We describe the regular pre-COVID mode of operations for the Canadian National Seismograph Network and the associated monitoring, alerting, and analysis for earthquakes in Canada; we describe how the current operational posture evolved and discuss the ways in which the posture was and was not suitable to respond to the challenges and constraints of the COVID-19 situation in Canada. We find that many of the design and operation decisions that have been taken over the last several decades for earthquake monitoring in Canada, collectively driven largely by considerations of resilience and cost-effectiveness and further refined after the experience of the H1N1 pandemic, resulted in a system that continued to function effectively under lockdown conditions. There were many earthquakes in Canada that required seismologist response during the lockdown, all of which were handled remotely without issue. Specific challenges and lessons learned from the first few months of the pandemic are noted.

The Canadian National Seismograph Network (CNSN) operated by Natural Resources Canada consists of approximately 200 stations. Data from this network are used to produce the national earthquake catalog, to provide alerts in the aftermath of an earthquake, to develop the national seismic hazard maps, and for research within Canada and internationally. A significant upgrade to the instrumentation and infrastructure of the CNSN, which began in 2014, is nearing completion. The newly refurbished network is uniform in terms of instrumentation with the remaining single-component short-period stations converted to three-component broadband stations and consistent sampling rates across the network. Strong-motion instruments are now collocated with weak-motion instruments at many sites in all regions of Canada, and there is also a significant increase in the number of stand-alone strong-motion sites. Improvements in telecommunications were aimed at improving reliability and decreasing latency. All upgraded stations undergo a series of quality checks before the data are approved for dissemination. Data from the CNSN are freely available to the seismological community and the general public.

Some of the most dramatic effects of climate change have been observed in the Earth's polar regions. In Greenland, ice loss from the Greenland ice sheet has accelerated in recent years [ Shepherd et al ., 2012]. Outlet glaciers are changing their behavior rapidly, with many thinning, retreating, and accelerating [ Joughin et al ., 2004]. The loss of ice weighing on the crust and mantle below has allowed both to rebound, resulting in high rock uplift rates [ Bevis et al ., 2012]. Changes in ice cover and meltwater production influence sea level and climate feedbacks; they are expected to contribute to increasing vulnerability to geohazards such as landslides, flooding, and extreme weather.

A spatial filter is often attached to a microphone or microbarometer in order to reduce the noise caused by atmospheric turbulence. This filtering technique is based on the assumption that the coherence length of turbulence is smaller than the spatial extent of the filter, and so contributions from turbulence recorded at widely separated ports will tend to cancel while those of the signal of interest, which will have coherence length larger than the spatial dimensions of the filter, will be reinforced. In this paper, the plane wave response for a spatial filter with an arbitrary arrangement of open ports is determined. It is found that propagation over different port-to-sensor distances causes out-of-phase sinusoids to be summed at the central manifold and can lead to significant amplitude decay and phase delays as a function of frequency. The determined spatial filter plane wave response is superimposed on an array response typical of infrasound arrays that constitute the International Monitoring System infrasound network used for nuclear monitoring purposes. It is found that signal detection capability in terms of the Fisher Statistic can be significantly degraded at certain frequencies. The least-squares estimate of signal slowness can change by up to 1.5° and up to 10 m/s if an asymmetric arrangement of low and high frequency spatial filters is used. However, if a symmetric arrangement of filters is used the least-squares estimate of signal slowness is found to be largely unaffected, except near the predicted null frequency.

More than 100 separate incidents of interactions between aircraft and volcanic ash were documented between 1973 and 2003. Incidents on international flight paths over remote areas have resulted in engine failures and significant damage and expense to commercial airlines. To protect aircraft from volcanic ash, pilots need rapid and reliable notification of ash‐ generating events. A global infrasound array network, consisting of the International Monitoring System (IMS) and other national networks, has demonstrated a capability for remote detection of Vulcanian to Plinian eruptions that can inject ash into commercial aircraft cruise altitudes (approximately 12 kilometers) near the tropopause. The identification of recurring sound signatures associated with high‐ altitude ash injection implies that acoustic remote sensing can improve the reliability and reduce the latency of these notifications.

On July 14, 2006, Tungurahua volcano in Ecuador initiated a dramatic eruptive sequence that caused the evacuation of villagers, the destruction of crops, and the reports of ash clouds above 15 km. Acoustic signals have been recorded by the prototype infrasound acoustic-surveillance system (ASHE) currently operating in Ecuador. The infrasonic signals recorded by the ASHE included explosions, pyroclastic flows, and a sustained low frequency vibration of the ground. We present the features of the different types of eruption signals, and discuss the physics of eruptive processes and volcanic ash injection into the atmosphere. We also compare the July 2006 ash-rich eruption sequence with the May 2006 eruptive episode, which injected very little ash into the atmosphere. Results are of significance for the future operational use of acoustic monitoring of volcanic activity for aviation safety.

