Focal depth distribution of the 1982 Miramichi earthquake sequence determined by modelling depth phases

Abstract

On 9 January 1982, in the Miramichi region of New Brunswick, Canada, an earthquake with body-wave magnitude (mb) 5.7 occurred, and extensive aftershocks followed. The mainshock was felt throughout Eastern Canada and New England, USA. The mainshock and several principal aftershocks were digitally recorded worldwide, but smaller aftershocks were digitally recorded only at regional stations. Digital stations were not yet popular in 1982; therefore, available regional digital waveform records for modelling are very limited. Fortunately, two Eastern Canada Telemetered Network (ECTN) stations, EBN and KLN, produced excellent waveform records for most of the aftershocks until their closure at the end of 1990. The waveform records can be retrieved from the archive database at the Geological Survey of Canada (GSC). Since EBN had clear sPmP records of the larger aftershocks (with magnitude mN ≥ 2.8), we were able to determine focal depths for these larger events. Most of the focal depth solutions for the 113 larger aftershocks were within a depth range of 3–6 km. The majority of the depths were at about 4.5 km. Some aftershocks had depths of about 1–2 km. The focal depth solutions for the shallow events were confirmed by the existence of prominent crustal Rayleigh waves. As the records for the foreshock and the mainshock at EBN were not available, we used the records at station LMN for the foreshock and a teleseismic depth phase for the mainshock. The teleseismic depth phase comparison shows that the mainshock and its three principal aftershocks migrated from a depth of about 7 km to near the Earth’s surface.