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The absolute gravity measurement by FG5 was performed for the first time after starting continuous gravity observation by iGrav SG (# 017) at the Teshikaga Observatory, Hokkaido University. From the comparison between the absolute gravity value obtained with FG5 and the output value of iGrav SG, the scale factor in Medium Feedback mode of iGrav SG was estimated to be -1998 μGal/V (×10-8 ms-2/V).

An M5.0 shallow earthquake occurred on 4 June 2015 in the eastern Hokkaido. In order to evaluate aftershock activity, three temporal seismic stations had been operated in the focal region from 4 June 2015 to 22 August 2015. Hypocenters calculated with a local seismic velocity structure indicated clear southwestern-dipping distribution. This alignment on a plane was consistent with one of nodal plane of mainshock focal mechanism. This earthquake and historical records suggested higher seismic activity in this region.

An earthquake swarm begun at 15 July 2012 in Nakagawa town of northern Hokkaido. The largest earthquake with MJMA4.3 occurred on 16 July 2012. We carried out temporal seismic observation with seven stations from 18 July 2012 to the last October 2012. Hypocenters were calculated using the Double-Difference hypocenter determination procedure with a local one dimensional P-wave velocity structure. Precise hypocenter data indicated that epicenters were distributed in very narrow area of 2 km×2 km with shallow depth from 4 km to 7 km. Earthquakes after middle of August occurred only in southern part of the region and depth had got shallower with time. Hypocenters indicated no clear alignment in consistent with any nodal planes of major earthquakes. An independent hypocenter cluster with shallower than 2 km was observed above the main activity area. A slow slip event (SSE) with Mw 5.4 coincidentally detected by GNSS crustal deformation data during the swarm. This swarm was situated at the southeastern end of the fault of SSE. This fact suggested that seismic swarm might triggered SSE or was induced by SSE.

Gravimetric connection between SapporoGS and TsukubaFGS via ChitoseGS and HanedaGS with SCINTREX CG5 gravimeter S/N130141025 was carried out for instrument calibration. Observed relative gravity between SapporoGS and TsukubaFGS was 526mGal, which was the same as that of value determined by Geospatial Information Authority of Japan in order of 10-4. This measurement suggested that scale constant of this gravimeter is valid for below 500mGal difference with correction coefficient of 1.0002.

Relative gravity measurements with SCINTREX CG5 gravimeter S/N130141025 for instrument calibration were carried out at fundamental, first order gravity benchmarks, and triangulation point in Hokkaido. Observed relative gravity value between ObihiroFGS and ShintotsukawaFGS by CG5 was 76 mGal, which is same as that of value determined by Geospatial Information Authority of Japan in the order of 10-4. Estimated correction coefficient was 1.00020. Simultaneous measurement by three gravimeters at Teine triangulation point showed 231 mGal relative gravity value from MinamiShinkawa benchmark within 0.120 mGal difference between instruments. This suggested that estimation of correction coefficients for several hundreds to a thousand mGal difference should be conducted for precise calibration. Drift rate fluctuation shown in continuous measurement data also should be evaluated in future.

On December 2, 2010, MJMA4.6 shallow felt earthquake occurred beneath southeastern part of Sapporo city. Three temporal seismic stations were installed near epicentral area. Reliable hypocenter data estimated by double difference procedure with local seismic velocity structure indicated clear southeastern-dipping distribution. Fault plane from focal mechanism estimated from P-wave polarization well agreed with this hypocenter distribution. Geometry of hypocenter distribution was also well consistent with an anticline structure and possible buried active fault estimated for earthquake disaster damage assumption by local government. Recalculation of hypocenters of the 1985, 1990, and 2010 felt earthquakes under equal condition implied that epicentral locations of these three events were approximately same. These facts implied possible stress concentration in epicentral region.

We have developed an easy-to-use Crustal Deformation Database (CDD) for strain, tilt and other long-term geophysical data. WWW-based user interface is employed for operations and the servers are connected to internet. Anyone who has internet connections and web browser, therefore, are easily possible to refer and download data from anywhere. We have adopted the WIN format (de facto standard format for the seismic data exchange in Japan) for data acquisition and management, because it can treat any sampling interval data without re-sampling procedure during data processing. This database has following functions; 1) data drawing on the display for selected period from 1 second to 10 years, 2) data download for selected channels and period, and 3) several data analyses, i.e., filtering, tidal analysis by BAYTAP-G program, plane strain analysis and its linkage to seismic data. Trial run of CDD is in operation from 2004 and 66 stations data operated by Hokkaido University, Tohoku University, University of Tokyo, Nagoya University, Kyushu University, Tono Research Institute of Earthquake Science, Geological Survey of Hokkaido and National Astronomical Observatory of Japan have been stored to CDD in real time and above functions are in use. We hope any institutions operating crustal deformation observation will join our CDD network to establish unified CDD of Japan.

