A densely distributed high-sensitivity seismograph network in Japan:Hi-net by National Research Institute for Earth Science and DisasterPrevention

Abstract

Seismic observations to retrieve various information from the Earth are the basis of seismology. A seismic observation system requires various technologies for vibration sensors, analog-and-digital measurement, data transmission, and computing for mass data analysis, for example. New developments in technology are adopted whenever possible in the construction of seismic observation systems. In Japan, after the disastrous Kobe Earthquake in 1995, a high-density and high-sensitivity seismograph network was constructed. The seismic network, called the National Research Institute for Earth Science and Disaster Prevention (NIED) Hi-net, uniformly covers the Japanese Islands with a spacing of 20–30km. As a result, the detection capability for microearthquakes has been greatly improved, and various research using Hi-net data has indicated that this seismic network has a great potential to resolve the underground structure and various geophysical phenomena as a radar-array oriented toward the Earth. Equipped with modern standard techniques, the Hi-net system is designed to be able to be smoothly upgradeable in the future.

Hydrological gravity response detection using a gPhone below- and aboveground

“…If such a borehole gravimeter were incorporated into a device such as a multi-component borehole instrument (Ishii et al, 2002), the cost of each observation item would decrease. Furthermore, if the borehole device housing the gravimeter were adopted as part of a nationwide observation infrastructure, such as Hi-net (Obara et al, 2005), many observation environments would become available without requiring the construction of local hydrological models, thus enabling the correction of the inland-water gravity effect. Such a gravity monitoring system would contribute not only to geophysical applications, but also to monitoring at geothermal power stations and geological disposal sites.…”

Correlation between Coulomb stress changes imparted by large historical strike-slip earthquakes and current seismicity in Japan

“…The completeness magnitude, above which all earthquakes are detected considered to be detected by a seismic network, has been estimated to be 1.0 for the Japanese mainland during this period (Ishibe, 2007;Schorlemmer et al, 2008;Ishigaki, 2009;Nanjo et al, 2010). The detection capability and accuracy of the hypocentral location have significantly improved since October 1997 due to the unification of earthquake observation data by JMA and the installation of a new high-sensitivity seismograph network in Japan (Hi-net) (Okada et al, 2004;Obara et al, 2005). The accuracy of the epicentral location is important for determining the spatial correlation between the current seismicity and CFF; hence, we use shallow earthquakes (hypocentral depth ≤ 30 km) with location uncertainties of <2 km.…”

CSEP Testing Center and the first results of the earthquake forecast testing experiment in Japan

“…Among the non-JMA networks is Hi-net, a borehole seismic network of about 700 stations deployed by the National Research Institute for Earth Science and Disaster Prevention (Obara et al, 2005). Nowadays, about 1200 seismometers are operating under the hybrid network and are detecting more than 100,000 events annually.…”