Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures

Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above∼15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient highfrequency radiation, and few early aftershocks. The slab interface at intermediate depths (∼15–35 km) is themain unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediateto highfrequency radiation. The deeper portion of the slab interface (∼35– 45 km) is seismically unstable, howeverwith small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.