Double-Difference Adjoint Tomography of the Crust and Uppermost Mantle Beneath Alaska

We perform adjoint waveform tomography to reveal the P-wave velocity structure of the crust and uppermost mantle in Alaska by using common-source double-difference traveltime data. Our underlying forward modeling tool is a 3D seismic-wave solver called SPECFEM3D_GLOBE. We select similar to 13,000 high-quality P-wave arrivals from 147 earthquakes recorded by more than 600 stations. The waveforms are filtered between 15 and 8 s and used to generate about 100,000 differential traveltimes via the cross-correlation technique. We invert these common-source differential traveltimes for a P-wave velocity model down to 150 km depth in Alaska. In the upper mantle, a high-velocity zone is imaged along the Aleutian volcanic arc, which represents the Pacific plate subducting beneath the North America plate. On the eastern edge of the subducting Pacific plate, the imaged high-velocity anomaly extends distinctly beyond the Wadati-Benioff zone, indicating an aseismic slab edge. We observe an absence of a low-velocity mantle wedge beneath the Denali Volcanic Gap (DVG) in our tomographic model. This finding suggests that minimal melt accumulation exists beneath the DVG, which may explain the cessation of magmatism there. Our model also reveals a high-velocity anomaly near the Wrangell Volcanic Field (WVF), suggesting the possible existence of the Wrangell slab or the high-velocity slab edge of the Yakutat slab. A potential slab gap shown as a low-velocity body is detected at 95-125 km depth near the WVF, which could act as a channel to transport mantle materials to feed the cluster of volcanoes in the WVF. Plain Language Summary Characterizing the subsurface structure of Earth's interior is achieved by recording how seismic waves-originating from earthquakes-travel different speeds depending on the materials through which they travel. We focus on using the traveltimes of seismic waves from earthquakes in Alaska to produce a tomographic image of the subsurface variations of Alaska. Our technique differs from previous approaches in that we rely on differences in traveltimes from earthquakes to stations that are close to each other, which can be calculated by correlating the shapes of recorded waveforms. Our resulting tomographic image reveals major plate tectonic features, such as the subducting Pacific plate and volcanic regions. Our technique and resulting images enable a better understanding and interpretation of the subsurface structure and geodynamics of Alaska.