S038-0010
Pushing the limit of single station: source characterization for two small earthquakes in Dartmouth, Nova Scotia, Canada
Friday, 11 December 2020
Poster
Miao Zhang, Dalhousie University, Department of Earth and Environmental Sciences, Halifax, NS, Canada, Min Liu, Dalhousie University, Department of Earth and Environmental Sciences, Halifax, Canada; China University of Geosciences (Beijing), Beijing, China, Alexandre P Plourde, Dalhousie University, Halifax, NS, Canada, Feng Bao, Institute of Geodesy & Geophysics, Hubei, China and Ruijia Wang, University of New Mexico, Earth & Planetary Sciences, Albuquerque, United States
Abstract:
A pair of small earthquakes (M
L2.4&2.6) surprised the city of Dartmouth, Nova Scotia, Canada in March 2020. Their reported epicenters were within several kilometers of the downtown area at depths of 2 km (Earthquake Canada) and they were widely felt by local residents. In addition, people heard “really loud bang” during both two earthquakes, confirming their shallow depths. The pair of events are recorded by three seismic stations within 200 km, but only one station is close enough (HAL, < 10 km) to offer high-quality broadband signals. High waveform similarity and identical P-S travel time differences indicate the two events are repeaters. In this study, we explore their source parameters using the nearest station through waveform modeling. A nearby quarry blast (M
L 2.0) with known GPS coordinates was also adopted as a reference to assist the analysis.
Strong surface waves of all three events suggest the existence of a regional sedimentary layer. We first build a half space velocity model by estimating the P-S travel time difference of the blast and determine the sedimentary layer through full-waveform modeling (i.e., comparing a set of synthetic waveforms with the observed blast). The velocity model is then used to evaluate the pair of earthquakes, where waveform fitting and Rg/S amplitude ratios suggest shallower depths (~0.6 km) than reported. The epicenters of these two earthquakes are determined toward the northwest of the Lake Micmac, where residents nearby felt strong ground shaking. Lastly, single station template matching finds no similar earthquakes in the past 10 years and only two aftershocks are detected following the two events in four months. Like many other intraplate earthquakes in eastern Canada, the two shallow earthquakes are likely caused by crustal stress perturbations associated with glacial isostatic adjustment. Our study demonstrates that we are able to build a detailed regional velocity model and determine accurate earthquake source parameters in regions where only single station is available.
