Aftershock sequences of the 1982 Miramichi, New Brunswick, earthquakes

Abstract

The mb 5.7 Miramichi, New Brunswick, earthquake of 9 January 1982 was located by comparison to the aftershock distribution in central New Brunswick, Canada, at 47.00°N, 66.60°W with a focal depth of 7 km and an uncertainty of ± 3 km on each of the spatial coordinates. The main shock was followed by a long and complex aftershock sequence with four events M ≧ 4.5 and more than 800 events M ≧ 1.0 until 30 June 1982. The aftershocks were studied by three temporary field networks set up in the epicentral areas from 10 to 20 January, 2 to 7 April, and 17 to 23 June 1982 and by the permanent Canadian Seismograph Networks. The aftershock activity was concentrated in a volume approximately 6 km NS × 6 km EW × 8 km deep and arranged in a north-trending conjugate “V” pattern. The Miramichi earthquakes were predominatly thrust earthquakes occurring on north-trending, mid- to high-angle faults in the upper 8 km of the crust in the presence of an east-west compressive, horizontal regional stress field. Detailed field surveys have not yet been able to associate the earthquakes with any known preexisting faults. The main shock had a moment of 2.2 ± 0.7 × 1024 dyne-cm and a stress drop of 35 to 70 bars. The rupture started at 7-km depth on a plane striking 195°, dipping 50° down to the west and propagated up to the east with a rake angle of 120°. Aftershocks deeper than 3.5 km had (composite) mechanisms very similar to the main shock's, but shallower events took place on more steeply dipping planes (up to 78° dip). The rupture planes of the main shock and its principal aftershocks may have been similarly curved. The magnitude-frequency relationship ( b = 0.8) of the aftershocks was comparable to the regional one. The aftershock activity decayed as t −0.8 and had the highest rate of activity following the principal aftershock on 11 January. That aftershock ( mb 5.4) appears to have ruptured the east dipping plane conjugate to the main shock's plane. Two other principal aftershocks ( mb 5.1 on 9 January and mb 5.0 on 31 March) were associated with the main shock rupture plane, while the fourth large event ( mb 4.7 on 16 June) occurred 30 km west of the other activity and was seemingly unrelated. However, all the earthquakes were apparently associated with the same granitic terrain. The Miramichi earthquakes did not produce any significant surface faulting. One small (25-mm) surface displacement was found but it was identified as a secondary stress-induced movement along a preexisting joint.