**6. Aftershock locations**

### **6.1 Location of the ME and the adjustment to the crustal model**

We selected a *m*N 3.5 aftershock (No. 36 in **Table 1**) as an ME. Its waveforms were presented in the upper three traces in **Figure 5**. This aftershock had very clear onsets


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



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


**Table 1.** *Catalog of the located 68 aftershocks.*

*by Saikia and Herrmann [6].*

*44 are used in the Section 6. The earthquake No. 69 is for a small aftershock, S1 for the mainshock, S2 and S3 for the two principal aftershocks [4]; and the last four are small aftershocks located* 

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

#### **Figure 6.**

*Regional depth phase sPg modeling at KLN (distance 23.6 km) for the ME. The top synthetic vertical trace U\_D/5.5 was generated using a depth of 5.5 km. Other synthetic traces were generated with a depth increment of 0.1 km. Trace KLN/SHZ is the observed vertical short period seismogram at KLN. The synthetic and the observed Pg are aligned. The time difference between sPg and Pg along trace U\_D/5.9 and the time difference along the observed trace is approximately equal. Therefore, the modeled depth for the ME is 5.9 km.*

of phases Pg, Sg, and Pn; the arrival time readings can be accurate. A focal depth of 4.5 km was previously estimated using the depth phase sPmP, recorded at EBN, and the epicenter (47.0°N, 66.6°W) of the mainshock [10]. However, the sPmP – PmP time could not be accurately measured at EBN (see **Figures 5** and **6** in [10]), resulting in an uncertainty of about 1.0 km in focal depth. In this article, we increased the depth accuracy by using the depth phase sPg at KLN. **Figure 6** demonstrates the depth phase sPg modeling for the ME. The top trace U\_D/5.5 was generated using a depth of 5.5 km; other synthetic traces were generated with a depth increment of 0.1 km. The sPg - Pg time along trace U\_D/5.9 and the time difference along the observed trace were approximately equal, so the modeled focal depth was 5.9 km. As the arrival times of Pg and sPg could be precisely compared, the uncertainty in the focal depth obtained using sPg was reduced.

After the focal depth for No. 36 was obtained, the SEISAN computer program was run at the newly obtained focal depth value. During the first trials of the epicentral location, the residuals between the arrival times of the observed and the calculated Pn phases at GGN were not small, so the P wave velocity value in the crustal model beneath the Moho was adjusted to reduce the residuals.

#### **6.2 Location of the 68 aftershocks**

The aftershocks with *m*<sup>N</sup> ≥ 2.8 that occurred after KLN installation usually had clear onsets of Pg and Sg phases at KLN, Pg at EBN, and Pn at GGN. Of the 113 aftershocks for which the focal depths were determined [10], 68 satisfied the requirements for using the ME relocation method. Therefore, 68 aftershocks were located at the focal depths determined using depth phase sPg. The epicenters of the 68 aftershocks are plotted in **Figure 2** and listed in **Table 1**.

## **6.3 Epicenter corrections**

Since the station coverage was poor, the available arrival time readings were limited, and a 1-D crustal velocity model used, an epicentral shift relative to its true location was unavoidable. To obtain an epicentral distribution with absolute errors as small as possible, we performed an epicentral shift correction.

The star labeled S2 in **Figure 2** marks the epicenter of the mb 5.0 aftershock, determined by Wetmiller et al. [4] using the centre of the small aftershocks they detected. It was assumed that the epicenter had smaller absolute errors compared to the epicenter for the same aftershock we obtained. The reason is that Wetmiller et al. [4] used the arrival times at portable stations which were less than 10 km from the earthquake sequence the absolute errors in the small aftershocks they detected were small. Accordingly, the absolute errors in the epicenter of the mb 5.0 they obtained were smaller. The differences between the two epicenters for the same mb 5.0 earthquake were subtracted from the epicenters of all the 68 aftershocks in **Table 1**. **Figure 2** shows the corrected epicentral distribution.

