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We investigated impact of dense seafloor observation network along the Japan trench, S-net, on tsunami generation and propagation modeling. We focused on the 2016 Mw 6.9 off-Fukushima earthquake, Japan, which generated the largest tsunami in this subduction zone after the S-net deployment. We inverted the offshore tsunami waveforms for the accurate tsunami source, and then used it in numerical tsunami simulation to clarify how well the tsunami waveforms recorded at the coastal sea-level stations can be reproduced. The results showed that the simulated tsunami waveforms matched the observations well for 90 min from the tsunami arrival time at each station, whereas the reproducibility in the temporal variation of tsunami energy over the long duration, including the decay process, varied from station to station. Because the discrepancies between the observations and calculations exceed the uncertainties due to the source estimation error, our results suggest the room for improvement in the propagation modeling. Another insight we found thanks to the offshore-data-constrained tsunami source is that the leading parts of the coastal tsunami waveforms might have non-negligible effect from the tsunami nonlinearity although they are often fed into linear tsunami inversion. This study has demonstrated that tsunami data from dense offshore network can contribute not only to accurate reproduction of targeted tsunami event, but also to the performance evaluation of the present source and propagation modeling technologies.We used offshore and coastal tsunami data provided by NIED, MLIT, GSI and JMA. This work was partially supported byJSPS KAKENHI Grant NumbersJP19H02409 and JP21K04621., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Hyogo-ken Nanbu earthquake (MJMA7.2), that took place on January 17th, 1995, in Kobe and Awaji area, caused tremendous disasters in the areas. After the earthquake, the Japanese University Consortium for GPS Research (JUNCO) deployed more than 30 GPS receivers around the hypocentral area to find co-seismic and to monitor post-seismic crustal deforma-tions related to the earthquake. Data have been archived first by on-site recordings and later by tele-communcations at the Disaster Prevention Research Institute, Kyoto Universsty. Temporary dense array terminated by the end of March, 1995. Further temporary occupa-tions were conducted in May, 1995, in November, 1995, and in March, 1996, as well as long term continuous monitorings at selected several sites. The monitorings ended in August, 1996. Co-seismic offsets were observed at several sites around the source area. The largest offsets amounted to 45cm at Iwaya site which is about 4 km east of the Nojima fault. These data were used for simultaneous inversion together with strong motion data to clarify slip dis-tribution on the buried rectangular faults. Post-seismic deformations were also found at all of sites. They mostly showed temporal decay and amounted to 2 to 3 cm. The largest one was observed at Iwaya site. The areal distri-bution of post-seismic displacement vectors seems to indicate afterslip on the fault planes, but not the areal visco-elastic readjustments. Relaxation processes at Iwaya and Kawaragi sites were fitted by a logarithmic curve. Application of a theory on the mechanics of afterslip based on the constitutive relationship of a fault surface indicated that the fitting give reasonable es-timate on the frictional rate parameter or the thickness of velocity-strengthning layer of the earth's surface. Assuming that the constitutive parameter is ranging from 0.001 to 0.005 based on the results of rock experiments, thicknesses of velocity-strengthning layer at Iwaya and Kawaragi were estimated to be ranging between 500m and 2 km, which is consistent with other seismological data. Monitoring of fault offsets using real-time kinematic GPS (RTK-GPS) were also conducted at two baselines crossing the Nojima fault and the Arima-Takatsuki tectonic line, respectively. Though significant deformation was not observed, it showed a potential capability of real time monitorings of ground deformations in a few centimeter accuracy.

Movements of the Yamasaki fault in the Kinki District, were monitored by employing precise geodetic measurements once a year during the period from 1975 to 1993.The measurements were performed on a small aperture baseline network across the fault. The Hyogo-ken Nanbu earthquake (M = 7.2) occurred on January 17, 1995 in southern part of Hyogo prefecture. Measurements on the network, which is located at the epicental distance of about 60 kim, were carried out in November 1995.Analyzed data for the period of 20 years from 1975 to 1995 show as follows: No remarkable crustal movement due to the earthquake was detected. On the other hand, the values of average secular strain and tilt rates decreased from 1.4 μstrain/year and 0.95 μrad/year in 1975-1986 to 0.5 and 0.16 in 1987-1995, respectively.

A 3-components small borehole strainmeter was installed at Rokko-Takao station, Kobe, Japan, and observation has been started at July 1993. The station is located at 240 m under the ground surface in the side tunnnel which joins two tunnels for automobiles through the Rokko mountain area (Shin-Kobe and Daini-Shin-Kobe tunnels).Geological features of the Rokko mountain area are mainly characterized by the Rokko granite containing many active faults.The aim of this study is to obtain useful information for crustal strain observation by using the simple borehole strainmeter at the tunnel, the observation in which is disturbed by changes of atmospheric pressure and room temperature and oscillation of the ground due to running automobiles.

GPS/levelling geoid undulation differences were determined at 37 stations in Chugoku, Shikoku and northern Kyushu districts of southwestern Japan.Since the measurement area was widely extended, five reference stations, coordinates of which were accurately determined, were used in GPS measurements.The geoid differences determined by the present study were approximately consistent with the preceding geoids, such as the JHD geoid and the ORI-89 geoid.

Daily measurements by using GPS had been carried out at Kyoto, Kochi, Aso and Tottori sites in Southwest Japan from September 1990 to September 1991. The long term repeatability of the baseline lengths determined was 0.26 ppm, which was comparable to the short term repeatability of them on even as long baseline as about 200 km. The baseline lengths showed annual changes, amplitude of which were about 1 ppm. It seems that such changes were caused by insufficient consideration of tropospheric delay in computation of baseline vector.

Daily measurements by using GPS have been carried out at Kyoto, Kochi, Aso and Tottori sites in Southwest Japan since September 1990. The aims of the present measurements are to estimate a long term repeatability of the baseline vectors determined, to investigate effects of error factors and finally to improve the accuracy of GPS measurements.Results for the first six months are as follows:(1) The long term repeatabilities of the baseline lengths of about 150 to 500km were 0.2 to 0.4 ppm.(2) The baseline length in summer was determined longer than that in other seasons.(3) There were positive correlations among the changes of the baseline vectors.