ALBERT

Description: We systematically relocated the 2012 Haida Gwaii aftershock sequence by re-examining continuous seismic waveforms of the Canadian National Seismographic Network (CNSN). Because of the sparse station density in the source area and the offshore location of the majority of events, precise determination of hypocenters is extremely challenging. Constraints on the back azimuth and incident angle of ray paths were derived from the cross correlation of three-component waveforms and incorporated into the locating process to improve the results. We located 1228 aftershocks for the first week after the mainshock, effectively double the size of the CNSN routine earthquake catalog. The distribution of aftershocks tends to form two linear trends roughly parallel to the strike of the plate margin in the northwest–southeast direction. The trend located updip from the mainshock within the subducting Pacific plate appears to have three clusters spanning a lateral distance of ~80 km. The other trend coincides with the surface trace of the Queen Charlotte fault (QCF), spanning the northern two-thirds of the main rupture zone. The overall area of aftershock distribution is ~120 km x ~40 km. Aftershocks overlap the southern part of the estimated rupture zone of the 1949 earthquake, with no activity in the seismic gap farther to the south. Therefore, it is likely that the 2012 earthquake only partially released the accumulated stress on the strike-slip transform fault system. In this scenario, the possibility of a major strike-slip earthquake along the southernmost part of the QCF should not be ignored. Online Material: Table of source parameters of relocated mainshock and aftershocks.

Description: All the significant ( M L ≥4) events in the 2012 Haida Gwaii earthquake sequence are systematically relocated, and their moment tensor solutions are determined from waveform inversion. The focal mechanism of the mainshock shows low-angle thrust faulting along a shallowly dipping plane with a strike parallel to the Queen Charlotte fault (QCF), consistent with the inference of Pacific plate underthrusting beneath the overriding North American plate. The epicenter of the mainshock is located ~5 km landward (northeast) of the surface trace of the QCF, suggesting the nucleation of the rupture was near the bottom of the seismogenic (locked) interface. Significant aftershocks appear to cluster on the periphery of the main rupture zone with most events located immediately seaward of the deformation front. The majority of these events show normal-faulting mechanisms that are probably associated with the bending stress within the Pacific plate near the deformation front. Several normal and strike-slip events at greater depths within the subducted Pacific slab show a consistent pattern of T -axis in the down-dip direction, implying the subducted plate is under a stress regime of down-dip extension. Only a few strike-slip events were observed along or near the QCF. The limited size and distribution of these events suggest that most of the elastic strain accumulated along the QCF was not released during the 2012 Haida Gwaii sequence. Major strike-slip earthquakes are likely to occur along the southernmost part of the QCF system in the future.

Description: 〈span〉〈div〉ABSTRACT〈/div〉On 30 November 2018, three felt earthquakes occurred in the Septimus region of northeast British Columbia in an area where hydraulic fracturing was in progress. The proximity of oil and gas activities to populated areas and to critical infrastructure including major dams raises significant concern regarding the seismic hazard posed by moderate induced events and motivates study of their ground motions. Here, we analyze the ground‐motion amplitudes from these events recorded between 3 and 400 km. We use three‐component waveforms from 45 seismometer and accelerometer sensors to analyze the observed ground motions. The moment magnitude (Mw) of the first event is estimated as 4.6 using the vertical pseudoresponse spectral acceleration (PSA) based on the relations provided by 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉. The Mw for the two smaller earthquakes are 3.5 and 4.0. The intensity of shaking from the Mw 4.6 and 4.0 events generally exceeded modified Mercalli intensity (MMI) VI at distances 〈6  km. The maximum duration above the MMI VI threshold at the closest station (3.5 km distance) from the mainshock is 1.6 s. The observed ground motions agree with the ground‐motion prediction equation (GMPE) of 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉 for induced events in Oklahoma, with attenuation modified to match that for the study region, assuming typical regional site amplification. The inferred value of stress drop for the mainshock and the largest aftershock is approximately 50 bars based on the agreement of observed PSA values with the 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉 GMPE.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉In this study, we take a close look at the constituents of the 〈a href="https://pubs.geoscienceworld.org/srl#rf20"〉Richter (1935)〈/a〉 relation for calculation of local magnitude (ML), which is the basis for magnitude determination by Natural Resources Canada (NRCan) in the western Canada sedimentary basin (WCSB). Using a comprehensive catalog of Wood–Anderson amplitudes from earthquakes in northeast British Columbia and western Alberta, we first compare the distance correction terms −log(A0) for the Richter magnitude scale previously obtained for WCSB and several other regions. We also formulate a new correction term specifically for NRCan’s routine ML calculation that better accounts for the attenuation of direct and refracted waves from events within WCSB. Based on a bilinear model for ground‐motion attenuation, our −log(A0) is {0.7974×log(Rhypo100)+0.0016×(Rhypo−100)+3.0 Rhypo≤85  km−0.1385×log(Rhypo100)+0.0016×(Rhypo−100)+3.0 Rhypo〉85  km,in which Rhypo is the hypocentral distance. Our −log(A0) results in lower ML by an average of 0.29, 0.27, 0.12, and 0.34 units, respectively, compared with those obtained by 〈a href="https://pubs.geoscienceworld.org/srl#rf21"〉Richter (1958〈/a〉; California), 〈a href="https://pubs.geoscienceworld.org/srl#rf15"〉Hutton and Boore (1987〈/a〉; California), 〈a href="https://pubs.geoscienceworld.org/srl#rf11"〉Brazier 〈span〉et al.〈/span〉 (2008〈/a〉; Ethiopian plateau), and 〈a href="https://pubs.geoscienceworld.org/srl#rf9"〉Bona (2016〈/a〉; Italy) over all distances, but gives higher ML values than those obtained by 〈a href="https://pubs.geoscienceworld.org/srl#rf30"〉Yenier (2017〈/a〉; WCSB), with an average of 0.12 unit over all distances. The difference between our ML calculation and 〈a href="https://pubs.geoscienceworld.org/srl#rf30"〉Yenier (2017)〈/a〉 is more significant for Rhypo≤50  km (0.27 unit) and varies slightly for larger Rhypo: 0.08 unit for 50  km〈Rhypo≤100  km, 0.12 for 100  km〈Rhypo≤200  km, and 0.10 for Rhypo〉200  km.〈/span〉

