Abstract
More than 10 years have passed since observations began to be recorded by Hi-net, a network of high-sensitivity seismometers located in Japan. Several large earthquakes, including the 2011 Tohoku-Oki earthquake, have been recorded by the network during this period. Age-related degradation and the strong ground motion of large earthquakes may change the instrument response of the high-sensitivity seismometers of Hi-net. Thus, we checked the natural frequency f and damping constant h for each Hi-net sensor and monitored the instrument response for 10 years from 2003 to 2013. Most of the sensors showed a stable instrument response over this period. More than 95 % of the sensors whose responses we could well estimate showed small fluctuations in their natural frequencies and damping constants of within 0.05 Hz and 0.05, respectively. We also found that many Hi-net sensors in northeastern Japan showed slight changes in the instrument response as a result of the 2011 Tohoku-Oki earthquake. Based on the assumption that the instrument responses remained unchanged, the fractional velocity reduction in the subsurface structure was reported by seismic interferometry analysis. To investigate how changes in the instrument response can cause errors in seismic interferometry analysis, we conducted a synthetic test. The results indicate that the instrument response did not result in systematic variation in the time delay observed in the interferometry analysis. This confirmed that the velocity decrease observed as a result of the 2011 Tohoku-Oki earthquake was not due to artificial instrument error.
Highlights
The National Research Institute for Earth Science and Disaster Prevention (NIED) has operated three nationwide seismic networks since the 1990s
We found that the Hi-net sensors are well controlled with small instrument-to-instrument variation; more than 95 % of the sensors show natural frequencies in the range of f = 1.0–1.2 Hz and damping constants in the range of h = 0.6–0.8
Each Hi-net sensor is very stable against the temporal change over a decade
Summary
The networks consist of approximately 80 broadband seismographs, 1700 strong-motion seismographs, and 800 high-sensitivity seismographs and are referred to as F-net, KiK-net/KNET, and Hi-net, respectively. These three seismic networks observe seismic motions for short to long periods and detect earthquakes of small to large magnitudes (Okada et al 2004). The sensor orientation at the bottom of the borehole cannot be observed by the naked eye but may be accurately estimated by analyzing the long-wavelength waveforms and ambient seismic signals (Shiomi 2013). The Hi-net records have been used to determine the accurate hypocenter locations and mechanisms of small earthquakes (e.g., Yukutake et al 2008) and have contributed to the new finding of a non-volcanic seismic tremor in southwestern Japan (Obara 2002).
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