**3. The geological background and seismic activities in the source region and its vicinity**

The *M*<sup>W</sup> 7.9 Rat Islands earthquake ruptured beneath the Rat Islands in the western Aleutians Islands, Alaska. **Figure 2** shows its geographic location and the seismicity in the epicenter region and its vicinity. This Mw 7.9 event happened within the subducting Pacific slab in a region where the Bowers Ridge and the Aleutian Trench (subduction zone) meet. The Aleutian Trench is the boundary between the Pacific Plate and the America Plate. In the Aleutian Trench region, the Pacific Plate is moving relative to the North American plate, which is relatively stationary, at a rate of about 7.5 cm/year [e.g., [9]].

The Aleutian trench is, along the Alaska-Aleutian arc, one of the largest active tectonic margins in the world, spanning nearly 4000 km from the Gulf of Alaska to the Kamchatka Peninsula, Russia. The arc is formed by a convergent plate boundary where the Pacific plate is subducted beneath the North American plate at a rate that varies between 5.4 cm/yr in the east to 7.8 cm/yr in the far west [9]. The Aleutian trench zone is very seismically active. In the Rat Islands region, there are 10 earthquakes with a magnitude ≥ 7 occurred since 1960 (IRIS earthquake catalog). The June 23, 2014 MW 7.9 earthquake occurred at a depth of about 100 km. It is often called an intermediate-depth event. In the following paragraphs, we analyze the seismicity that occurred about seven and a half years before and after the *M*<sup>W</sup> 7.9 earthquake in the source region and its vicinity.

**Figure 3** shows the epicenter distribution of earthquakes with magnitudes ≥ 4.0, which occurred about seven and half years before and after the Mw 7.9 mainshock in the mainshock region and its vicinity. A solid circle shows the epicenter of an earthquake. It was color-coded with focal depth; its size is proportional to the magnitude.

#### **Figure 2.**

*The seismicity and the geological background in the vicinity of the 23 June 2014 Rat Islands, Alaska,* M*<sup>W</sup> 7.9 earthquake sequence. A color-coded solid circle shows an epicenter. A deeper color shows a deeper focal depth. The catalog, retrieved from the IRIS, contains the earthquakes with magnitude* ≥ *4, which occurred from 2014-0101 to 2022-0220. The star shows the epicenter of the* M*<sup>W</sup> 7.9 mainshock; the diamond approximately shows the location of the Rat Islands (the new name is Hawadax Islands); the triangles show the locations of the four seismic stations used in the aftershock relocation; the headed-on arrows show the compressive force directions. The earthquake sequence occurred in the region where the Bowers Ridge and the Aleutian Trench (subduction zone) meet.*

The left panel (a) shows the earthquakes that occurred seven and half years before the occurrence of the Mw 7.9. The events within the rectangular are those that occurred in the source region. The right panel (b) shows the earthquakes that occurred after the occurrence of *M*<sup>W</sup> 7.9. The aftershocks formed a trend in the northwest direction.

To analyze the seismicity along the vertical direction, we simulated a spatial plane using the hypocenters of 90 earthquakes with magnitudes ≥ 4.0, which occurred below the depth of 80 km from 2007-0101 to 2014-0622. The simulated strike is Az 276.8°; the dip angle is 47.7°. The plane dips at Az 6.8°. Then we projected the hypocenters of earthquakes onto a vertical plane that is parallel to the dipping direction. **Figure 4** shows the hypocenter projection comparison for the same earthquakes in **Figure 3**. The left panel (a) shows that the hypocenter projections beneath about 90 km are along with a dipping of 47.7° direction. Coincidently the hypocenter distribution trend of the aftershocks in the right panel (b) is also approximately along this dipping direction. The trend is neither in the dipping direction of the steep-dip plane nor in the dipping direction of the shallow-dip plane.

To observe the spatial distribution features of the aftershocks of the *M*<sup>W</sup> 7.9, we simulated a spatial plane using the hypocenters of 184 aftershocks, of which the magnitudes ≥ 4.0. The simulated strike is 201.7°; the dip angle is 39.4°. The plane dips at Az 291.7° (from north to west 68.3°). These parameter values are close to those of the nodal plane P1 (strike Az 207.4°, dip angle 27.1°, dips at 297.4°; **Table 1**). **Figure 5** shows the hypocenter projections onto a vertical plane, which is perpendicular to the strike of the simulated plane. It is found that the hypocenters of the aftershocks were distributed approximately along the simulated dipping direction (39.4°).