Description: Offshore seismicity at the Cascadia margin is poorly constrained because nearly all previous recordings of earthquakes were made using land-based networks. We conducted earthquake monitoring off Vancouver Island in northern Cascadia using ocean-bottom seismographs. Our results show that most of the offshore seismicity is concentrated along the Nootka fault zone. Otherwise seismicity is extremely low, with no earthquakes located along the shallow, seismogenic part of the megathrust. The lack of interplate seismicity may indicate complete healing and locking of the megathrust over three centuries after the great earthquake of 1700 and a somewhat lower degree of structure heterogeneity, such as subducting seamounts. Events along the Nootka fault zone occur over a 10–15 km depth range. This wide distribution and the previously reported overall moment release rate suggest that a significant part of deformation of this fault zone is aseismic. Several earthquakes beneath the continental shelf may be related to faults dividing tectonic terrains within the overriding plate.

Description: 〈span〉〈div〉ABSTRACT〈/div〉On 30 November 2018, three felt earthquakes occurred in the Septimus region of northeast British Columbia in an area where hydraulic fracturing was in progress. The proximity of oil and gas activities to populated areas and to critical infrastructure including major dams raises significant concern regarding the seismic hazard posed by moderate induced events and motivates study of their ground motions. Here, we analyze the ground‐motion amplitudes from these events recorded between 3 and 400 km. We use three‐component waveforms from 45 seismometer and accelerometer sensors to analyze the observed ground motions. The moment magnitude (Mw) of the first event is estimated as 4.6 using the vertical pseudoresponse spectral acceleration (PSA) based on the relations provided by 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉. The Mw for the two smaller earthquakes are 3.5 and 4.0. The intensity of shaking from the Mw 4.6 and 4.0 events generally exceeded modified Mercalli intensity (MMI) VI at distances 〈6  km. The maximum duration above the MMI VI threshold at the closest station (3.5 km distance) from the mainshock is 1.6 s. The observed ground motions agree with the ground‐motion prediction equation (GMPE) of 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉 for induced events in Oklahoma, with attenuation modified to match that for the study region, assuming typical regional site amplification. The inferred value of stress drop for the mainshock and the largest aftershock is approximately 50 bars based on the agreement of observed PSA values with the 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉 GMPE.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The availability of abundant digital seismic records and successful application of deep learning in pattern recognition and classification problems enable us to achieve a reliable earthquake detection framework. To overcome the limitations and challenges of conventional methods, which are mainly due to an incomplete set of template waveforms and low signal‐to‐noise ratio, we design a generalized model to improve discrimination between earthquake and noise recordings using a deep convolutional network (ConvNet). Exclusively based on a dataset of over 4900 earthquakes recorded over a period of 3 yrs in western Canada, a multilayer ConvNet is trained to learn general characteristics of background noise and earthquake signals in the time–frequency domain. In the next step, we train a secondary network using the wavelet transform of the major seismic arrivals to separate 〈span〉P〈/span〉 from 〈span〉S〈/span〉 waves and estimate their approximate arrival times. The results of validation experiments demonstrate promising performance and achieve an average accuracy of nearly 99% for both networks. To investigate the applicability of our algorithm, we apply the trained model on an independent dataset recently recorded in northeastern British Columbia (NE BC). It is found that deep‐learning‐based methods are superior to traditional techniques in detecting a higher number of seismic events at significantly less computational cost.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We investigate the spatiotemporal pattern of crustal anisotropy in the source area of the 2004 Niigata earthquake (〈strong〉M〈/strong〉 6.8) that occurred in the northern segment of the Niigata–Kobe tectonic zone, central Japan, by measuring shear‐wave splitting parameters from waveform data of local earthquakes. Our results show that the fast polarization directions in the upper crust have spatial variations across the region of the earthquake that are likely caused by both structural and stress field effects. The northwest–southeast direction near the northeastern end of the source zone (beneath station N.NGOH) and the east–west direction to the southwest (beneath station N.KWNH) are consistent with the spatial variation of the orientation of the maximum compression of the local stress field. Fast polarization directions at other stations tend to align in the directions of active faults and folds and thus are considered to be structure induced. These spatial patterns were unaffected by the earthquake. However, at two stations (N.NGOH and N.KWNH) we observe an increase in both the average and scatter of the normalized delay times (δt) during the aftershock period. In addition, two stations (HIROKA and N.YNTH) that are located in the strike‐normal direction east of the source area show an increase in the average of the normalized δt and a rotation of up to 90° of the fast direction immediately after the mainshock. We also notice that stations located very close to the source fault (DP.YMK and DP.OJK) show larger average delay times compared with stations farther away (HIROKA and N.YNTH) during the postseismic stage. To explain the temporal changes in the strength of the anisotropy, we speculate that spatiotemporal variations in microcrack development in and around the source area could be caused by static stress changes due to tectonic deformation and the earthquake rupture.〈/span〉

