Ambient seismic noise tomography of Canada and adjacent regions: Part I. Crustal structures

This paper presents the first continental-scale study of the crust and upper mantle shear velocity (V_s) structure of Canada and adjacent regions using ambient noise tomography. Continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in Canada and adjacent regions were used in the analysis. The higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms. Prominent low velocity anomalies are observed at shallow depths (<20 km) beneath the Gulf of St. Lawrence in east Canada, the sedimentary basins of west Canada, and the Cordillera. In contrast, the Canadian Shield exhibits high crustal velocities. We characterize the crust-mantle transition in terms of not only its depth and velocity but also its sharpness, defined by its thickness and the amount of velocity increase. Considerable variations in the physical properties of the crust-mantle transition are observed across Canada. Positive correlations between the crustal thickness, Moho velocity, and the thickness of the transition are evident throughout most of the craton except near Hudson Bay where the uppermost mantle V_s is relatively low. Prominent vertical V_s gradients are observed in the midcrust beneath the Cordillera and beneath most of the Canadian Shield. The midcrust velocity contrast beneath the Cordillera may correspond to a detachment zone associated with high temperatures immediately beneath, whereas the large midcrust velocity gradient beneath the Canadian Shield probably represents an ancient rheological boundary between the upper and lower crust

The October 2012 magnitude (Mw) 7.8 earthquake offshore Haida Gwaii, Canada

Alison L. Bird et al. report on the Mw 7.8 earthquake offshore Haida Gwaii, Canada, from 2012 for the Summary of the Bulletin of the International Seismological Centre

A High-Resolution Study of Seismogenic Structures: The Making of a Double Seismic Zone

170 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.Precise source parameters, including focal depths and mechanisms, of 74 large to moderate-sized (m\sb{b} \ge 5.5) earthquakes that occurred along the Kuril-Kamchatka arc between 1963 and 1991 were determined by inverting teleseismic P and SH waveforms. Based on these results, I delineated seismogenic structures at depths shallower than 200 km and proposed interpretations for systematic variation of such structures with depth and along the strike of the arc.Along the down-dip direction in central Kuril, a transitional thrust zone occurs between depths of $\sim$30-50 km, connecting a classic double seismic zone with the interplate thrust zone. The transitional zone is characterized by earthquakes of thrust faulting with a westerly dipping nodal plane of $\sim$40\sp\circ, $\sim$10-15\sp\circ steeper than that of typical low-angle thrust earthquakes along the plate interface. Waveform data suggest that the source region of transitional thrust faulting is within a crust-like low velocity zone of $\sim$10 km in thickness. I interpreted that the transitional thrust earthquakes occurred beneath the plate interface, within subducted crust of the Pacific plate, and that these events may indicate a locked plate interface at depths of 30-50 km.Northeast of 53\sp\circN, the double seismic zone turns into a single zone under compression. The changeover is caused by disappearing of the lower (extensional) layer of the double zone. Similarly, down-dip compressional earthquakes were not found southwest of 46\sp\circC. It is uncertain, however, whether the single extensional seismic zone is a southwestern extension of the lower layer of the double seismic zone. Lateral variation of this Wadati-Benioff zone can be explained by a superposition of two factors: A conventional model for double seismic zones and an additional component of stress transmitted from deep slab. Toward the northern end of the arc, compression transmitted from below overshadows extension in the lower layer of the double seismic zone caused by effects such as unbending. A similar reasoning applies to the southwestern termination of the double seismic zone.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Moment-tensor inversion for offshore earthquakes east of Taiwan and their implications to regional collision

[[sponsorship]]地球科學研究所[[note]]已出版;[SCI];有審查制度;具代表性[[note]]http://gateway.isiknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=Drexel&SrcApp=hagerty_opac&KeyRecord=0094-8276&DestApp=JCR&RQ=IF_CAT_BOXPLO

Imaging the Source Region of Deep Low-Frequency Events using the Source-Scanning Algorithm

