Source processes of injection‐induced earthquakes involve complex fluid‐rock interaction often elusive to regional seismic monitoring. Here we combine observations from a local seismograph array in the Montney Formation, northeast British Columbia, and stress modeling to examine the spatiotemporal evolution of the 30 November 2018 Mw 4.2 (ML 4.5) hydraulic fracturing‐induced earthquake sequence. The isolated occurrence of the mainshock at a depth of ∼4.5 km in the crystalline basement 2 days following injection onset at ∼2.5 km depth suggests direct triggering by rapid fluid pressure increase via a high‐permeability conduit. Most aftershocks are in the top 2 km sedimentary layers, with focal mechanisms indicating discrete slip along subvertical surfaces in an ∼1 km wide deformation zone. Aftershock distribution is consistent with static stress triggering from the Mw 4.2 coseismic slip. Our analysis suggests that complex hydraulic and stress transfer between fracture networks needs to be considered in induced seismic hazard assessment.

Stress drop scaling relations of earthquakes in intraplate seismic zones are less well constrained, partly due to less dense instrumentation and lower seismicity rate. Here we use new data to estimate the static stress drop values of earthquakes in the Charlevoix Seismic Zone in eastern Canada (MN < 4.5), June 2012 to July 2017. We first perform double‐difference relocation to obtain the hypocentral distribution of 518 events, which highlights a diffuse distribution within the Charlevoix Seismic Zone, probably related to the highly fractured crust by a Devonian meteorite impact. Using spectral ratios, we obtain stress drop values of 47 events ranging from ~2 to 200 MPa, typical of intraplate earthquakes, and observe an invariant stress drop scaling in the magnitude range Mw 2.2–3.8. Events within the impact structure have higher stress drops than those outside, which may indicate differences of fault maturity of the St. Lawrence paleorift system and the presence of a distributed fracture network.

This paper presents application of microtremor (ambient vibration) and surface wave field techniques for post-earthquake geotechnical reconnaissance purposes in Kathmandu, Nepal. Horizontal-to-vertical spectral ratios (HVSR) are computed from microtremor recordings at 16 individual measurement locations to obtain an estimate of fundamental frequency (site period) of the subsurface soils. A combination of active- and passive-source surface wave array testing was accomplished at five key sites including Kathmandu's Durbar Square and Airport. Joint inversion of each site's higher frequency dispersion and lower frequency HVSR data sets provides an estimate of subsurface material stiffness [i.e., shear wave velocity ( V S) depth profiles]. Direct comparison of our V S profiling at Kathmandu Durbar Square and that accomplished by downhole V S and/or standard penetration testing (SPT) profiling yield similar results. Classification of the five sites based on average V S, site period, and/or basin depth is presented. There is little differentiation in these site classification designations amongst the five sites, which does not capture significant differences in observed earthquake damage.

No abstract

Intraplate earthquakes occur in stable plate interiors away from tectonic plate boundaries. The typical strain rates (≤10 −10 yr −1 ) within intraplate seismic zones are two or more orders of magnitude lower than the average strain rates (≥10 −8 yr −1 ) reported for seismogenic plate boundary faults (e.g., Gordon, 1998;Mazzotti & Adams, 2005;Mazzotti & Gueydan, 2018). Consequently, intraplate fault zones produce moderate-to-large earthquakes (e.g., 2001 M 7 Bhuj earthquake, 1811-1812 ∼M 7 New Madrid earthquakes) less frequently than their plate boundary counterparts (Bendick et al., 2001;Hough et al., 2004), but their physical mechanisms remain poorly understood. Steady tectonic loading in plate interiors can be attributed to basal traction, gravitational body forces, and plate boundary forces (Liu & Stein, 2016). However, these mechanisms are not always sufficient to elevate stresses to levels that can trigger failure on intraplate faults, and are unlikely to be fully responsible for the stress budget within intraplate seismic zones. Several studies show that earthquakes within plate interiors concentrate within zones of inherited crustal weaknesses due to factors such as, tectonics, volcanism, and meteorite impacts (e.g.,