Naoki Suda

Incessant excitation of the Earth’s free oscillations

We, for the first time, report the evidence of incessant excitation of the Earth’s free oscillations, mainly the fundamental spheroidal modes in a frequency range from 0.3 to 5 mHz, based on the three year record of a superconducting gravimeter at Syowa Station, East Antarctica. The frequency-time spectrogram of this record is striped by more than 30 lines at nGal level parallel to the time axis, mostly corresponding to the fundamental spheroidal modes. This spectrogram is characterized by relatively efficient excitation of gravest fundamental modes, enhancement of signal intensities in the austral winter and amplification of signal in the frequency band from 3 to 4 mHz. Assuming that earthquakes are only the sources for the free oscillations, we calculate the synthetic spectrograms, which have not shown such a series of parallel lines as observed. The result of this synthetic test and characteristics of the observed spectrogram suggest that the mode signals we found are not of earthquake origin. We tentatively suggest atmospheric or oceanic origin for this newly discovered phenomenon of the solid Earth.

Degree‐2 pattern of attenuation structure in the upper mantle from apparent complex frequency measurements of fundamental spheroidal modes

The authors present preliminary results of an analysis of global lateral variations in elastic and anelastic structure of the upper mantle. Using the Sompi method, they have analyzed IDA records to measure apparent complex frequencies of fundamental spheroidal modes {sub 0}S{sub 21}-{sub 0}S{sub 34}, which are primarily sensitive to the structure of the transition zone. The apparent complex frequencies have then been inverted into local frequency and attenuation maps using the new inversion method. The authors have obtained consistent degree-2 patterns in the local attenuation maps as well as in the local frequency maps. The high-attenuation anomalies correlate with regions of slow P-wave velocity in the lower mantle and with regions of high hotspot density. This may represent a continuation of lower-mantle upwelling into the transition zone.

Loading and Gravitational Effects of the 2004 Indian Ocean Tsunami at Syowa Station, Antarctica

The 2004 Indian Ocean tsunami reached Syowa Station, Antarctica, approximately 12.5 hr after the December Sumatra–Andaman earthquake. We have analyzed the tsunami signals recorded on ocean-bottom pressure gauges, broadband seismometers (sts-1), and a superconducting gravimeter (sg). We calculated the sea level variation, tilt, and gravity changes induced by the tsunami and compared these results to observations. From this comparison we confirmed the loading and gravity effects of the tsunamis on the sts-1 (horizontal components) and the sg records at Syowa Station. The magnitudes of these effects given as root mean square amplitudes are as follows: for the tilt effects obtained from 20-hr-long sts-1 records at frequencies in the range 0.3–0.6 mHz, 5 and 8 μ Gal (10 −8 m/sec 2 ) in the east–west and north–south directions, respectively; and for the gravity effect obtained from the sg records for the same time period of 20 hr at frequencies in the range 0.1–0.2 mHz, 0.2 μ Gal. By using detailed bathymetry around Syowa Station, the synthetic amplitudes similar to the observed were obtained, although the waveforms of synthetic and observation are not always consistent.

Incessant excitation of the Earth's free oscillations: global comparison of superconducting gravimeter records

Abstract Records of superconducting gravimeters (SGs) at Canberra (Australia), Esashi (Japan), Metsahovi (Finland) and Syowa Stations (Antarctica) were analyzed to search for further evidence of background free oscillations of the Earth. Spectrograms for 1-year period and averaged power spectra for seismically quiet periods were obtained for each of the stations. Anomalous features of the oscillations observed at Syowa Station, such as an apparent seasonal variation and a high intensity at frequencies between 3 and 4 mHz, were absent at the other SG stations. Among the SG stations used in this study, the background free oscillations were detected most consistently and distinctly at Canberra, where the noise level was comparable to that at the IDA quietest station, while that at Syowa Station was close to the critical limit for detecting the oscillations. The background free oscillations provide a good reference to evaluate the noise level in the milliHertz band.

