S U M M A R YW e developed a new inversion method t o reconstruct static images of seismic sources from geodetic data, using Akaike's Bayesian Information Criterion (ABIC). Coseismic surface displacements are generally related with a slip distribution o n a fault surface by linear integral equations. Parametric expansion of the fault slip distribution by a finite number of known basis functions yields a set of observation equations expressed in a simple vector form. Incorporating prior constraints o n the smoothness of slip distribution with the observation equations, we construct a Bayesian model with unknown hyperparameters.T h e optimal values of the hyperparameters, which control the structure of the Bayesian model, are objectively determined from observed data by using ABIC. Once the values of hyperparameters are determined, we can use the maximum likelihood method t o find the optimal distribution of fault slip. W e examined the validity of this method through a numerical experiment using theoretical data with random noise. We analysed geodetic data associated with the 1946 Nankaido earthquake (Ms = 8.2) by using this method. T h e result shows that the fault slip distribution of this earthquake has two main peaks of 4 and 6 m, located off Kii Peninsula a n d Muroto Promontory. These two high-slip areas are clearly separated by a low-slip zone extending along Kii Strait. Such a slip distribution corresponds with the fact that the rupture process of this earthquake in the western part is notably different from that in the eastern part.

We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method is adopted to determine the position of seafloor stations from coordinates of a moving survey vessel and measured travel times of acoustic waves in seawater. The positioning accuracy is substantially improved by estimating the temporal variation of the acoustic velocity structure. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a huge earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from seven campaign observations for the period 2002-2005 exhibits a linear trend with a scattering rms of about 2 cm. A linear fit to the time series gives an intraplate crustal velocity of more than several centimeters per year towards the WNW, which implies strong interplate coupling around this region. The precision of each campaign solution was examined at MYGI and other seafloor reference points along the Nankai Trough through comparison of independent one-day subset solutions within the campaign. The resultant repeatability looks to be well-correlated with the temporal and spatial stability of the acoustic velocity structure in the seawater depending on the region as well as the season.

SUMMARY The spatial distribution of the strength of interplate coupling between the subducting Philippine Sea and overlying continental plates in the Tokai district, central Japan, was investigated in detail through the inversion analysis of geodetic data using Akaike's Bayesian Information Criterion (ABIC). The geodetic data used for the analysis are annual rates of level changes (1972‐1984) and horizontal length changes (1977‐1988), which presumably represent average crustal movements during the interseismic period. The result of the inversion analysis shows the existence of a strongly coupled region extending from 10 to 30 km in depth. The total seismic moment accumulated in this area since the last event (the 1854 Ansei earthquake) is roughly estimated to be 5.5 × 1027 dyne cm, which corresponds to Ms= 7.8. The interplate coupling becomes weaker toward the shallower and deeper portions. This is consistent in general tendency with a rheological model inferred from petrological viewpoints. The strength of coupling also tends to decrease toward northeast over the west coast of Suruga Bay. The direction of plate convergence inferred from the inversion analysis is oriented N30°W. This is significantly different from the direction of relative plate motion between the Philippine Sea and Eurasian plates but concordant with that between the Philippine Sea and North American plates.

Abstract:This Introduction. The space geodetic techniques have already been used in the monitoring of crustal deformations on land not only with high spatial and temporal resolutions but also with centimeter-level accuracy. In Japan, the continuous observation with the Global Positioning System (GPS) Earth Observation Network (GEONET) has been producing amazing results. These results have contributed significantly to the understanding of the complex crustal deformation processes.' The remote sensing from flying vehicles with SAR (Synthetic Aperture Radar) Interferometry technique also achieved great success with its high performance.2~ However, these space techniques are only available on land. Currently, about 70% of the solid earth surface is under the veil of sea water.Around Japan, many great earthquakes have occurred repeatedly beneath the deep ocean floor, at the seismic coupling zone between the descending oceanic crust and overhanging continental crust along the Kuril Trerlcli, the Japan Trench, the Nankai trough, etc. When little or no information is known as to what will happen at the ocean bottom crust, an unexpected attack of a tsunami generated by a great earthquake can develop sea-surface precise positioning with GPS and precise acoustic ranging between sea-surface platform and sea-bottom acoustic transponders (Fig. 1). This is analogous to GPS geodesy on land. In this GPS/Acoustic method, the precise positioning for the platform and the buoy on the sea-surface is realized by kinematic GPS technique which uses phase measurement of carrier Centimeter-level positioning on the seafloor 9

We have been carrying out GPS/acoustic seafloor geodetic observations at several reference points situated along the Japan Trench, a major plate boundary of subduction. A time series of horizontal coordinates of one of the seafloor reference points, located off Fukushima, obtained from seven campaign observations for the period 2002-2008, exhibits a linear trend with a scattering root mean square of about 3 cm. A linear fit to the time series gives an intraplate crustal movement velocity of 3.1 cm per year in a westerly direction, which is significantly smaller than that at the other seafloor reference point 120 km away along the trench axis. This result implies weak interplate coupling in this region.