Kiwamu Nishida

Continuous excitation of planetary free oscillations by atmospheric disturbances

Seismology provides a powerful tool for probing planetary interiors,, but it has been considered inapplicable to tectonically inactive planets where earthquakes are absent. Here, however, we show that the atmospheres of solid planets are capable of exerting dynamic pressure on their surfaces, thereby exciting free oscillations with amplitudes large enough to be detected by modern broad-band seismographs. Order-of-magnitude estimates of these forces give similar amplitudes of a few nanogals for the Earth, Venus and Mars despite widely varying atmospheric and ambient conditions. The amplitudes are also predicted to have a weak frequency dependence. Our analysis of seismograms, recorded continuously from 1992 to 1993 at 13 globally distributed stations, shows strong evidence for continuously excited fundamental-mode free oscillations on the Earth. This result, together with other recent studies, is consistent with our estimate of atmospheric forcing and we therefore propose that it may be possible to detect atmospheric excitation of free oscillations on Venus and Mars as well.

Earth's continuous oscillations observed on seismically quiet days

Analysis of IDA gravimeter data between 1983 and 1994 and GEOSCOPE data between 1988 and 1994 show that fundamental modes of the Earth, for frequencies between 2 and 7 mHz, are excited even on seismically quiet days. Amplitudes of acceleration are slightly less than one ngal(10−9gal). Examination of a sequence of shorter time interval records suggests that the Earth is oscillating continuously. Currently, both atmospheric excitation and tectonic motions are possible cause(s) of these oscillations.

Three‐dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi‐net tiltmeters

[1] We developed a three-step method for three-dimensional (3-D) S wave velocity tomography by fitting synthetic cross spectra to the observed ones of ambient seismic noise. We applied this method to the recording of Hi-net tiltmeters in Japan at 679 stations from June 2004 to December 2004. First, we calculated normalized cross spectra between radial components and those between transverse components for every pair of stations. The first step is local 1-D S wave velocity inversion for each station assuming small lateral heterogeneity under a 100-km circle of a station. We measured the dispersion curves of fundamental Rayleigh waves, fundamental Love waves, and first overtone of Love waves by fitting the synthetic cross spectra to the observed ones between pairs of stations within the circle. We inverted the measured dispersion curves for obtaining a 1-D S wave velocity model. The second step is the inversion of the observed cross spectra for obtaining path-averaged 1-D S wave velocity structure. The third step is the inversion of the resultant path-averaged structures for obtaining 3-D S wave velocity structure (0.1 � 0.1 � 1 km grid from the surface to a depth of 50 km) using ray approximation. The resultant S wave velocity structures show clear low-velocity anomalies along tectonic lines from the surface to a depth of 20 km. In particular, along the Hidaka mountain range, we observed S wave perturbation more extreme than � 20%. They also show low-velocity anomalies under volcanoes in Kyusyu and Tohoku. In the southwestern part of Shikoku, our results show a clear low-velocity anomaly corresponding to an accretional belt (Shimanto belt). Below 20 km, we observe a low-velocity anomaly in the center of Japan, which suggests a thick crust. Citation: Nishida, K., H. Kawakatsu, and K. Obara (2008), Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi-net tiltmeters, J. Geophys. Res., 113, B10302, doi:10.1029/2007JB005395.

Global Surface Wave Tomography Using Seismic Hum

Long-period surface waves from oceanic or atmospheric disturbances can be used for seismic mapping of the upper mantle. The development of global surface wave tomography using earthquakes has been crucial to exploration of the dynamic status of Earth’s deep. It is naturally believed that only large earthquakes can generate long-period seismic waves that penetrate deep enough into Earth for such exploration. The discovery of seismic hum, Earth’s background free oscillations, which are randomly generated by oceanic and/or atmospheric disturbances, now provides an alternative approach. We present results of global upper-mantle seismic tomography using seismic hum and without referring to earthquakes. At periods of 100 to 400 seconds, the phase-velocity anomalies of Rayleigh waves are measured by modeling the observed cross-correlation functions between every pair of stations from among 54 globally distributed seismic stations. The anomalies are then inverted to obtain the three-dimensional S-wave velocity structure in the upper mantle. Our technique provides a new means for exploring the three-dimensional structure of the interior of terrestrial planets with an atmosphere and/or oceans, particularly Mars.

