**1. Introduction**

On June 23, 2014, an *M*<sup>W</sup> 7.9 earthquake occurred in the central Aleutians near the Rat Islands, Alaska (**Figure 1**). This earthquake was one of the largest seismic events along the boundary between the Pacific and the North American plates in a century. The focal depth of this earthquake was about 100 km; it is an intermediate-depth event. In most cases, intermediate-depth events are not followed by many aftershocks [1]. However, this mainshock was followed by a large number of aftershocks. The Alaska Earthquake Centre (AEC) located more than 1800 aftershocks within 1 month after the mainshock, and more than 40 of them were larger than magnitude 4 [2].

#### **Figure 1.**

*The location of the 23 June 2014* M*<sup>W</sup> 7.9 earthquake and the study region in the global setting. The red solid circle at the center shows the epicenter. The trapezoid shows the study region. The triangles show the positions of the seismic stations, at which the recorded mantle Rayleigh-wave seismograms were selected for the moment tensor inversion. The cross sign shows the location of the north pole.*

Intermediate-depth earthquakes with *M*<sup>W</sup> ≥ 7.5 are rare. The Global Centroid Moment Tensor (G-CMT) catalog lists only 14 with magnitude *M*<sup>W</sup> ≥ 7.5, which occurred between depths of 70–200 km [2]. The occurrence of this large earthquake with many aftershocks is a very rare case.

The mainshock caused an intention to the seismological society. Some seismologists studied it and published papers on or related to this Rat Islands earthquake sequence.

Ye et al. [3] modeled the source ruptures using both nodal planes. They found the shallow-dip fault plane with strike azimuth Az 205.9° and dip angle 23.6° toward the northwest provides better matches to *P* waveforms at azimuths from 300° to 340° than does the steep-dip fault plane (strike azimuth Az 308° and dip angle 84°). However, the overall waveform mismatch is comparable between the two models, and some signals are better fit using the steep-dip fault plane, so their preference for selecting the shallow-dip plane as the rupture plane is mild. In another word, for this *M*<sup>W</sup> 7.9 earthquake, its causative plane cannot be 100% determined using the fit

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

between the observed and the synthetic waveforms. The maximum slip they obtained is 10.3 m; the average slip is 3.9 m. The rupture velocity they used is 1.5 km/s.

Macpherson and Ruppert [2] relocated the aftershocks. To attempt to determine the correct rupture plane, they plotted cross-sections parallel to the dipping directions of the nodal planes as determined by the Global Centroid Moment Tensor (gCMT) project. The gCMT solutions found one nodal plane with subvertical dip (84°) and a strike of 308° and the other with a moderate dip of 26° and strike of 207°. They found that the seismicity is dipping at a moderate angle to the northwest and does not align well with the dips from either of the gCMT nodal planes. They also found the shallowdip plane does align well with the mainshock hypocenter and a group of unusual oceanic mantle seismicity to the south of the mainshock. They interpret that alignment as delineating the mainshock fault plane, and thus, they prefer the moderately dipping nodal plane as the rupture plane of the *M*<sup>W</sup> 7.9 mainshock. They used the double-difference relocation method [4], and the catalog travel time picks from 29 Broadband and short-period stations to relocate over 2500 earthquakes, which occurred from June 23, 2014, to the end of the year in the mainshock epicenter region (50°N–53°N; 176°E–178°W). As those smaller aftershocks were included; the catalog travel time picks for smaller earthquakes may not be accurate, and the relocation results may or may not be reliable.

Twardzik and Ji [5] first performed a set of finite-fault inversions to invert the slip history of the *M*<sup>W</sup> 7.9 earthquake. They found they cannot identify the causative fault plane by comparing the misfits between observed and the synthetic seismograms. As such, they relocated aftershocks and used the relocated hypocenters to determine the causative plane. They used the Joint hypocenter determination (JHD) method and the arrival times of seismic phases (P, S, pP, sP, PcP, and ScP) reported by the International Seismological Centre (ISC). They selected a dataset, including 19 earthquakes with mb ≥ 4; 17 earthquakes with (3 ≤ mb ≤ 4), occurred from June 23 to September 23, 2014, within 60 km from the epicenter of the mainshock. The mean horizontal error of those JHD locations is 6.2 km; the mean vertical error is 19.8 km. They found that most relocated aftershocks distribute along a 40 km long linear segment orienting northeast and that coincides with the width of the surface projection of the steep-dip fault plane. Vertically, the relocated aftershocks span a depth range from 70 km to 150 km. It is noteworthy that the depth extension of the relocated aftershock distribution is nearly double of its horizontal extension. They concluded that the relocated aftershocks tend to align preferentially along with the fault plane that has a dip of 87° and a strike of 308°. Since the mean vertical error is 19.8 km in the relocated aftershocks, the hypocenter distribution may have some uncertainties. The maximum slip they obtained is about 3.5 m; the rupture velocity VR is about 2.0 km/s.

Miyazawa [6] studied the remote and dynamic earthquake triggering phenomena caused by global transient stress changes generated from seismic waves' propagation of other large earthquakes at a great distance. The author calculated the dynamic changes beneath station ADK and AMKA in the Coulomb Failure Function (ΔCFF) for the *M*<sup>W</sup> 7.9 mainshock. The focal mechanism from the Global CMT (strike 207°, dip 26°, and rake/slip �13°) and focal depth 109 km were used. It was found that the ΔCFF varies within roughly 10 Pa when the Lame's parameters, λ and μ, are 66 GPa and the effective friction coefficient μ<sup>0</sup> is assumed to be 0.4 at the depth. The stress changes varying within at most 10 Pa at the hypocenter region probably caused a reduction in the fault's strength by cyclic fatigue and eventually triggered the fault failure and released energy in the form of an *M*<sup>W</sup> 7.9 earthquake. Unfortunately, the author did not provide the results from the steep-dip plane.

Florez and Prieto [7] introduced a relative earthquake depth determination algorithm using depth phases. They applied their method to determine focal depths for 17 larger aftershocks of the *M*<sup>W</sup> 7.9 earthquake. They projected their relocated hypocenters onto two vertical planes. One is parallel to the steep-dipping direction; the other to the shallow-dipping direction. They found that their results are not consistent with a shallow-dip nodal plane. Therefore, they can confidently assign the causative fault plane to the steep-dip plane. It seems that the catalog epicenters were used when projections of the hypocenters were plotted. It is noticed that their hypocenter projections on the vertical plane that is parallel to the steep-dipping direction did not form a linear trend along with the 84° dipping. The majority are along with about 60° dipping (their Figure 2c).

The above authors made contributions to the studies of this *M*<sup>W</sup> 7.9 sequence. For example, they found that the causative plane of the mainshock cannot be determined using the misfit between observed and synthetic waveforms. Some phenomena related to the mainshock are still not very well emphasized. Such as, the majority of the aftershocks were distributed along with a moderate-dipping trend neither along with the shallow dip nor the steep-dip nodal plane. We have been studying this very rare event from its occurrence; we want to present our results and confirm some phenomena we found. We organized a chapter, which covers parts of our results.

	- nodal plane 1, *P*1: strike 207.4°, dip 27.1°, rake/slip -12.7°, dipping at Az 297.4° (shallow-dip);
	- nodal plane 2, *P*2: strike 308.7°, dip 84.2°, rake/slip -116.5°, dipping at Az 38.7° (steep-dip).
