**1. Introduction**

The 9 January 1982 Miramichi, New Brunswick, magnitude (mb) 5.7 earthquake was a rare case in North America (**Figure 1**). It was felt throughout eastern Canada and northeastern USA and intrigued scientists and the public as it was the largest one in eastern Canadian and eastern US in recent 100 years. The mainshock (mb 5.7; *M*<sup>W</sup> 5.6) occurred at 12:53 UT on the 9th January, followed 3.5 hours later by a large mb 5.1 (*M*W 4.9) aftershock. On the January 11th the largest aftershock (mb 5.4, *M*W 5.0) followed, then on March 31st another large aftershock (mb 5.0, *M*W 4.9) occurred. The above three large aftershocks were called principal aftershocks. The mb magnitude was used for the mainshock and the three principal aftershocks in the majority of the publications and the media for many years. The moment magnitudes can be found in the report by Bent [1].

#### **Figure 1.**

*The location of the 1982 Miramichi earthquake sequence, the seismicity occurred from 1980 to March 2022, and the geological background in its surrounding regions. The triangles show the locations of seismic stations, KLN, EBN, GGN, LMN, LPQ, and GSQ. The diamonds show the locations of cities or towns. The symbols CSZ and LSZ are abbreviations for the Charlevoix seismic zone, and lower St. Lawrence seismic zone. A solid circle color coded with focal depth shows an earthquake epicenter, which were retrieved from the incorporated research institutions for seismology (IRIS). A focal depth value is indicated by the depth scale on the right. The color scale at the bottom shows the height of the topographic locations above the sea level. The St. Lawrence River runs through Quebec City, Charlevoix seismic zone, and lower St, Lawrence seismic zone. The St. Lawrence faults system also runs along this trend (e.g., [2]); for clarity it is not plotted. At the north of CSZ is the Saguenay Graben. The 1988 MW 5.9 earthquake occurred along this Graben. Figure 1 was plotted using the GMT program [3].*

Three field surveys were conducted in 1982 by the Geological Survey of Canada (GSC) to investigate the aftershock sequence. In the January survey (S1), the most detailed coverage of the aftershock activity was from 19 to 22 January (the temperatures were below -25C°) when aftershocks were recorded by analog MEQ-800 seismographs at four sites within 10 km of the active zone. The hypocenter of the mainshock was estimated using the hypocenters of the detected small aftershocks. The April survey (S2) was conducted in response to the 31 March mb 5.0 aftershock, whose hypocenter was also estimated using the hypocenters of the detected small aftershocks. The survey in June (S3) followed the 16 June mb 4.7 earthquake (**Figure 1**). As this event was located about 30 km west of the Miramichi mainshock (e. g. Wetmiller et al., [4]), it is not discussed in this article.

Responding to a request from Canada, the U.S. Geological Survey (USGS) installed a portable digital network. This network located about 40 aftershocks between the 15 and 22 January 1982 [5]. Among the 40 aftershocks, 4 larger ones were relocated and their focal mechanisms were studied by Saikia and Herrmann [6].

*Locations of the 1982 Miramichi (Canada) Aftershocks: Implication of Two Rupture Regions… DOI: http://dx.doi.org/10.5772/intechopen.108195*

#### **Figure 2.**

*The shift-corrected epicentral distribution of the located 68 aftershocks. The size of a solid circle is proportional to the magnitude, while the color matches the focal depth (see the depth scale on the right). Star S1 marks epicenter of the mainshock and star S2 marks the epicenter of the mb 5.0 aftershock, determined by Wetmiller et al. [4]. The coordinate point (0, 0) is at (47.0°N; 66.6°W). The aftershocks in the upper part of the figure were separated into two groups (the left and right groups) by a gap region indicated by a dashed-line with an arrow at Az 38°. The aftershocks in the lower part were included in the bottom group. The diamond shows the epicenter of the mb 5.4 aftershock, located using phase readings at stations EBN, GGN, LPQ, and GSQ. The triangles indicated with 5.7, 5.1, 5.4, and 5.0 show the epicenters of the mainshock and its three principal aftershocks, located by Choy et al. [8]. The epicenter of the mainshock was calibrated to that obtained by Wetmiller et al. [4]; accordingly, the epicenters of the 3 principal aftershocks were moved with the same amount of the distance and direction as those of the mainshock. The "beachball" shows focal mechanism of the mainshock calculated from the moment tensor solution (the global CMT project; globalcmt.org). The nodal plane indicated with p2 is the inferred the rupture plane. The two arrows pointed to the "beachball" show the compressive force direction in the source region.*

