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The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (~20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano., The activity of volcanoes has been conventionally monitored with short-period seismometers (>1 Hz) (1) and with geodetic instruments including strain- and tilt-meters (2). Short-period records of volcanic earthquakes and tremors [...]

The article discusses geophysics research by N. Schmerr and colleagues within the issue. The author presents a brief overview of the theory of plate tectonics which posits that horizontal plates below the Pacific Ocean are moving laterally at a rate of approximately 10 centimeters (cm) per year above a weak layer called the asthenosphere. He notes that scientists are puzzled by how the plates, which are as large as 10 thousand kilometers (km), can maintain their rigidity while causing earthquakes at their edges. Topics include the use of high-resolution seismic maps by Schmerr's team to observe the process of plate tectonics, the role of the asthenosphere, which is a viscous layer below the lithosphere, in plate movement, and a diagram illustrating plate tectonics in the ocean.

The article is a report on global surface wave telegraphy using seismic hum. Earth's seismic hum signals, which are background free oscillations, are continuously excited by the ocean and atmosphere. The authors used cross-correlation (CC) analysis of hum signals that showed background Rayleigh waves which were propagated globally. Topics include how the waves open the possibility for global surface wave tomography without references to earthquakes.

We recorded P and S waves traveling through the oceanic lithosphere-asthenosphere system (LAS) using broadband ocean-bottom seismometers in the northwest Pacific, and we quantitatively separated the intrinsic (anelastic) and extrinsic (scattering) attenuation effects on seismic wave propagation to directly infer the thermomechanical properties of the oceanic LAS. The strong intrinsic attenuation in the asthenosphere obtained at higher frequency (~3 hertz) is comparable to that constrained at lower frequency (~100 seconds) by surface waves and suggests frequency-independent anelasticity, whereas the intrinsic attenuation in the lithosphere is frequency dependent. This difference in frequency dependence indicates that the strong and broad peak dissipation recently observed in the laboratory exists only in the asthenosphere and provides new insight into what distinguishes the asthenosphere from the lithosphere., (Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)