UAV-LiDAR-Photogrammetry analyses of Stress-Release Structures in Southern Ontario, Canada: Implications for Regional Seismic Hazard Assessment

Seismicity in eastern North America occurs in sporadic clusters distal from plate boundaries throughout western Quebec and continues with typically lower magnitude and frequency events in southern Ontario and the Great Lakes region. Although M4-5 earthquakes have been recorded in southern Ontario, there is limited understanding of regional seismogenic structures, the state of stress, and reactivation potential of basement faults. Stress-release structures, such as ‘pop-ups’, whilst somewhat rare and poorly documented, have been previously reported across the region. These structures can be produced by far-field intraplate tectonic processes far from plate boundaries, and thus can be used infer stress states and assess seismic hazard potential. 
This study aims to document, analyse, and interpret potential stress release features, including pop-ups, in southern Ontario, Canada. Employing a DJI Matrice 350 RTK with an L2 LiDAR payload and Emlid RS3 DGPS, we conducted a high-resolution (sub-cm) LiDAR and photogrammetry survey of well-exposed pop-ups at Wainfleet Wetlands, a former aggregate quarry located ~4 km west of Port Colborne, Ontario. 250 MHz ground-penetrating radar (GPR) profiles were also collected along several transects across the folds. Previous work here had identified at least two ~NW-SE oriented curvilinear pop-up structures ~100 m each in length within Devonian dolomitic limestones of the Onondaga Formation. The features exhibit en-echelon fractures with stepovers, indicating complex fault geometries and reactivation history. 
Regional estimates of the maximum horizontal stress (σH) suggest σH is ~NE-SW, consistent with the pop-up orientations and formation by far-field intraplate stresses.  FracPaQ analysis of fracture orientation, density (P20) and intensity (P21) on UAV-orthomosaics reveals deviations from regional fracture orientations and an increase in P20 and P21 proximal to pop-ups compared to nearby outcrops on the Lake Erie shoreline. GPR profiles imaged the internal geometry of fold structures to a depth of > 5 m.
The pop-ups are interpreted as stress-release buckles triggered by local overburden removal during quarrying. This initial work indicates that stress-release structures are perhaps more widespread, and structurally complex, in southern Ontario than previously considered, and that they may inherit complex geometries from deep-seated faults. Our work underscores the need to seek out and document other potential stress-release structures elsewhere in the region to elucidate their implications for intraplate stress and thus seismic hazards.