Locations of the 1982 Miramichi (Canada) Aftershocks: Implication of Two Rupture Regions Activated

On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with mb 5.7 occurred. It was followed by extensive aftershocks and felt throughout eastern Canada and north-eastern USA. Since this earthquake occurred in an uninhabited region, the damage was minor. Due to an mb 5.7 event is rare in north-eastern America, investigating it and its aftershocks is important for understanding intraplate seismicity. Digital seismic stations were not yet common by 1982. Fortunately, four seismic phases at three stations could be used to locate larger aftershocks. A simplified master-event location method combined with regional depth-phase modeling was used to locate aftershocks. For each aftershock its focal depth was first determined using a depth phase; then, with the depth fixed, the epicenter was determined using the four arrival time readings measured at the same three stations. The located aftershocks were divided into three groups. In each group the earthquake numbers are similar, but the majority of the energy was released in one group. The epicenters formed two trends in the NE–SW direction, implying that the Miramichi earthquake sequence activated two rupture regions