深部低周波(DLF) イベントの震源域をイメージングするために， Source-ScanningAlgorithm (SSA法)の適用を検討した。SSA法では，考察している時空間内に格子点を設定し，観測された地震波形データをある条件で足し合わせることにより，各格子点で発生した地震波の振幅の分布状態，すなわち震源域のイメージングを行なう。この方法では，P波やS波等の特定の位相を同定する必要がないため，これらの検測が困難なDLFイベントに効果的である。本稿では，テストデータや通常の地震，さらに実際のDLFイベントにSSA法を適用して検討を行なった。We demonstrated the Source-Scanning Algorithm (SSA) to image the source region of the deep low-frequency (DLF) events. It is difficult to locate DLF events precisely using conventional hypocenter determination method such as using P- and S-wave arrivals, because of emergent onsets of those phases. The SSA method just stacks observed seismograms to locate source region of the events, without using the arrival time information of particular phases such as P or S. Therefore, the SSA method is a powerful tool to locate the events with emergent onsets such as DLF events. We applied the method to the synthetic and real seismic data to demonstrate the potential of the method.深部低周波(DLF) イベントの震源域をイメージングするために， Source-ScanningAlgorithm (SSA法)の適用を検討した。SSA法では，考察している時空間内に格子点を設定し，観測された地震波形データをある条件で足し合わせることにより，各格子点で発生した地震波の振幅の分布状態，すなわち震源域のイメージングを行なう。この方法では，P波やS波等の特定の位相を同定する必要がないため，これらの検測が困難なDLFイベントに効果的である。本稿では，テストデータや通常の地震，さらに実際のDLFイベントにSSA法を適用して検討を行なった。We demonstrated the Source-Scanning Algorithm (SSA) to image the source region of the deep low-frequency (DLF) events. It is difficult to locate DLF events precisely using conventional hypocenter determination method such as using P- and S-wave arrivals, because of emergent onsets of those phases. The SSA method just stacks observed seismograms to locate source region of the events, without using the arrival time information of particular phases such as P or S. Therefore, the SSA method is a powerful tool to locate the events with emergent onsets such as DLF events. We applied the method to the synthetic and real seismic data to demonstrate the potential of the method

Significant geometric variation of the subducted plate beneath the northernmost Cascadia subduction zone and its tectonic implications as revealed by the 2014 M 6.4 earthquake sequence

Highlights • Hypocenters within the subducted Explorer plate indicate slab deformation. • The oceanic slab is bending downward toward the northwest. • A complex sequence of focal mechanisms also indicates plate deformation. • Decreased seismic activity in the overriding plate indicates decoupling to the NW. • Deformation and decoupling could limit megathrust rupture propagation. Abstract At the northernmost extent of the Cascadia subduction zone, the Explorer plate subducts at approximately 2 cm/yr, less than half the rate of the Juan de Fuca plate to the south. The boundary between these two plates is known as the Nootka fault zone, which is one of the focuses of the Seafloor Earthquake Array Japan-Canada Cascadia Experiment (SeaJade). During this survey, an 6.4 earthquake occurred on 24 April 2014. This event and the subsequent aftershocks (referred to as the Nootka Sequence) reveal an approximately 40-km-long subducted fault within the Explorer Plate to the north of the Nootka fault zone. We infer that the fault is a subducted conjugate fault because of its nearly identical orientation to those seaward of the subduction front within the Nootka fault zone. The depth distribution and focal mechanisms of the aftershocks indicate significant margin-parallel deformation of the subducting plate. The subduction interface at the Nootka Sequence fault has been deflected downward to the northwest from a depth of approximately 15 – 25 km over a distance of 25 km. We propose two possible scenarios that are modified from previously suggested slab-tear model with induced margin-parallel mantle flow to explain the significant deformation of the young, warm subducting Explorer plate. To the northwest of this change in slab geometry, a lack of seismic activity above the plate interface indicates that the Explorer plate has partially decoupled from the overriding North America plate. We conclude that the geometric variation separating the southern Explorer plate from the north, along with decoupling and a possible intraslab tear, may be a significant combination to resist the propagation of a megathrust rupture across this boundary