### **6.4 Location of the mb 5.4 aftershock which did not have KLN record**

Since the mb 5.4 aftershock was strong, it had clear Pg and Sg at EBN (**Figure 4**), clear Pn at GGN, LPQ, and GSQ, and it already had an accurate focal depth solution [10], so it can be located without a record at KLN. The aftershock mb 5.0 also had common phase arrival time reading as those of the mb 5.4, and had an accurate focal depth solution, so the mb 5.0 can also be located with the same precision as that of the mb 5.4. After the two epicenters were obtained using the arrival time readings at the above 4 stations, the epicenter of the mb 5.0 was corrected to that obtained by Wetmiller et al. [4]; accordingly, the epicenter of the mb 5.4 was moved the same amounts in latitude and longitude as did for the mb 5.0. The diamond symbol indicated with 5.4 in **Figure 2** shows the corrected epicenter for the mb 5.4 aftershock.

#### **6.5 Distribution features of the located hypocenters**

After the epicenter shift correction, the mainshock is located within the southern part of the located aftershock cluster (**Figure 2**). Most aftershocks occurred within a 5 × 5 km2 area, with the remaining ones scattering to the southwest. Overall, the aftershocks trend in a northeasterly direction (about Az 38°). This trend is close to the strike of one of the nodal planes for the mainshock obtained by the CMT group (202°) and by Choy et al. ([8]; 195°). Within the overall trend, the located aftershocks appear to form a pair of northeast trends separated by a gap region indicated by a dashed-line at Az 38°.

To observe more distribution features, the epicenters were divided into three groups: the left group, right group, and the bottom group (**Figure 2**). The hypocenters in the left and right groups were projected onto a vertical plane at Az 128° (**Figure 7**). The gap region indicated by a vertical dashed line in the figure separates the aftershocks into clearly two groups. The hypocenters on the left side were clustered together, and most were in a depth range of 4 to 6 km. In this left group only one aftershock with magnitude ≥5.0, of which the epicenter (the triangle) was determined by Choy et al. [8]. This epicenter and its focal depth have been corrected for comparison. The hypocenters on the right side were distributed from about depth 7 km to about 2.5 km. In this right group the mainshock and aftershocks mb 5.4 and mb 5.0 located. The triangles show the projections of hypocenters determined by Choy et al. [8].

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

#### **Figure 7.**

*Projections of the shift-corrected hypocenters in the left and right groups onto a vertical plane, striking at Az 128° (NW–SE) indicated in Figure 2. The diamond shows the projection of the hypocenter for the mb 5.4 aftershock, determined in this article. Most of the aftershocks occurred at depths between 3 km and 6 km. Stars S1 and S2 represent the hypocenter projections of the mainshock and the mb 5.0 aftershock, respectively, obtained by Wetmiller et al. [4]. The triangles indicated with 5.7, 5.1, 5.4, and 5.0 show the projections 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]; the epicenters of the 3 principal aftershocks were moved with the same amount of the distance and direction as those of the mainshock, accordingly. The 4 focal depths were calibrated to those obtained by Ma and Motazedian [10].*

#### **Figure 8.**

*Projection of the located 68 hypocenters (the epicenters were shift-corrected) onto a vertical plane, striking at Az 38° (NE–SW), indicated by a dashed line with an arrow in* **Figure 2***. Most of the aftershocks occurred at depths between 2 km and 6 km. Stars S1 and S2 represent the hypocenter projections of the mainshock and the mb 5.0 aftershock, obtained by Wetmiller et al., [4]. The triangles indicated with 5.7, 5.1, 5.4, and 5.0, show the hypocenter projections of the mainshock and its 3 principal aftershocks, relocated by Choy et al., [8]. The aftershocks were separated into two groups by a gap region indicated by a vertical dashed line. Most of the aftershocks occupy a region of around 5 × 5 km<sup>2</sup> .*

We also projected all the corrected hypocenters onto a vertical plane at Az 38° (**Figure 8**). The aftershocks were separated into two groups by a gap region indicated by a vertical dashed line. The aftershocks at the left side of the vertical line are those

in the bottom group in **Figure 2**. The aftershocks at the right side are those in the left and right group. They occupied a region of around 5 × 5 km2 . Comparing to the region the left and right groups occupied in **Figure 2**, it can be inferred that the source volume formed by the left and right group is about 5 × 5 × 5 km3 .
