[S-SS04] Seismological advances in the ocean

[SSS04-08] Fluids control along-strike variations in the Alaska seismogenic zone and mantle wedge processes

*Fan Wang¹, Songqiao Wei¹, Connor Drooff¹, Julie Elliott¹, Jeffrey T Freymueller¹, Natalia A Ruppert², Haijiang Zhang³ (1.Michigan State University, 2.University of Alaska Fairbanks, 3.University of Science and Technology of China)

Keywords:Alaska megathrust, Fluids, Slip deficit variations

The seismogenic zone usually displays high interseismic coupling between the downgoing slab and the overriding plate. The highly coupled seismogenic zone is bounded by creeping transition zones in both up-dip and down-dip directions. However, more complexities beyond this 2D model are observed, suggesting varying sizes and properties of seismic asperities on the plate interface. The Alaska Peninsula shows large along-strike variations from the fully coupled Kodiak segment in the northeast to the lowly coupled Shumagin segment in the southwest. Changes in seafloor fabrics, slab hydration states, and sediment thickness have been proposed to explain the variations of slab coupling along the trench. However, the controlling factors of the interface slip behaviors have not been fully understood. Here we image high-resolution 3D Vp/Vs structures along the Alaska Peninsula using the newly available onshore/offshore seismic data. The overriding plate and the plate interface display distinct changes in the Vp/Vs signature across different segments. A very high Vp/Vs (~1.88) anomaly at the plate interface at 20-30 km depths in the eastern Shumagin segment indicates the migrated free fluids released from deep sources and ponding beneath the overriding plate Moho. The 2020 M7.8 Simeonof earthquake nucleated from this high Vp/Vs anomaly. In contrast, the Vp/Vs ratio is much lower (~1.72) at 20-30 km depths in the western Semidi segment, adjacent to higher Vp/Vs anomalies (~1.82) in the up-dip and down-dip directions. The 2021 M8.2 Chignik earthquake nucleated from the down-dip high Vp/Vs anomalies and ruptured across the low Vp/Vs anomaly. The Vp/Vs ratio in the overriding plate is anti-correlated with the variations of slab coupling. This striking anti-correlation suggests that free fluids migrated from deep sources play an important role in controlling slab slipping behaviors. In addition, we image low Vp and high Vp/Vs anomalies in the mantle wedge, indicative of sub-arc melting. The Vp/Vs variations in the mantle wedge can shed light on the distribution of free fluids and help us better understand mantle wedge processes.