Description: On 9 October 2007, an unusual sequence of earthquakes began in central British Columbia about 20 km west of the Nazko cone, the most recent (circa 7200 yr) volcanic center in the Anahim volcanic belt. Within 25 hr, eight earthquakes of magnitude 2.3-2.9 occurred in a region where no earthquakes had previously been recorded. During the next three weeks, more than 800 microearthquakes were located (and many more detected), most at a depth of 25-31 km and within a radius of about 5 km. After about two months, almost all activity ceased. The clear P- and S-wave arrivals indicated that these were high-frequency (volcanic-tectonic) earthquakes and the b value of 1.9 that we calculated is anomalous for crustal earthquakes but consistent with volcanic-related events. Analysis of receiver functions at a station immediately above the seismicity indicated a Moho near 30 km depth. Precise relocation of the seismicity using a double-difference method suggested a horizontal migration at the rate of about [IMG]/medium/1732eq1.gif" ALT="Formula "〉, with almost all events within the lowermost crust. Neither harmonic tremor nor long-period events were observed; however, some spasmodic bursts were recorded and determined to be colocated with the earthquake hypocenters. These observations are all very similar to a deep earthquake sequence recorded beneath Lake Tahoe, California, in 2003-2004. Based on these remarkable similarities, we interpret the Nazko sequence as an indication of an injection of magma into the lower crust beneath the Anahim volcanic belt. This magma injection fractures rock, producing high-frequency, volcanic-tectonic earthquakes and spasmodic bursts.

Description: 〈span〉〈div〉ABSTRACT〈/div〉In this study, we take a close look at the constituents of the 〈a href="https://pubs.geoscienceworld.org/srl#rf20"〉Richter (1935)〈/a〉 relation for calculation of local magnitude (ML), which is the basis for magnitude determination by Natural Resources Canada (NRCan) in the western Canada sedimentary basin (WCSB). Using a comprehensive catalog of Wood–Anderson amplitudes from earthquakes in northeast British Columbia and western Alberta, we first compare the distance correction terms −log(A0) for the Richter magnitude scale previously obtained for WCSB and several other regions. We also formulate a new correction term specifically for NRCan’s routine ML calculation that better accounts for the attenuation of direct and refracted waves from events within WCSB. Based on a bilinear model for ground‐motion attenuation, our −log(A0) is {0.7974×log(Rhypo100)+0.0016×(Rhypo−100)+3.0 Rhypo≤85  km−0.1385×log(Rhypo100)+0.0016×(Rhypo−100)+3.0 Rhypo〉85  km,in which Rhypo is the hypocentral distance. Our −log(A0) results in lower ML by an average of 0.29, 0.27, 0.12, and 0.34 units, respectively, compared with those obtained by 〈a href="https://pubs.geoscienceworld.org/srl#rf21"〉Richter (1958〈/a〉; California), 〈a href="https://pubs.geoscienceworld.org/srl#rf15"〉Hutton and Boore (1987〈/a〉; California), 〈a href="https://pubs.geoscienceworld.org/srl#rf11"〉Brazier 〈span〉et al.〈/span〉 (2008〈/a〉; Ethiopian plateau), and 〈a href="https://pubs.geoscienceworld.org/srl#rf9"〉Bona (2016〈/a〉; Italy) over all distances, but gives higher ML values than those obtained by 〈a href="https://pubs.geoscienceworld.org/srl#rf30"〉Yenier (2017〈/a〉; WCSB), with an average of 0.12 unit over all distances. The difference between our ML calculation and 〈a href="https://pubs.geoscienceworld.org/srl#rf30"〉Yenier (2017)〈/a〉 is more significant for Rhypo≤50  km (0.27 unit) and varies slightly for larger Rhypo: 0.08 unit for 50  km〈Rhypo≤100  km, 0.12 for 100  km〈Rhypo≤200  km, and 0.10 for Rhypo〉200  km.〈/span〉