Focal depth distribution of the 1982 Miramichi, New Brunswick, earthquake sequence, retrieved by analyzing regional depth phases

English Abstract:

A magnitude mb 5.7 earthquake occurred on 9 January 1982 in the Miramichi region, New Brunswick, Canada. The earthquake was felt throughout much of the Maritime Provinces, Canada and the New England States, United States. The mainshock was followed by an extensive aftershock sequence. The 1982 Miramichi mainshock is the largest earthquake in eastern Canada since the Cornwall-Massena earthquake of 1944. It is the first earthquake in eastern North America recorded on modern seismograph networks world-wide, so its source parameters can be determined by many modern techniques. Focal depth is an important source parameter. As the contribution from forcal depth to travel times are much smaller than that from the distance, the error in focal depth is much larger than that in epicenter obtained by conventional locating method. Regional depth phases are often detectable at regional distances. As the epicentral region of the Miramichi earthquakes lies within the northern Appalachian Mountains, regional depth phases are very well developed and recorded at several seismic digital stations. The waveforms generated by the aftershock sequence are very well saved at GSC database, so these regional depth phase records can be used to retrieve focal depths for the sequence by modeling method which has not been used before. In the epicentral region of the mainshock the documented aftershock number is about 700. To abstract main features we first retrieved waveform records for the aftershocks with magnitude mN >= 2.8 (about 100). These earthquakes are representative and have clear depth phase records. Then we performed regional depth phase modeling. We found the dominant focal depth range is from 5 km to 7 km. About one third of the analyzed aftershocks occurred at about 6 km. Another small group has focal depths at about 3 km. Some aftershocks have very shallow focal depths, almost near the ground surface, which was confirmed by strong Rg phase records. The deepest is at about 10 km.