conditions prevailing in thelower crust and upper mantle of the Yakutat plate.

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Slip behaviour of the Queen Charlotte plate boundary before and after the 2012, Mw 7.8 Haida Gwaii earthquake : evidence from repeating earthquakes (2017)

The Queen Charlotte plate boundary, near Haida Gwaii, B.C., includes the dextral, strike-slip, Queen Charlotte Fault (QCF) and the subduction interface between the downgoing Pacific and overriding North American plates. In this dissertation, we present a comprehensive repeating earthquake catalogue that represents an effective slip meter for both faults in the area. The catalogue comprises 730 individual earthquakes (0.3
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Crustal anisotropy in a subduction zone forearc : Northern Cascadia (2014)

S-wave splitting analyses using high signal-to-noise ratio low frequency earthquake (LFE) templates at 3-component stations across southern Vancouver Island (SVI) and northern Washington indicate the presence of a heterogeneous distribution of crustal anisotropy in the North American plate. For SVI, we investigate the contribution to anisotropy from the Leech River Complex (LRC), an allochthonous terrane comprised of strongly foliated greenschist facies phyllites and amphibolite facies schists with steeply dip- ping foliations striking E-W. On SVI, estimates of initial S-wave polarization direction are consistent with predictions from radiation patterns generated by LFE focal mechanisms, providing corroboration for thrust mechanisms at the plate boundary. Fast directions across mainland SVI are subparallel to the dominant foliation direction in the LRC. Increases in depth normalized delay times from east to west, combined with small-scale azimuthal varia- tions in fast directions suggest a heterogeneous distribution of anisotropy. We test azimuthally anisotropic LRC models based upon analyses of geological fabric and geometrically constrained by reflection studies, through forward modeling using 3D spectral element method (SEM) simulations. The preferred model of a north/northeast shallowly dipping wedge of LRC material with varying orientations of anisotropy terminating at mid crustal levels is able to recreate mean and azimuthal variations in fast directions along with variations in delay times, thereby supporting the hypothesis of the LRC as a primary contributor to crustal anisotropy beneath SVI. For select stations where anisotropic LRC models do not recreate observations, fast directions are subparallel to local estimates of maximal compressive horizontal stress, suggesting fluid-filled cracks could be a source of anisotropy. We refute the idea that anisotropy along mainland SVI is primarily due to stress related cracks as has been suggested by prior studies. Fast directions at stations on northern Washington exhibit variations with azimuth and incidence angle suggesting complex anisotropy interpreted as due to a combination of cracks and preferred mineral orientation of metamorphosed slates of the Olympic core rocks. These slates may also underlay stations on SVI and represent another source of anisotropy.

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Nature of the low velocity zone in Cascadia from receiver function waveform inversion (2011)

Dipping low-velocity zones (LVZs) are a ubiquitous structural element of subduction zones worldwide. In this study we map seismic attributes characterizing the LVZ beneath the Cascadia subduction zone from northern Vancouver Island to northern California using receiver function waveform inversion. Throughout this region, the LVZ is characterized by high Vp/Vs ratios (mean=2.77), strong S-velocity contrasts (~50%) and thicknesses averaging 3.38 km. The LVZ is immediately underlain by a second, weaker layer exhibiting moderate Vp/Vs ratios (mean=1.85) with mean thickness of 4.62 km. We interpret the combined structure in terms of subducting oceanic crust, based on classical structural/petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores. The LVZ is identified with pervasively hydrated, high porosity pillow basalts and sheeted dikes of Layer 2 with possible contributions from sediments (Layer 1). Fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above, and a low porosity, gabbroic/mafic-cumulate dominated Layer 3 below.

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