**9. Discussion and conclusion**

More than 40 years ago, on 9 January 1982 in the Miramichi region of north-central New Brunswick, an earthquake with magnitude mb 5.7 occurred. Since the digital seismographs had not been widely deployed at that time, the source parameters of the mainshock and its aftershocks were not well determined. We analyzed the seismograms and found that at station KLN, there were very clear onsets of Pg- and Sg-waves, at EBN, clear onsets of Pg-waves, and at GGN, clear onsets of Pn phase for the larger aftershocks. We also unexpectedly found that the depth phase sPg was well developed and recorded at KLN. Once the velocity records were converted into displacement records, the onsets of the depth phase could be read correctly and accurately. The depth phase information can be used to determine focal depth accurately, and the Pg, Sg, and Pn arrival time readings at the three stations can be used to stably determine epicenters and the origin times at fixed focal depth using a conventional location method. To obtain a reliable epicentral distribution pattern, the uncertainties (errors) in the relative locations of the epicenters need to be small. To reduce errors in the relative locations, we used a simplified masterevent method; specifically, we used the elite part in the master-event method. By using arrival time readings of the common four phases at the same three stations, the errors in the relative locations of adjacent aftershocks can be small.

In our study on the locations of larger aftershocks in the Miramichi sequence, we made a great effort to reduce the errors in the relative hypocenters. The errors in the relative locations between an epicenter and its closest neighbor event are listed in **Table 1**. Most of the values in the modu column are less than or equal to 0.3 km, which is smaller than the narrowest part of the gap indicated by the dashed vertical line in **Figure 7**. In another word if any epicenter in the left group is moved by 0.3 km in any direction, it cannot go into the right group. This implies that the pattern of two groups is reliable.

The left and right group phenomenon could be observed by arranging the waveform records at stations KLN and EBN. **Figure 3** shows the vertical component seismograms recorded at KLN, generated by aftershocks along a line approximately running through KLN at about Az 128°. The top 5 records were generated by the aftershocks in the right group, while the bottom 5 records by aftershocks in the left group (see **Figure 2**). The TSg – TPg times along the top 5 records are shorter than those along the bottom 5 records; due to those epicenters to KLN are shorter. The time δt indicated along the bottom trace in **Figure 3** corresponds to the spatial gap between the left and right group. Similarly **Figure 4** shows vertical component seismograms recorded at EBN generated by 9 aftershocks, occurred along a line running through EBN at about Az 128°. The TSg – TPg times (about 16.54 s) along the traces 2, 3, 4, and 5 in the top panel (left group) are shorter than those (about 17.04 s) along the bottom 5 traces (right group). The time difference 0.5 s (17.04–16.54) also corresponds to the spatial gap between the left and right group in **Figures 2** or **7**.

In **Figure 2** the located aftershocks were divided into three groups. The mainshock, its largest (mb 5.4), and the mb 5.0 aftershocks are in the right group. In the left group only one principal aftershock (the mb 5.1) occurred there. The total energy released in the right group is more 10 times than that released in the left group. The fault in the right group is larger than that in the left group. However, the located aftershock number is 26 in the left group, more than that (No. 23) in the right group (**Figure 9**). The left group looks like an earthquake swarm.

The aftershocks we located were in about 8 years when KLN station was operated. Since the mainshock occurrence more than 40 years has past, the aftershock activity in the source region still continues. The mystery related to the Miramichi earthquake sequence, such as why there are so many aftershocks, and several principal aftershocks followed the mainshock, is still waiting for being explored.

Based on the analyses of the located aftershocks in previous sections, the following can be concluded: (1) the major source volume was about 5 × 5 × 5 km3 ; (2) the focal depths ranged from about 2 km to 7 km; (3) two separate fault systems (the left and right group in **Figure 2**) were activated; the right one, activated by the mainshock and its two principal aftershocks, was large, and most energy was released there; the left one was small; (4) the trend in the aftershock epicenters was close to the northeast strike of the nodal plane for the mainshock; (5) the epicenter distribution trend was parallel to the trend of the Appalachian Mountain range (NE–SW); and (6) for more than 40 years the aftershocks have been occurring in the mainshock source region.

The procedure used to locate the Miramichi aftershocks has been successfully used for other earthquakes (e.g., [28, 29]). It is applicable for any earthquake sequence that has depth phase records.

The reliable epicentral distribution trends we obtained are crucial information for the seismic hazard assessment in the source region and its vicinity.
