**4. Methods**

#### **4.1 A simplified master-event relocation method**

Master-event relocation methods have been studied and used by many scientists (e.g., [11, 14–16]). The steps in the master-event method described by Havskov and Ottemöller [11] are as follows: (1) Locate the master-event (ME) using a conventional location method; (2) Select stations and phases which are common to the ME and the slave-events (SEs); (3) Calculate the residuals at the selected stations for the ME; (4) Add the residual of a certain phase to the arrival time readings of the same phase for the ME and SEs; and (5) Relocate all events (the ME and the SEs) using a conventional event locating program.

The step (2) is necessary. We tested that once this necessary step was kept the uncertainty between two adjacent epicenters can be small, and the epicenter distribution pattern can be reliable, so we only kept this step in our work. We used the program in SEISAN (e.g., [17] to locate the aftershocks.

#### **4.2 A depth phase modeling procedure to determine a focal depth**

When the Pg- and Sg-phase arrival times at KLN, Pg at EBN, and Pn at GGN are available for an aftershock, theoretically the four source parameters (origin time, latitude, longitude, and focal depth) can be determined using the four time readings. However, the uncertainty in the four parameters could be large, especially that in focal depth. In practice, if only a few arrival time readings are available or station coverage is poor, the focal depth is assigned a nominal value to stabilize the location when locating an event.

To reduce uncertainties in epicenters due to uncertainty in focal depth, the time difference along a trace between depth phase sPg and its reference phase Pg (**Figure 3**) is used to first retrieve a reliable focal depth for the aftershock that generated the trace; then, the epicenter of the aftershock is located at the focal depth retrieved. In this way, the trade-off between the epicenter and the focal depth is removed, so the uncertainty in the epicenter can be dramatically reduced.

The crucial step in the procedure to retrieve a focal depth using a depth phase is the generation of the synthetic traces along which the depth phase appears. In generating synthetic waveforms, a crustal model, station distance, focal mechanism, and focal depth are needed input parameters. Since the crustal structures through which the waves travel are related to travel times, the crustal model is a key input parameter. The reflectivity method [18], the centroid moment tensor solution for the Miramichi mainshock from the gCMT Catalog (see data and resources section), and the crustal model introduced in next section were used to generate the synthetic traces. The details of the depth phase studies can be found in e.g., [19–22].