The potential of using infrasound to rapidly identify explosive volcanic eruptions has been discussed in the environmental acoustics and aviation safety communities for some time. The ability of sounds in the 0.01–10 Hz range to propagate for long distances with little attenuation suggests broad-area regional monitoring with a modest number of observing sites is possible. The ASHE experiment tests both the practical utility of infrasound as a regional-scale volcanic eruption detection tool, and the feasibility of using such an infrasound system to provide timely operational alerts to aviation through Volcanic Ash Advisory Centres (VAACs). Several infrasound arrays are deployed in a volcanic region, sending data in real time to a central detector, and onward to participating VAACs for comparison with existing warning systems. The ASHE experiment will determine if infrasound can complement both seismic and satellite observations to improve monitoring of volcanic hazard. Continuous acoustic surveillance can reduce the ambiguity between eruptive and purely seismic activity in an active volcano and provide additional estimates for the onset time of an eruption. The onset time estimates can be used as triggers for ash transport models.

Recent global events have renewed interest in acoustic observations of large earthquakes and associated phenomena such as landslides and tsunamis. In addition, the proliferation of global networks of atmospheric acoustic (infrasound) and hydroacoustic observing systems mean that signals from such events are captured in close to real-time more frequently and with better resolution than ever before. Such data provide a useful complement to more traditional seismological observations of such events. In this paper, recent acoustic and hydroacoustic observations from large earthquake events are presented. As well as the scientific insights provided by use of such monitoring systems, operational applications useful for hazard mitigation or hazard alerting are considered. Current plans to develop observing systems to perform systematic studies of areas susceptible to earthquake and tsunami hazard offshore western Canada, and the use of such proposed systems to make systematic studies of hydroacoustic signals from seismic events, underwater slumping and landslides are described.

The Racha-Dzhava earthquake (Ms = 7.0) that occurred on 1991 April 29 at 09: 12:48.1 GMT in the southern border of the Great Caucasus is the biggest event ever recorded in the region, stronger than the Spitak earthquake (Ms= 6.9) of 1988. A field expedition to the epicentral area was organised and a temporary seismic network of 37 stations was deployed to record the aftershock activity. A very precise image of the aftershock distribution is obtained, showing an elongated cloud oriented N105 degrees, with one branch trending N310 degrees in the western part. The southernmost part extends over 80 km, with the depth ranging from 0 to 15 km, and dips north. The northern branch, which is about 30 km long, shows activity that ranges in depth from 5 to 15 km. The complex thrust dips northwards. A stress-tenser inversion from P-wave first-motion polarities shows a state of triaxial compression, with the major principal axis oriented roughly N-S, the minor principal axis being vertical. Body-waveform inversion of teleseismic seismograms was performed for the main shock, which can be divided into four subevents with a total rupture-time duration of 22 s. The most important part of the seismic moment was released by a gentle northerly dipping thrust. The model is consistent with the compressive tectonics of the region and is in agreement with the aftershock distribution and the stress tensor deduced from the aftershocks. The focal mechanisms of the three largest aftershocks were also inverted from body-wave records. The April 29th (Ms = 6.1) and May 5th (Ms = 5.4) aftershocks have thrust mechanisms on roughly E-W-oriented planes, similar to the main shock. Surprisingly, the June 15th (Ms = 6.2) aftershock shows a thrust fault striking N-S. This mechanism is explained by the structural control of the rupture along the east-dipping geometry of the Dzirula Massif close to the Borzhomi-Kazbeg strike-slip fault. In fact, the orientation and shape of the stress tensor produce a thrust on a N-S oriented plane. Nappe tectonics has been identified as an important feature in the Caucasus, and the source mechanism is consistent with this observation. A hidden fault is present below the nappe, and no large surface breaks were observed due to the main shock. The epicentral region is characterized by sediments that are trapped between two crystalline basements: the Dzirula Massif, which crops out south of Chiatoura, and the Caucasus Main Range north of Oni. Most, if not all, of the rupture is controlled by the thrusting of overlapping, deformed and folded sediments over the Dzirula Massif. This event is another example of blind active faults, with the distinctive feature that the fault plane dips at a gentle angle. The Racha Range is one of the surface expressions of this blind thrust, and its growth is the consequence and evidence of similar earthquakes in the past.