Statistical features of Earth's continuous free oscillations

Ensemble averages of spectra on seismically quiet days exhibit fundamental spheroidal modes of Earths free oscillations from 3 to 7 mHz at 14 stations. We obtain acceleration amplitudes for these modes of ∼0.4 ngal (1 ngal = 10−11 ms−2) by fitting a spectral model to the ensemble averages of the spectra. The following three statistical features are characteristic to these spectra: (1) standard deviations of the spectra show that the amplitude of each mode fluctuates with time, (2) total modal signal power of the spectra shows that the oscillations are excited continuously, and (3) cross-correlation coefficients between the modal amplitudes show that the modes do not correlate even with adjacent modes. These features suggest a random excitation mechanism. Assuming random excitation, we estimate the correlation length of the source to be shorter than 600 km, the source area to extend over the whole Earths surface, and the repeat time of excitation to be shorter than the damping time of mode. Thus the sources must be incessant random disturbances on the whole surface of the Earth. On the basis of these results, four possible mechanisms are discussed. Atmospheric turbulent motions are efficient in exciting the oscillations and explains the statistical features, whereas the efficiencies of processes in the ocean remain questionable. The cumulative effects of earthquakes with seismic moments smaller than 1017 Nm cannot explain the observed amplitudes because the observed amplitudes at 3 mHz would require a moment release rate of 1019 Nm d−1. We also argue that slow earthquakes cannot explain the stochastic properties of the source.

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.

Atmospheric pressure change associated with the 2003 Tokachi‐Oki earthquake

Clear atmospheric pressure changes associated with the 2003 Tokachi-Oki, Japan, earthquake with Mw 8.3 were recorded with the microbarographs distributed in Japan. The pressure change starts at the arrival of seismic waves and reaches its maximum amplitude at the arrival of Rayleigh waves, suggesting that the observed pressure change was driven by the ground motion of seismic waves passing by the site. We computed the seismic-to-pressure transfer function (i.e., the spectral ratio of the pressure change to the vertical ground motion velocity) for periods between 10 to 50 s from the co-located barograph and seismograph records. Comparison of the observed transfer function with the theoretical one including the finite frequency and wavelength effects for a gravitationally stratified isothermal atmosphere confirms that the observed amplitude and phase of the pressure change are explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors.

Temporal change of phase velocity beneath Mt. Asama, Japan, inferred from coda wave interferometry

[1] We estimated the temporal changes of phase velocity of Rayleigh waves extracted from cross correlations of S-coda waves recorded at 12 stations around Mt. Asama, Japan. First, we extracted a Rayleigh wave by taking cross correlations of S-coda waves for 315 regional earthquakes between October 2005 and February 2009. The dispersion curve of the Rayleigh wave was measured and compared with the one extracted from 18 days of ambient seismic noise. We found that both dispersion curves are consistent with each other, demonstrating the dominance of the fundamental Rayleigh waves. We then divided the entire time period into sub-periods, each of which consists of 80 earthquakes, to measure the temporal changes at frequencies between 0.3 and 0.6 Hz. The result shows the reduction of phase velocity by 1.5 % and the subsequent recovery before the eruption of Mt. Asama in 2008.

Magma pathway and its structural controls of Asama Volcano, Japan

Abstract Asama Volcano, Japan, is one of the most active volcanoes in the Japanese islands. Recent development of geophysical monitoring in Asama Volcano allows us to infer the magma pathway and its structural controls beneath the volcano. Combining geodetic data and precise earthquake locations during recent eruptions suggests that the magma intrudes several kilometres to the west of the summit to a depth of about 1 km below sea level as a nearly east–west-trending dyke. The vertically intruded magma then moves horizontally by several kilometres to beneath the summit before it ascends vertically to make the surface. Combining the P-wave velocity and the resistivity structure shows that the intrusions are under structural controls. Frozen and fractureless magma associated with volcanic activity until 24 000 years ago impedes the ascent of rising magma on its way to the surface. The S-wave velocity structure inferred from ambient noise tomography reveals a low-velocity body beneath the modelled dyke. From independent information, we have inferred that this low-velocity body is likely to be a magma chamber.

Atmospheric excitation of planetary free oscillations

Seismology is a powerful tool for probing planetary interiors, and provides us with information on planetary evolution and material properties under high pressure and high temperature. However, seismology has been considered inapplicable to tectonically inactive planets. We, however, show that the atmospheres of solid planets are capable of exerting dynamic pressure on their surfaces, thereby exciting free oscillations with amplitudes that are large enough to be detected by broad-band seismographs. An order-of-magnitude estimate gives amplitudes of a few nanogals for the Earth, Venus and Mars despite the widely varying ambient conditions. The amplitudes are predicted to have weak frequency dependence. Continuous excitations of the Earths free oscillations having the features predicted by our above theory have recently been confirmed by observations. Seismology promises to provide us with the internal structure of Venus and Mars.