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Abstract Beneath oceanic spreading centres, the lithosphere–asthenosphere boundary (LAB) acts as a permeability barrier that focuses the delivery of melt from deep within the mantle towards the spreading axis¹. At intermediate-spreading to fast-spreading ridge crests, the multichannel seismic reflection technique has imaged a nearly flat, 1–2-km-wide axial magma lens (AML)²that defines the uppermost section of the LAB³, but the nature of the LAB deeper into the crust has been more elusive, with some clues gained from tomographic images, providing only a diffuse view of a wider halo of lower-velocity material seated just beneath the AML⁴. Here we present 3D seismic reflection images of the LAB extending deep (5–6 km) into the crust beneath Axial volcano, located at the intersection of the Juan de Fuca Ridge and the Cobb–Eickelberg hotspot. The 3D shape of the LAB, which is coincident with a thermally controlled magma assimilation front, focuses hotspot-related and mid-ocean-spreading-centre-related magmatism towards the centre of the volcano, controlling both eruption and hydrothermal processes and the chemical composition of erupted lavas⁵. In this context, the LAB can be viewed as the upper surface of a ‘magma domain’, a volume within which melt bodies reside (replacing the concept of a single ‘magma reservoir’)⁶. Our discovery of a funnel-shaped, crustal LAB suggests that thermally controlled magma assimilation could be occurring along this surface at other volcanic systems, such as Iceland. 

Abstract On 8 July 2021 a M6.0 normal faulting earthquake rocked the community of Walker and the surrounding region near the California‐Nevada border. In the 1990s, field surveys of nearby Meadowcliff Canyon identified numerous precarious rocks deemed likely to topple in the event of strong shaking. Despite their proximity (∼6 km) to the 2021 earthquake, the precarious rocks still remain standing. In this work, we combine advanced source and ground motion characterization techniques to help unravel this mystery. High‐precision hypocentral locations reveal a clear north/south‐striking, east‐dipping rupture plane along the southern extension of the Slinkard Valley fault. The mainshock nucleated near the base of the fault, triggering thousands of aftershocks. Bayesian source spectral analyses indicate that the mainshock had a moderately‐high stress drop (∼17 MPa), and that aftershocks with deeper hypocenters have higher stress drops. Peak Ground Acceleration (PGA) recordings at regional stations agree well with existing ground motion models, predicting PGA of ∼0.3 g in Meadowcliff Canyon, a level sufficient to topple precarious rocks based on PGA‐derived stability criteria. We demonstrate that despite these large ground accelerations, the pulse duration in Meadowcliff Canyon is too short to supply the impulse necessary to damage these features, observations which support the application of dynamic toppling models that account for the joint effects of pulse amplitude and duration when assessing rock fragility. This study provides a unique vantage point from which to interpret rarely‐observed strong‐motion recordings from close to an active normal fault, one of many that dominate hazard along the eastern Sierra.