Naoki Suda

Hiroshima University | HU · Department of Earth and Planetary Systems Science

Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab, spanning ~50 major deep slow slip with tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of in-slab seismicity are temporally associated with the episodes. F...

Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab under Kii Peninsula, Japan, spanning nearly 50 major deep slow slip and tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of inslab seismicity are temporally...

Low frequency tremor is one of major seismic activity in specified subduction area. Western Japan subducted by Philippine Sea plate is found the variety of low and slow tectonic phenomena. Very low frequency (VLF) seismic events at low frequency (LF) tremor genetic zone are also identified by tiltmeter and broadband seismic data. Ito et. al. (2007...

Swarms of deep non-volcanic tremors occur with a recurrence interval of 2-6 months along the subduction zone of the Philippine Sea plate in southwest Japan. Space-time plots for tremor sources show that the Shikoku area can be divided into three major segments of tremor sources: the eastern, central, and western segments. Furthermore, the eastern a...

Magnitude-frequency distribution of earthquakes follows the Gutenberg-Richter law. The slope of this law, b value, represents the relative occurrence of large and small earthquakes. Since magnitude is defined as corrected logarithmic amplitude, amplitude-frequency distribution of earthquakes is linear on log-log graph. On the other hand, that of no...

The recent development of a system for exchanging and distributing seismic waveform data over high-speed networks enables seismic events to be monitored in real time throughout Japan. In the present study, we have developed an automatic real-time monitoring system for deep nonvolcanic tremors in southwest Japan. The system automatically detects the...

Coseismic gravity change, correlated with the 2004 off the Kii peninsula earthquakes (∼M7), was detected with a superconducting gravimeter installed at the Inuyama station of Nagoya University, central Japan, where distances from epicenters were ∼250 km. The influence of precipitation in the vicinity of the site just before the earthquakes was incl...

Slow slip events are accompanied by swarms of non-volcanic tremor along the subduction zone of the Philippine Sea plate in southwest Japan: the swarms often occur with a periodicity of about 12 or 24h (refs 4, 5). These episodic events are considered to be a manifestation of stress relaxation at the subducting plate interface. Here, we analyse seis...

The matrix element, which is necessary for evaluating the effect of aspherical perturbations on the splitting and coupling of modes of the Earth's free oscillations, is derived for a transversely isotropic earth model. the terms associated with the hydrostatic ellipticity are expressed in a form which does not include the derivatives of model param...

Core modes are spheroidal modes of the Earth's free oscillations whose energy is dominantly partitioned into the inner core as shear energy. To detect low-frequency core modes a new autoregressive technique of spectral analysis, which is called the Sompi method, has been used. It is shown that the Sompi method can unambiguously recover core-mode si...

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 i...

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 obs...

Swarms of deep low-frequency tremors occur with slow slip events along the subduction zone of the Philippine Sea plate in southwest Japan. These episodic events are considered to be linked in a stress relaxation process in the transition zone of the subducting plate interface. Tremor swarms often exhibit occurrence with a period of about 12 or 24 h...

Low-frequency tremors (LFT) occur in a non-volcanic region in SW Japan. In Eastern Shikoku the occurrence of LFT shows a quasi-periodicity with the interval of about 3 months. In addition to such a long-term periodicity, we can easily observe a short-term periodicity with the periods of 12 and 24 hours from analyses using the periodgram and the AR...

The Indian Ocean Tsunami reached Syowa Station, Antarctica, in approximately 12.5 h after the 2004 Sumatra earthquake. We have analyzed the tsunami records of the tide gauge, including the superconducting gravimeter (SG) at Syowa Station. The synthetic tsunami and the induced gravity variations were calculated in order to compare with observations....

Activities of low-frequency tremors, which occur in the non-volcanic region of southwest Japan along the subduction zone of the Philippine Sea plate, often show slow migration of tremor sources with a speed of about 10 km/day and quasi-periodicity of occurrence with an interval of 2--6 months. Such characteristics seem to be related to fluid libera...