The focal mechanism of the mainshock was a thrust type (e.g., [7]). The rupture was inferred to be updip on a west dipping NNE striking fault plane (Choy et al., [8]). The "beach-ball" is plotted using gCMT data (the global CMT project; globalcmt.org), and the inferred rupture plane is labeled p2 (see **Figure 2**).

As this earthquake sequence occurred in an almost completely uninhabited region, the damage was minor. However, investigating this mainshock and its aftershocks is important for understanding intraplate seismic activity, and assessing the seismic hazard in the source region and its vicinity for the future.

Since there was no close Canadian digital seismic station, a new station (KLN) was installed by GSC on 23 January 1982, 14 days after the mainshock, to better monitor the sequence. The station belonged to the Eastern Canadian Telemetered Network (ECTN). KLN recorded hundreds of aftershocks, and the waveform record quality was excellent. Two existing ECTN stations, EBN and GGN, also had clear records for the larger aftershocks (*m*<sup>N</sup> ≥ 2.8; the magnitude *m*N was defined by Nuttli, [9]). **Figure 1** shows the locations of these three stations, as well as those of stations LPQ and GSQ.

Between latitudes 46.88° N – 47.16° N and longitudes 66.35° W–66.80° W, there were about 700 aftershocks (the smaller aftershocks detected in the field surveys are not included) in the Natural Resources Canada (NRCan) catalog database. Ma and Motazedian [10] determined the focal depths for more than 100 aftershocks with

*m*<sup>N</sup> ≥ 2.8, using depth phase sPmP recorded at EBN, but left the epicenters unchanged. Most of the aftershocks in the database were assigned the same epicenter (47.00° N, 66.60° W), which is the epicenter of the mainshock, determined by Wetmiller et al. [4].

For an earthquake with Pg and Sg arrival readings at KLN, Pg at EBN, and Pn at GGN, a conventional location method can in principle be used to determine its hypocenter. However, the value of an earthquake's focal depth is usually much smaller than those of the station distances; for a seismic phase recorded at a station, the depth typically has a much smaller contribution to the travel time than that from the station distance. As such, the error (uncertainty) in the depth is much larger than the uncertainties in the latitude and longitude of an epicenter. To reduce the error in an epicenter, a focal depth can be first determined using a depth phase; then, the epicenter is located at the depth determined.

Since the station coverage for the sequence was not good (**Figure 1**), and regional velocity models are not good either, it was not possible to determine an epicenter for an aftershock with small absolute errors. However, errors in the relative locations in a small aftershock group should be smaller and can be obtained using a master-event method (e.g., [11]).

Since the energy released by the mb 5.4 aftershock is of the same order as that of the mainshock, the Miramichi earthquake is also called a double-earthquake. The mainshock and its 3 principal aftershocks were relocated by Choy et al. [8]. The 4 focal depths were determined using a waveform modeling method [10]. The depth of mb 5.7 was 6.8 km, mb 5.1, 5.5 km, mb 5.4, 5.2 km, and mb 5.0, 2.0 km. The 4 depths were progressively shallower with occurrence times.

The durations of the surveys conducted by GSC or USGS were shorter than one week; the epicenters of the aftershocks available from IRIS database have large uncertainties. This implies that no clear patterns for the Miramichi earthquake sequence are available yet.

Our motivation was to obtain a reliable pattern of the hypocentral distribution by locating larger aftershocks. In the following sections we briefly introduce the seismicity and geological background in the vicinity of the Miramichi earthquakes; analyze the waveform data; briefly introduce the methods for locating aftershocks; present the hypocentral distribution features of the located aftershocks; display the time series and strength of the three earthquake groups obtained; analyze the errors in the relative locations between two adjacent aftershocks; and discuss some related issues.
