Some seismic waves excited near the low-velocity subducting oceanic crust are trapped in the low-velocity layer and propagate as guided waves, which are recorded by forearc seismic stations and can be used to image oceanic crustal structures. The subducting oceanic crust plays a critical role in the exchange of material and energy between the Earth’s surface and its interior. The subducting oceanic crust transports water into the Earth’s interior, contributing to partial melting of the mantle wedge and island arc magmatism, and is therefore characterized by a low-velocity layer in its velocity structure. Studies of the subducting oceanic crustal structure can enhance our understanding the processes of subduction metamorphism and subduction dynamics.

When seismic waves excited by the seismic source near the low-velocity oceanic crust at the top of the subducting slab, some waves are trapped in the low-velocity layer and propagate as guided waves. Meanwhile, some seismic waves leak into the high-velocity slab mantle and propagate as head waves. The guided wave phases continue to interact with the oceanic crustal structure during propagation, and their arrivals and waveform characteristics carry information about the velocity and geometry changes of the oceanic crust. After propagating a certain distance, the guided waves can leak out to the Earth’s surface due to the curvature of the slab and can be recorded at forearc seismic stations. In recent years, guided wave information, which is more sensitive to the structure of the low-velocity layer, has been widely used to image the seismic wave velocity structure. Structural characterization studies of the oceanic crust with guided waves have been conducted in many subduction zones around the world, including Japan trench, Alaska-Aleutian subduction zone, Nicaraguan subduction zone, Peru-Chile trench （or Andean subduction zone）, and Hellenic subduction zone.

The Alaska subduction zone, with abundant seismic events recorded by permanent seismic stations, is a natural testing laboratory for imaging oceanic crustal structure with guided waves.

Waveforms from two intermediate-depth earthquake events, located at the top of the subduction seismic zone at the eastern and western sides of the Yakutat terrane, are used to analyze the propagation characteristics of guided wave phases in this paper. Clear guided wave arrivals recorded by regional stations for both events show the significant influence of the subducting oceanic crust. When the epicentral distance is greater than 200 km, the amplitudes of the guided wave phases are often much larger than those of other phases in the wave trains, indicating that the low-velocity oceanic crustal structure in this area is very conducive to the generation and propagation of guided waves.

The arrival delays of the guided waves increase with propagation distance in the Yakutat area at the eastern end of Alaska subduction zone, while no usual guided wave is observed in the Pacific slab area to the west of the Yakutat terrane. This implies that the Yakutat and Pacific oceanic crusts do not directly connect to each other and that the guided waves cannot effectively propagate from the former to the latter. Thus, the propagation characteristics of the guided waves effectively indicate the boundary between the Yakutat terrane and the Pacific slab.

To understand the propagation characteristics of guided wave phases in the subducting Yakutat oceanic crust among ambient mantle, we additionally selected small earthquake events with focal depths ＞70 km at the top of the subduction seismic zone on both sides of the Yakutat terrane. In the records of these earthquake events observed at the same station, the arrival time difference between the P-P guided wave and the S-S guided wave phases increases with the focal depth, indicating that the Yakutat oceanic crust is continuously distributed and subducts into the Earth’s interior.

The arrival times of the seismic phases were used to infer the apparent velocity anomalies of the guided wave phases on four profiles in the region. P and S guide wave apparent velocity anomalies of −19% and −15% are commonly inferred in the Yakutat area. Similar apparent velocity anomalies are found in the guided waves from seismic events at depths of 70−100 km on both the eastern and western sides of the Yakutat terrane, with the apparent velocity anomalies on the western side of the Yakutat terrane are slightly smaller than those on the eastern side. These apparent velocity anomalies are reduced to −6% in the seismic events recorded at depths of 100—130 km on the eastern side of the Yakutat terrane.

The above results indicate that the Yakutat oceanic crust subducts into deep Earth in the region, and the velocity anomalies are reduced due to the dehydration of the Yakutat oceanic crust during its subduction.