Ryota Takagi

Teleseismic S wave microseisms

A seismic “weather bomb” detector Seismic tomography is like an x-ray of Earths interior, except that it uses earthquakes for the illumination. Earthquakes are imperfect illuminators because they are clustered on plate boundaries, leaving much of the interior in the shadows. Using a seismic array in Japan, Nishida and Takagi detected seismic waves that they attribute to a severe and distant North Atlantic storm called a “weather bomb” (see the Perspective by Gerstoft and Bromirski). The seismic energy traveling from weather bombs through the Earth appears to be capable of illuminating the many dark patches of Earths interior. Science, this issue p. 919; see also p. 869 Detection of microseisms from a severe distant storm provides a new path for seismic structure determination. Although observations of microseisms excited by ocean swells were firmly established in the 1940s, the source locations remain difficult to track. Delineation of the source locations and energy partition of the seismic wave components are key to understanding the excitation mechanisms. Using a seismic array in Japan, we observed both P and S wave microseisms excited by a severe distant storm in the Atlantic Ocean. Although nonlinear forcing of an ocean swell with a one-dimensional Earth model can explain P waves and vertically polarized S waves (SV waves), it cannot explain horizontally polarized S waves (SH waves). The precise source locations may provide a new catalog for exploring Earth’s interior.

Separating body and Rayleigh waves with cross terms of the cross‐correlation tensor of ambient noise

We develop a novel method to separate body and Rayleigh waves with the vertical-radial (ZR) and radial-vertical (RZ) components of the cross-correlation tensor of ambient noise. Furthermore, analyzing ambient noise records observed at a seismic array, we validate the method. For the separation, we utilize the difference in polarizations between the rectilinear P and the elliptic Rayleigh waves. Assuming the two-dimensional surface and three-dimensional body waves are the superposition of random uncorrelated plane waves, we derive two fundamental characteristics of the ZR and RZ correlations. One is that between the ZR and RZ correlations, Rayleigh wave contributions have the opposite signs, and P waves have the same signs. The other is that for both ZR and RZ correlations, Rayleigh wave contributions are time symmetric, and P waves are time antisymmetric. Accordingly, we can separate P and Rayleigh waves by just taking the sum and difference between ZR and RZ correlations and by just taking the time-symmetric and time-antisymmetric components. This method can be performed (1) without any knowledge of velocity structure, (2) using only two stations with three-component sensors on a ground surface, (3) even in the case of anisotropic wave incidence, and (4) with the quite simple procedure. We consider that the developed method can make better use of three-component observations of ambient noise for evaluating the cross-correlation tensor accurately, for improving deep velocity structure using both of extracted body and surface waves, and, more fundamentally, for understanding the composition of ambient noise.

Erratum to: Reconstruction of a 2D seismic wavefield by seismic gradiometry

We reconstructed a 2D seismic wavefield and obtained its propagation properties by using the seismic gradiometry method together with dense observations of the Hi-net seismograph network in Japan. The seismic gradiometry method estimates the wave amplitude and its spatial derivative coefficients at any location from a discrete station record by using a Taylor series approximation. From the spatial derivatives in horizontal directions, the properties of a propagating wave packet, including the arrival direction, slowness, geometrical spreading, and radiation pattern can be obtained. In addition, by using spatial derivatives together with free-surface boundary conditions, the 2D vector elastic wavefield can be decomposed into divergence and rotation components. First, as a feasibility test, we performed an analysis with a synthetic seismogram dataset computed by a numerical simulation for a realistic 3D medium and the actual Hi-net station layout. We confirmed that the wave amplitude and its spatial derivatives were very well-reproduced for period bands longer than 25 s. Applications to a real large earthquake showed that the amplitude and phase of the wavefield were well reconstructed, along with slowness vector. The slowness of the reconstructed wavefield showed a clear contrast between body and surface waves and regional non-great-circle-path wave propagation, possibly owing to scattering. Slowness vectors together with divergence and rotation decomposition are expected to be useful for determining constituents of observed wavefields in inhomogeneous media.