During the period from 2004 to 2008, we established the broadband seismic network consisting of 8 stations in Far Eastern Russia, in order to detect seismic waves passing through the stagnant slab distributed in depth range from 660 to 900 km and to clarify the feature of the stagnant slab beneath Far Eastern Russia. At each seismic station, STS-2 broadband seismometer and a short period seismometer are installed using 24 bits analog/digital converter with 100Hz sampling and the records are stored continuously on a nonvolatile memory card. In this paper, we report the seismic network in detail.

From April 27, 2004, Mt. Syari-dake earthquake swarm occurred in the eastern part of Hokkaido, Japan. The activity began at 4:19 on April 27, 2004. We installed nine temporary seismic stations around the seismic active area immediately after its occurrence and had continued the observation for about seven months. We determined 2228 hypocenters of earthquakes. It is found that the seismic active area is clearly distributed on the plane having southwest-northeast strike with dip angle of about 80 degrees. This plane well agrees with one of the nodal planes of the focal mechanisms of several earthquakes determined by P-wave first motions. Hypocenters migrate northeastward at a rate of 96 m/day and westward at a rate of 51 m/day. We also investigated hypocenter distribution before the installing of the temporary seismic stations. To improve accuracy of hypocenter determination we applied travel time correction for every permanent seismic stations. Relocated precise hypocenters demonstrate that the main seismic activity started to expand eastward just after the begining of the swarm.

Secular and short term strain variations before and after the 2003 Tokachi-Oki earthquake, Mw8.0, recorded at five continuous crustal movement stations located along the Pacific coast are reported. There is no obvious forerunner movement at all stations. However, clear co-seismic and post-seismic strain changes are recorded at all stations. Strain data are digitized by 18 bits with 1 Hz sampling rate so that the complete strain seismogram of Mw8.0 event were recorded at MYR station nearest the source of the event. Strain seismograms recorded at all stations in Hokkaido are also reported.

On December 14, 2004, an M 6.1 earthquake occurred in the northwestern part of Hokkaido, Japan. We installed nine temporal seismic stations around the source area immediately after the occurrence and had continued the observation for about two months after the main shock. We determined 823 hypocenters of aftershocks and one dimensional P-wave velocity structure model from the travel time data. It is found that aftershocks are clearly distributed on an eastward dipping plane with a dip angle of about 25 degrees. This plane agrees well with one of the nodal planes of the focal mechanism determined by P-wave first motions in this study. Next, we investigated a position of the main shock relative to the aftershock distribution. The station correction values for permanent stations were determined from the data during the period of the temporary observation. Estimated main shock hypocenter was confirmed to be situated on the aftershock plane. Finally, we estimated three-dimensional P-wave velocity structure based on the temporary travel time data. Relocated aftershocks using the three dimensional velocity structure were mainly distributed along the boundary between the high and low velocity zones.

An earthquake (MJMA 6.1), occurred in December 14, 2004, at southern area of Rumoi sub-prefecture, northern Hokkaido, Japan. The earthquake intensity was assumed to 6 in Obira town. In Hokkaido, this is the first inland earthquake larger than M6 since Teshikaga earthquake (MJMA 6.5, 1967). Detailed aftershock distribution shows that the aftershocks occur on an eastward dipping plane of 10 km square, depth of which 5 km. The mainshock assumed to have occurred near the southern end of this plane. Despite the sparseness of existing gravity stations over the focal region, it was found that the aftershocks occur just above the gravitational rise of about 20 mGals. So the gravity survey was operated in Nov. 2005 to Aug. 2006, to make a detailed Bouguer anomaly map and to explicate the subsurface structure of the focal region. The measurement was done with CG-3 type gravimeter. Measurement positions were carefully decided by operating differential GPS observation. The accuracy of altitude decision is within 1 meter in all of the measurement points. Besides, data provided by Japan Petroleum Exploration Co. Ltd. was very helpful. The appropriate assumed density was detected to 2300 kg/m3, using density log of deep borehole near the focal region, and the correlations of gravity correction terms. The Bouguer anomaly map by current dataset also shows good coincidence with aftershock distribution.