Takashi Okuda

Error evaluation in acoustic positioning of a single transponder for seafloor crustal deformation measurements

The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its “true” position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.

Continuous long-term array analysis of seismic records observed during the 2011 Shinmoedake eruption activity of Kirishima volcano, southwest Japan

We deployed a seismic array at a site 5 km east of Shinmoedake volcano, in the Kirishima volcanic complex of southwest Japan, five days after the sub-Plinian eruption on 26 January, 2011. The array record between February and September 2011 included explosion earthquakes and episodes of weak continuous tremor during eruption periods. We estimated slownesses and back azimuths of seismic waves on a sliding 1-min window using the semblance method. The slownesses of the weak continuous tremor clustered within the range 0.2–0.8 s/km, consistent with a mix of body and surface waves. A probabilistic approach based on a grid search was used to estimate the source locations of the explosion earthquakes and weak continuous tremor. The sources of the explosion earthquakes were beneath the crater at depths of −0.5–1 km above sea level, while the source of the weak continuous tremor was beneath the northern part of Shinmoedake at depths between 1 km below sea level and 1 km above sea level. This latter region corresponds to a shallow low-resistivity layer, suggesting that hydrothermal processes are more plausible than magmatic processes as the generating mechanism of the weak continuous tremor.

Interplate locking condition derived from seafloor geodetic data at the northernmost part of the Suruga Trough, Japan

We observed seafloor crustal deformation at two observation sites on opposite sides of the Suruga Trough off Japan from 2005 to 2011 to investigate the interplate locking condition at the source region of the anticipated great subduction earthquake, named Tokai earthquake. We estimated the displacement velocity vectors relative to the Amurian Plate on the basis of repeated observations. Our results at the two points, Suruga northeast and Suruga northwest (SNW) were 42 ± 8 mm/yr toward N94 ± 3°W and 39 ± 11 mm/yr toward N84 ± 9°W, respectively. These directions are the same as those measured at on-land GPS stations. The magnitudes of the velocity vectors indicate a significant shortening of approximately 4 mm/yr between SNW and on-land GPS stations located to the west of the Suruga Trough. The results show that the plate interface is strongly locked (no slip) shallower than the source region of the anticipated Tokai earthquake.

Repeatable Measurements of Baseline Length by Global Positioning System in Central Japan

An accurate determination of baseline length is of basic importance in the interpretation of crustal motions in earth’s interior for the study of earthquake prediction. A complete understanding of the crustal motion of a particular region requires a dense set of geodetic measurements in space and time. During the past decade, Global positioning System (GPS) at the United State National Geodetic Service has been developed chiefly for instantaneous positioning and dynamic navigation. This system has developed further with the capability of measuring baseline length on the order of 1 to 2 ppm accuracy for distances of several ten to several hundred kilometers in length, by using phase interferometric method with the GPS carrier wave length (e.g,, Hothem and Fronczek, 1983).

Combined Use of a Superconducting Gravimeter and Scintrex Gravimeters for Hydrological Correction of Precise Gravity Measurements: A Superhybrid Gravimetry

A variant of hybrid gravimetry using both a superconducting gravimeter and Scintrex gravimeters is proposed. One of the main factors limiting the accuracy of time lapse gravity measurements is the instrumental drift of spring-type gravimeters. Running the Scintrex CG-5 gravimeter in the nighttime on the same pier as the superconducting gravimeter allows us to model the long-term behavior of the former and to remove efficiently the effect of irregular drift on measured gravity. Initial tests performed at Ishigakijima, Japan, proved that accuracy of a few μGal level can be achieved with this method. This will help us precisely correct for the effect of underground water on superconducting gravimeters with 2-dimensional local gravity survey.

Pressure Sources of the Miyakejima Volcano Estimated from Crustal Deformation Studies During 2011–2013

Following the eruption of the Miyakejima Volcano in the Izu Islands, Japan, in the year 2000, a continuous GPS network observed the ongoing contracting crustal deformation. Subsequently, a slight inflation of the island was detected from around 2006, and we initiated a campaign of dense GPS observations around the volcano from 2011. Precise crustal deformation studies indicated inflation in the southern part of the island and deflation around the center of the crater. Using these observations, we estimated that three magma sources (a shallow deflation sill under the crater, a southern inflation dyke, and a deep inflation spherical source) were activated during 2011–2013. In particular, the presence of an inflation dyke at an intermediate depth had not been inferred by previous studies. Accordingly, we posit that the supply of magma from a deep spherical source to the new dyke source has been initiated only recently.

Observation of Seafloor Crustal Deformation at the Nankai Margin, Japan

We developed an observation system for measuring seafloor crustal deformation composed of precise acoustic ranging and kinematic positioning technique. The repeatability of the system is 3 cm in both horizontal components. We observed the coseismic crustal deformation caused by the two Ml class offshore earthquakes at the Kumano Bay, Japan. From the repeated observation at the Suruga Bay, we obtained the horizontal velocity field that is consistent to the vectors observed by the nationwide GPS observation network. The observation of seafloor crustal deformation can provide better information of seafloor crustal deformation associated with crustal activities, such as coseismic displacement and plate convergence.

Gamma Ray Intensity Surrounding the Fukouzu Fault

Surveys of y-ray intensity around the Fukouzu fault were carried out by automatic measuring systems with a GM-survey meter and microcomputer. The Fukouzu fault, and its secondary fault were formed at the time of the Mikawa earthquake in 1945. These are reverse faults. We selected observation sites along these faults where the crustal deformation resulting from the event is well preserved. The survey was conducted by several observation systems of 7-ray intensity on a line across the faults at intervals of 5 to 280m. The results of the survey show the following for almost all of the sites: (1) the intensity of y-rays at the upheaval sites is greater than that of others; (2) peak y-ray intensity is seen at the upheaval sites; and (3) as thickness of alluvion increases, peak value of intensity decreases. We concluded that radioactive elements are condensed at the upheaval sites along the faults where many fissures are produced.