No abstract

Robust determination of earthquake source parameters over a continuous depth range is central to inferring rupture mechanisms dominant at different depths. We employed a cluster‐event method to constrain the source parameters as well as along‐path attenuation for earthquakes over 0–150 km depths and 4 orders of seismic moments in the Japan subduction zone. We found that corner frequency and stress drop increase with depth, whereas the radiated energy scaled by seismic moment declines with depth slightly. As a result, the radiation efficiency exhibits a notable deficit for events deeper than 60 km. Together these suggest an increased energy dissipation during faulting in ductile deformation regime, consistent with shear heating instability as an important faulting mechanism for intermediate‐depth earthquakes.

The morphologies of subducted remnants in the lower mantle are essential to our understanding of the history of plate tectonism. Here we image a high‐velocity slab‐like (HVSL) anomaly beneath the southeastern U.S. using waveforms from five deep earthquakes beneath South America recorded by the USArray. In addition to travel time anomalies, the multipathing of S and ScS phases at different distances are used to constrain the HVSL model. We jointly invert S and ScS traveltimes, amplitudes, and waveform complexities to produce a best fitting block model characterized by a rectangular shape with a 2.5% S wave velocity increase and tapered edges. While the Farallon slab is expected to dip primarily eastward, the HVSL structure apparently dips 40° to 50° to the SE and appears to be related to the eclogitized Hess conjugate.

The Tatun Volcanic Group (TVG) is proximal to the metropolis of Taipei City (population of ca. 7 million) and has long been a major concern due to the potential risks from volcanic activity to the population and critical infrastructure. While the TVG has been previously considered a dormant or extinct volcano, recent evidence suggests a much younger age of the last eruption event (~ 6000 years) and possible existence of a magma reservoir beneath the TVG. However, the location, dimension, and detailed geometry of the magma reservoir and plumbing system remains largely unknown. To examine the TVG volcanic plumbing structure in detail, the local P-wave travel time data and the teleseismic waveform data from a new island-wide Formosa Array Project are combined for a 3D tomographic joint inversion. The new model reveals a magma reservoir with a notable P-wave velocity reduction of 19% (ca. ~ 19% melt fraction) at 8–20 km beneath eastern TVG and with possible northward extension to a shallower depth near where active submarine volcanoes that have been detected. Enhanced tomographic images also reveal sporadic magmatic intrusion/underplating in the lower crust of Husehshan Range and northern Taiwan. These findings suggest an active volcanic plumbing system induced by post-collisional extension associated with the collapse of the orogen.