Monitoring the instrument response of the high-sensitivity seismograph network in Japan (Hi-net): effects of response changes on seismic interferometry analysis
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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.
KeywordsHi-net sensor Test coil Instrument response Seismic interferometry
The National Research Institute for Earth Science and Disaster Prevention (NIED) has operated three nationwide seismic networks since the 1990s. 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/K-NET, 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).
Hi-net is designed for the detection of small earthquakes and very weak signals from the deep crust. At a Hi-net station, a sensor characterized by a natural frequency of 1 Hz is installed at the bottom of a borehole of 100 m or more in depth to reduce background noise (Obara et al. 2005). The performance and limitations of the sensor dynamic range were investigated by Shiomi et al. (2005). 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). Additionally, the records have been used for the construction of subsurface structure models by seismic travel-time tomography (e.g., Matsubara et al. 2009), receiver function analysis (e.g., Shiomi et al. 2004; Ueno et al. 2008), and scattering-wave analysis (e.g., Takahashi et al. 2009). Although the Hi-net sensors were originally designed for regional or small earthquakes, high-quality observations can be successfully achieved using a simulation filter from the Hi-net sensors to the F-net broadband sensors for far-field large earthquakes (Maeda et al. 2011). The waveforms recorded by the Hi-net stations are available on the NIED Hi-net website (http://www.hinet.bosai.go.jp/?LANG=en).
Wegler and Sens-Schönfelder (2007) proposed a method referred to as passive image interferometry (PII), in which fractional changes in the background seismic velocity structure (less than approximately 1 %) can be identified by monitoring a correlation function of the ambient seismic noise. This method has successfully detected temporal changes in the subsurface structure associated with a large earthquake (e.g., Brenguier et al. 2008a), a volcanic eruption (e.g., Brenguier et al. 2008b), and Earth tides (Takano et al. 2014). Since Hi-net provides continuous high-quality seismograms, Hi-net records are suitable for the PII. For example, the PII of the Hi-net records revealed a significant velocity decrease of more than 0.3 % for the 2004 Mw 6.6 mid-Niigata earthquake (Wegler et al. 2009), the 2007 Mw 6.6 Noto Hanto earthquake (Ohmi et al. 2008), and the seismic swarm activity by magma intrusions in the Izu Peninsula, Japan (Ueno et al. 2012). Additionally, for the 2011 Tohoku-Oki earthquake, a significant velocity decrease of approximately 1.5 % was reported based on monitoring by the autocorrelation function (ACF) using Hi-net records (Minato et al. 2012).
This study thoroughly investigates the instrument responses of approximately 800 Hi-net sensors using a daily test coil signal over a period of 10 years that includes the 2011 Tohoku-Oki earthquake. Analyzing the test coil signal, we estimated the natural frequency and damping constant as the instrument responses for each sensor of Hi-net. Additionally, we examined how changes in the instrument response can affect the subsurface structure changes detected by PII for the 2011 Tohoku-Oki earthquake.
Evaluation of instrument response
Therefore, we obtain the output voltage, in other words, an instrument response, with respect to the force given by the right-hand side of Eq. (2) by performing the inverse Fourier transform given by Eq. (4).
Estimating the parameters of instrument response using a test coil record
Results and discussion
Influence of changes in instrument response on crustal monitoring by interferometry analysis
Passive image interferometry of the 2011 Tohoku-Oki earthquake
During the 2011 Mw 9.1 Tohoku-Oki earthquake, a large-amplitude seismic wave propagated through eastern Honshu (e.g., Suzuki et al. 2011). Fractional subsurface velocity change after the earthquake was reported associated with the strong motion and the large stress/strain change (e.g., Brenguier et al. 2014; Nakata and Snieder 2011). We conducted PII using the records of the vertical components of Hi-net. Following a stretching method proposed by Wegler et al. (2009), we estimated the time delay of the ACF from the reference ACF in the lag time of 4–15 s for the frequency range of 1–3 Hz and detected the changes in the subsurface seismic velocity structure that occurred during the 2011 Tohoku-Oki earthquake.
Changes in instrument response
Since the ground shaking caused by the 2011 Tohoku-Oki earthquake was too strong for a high-sensitivity seismograph, an instrument response change must be considered. After the earthquake, in fact, a very small change of the f and the h seems to occur at N.KMIH as shown in Fig. 8c. We may have misidentified the change in the instrument response as corresponding to a change in the subsurface velocity structure resulting from the earthquake. Therefore, we investigate whether the change in the instrument response can misrepresent the subsurface velocity change.
Apparent temporal changes in subsurface structure due to changes in instrument response
We monitored the instrument responses for Hi-net sensors for 10 years from 2003 to 2013 by estimating their natural frequencies f and damping constants h from the test signal. 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. Furthermore, each Hi-net sensor is very stable against the temporal change over a decade. Their standard deviations of the natural frequency and damping constant were within 0.05 Hz and 0.05, respectively. During the 2011 Tohoku-Oki earthquake, large seismic waves exceeding the dynamic range of the Hi-net sensor were observed at the stations near the earthquake focal area. Small changes in the instrument response were also recognized in the Hi-net sensors after the earthquake. Thus, we conducted a synthetic test to investigate how changes in the instrument response can cause errors in seismic interferometry analysis. The test indicated that the instrument response change did not result in systematic variation in the time delay obtained from the PII of the 2011 Tohoku-Oki earthquake. This supports the conclusion that the 0.5 % decrease in the velocity at the N.KMIH station after the 2011 Tohoku-Oki earthquake was not due to artificial error caused by the large ground shaking.
We thank K. Nomura for his helpful comments about the Hi-net records and the sensor. M. Yamamoto suggested the damage of Hi-net sensors by large ground shaking. We also thank two anonymous reviewers for useful comments. We used the Generic Mapping Tools (GMT) (Wessel and Smith 1998) to make the figures in this paper.
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