**6. Discussion and conclusion**

This *M*<sup>W</sup> 7.9 event intrigued scientific interests and generated issues. In this section, we discuss some issues and provide some conclusions.

Beneath the Rat Islands at a depth of 100 km, the shear wave velocity is about 4.5 km/s. Using the PREM earth model and the assumed rupture velocity formula *V*<sup>R</sup> = 0.7*β*, *V*<sup>R</sup> = 0*:*7 4*:*5 = 3.1 km/s. This value is much larger than what was obtained using the back-projection method (1.5 km/s) by Ye et al. [3]. The average rupture velocity obtained by Twardzik and Ji [5] was 2.4 km/s from modeling the steep-dip plane and 2.3 km/s from the shallow-dip plane. The optimal rupture velocity we obtained from trial inversion tests at the initial depth of 92 km was 2.0 km/s. For the trial tests with an initial depth of 84 km, the optimal rupture velocity was 1.8 km/s, and 2.1 km/s at the initial depth of 105 km. Therefore, the rupture velocities of around 2.0 km/s may be reasonable for the Rat Islands *M*<sup>W</sup> 7.9 earthquake.

#### **Figure 18.**

*(a) Lower hemispherical projection of the simulated focal mechanism. (b) Source time function. (c) Distribution of the slip on the simulated plane. The star sign \* with "start point" shows the assigned location of the initial rupture. The arrow at a sub-fault shows the direction and the amount of the slip. The obtained maximum slip is about 3.36 m, occurred at about a depth of 70 km. The used rupture velocity VR = 1.5 km/s. The dashed circles show the rupture propagation.*

Two nodal planes can be retrieved from an earthquake moment tensor solution. One of them is assumed to be close to the rupture plane and used for establishing a rupture slip model. Ye et al. [3] preferred the shallow-dip plane for rupture modeling. Twardzik and Ji [5] relocated larger aftershocks. Based on the relocated hypocenters they selected the steep-dip plane as the rupture plane. When Miyazawa [6] calculated the dynamic changes in the Coulomb Failure Function for the *M*<sup>W</sup> 7.9 mainshock, the shallow-dip plane of the G-CMT was used. Macpherson and Ruppert [2] relocated the aftershocks. They found that the seismicity is dipping at a moderate angle to the northwest and does not align well with any dip. They also found the shallow-dip plane *Studies on the Source Parameters of the 23 June 2014 Rat Islands, Alaska… DOI: http://dx.doi.org/10.5772/intechopen.104600*

**Figure 19.**

*Comparison between the 27 observed and synthetic seismograms. For each pair of waveforms, the upper trace is the observed (solid-line); the lower trace is the synthetic (dashed-line), generated with the slip distribution in Figure 18c. The observed waveforms are exactly the same as those in Figure 16. The fits at stations AAK, KIP, and MIDW are not good; the maximum amplitudes ratio at several stations is not close to 1 (AAK 2.58; BFO 0.37; KBS 0.47; MIDW 0.46; TARA 0.46). All these numbers are far from the ideal ratio (1). The average variance (0.2720) is also larger than those in Figure 14 (0.1570) and Figure 16 (0.1545).*

does align well with the mainshock hypocenter, so they preferred the moderately dipping nodal plane as the rupture plane of the *M*<sup>W</sup> 7.9 mainshock. Florez and Prieto [7] recalculated the focal depths for a subset of 17 *M*<sup>W</sup> > 4.9 aftershocks using the time difference between a tele-depth phase pP and direct P phase. Based on their results they confidently assigned the causative fault plane to the steep one. To identify which nodal plane is close to the rupture plane we also relocated the larger aftershocks. We carefully recalculated the focal depths using pP-P times and relocated the epicenters at the recalculated depths for 23 aftershocks with mb > 4.5, which occurred within 20 days after the mainshock. We found a linear segment about 15 km long formed by 11 aftershocks in the deeper group (**Figure 9a**) is approximately parallel to the dipping of the steep-dip plane, but no linear segment along the dipping of the shallow-dip plane was formed (**Figure 9b**). Based on the above features we deduced that the steep-dip nodal plane is close to the rupture plane of the mainshock.

When the steep-dip plane was used as the rupture plane, the major rupture patch we retrieved was distributed in a depth range from about 80 km to 140 km (**Figure 13**, the largest patch). The maximum slip we obtained was about 3.5 m, which was well consistent with that (3.7 m) obtained by Twardzik and Ji [5].

We also performed trial inversion using the shallow-dip plane as the rupture plane and found the average variance (0.1545) is almost the same as that (0.1570) obtained using the steep-dip plane. This implies that the rupture plane indeed cannot be identified using the mismatch between the observed and synthetic seismograms.

Since the majority of aftershocks are distributed along a moderate-dipping plane, it may be thought that the mainshock ruptured along the moderate-dipping plane. Test inversions using the simulated plane as the rupture plane were performed. It was found that the waveform fits at stations AAK, KIP, and MIDW are not good; and the ratio of the maximum amplitudes at several stations is far from the ideal ratio. The average variance (0.2720) is much larger than those in **Figures 14** and **16**; so, the simulated moderate-dipping plane was denied to be the rupture plane of the mainshock.

Based on the assumption that the immediate aftershocks occurred on the rupture plane of the mainshock or near the edges of the rupture [27], aftershock distributions are often used to select the rupture plane from the two nodal planes. When Kikuchi and Kanamori [28] studied the 1994 Shikotan *M*<sup>W</sup> 8.2 earthquake, they found the aftershocks seem to favor the steep nodal plane as the fault plane. The steep-fault model resulted in a better waveform match than the shallow-dip fault model. Delouis and Legrand [29] found that the aftershocks of an intermediate-depth large earthquake delineate a low angle plane, and the low angle fault model provides a much better fit for the strong-motion waveforms. However, for this *M*<sup>W</sup> 7.9 earthquake, the majority of aftershocks were distributed neither along the steep-dip, nor the shallowdip nodal plane. The waveform fits for both nodal planes are almost the same.

A hypothesis may be able to explain that the majority of aftershocks occurred along a moderate-dipping plane, which may be close to the boundary between the Pacific plate and North American plate beneath the Rat Islands region—most parts of the huge rupture fault were immediately locked under a tremendous pressure blow about 80 km of the depth after the occurrence of the mainshock, the stress in the source region was re-distributed, and migrated to the boundary region beneath the Rat Islands region, so most aftershocks distributed along that boundary, rather than the rupture plane of the mainshock.

This huge earthquake is very unique. For example, it had a vigorous aftershock sequence; other intermediate-depth earthquakes were usually followed by few or no aftershocks [1]. Solve the mysteries behind the observed phenomena requires more studies.