Tele-seismic body wave analysis is one of most efficient method to study source rupture process for ordinary earthquake. However in case of 2004 West coast off Northern Sumatra earthquake, excited seismic wave is dominant of long period component and shows long duration P and S wave records. Therefore we can_ft extract pure direct body wave records...

In an eventual aim to detect background long-period acoustic waves in the atmosphere that have to be excited incessantly by the same sources as for background free oscillations of the solid Earth, we installed a cross array of barometers in a 10-km-wide university forest in central Honshu. The array has 28 microbarometers employing quartz crystal r...

We analyze sea level variation data from a differential GPS and gravity data from a superconducting gravimeter (SG) at Syowa Station, East Antarctica (69.0°S, 39.6°E), in an eight month period of 1998. At frequencies between 0.2 and 2.5~mHz in seismic normal mode band we observe similar spectral peaks in both of the data. Power spectral densities o...

Recently some groups reported Earth's background free oscillations even on seismically quiet days [e.g. Nawa et al., 1998]. Statistical features of them and annual variations of their amplitudes with a peak in July suggest that atmospheric disturbance is the most probable excitation source [Nishida and Kobayashi, 1999; Nishida et al., 2000]. If the...

It has recently been established that the Earth's free oscillations are continuously excited by phenomena other than earthquakes and that these oscillations constitute the background noise in the normal mode band at quiet sites. On the basis of evidence that the excitation source is at or just above the Earth's surface, a normal mode theory of the...

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 modulato...

It has been suggested that the excitation source of the Earth's background free oscillations (BFO) is the random force exerted on the surface due to atmospheric disturbance (e.g. Kobayashi and Nishida, 1998). The previous studies also suggested a possibility for detection of the BFO on Mars. In this study we estimate the excitation level of the Mar...

It has been shown that Earth's background free oscillations (BFO) are continuously excited by random forces exerted on the surface due to atmospheric and/or oceanic disturbances. Beyond amplitude level of BFO, free oscillations could be excited transiently by exotic events, such as slow earthquakes and unknown transient phenomena in fluid parts of...

Superconducting gravimeter (SG) records of GGP (Global Geodynamics Project) and~OHP (Ocean Hemisphere Project) networks are now available for investigating global gravity change at very broadband frequencies. Using these records we can study not only Earth and ocean tides at periods longer than several hours but also Earth's free oscillations at pe...

Records of superconducting gravimeters (SGs) at Canberra (Australia), Esashi (Japan), Metsähovi (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 sta...

Earth's free oscillations were considered to be transient phenomena occurring after large earthquakes. An analysis of records of the IDA (International Deployment of Accelerometers) gravimeter network shows that Earth is freely oscillating at an observable level even in seismically inactive periods. The observed oscillations are the fundamental sph...

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...

Free oscillations excited by the 1989 Macquarie Ridge earthquake were recorded by a local broadband seismograph network. Amplitude patterns of the earth's free oscillations vary rapidly across short distances and provide information about seismic sources. The apparent centroid location for the Macquarie Ridge earthquake was estimated by searching a...

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 Earth's free oscillations 1S8, 1S9, 1S10 and 2S16, which are primarily sensitive to the structure adjacent to the core-mantle boundary (CMB). The observed central frequencies...

Analysis of the Earth's free oscillations using high-quality digital seismograms accumulated over the past 15 years has revealed the large-scale aspherical structure of the Earth. Recently modes of low harmonic degree have been analyzed using a technique of iterative inversion in which spectra of a narrow frequency band which contains a mode of int...

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 âSââ-âSââ, which are primarily sensitive to the structure of the transition zone....

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 predict...

Core modes are spheroidal modes of the Earth's free oscillations whose energy is dominantly partitioned into the inner core as shear energy. We analyzed a total of 87 IDA records of the Earth's free oscillations to detect three core modes 2S2, 6S2 and 7S3 at periods of about 1050, 410 and 320s, respectively. We used a new autoregressive technique o...