Eikichi Tsukuda

Preseismic changes in groundwater level and volumetric strain associated with earthquake swarms off the east coast of the Izu Peninsula, Japan

We have observed the groundwater level at Omuroyama-kita (OMR) well on the Izu Peninsula, Japan since October 1994. During the observation period, there were four earthquake swarms. We detected one or two preseismic groundwater level changes together with preseismic crustal deformation recorded at the other nearby stations. Therefore those preseismic groundwater level changes are considered to be caused by crustal deformation or volumetric strain changes. The crustal deformations are inferred to be caused by the magmatic dike intrusion, which is a possible source of the earthquake swarms off the east coast of the Izu Peninsula.

Preliminary results from permanent GPS array by the Geological Survey of Japan in conjunction with groundwater-level observations

A regional continuous Global Positioning System (GPS) array, consisting of uniform antenna type, has been established by the Geological Survey of Japan (GSJ) mainly around the Kansai district. Geodetic results for 2.5 years data are as follows. In comparison with the solutions obtained from the Geographical Survey Institute (GSI) continuous GPS array which employs different type of antenna from the GSJ’s array, it was found that the agreements between the daily positions were about 10 mm in horizontal and 15 to 20 mm in vertical components. This result indicates that the influence of the antenna type difference is at this level. The GSJ’s GPS stations around the Kansai district were found to have a common annual variation in the vertical component, whose amplitude was different from site to site. There were small correlations with in situ measurements of groundwater-level variation, except for the two stations whose annual variations of the GPS vertical component were the largest. The vertical displacements due to elastic deformation by groundwater-pressure changes in the aquifers were estimated using a simple model, but they were too small to account for the observed variations.