Toshikazu Tanada

Crustal deformation associated with the 2011 Shinmoe-dake eruption as observed by tiltmeters and GPS

The National Research Institute for Earth Science and Disaster Prevention (NIED) developed volcano observation stations at the Kirishima volcanic group in 2010. The stations observed remarkable crustal deformation and seismic tremors associated with the Shinmoe-dake eruption in 2011. The major eruptive activity began with sub-Plinian eruptions (January 26) before changing to explosive eruptions and continuous lava effusion into the summit crater (from January 28). The observation data combined with GEONET data of GSI indicated a magma chamber located about 7 km to the northwest of Shinmoe-dake at about 10 km depth. The tiltmeter data also quantified detailed temporal volumetric changes of the magma chamber due to the continuous eruptions. The synchronized tilt changes with the eruptions clearly show that the erupted magma was supplied from the magma chamber; nevertheless, the stations did not detect clear precursory tilt changes and earthquakes showing ascent of magma from the magma chamber just before the major eruptions. The lack of clear precursors suggests that magma had been stored in a conduit connecting the crater and the magma chamber prior to the beginning of the sub-Plinian eruptions.

Detectability for GPS, EDM and tiltmeters of the Hot Springs Research Institute in the western Kanagawa prefecture, Japan

To better understanding the observation data, we attempt to assess the detectability for the crustal deformation observation network of the Hot Springs Research Institute of Kanagawa prefecture (HSRI). The HSRI has carried out continuous monitoring of crustal deformation with GPS, EDM, borehole tiltmeters, groundwater leveling since 1989. Our observation purpose is to detect the pre-, co- and post-seismic deformations of the hypothetical M7 class earthquake designated as the “Western Kanagawa Prefecture Earthquake (WKE)”. In order to estimate the detectability, weapplya simple calculation method, thatiscomparing between co-seismic deformation (displacement and tilt) and the accuracy of observation instruments. Co-seismic deformation is calculated using the Okada’s method, which is a formulation of the expected crustal deformation due to seismic faulting as a function of an earthquake magnitude and a hypocentral distance without assuming focal mechanism. It is indicated that the highest detectability area is located in the center of our HSRI observation network. Comparing detectability contour maps for several kinds of networks, we can find that the detectability of the co-seismic deformation by the tiltmeter network is higher than those by the GPS and EDM networks in the case of the M5 class earthquake with a focal depth of 10 km, and that the detectability by the GPS network is almost equal to that by the EDM network in the case of the M6 class earthquake with a focal depth of shallower than 15 km. For M4 class earthquakes, it is difficult for the GPS, EDM and tiltmeter networks to detect co-seismic deformation. The present result shows advantageous characteristics of the HSRI geodetic network and an important constraint on investigating the pre-, co- and post-seismic deformation of the WKE. To monitor the pre-, co- and post-seismic deformation of the WKE, it is necessary to combine these observations in the most effective way.