*Studies on the Source Parameters of the 23 June 2014 Rat Islands, Alaska… DOI: http://dx.doi.org/10.5772/intechopen.104600*

#### **Figure 3.**

*The epicenter comparison for earthquakes with magnitudes* ≥ *4.0, occurred about 7 and half years before and after the Mw 7.9 mainshock, in the mainshock region and its vicinity. (a) the earthquakes occurred 7 and half years before the occurrence of the Mw 7.9 (between 2007-0101 and 2014-0622). (b) the earthquakes occurred after the occurrence of the Mw 7.9 (between 2014-0623 and 2022-0220). The earthquakes in the rectangle of the left panel are in the source region of the mainshocks. The star with the number 7.9 shows the initial location of the* M*<sup>W</sup> 7.9 mainshock. The catalog was retrieved from IRIS.*

#### **Figure 4.**

*The hypocenter projection comparison for the same earthquakes in Figure 3. (a) The hypocenter projections show the seismic activity along the dipping direction of a simulated spatial plane. The simulation was performed using the hypocenters of 90 earthquakes with magnitudes* ≥ *4.0, which occurred below the depth of 80 km from 2007-0101 to 2014-0622. The simulated strike is Az 276.8°; the dip angle is 47.7°. The plane dips at Az 6.8°. The relatively narrow seismicity belt below the red star may be assumed to be close to the boundary between the Pacific Plate and the North American plate beneath the Rat Islands region. The red star shows the initial location of the mainshock. (b) The hypocenter projections of earthquakes occurred between 2014-0623 and 2022-0220.*

To further observe the spatial distribution trend of the aftershocks, we projected the hypocenters onto two vertical planes. The left panel of **Figure 6** a shows the hypocenters projected onto a vertical plane at a steep-dipping direction. The tilted


*Note: prin. val. refers to the principal axis value (1020Nm); dev. part refers to the deviatoric part (1020Nm); P,T, and N mean the compressional, tensional, and null components.* P*1 and* P*2 mean nodal planes 1 and 2. The unit for the strike, dip, slip, and dipping is the degree (°), and for the moment is 1020Nm. Compared to the scalar moment of the major double couple, the isotropic part (ISO) is 2.88%, and the minor double couple is 3.64%. The inversion was performed using a depth of 105 km.*

#### **Table 1.**

*A full moment tensor solution for the Rat Islands* M*<sup>W</sup> 7.9 mainshock.*

#### **Figure 5.**

*Hypocenter projections onto a vertical plane, perpendicular to the simulated spatial plane using the hypocenters of 184 aftershocks, of which the magnitudes* ≥ *4.0. The red star shows the initial location of the mainshock. The simulated strike is 201.7°; the dip angle is 39.4°. The plane dips at Az 291.7° (from north to west 68.3°). These parameter values are close to those of the nodal plane P1: strike Az 207.4°, dip angle 27.1°, dips at 297.4° (Table 1).*

dashed line indicated with the P2 projection shows the projection of the steep-dip plane (P2). Generally, those aftershocks should form a linear trend around that tilted line, if the plane P2 is the real rupture plane. The right panel (b) shows the hypocenters on a vertical plane in a shallow-dipping direction. In the same sense, those aftershocks should form a linear trend around that tilted line indicated with P1

*Studies on the Source Parameters of the 23 June 2014 Rat Islands, Alaska… DOI: http://dx.doi.org/10.5772/intechopen.104600*

#### **Figure 6.**

*The comparison between hypocenters projected onto dipping planes for the same earthquakes is in Figure 3 (b). (a) The hypocenter projections on a vertical plane at a steep-dipping direction. The tilted dashed line indicated with the P2 projection shows the projection of the steep-dipping plane (P2). Ideally, those aftershocks should be approximately aligned around that tilted line, if the plane P2 is the rupture plane. (b) The hypocenter projections on a vertical plane at a shallow-dipping direction. The tilted dashed line indicated with the P1 projection shows the projection of the shallow-dip plane (P1). The trend of the belt formed by the aftershocks is not consistent with the P1 projection; the trend has a moderate dipping. Both panels show that most aftershock activities were from a depth of about 80 km to 150 km.*

projection, if P1 is the real rupture plane. The trend of the belt formed by the aftershocks in (a) or (b) is not consistent with the P2 projection or P1 projection.
