The 2015 Canada Day,Mw 3.8, Earthquake in Nova Scotia

Abstract

ABSTRACT On 1 July 2015, a magnitude ( M N , M w ) 3.8 earthquake occurred north of Yarmouth in southwestern Nova Scotia, a region of extremely low seismicity. Prior to this event, only five earthquakes had been recorded within 50 km of its epicenter since 2000. The earthquake caused no damage but was widely felt in southwestern Nova Scotia with nearly 600 people filling out the Geological Survey of Canada’s online “Did You Feel It?” form. Good‐quality regional seismograms enabled us to determine the source parameters of the earthquake. The focal mechanism determined by a regional centroid moment tensor inversion is indicative of oblique‐thrust faulting. We believe this to be the first focal mechanism solution for a Nova Scotian earthquake. It is consistent with those of earthquakes in the neighboring regions of New Brunswick and Maine and with the regional stress field. Depths were obtained by the moment tensor inversion and by regional depth phase modeling, with consistent results indicative of a focal depth of 9–10 km. The M w 3.8 is larger than average based on the M N value but within the range of previous observations for eastern Canada. A magnitude ( M N ) 2.2 aftershock occurred ∼7 min after the mainshock but was not reported as felt. A focal depth of 8 km was obtained via the regional depth phase method. The focal mechanism of the aftershock was not determined directly, but a comparison of the waveforms to those of the mainshock suggests a similar source.