Iodine-stabilized Nd:YAG laser applied to a long-baseline interferometer for wideband earth strain observations

We stabilized a frequency-doubled Nd:YAG laser (λ=532 nm) with reference to an iodine absorption line, and applied it to a long-baseline interferometer for earth strain observations. To obtain unmodulated light, saturated absorption signals of an external iodine cell were detected by the modulation transfer technique using an acousto-optic modulator working as both amplitude and frequency modulators. Two feedback loops, which could control the laser frequency by a piezo-electric actuator and a thermal actuator, realized fast and wide-range frequency stabilization, and ensured long-term stable operation. From a beat-note measurement between two identical systems, we obtained a frequency stability (in Allan variance) of ≲2×10−13 for time intervals of 10 to 1000 s. By applying the stabilized Nd:YAG laser to a light source of a 10 m interferometer, we successfully observed earth tides and earthquakes in strain variations. Other geophysical signals, detectable by this strainmeter, are also discussed.

Structure of the D″ layer inferred from the Earth's free oscillations

Abstract We detected, by applying the Sompi method of spectral analysis to the long-period records of the International Deployment of Accelerometers (IDA), the spheroidal modes of the Earths free oscillations 1 S 8 , 1 S 9 , 1 S 10 and 2 S 16 , which are primarily sensitive to the structure adjacent to the core-mantle boundary (CMB). The observed central frequencies are systematically lower than those predicted from the standard Preliminary Reference Earth Model (PREM). These frequencies and those of several other lower-mantle modes were inverted into a spherical structure of the D″ layer. A global feature resolvable from the data is a decrease in P velocity gradient in the D″ layer. A possibility of anomalous density increase in the D″ layer is suggested but a trade-off between density and S velocity hampers a definitive conclusion. There is no indication of a strong decrease in Q in the D″ layer as might be expected from a simple thermal boundary layer model. We retrieved the splitting functions up to spherical harmonic degree four from the frequencies of apparently split peaks of 0 S 7 , 1 S 8 and 1 S 10 . The splitting functions of these modes consistently show negative anomalies in the Pacific and in Africa and positive anomalies in the circum-Pacific region, in good agreement with body wave tomographic models. Their intensities increase significantly with the increasing sensitivity to the structure near the CMB, indicating enhancement of lateral heterogeneity in the lowermost mantle, a result difficult to explain by a simple thermal boundary layer model, in which the isothermal CMB tends to minimize the thermally induced heterogeneity of the D″ layer.

use of apparent complex frequencies of free oscillations to estimate aspherical structure of the Earth

A formula is developed to relate observed apparent complex frequencies of a multiplet to the interaction coefficients of the splitting matrix. This formula is then incorporated into an inversion scheme: estimating the interaction coefficients from the observed apparent complex frequencies. The numerical experiments demonstrated an excellent predictability of our method both in data (apparent complex frequencies) space and in model (aspherical structure of elasticity and anelasticity) space.

LOCAL NETWORK OBSERVATION OF THE EARTH'S FREE OSCILLATIONS : APPARENT CENTROID OF THE 1989 MACQUARIE RIDGE EARTHQUAKE

A local broadband seismograph network in Honshu Island, Japan, recorded the free oscillations excited by the 1989 Macquarie Ridge earthquake, one of the largest strike-slip events in the last decade. The initial amplitude pattern of the spheroidal modes varies from station to station in the network in spite of the small differences in epicentral distance and epicenter-to-station azimuth. Theoretical amplitude of each mode depends on the earthquake mechanism and distance. The most dramatic change occurs for a pure strike-slip event near the distance of 90° where the initial amplitude of every odd order multiplet becomes exactly zero. This is the case for the Macquarie Ridge earthquake observed in Japan. From this sensitivity of the amplitude pattern to the epicentral distance, we can estimate the apparent centroid location by searching a minimum misfit between the observed and theoretical initial amplitudes. The apparent centroid, which is a projection of the real centroid onto the great circle path, is estimated to be 49 km from the epicenter in an azimuth of N19°W.

Sea level and gravity variations after the 2004 Sumatra Earthquake observed at Syowa Station, Antarctica

The Indian Ocean Tsunami reached Syowa Station, Antarctica, in approximately 12.5 hours after the 2004 Sumatra-Andaman Earthquake. We have analyzed the tsunami records of the tide gauge, including the superconducting gravimeter (SG) at the station. The synthetic tsunami and the induced gravity variations were calculated in order to compare with observations. It was found that the gravity effects of the tsunami exhibited an amplitude of microGal (10−8 m/s2); obtained from the Syowa SG. Furthermore, the effects of the tsunami on the Earth’s free oscillation records of the SG were subtracted by applying a transfer function method, using the tide gauge records as input. The improvement of S/N at frequencies of 0.3 mHz is remarkable.