## 4 Discussion
### Slab-Derived Fluids and Earthquakes  
Likewise, we compared the slab water content with the M \(>\) 3 earthquakes near the interplate surface from 1900 to 2010, plotted as colored spheres in Figures 4 and 5. The regular earthquakes with recorded magnitudes of M3-M5.5 are mostly distributed along the dewatering zone (Figure 5a). Most of the M \(>\) 5 earthquakes are located in the updip part of the dewatering zone. The SSEs beneath the Upper Cook Inlet are located in the dewatering zone (yellow dashed circle, Figure 5a), whereas the SSEs beneath the Lower Cook Inlet are located partly in the downdip part of the dewatering zone (gray dashed circle, Figure 5a). The temperature variations along the arc control the fluid's release of hydrous minerals, resulting in a heterogeneous distribution of tremor events beneath the Unalaska and Akutan Islands [20; 56]. The great Mw7.1 (Anchorage, 2018) earthquake ruptured beneath the Upper Cook Inlet, and the Mw9.2 Great Alaskan Earthquake (1964) ruptured from the Copper River Delta to Kodiak Island above the subduction. [PERSON] et al. [57] have suggested that high pore pressures do not exist on the surface slab beneath Kodiak Island at a depth of \(\sim\)8-20 km. As we thought, the dehydration should occur deeper in the slab. From our results, we found that the distribution of fast and slow earthquakes coincides with the dewatering zone both on the slab's upper surface and inside the slab, likely due to the dehydration front variation with depth.  
The slab water content heterogeneity usually leads to the release of large volumes of fluid from the oceanic plate when particular subduction pathways experience sufficiently high temperatures and pressures [58; 59]. The area where the water content of the rock changes significantly from the plate interface to the intraslab at 8 km (Figure 4a,b) indicates that abundant fluids are released from the dehydrated slab under notable temperature and subsequent viscosity differences. Slab-derived fluids are a crucial part of flux melting. High-permeability systems are dominated by buoyancy, and fluid flow is primarily vertical, but if compaction pressure is included, the upslope flow within high-permeability layers in the slab produced by local dehydration reactions is enhanced [60]. Explicitly modeling water migration in subduction dynamic models through a compacting and decompacting solid matrix shows that a shallow release depth (e.g., 40 km) leads the water mainly through the trenchward (upslab) pathway [61]. Thus, we suggest that the fluids released from the slab beneath south-central and southwestern Alaska can be transported to shallow-medium depths.  
Furthermore, if enough fluid is released from the dipping oceanic lithosphere into the mantle wedge, the forearc mantle can constitute an important reservoir of water (e.g., [62]). In this case, such a reservoir hydrates the plate interface and increases the pore fluid pressure (e.g., [63]). Hydrous phases such as serpentine progressively dehydrate, release fluids and cause dehydration embrittlement [64; 65; 66]. Then, when the effective normal stress is reduced, the occurrence of earthquakes will be facilitated. In other cases, rheological weakening [67] and the chemical reaction kinetics of self-sustained oscillations of serpentinite [68] are other candidates used to interpret the relations between fluids and slow earthquakes, but these processes are also based on a hydrous wedge. Consequently, we consider that the occurrence of interplate earthquakes, such as those recorded along the coast and the SSEs detected in and around the Upper Cook Inlet, is possibly facilitated by fluid migration and dehydration embrittlement. In a recent study, [PERSON] et al. [29] inferred that the released water beneath south-central Alaska may be transported to the plate boundary and cause the occurrence of tectonic tremors, which coincides with our conclusion. Moreover, [PERSON] et al. [6] also suggested that the slab beneath southwestern Alaska is more hydrated, which is consistent with our results.