Micro-Tremors and Aftershocks Following the March 2019 ML 4.2 Earthquake near Red Deer, Alberta

Abstract:

On 4 March 2019, a moderate-magnitude (ML 4.18) earthquake occurred near Red Deer, Alberta. This event and a number of aftershocks were suspected to be associated with nearby hydraulic fracturing (HF) operations. In this study, we analyze continuous data collected from a temporary array of 16 nodal geophones and identify persistent low-frequency (1-2 Hz) noise tremors. Waveform cross-correlation approaches and source migration reveal prominent signals in connection with nearby industrial activities, river/lake ice-breaking events, and local traffic. Aseismic creep is further detected and spatially collocated with the two HF wells, particularly during relatively quiet evening hours.

We explore the nature of these micro-tremors using (1) high-precision earthquake relocation to examine the spatial relationship and (2) focal mechanisms to assess source characteristics. Complemented by regional broadband data, a catalog of 417 events was constructed using a machine learning phase picker and an earthquake association and location algorithm. The spatiotemporal correlation between the aftershock activity and normalized tremor energy indicates that near-field ground motions due to aftershocks consistently produce noise tremors. In addition, event distribution and the resolved focal mechanisms reveal a NE-trending rupture area with two strike-slip fault planes. Reactivation of pre-existing faults by pore pressure diffusion is likely responsible for the occurrence of the earthquake sequence. The temporal variations of reactivated fault orientations suggest apparent stress perturbations after the mainshock, which are also responsible for a remotely triggered cluster one month after the mainshock. In summary, our tremor migration and quick-response nodal deployment highlight the need to incorporate both seismicity (e.g., aftershocks) and aseismic noise in risk assessment.