The 2011 Mw 9 Tohoku‐Oki earthquake, recorded by over 1000 near‐field stations and multiple large‐aperture arrays, is by far the best recorded earthquake in the history of seismology and provides unique opportunities to address fundamental issues in earthquake source dynamics. Here we conduct a high resolution array analysis based on recordings from the USarray and the European network. The mutually consistent results from both arrays reveal rupture complexity with unprecedented resolution, involving phases of diverse rupture speed and intermittent high frequency bursts within slow speed phases, which suggests spatially heterogeneous material properties. The earthquake initially propagates down‐dip, with a slow initiation phase followed by sustained propagation at speeds of 3 km/s. The rupture then slows down to 1.5 km/s for 60 seconds. A rich sequence of bursts is generated along the down‐dip rim of this slow and roughly circular rupture front. Before the end of the slow phase an extremely fast rupture front detaches at about 5 km/s towards the North. Finally a rupture front propagates towards the south running at about 2.5 km/s for over 100 km. Key features of the rupture process are confirmed by the strong motion data recorded by K‐net and KIK‐net. The energetic high frequency radiation episodes within a slow rupture phase suggests a patchy image of the brittle‐ductile transition zone, composed of discrete brittle asperities within a ductile matrix. The high frequency is generated mainly at the down‐dip edge of the principal slip regions constrained by geodesy, suggesting a variation along dip of the mechanical properties of the mega thrust fault or their spatial heterogeneity that affects rise time.

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We report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Piñon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ. Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity Sθ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low‐velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of Sθ≈5 × 103 and a stress sensitivity of 0.5 MPa−1. The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here.

Earthquakes get a more flexible source Earth's surface deforms in part as a result of ruptures along brittle crustal faults that generate earthquakes. Understanding rock deformation in the ductile lower crust and mantle is challenging. Using the densest seismic arrays in the world, Inbal et al. have found an unexpected localization of seismicity at these depths under the Newport-Inglewood fault in southern California. The seismicity points to a type of earthquake that may help us understand how ductile deformation operates in this region of Earth. Science , this issue p. 88