First signal test
The first test operation began on 12 June 2012 and continued until 17 September 2012, for 97 days. During this test period, one vibrator was operated with a single frequency at 10.01 Hz, and the other was operated with FM from 10 to 15 Hz and a modulation period of 50 s. We chose 10.01 Hz for the single frequency so as not to overlap with the frequency components produced by the other vibrator in FM operation. The FM operation produces a series of sinusoids from 10.00 to 15.00 Hz with a 0.02-Hz interval. The rotation direction was switched every 2 h to synthesize two independent linear excitation forces. The received signals of two sequential operations, one of which is clockwise and the other is anticlockwise rotation of the source, are linearly combined with an appropriate phase shift. This operation synthesizes the signals at the seismic stations by radial and transverse linear excitations. The rate of operation in the first test period was 88%. The main cause of the suspension was power instability or failure due to lightning or storm around the site. In most cases, the system was restarted remotely.
The signal from the ACROSS sources is recorded by seismic stations that are routinely operated for monitoring seismic and volcanic activity. Figure 2 shows the location of seismic stations that are used for receiving the signal from the ACROSS source. We used nine stations in Sakurajima Island and six stations in the surrounding region, which are operated by the Disaster Prevention Research Institute (DPRI) of Kyoto University, Faculty of Science of Kagoshima University and National Research Institute of Earth Science and Disaster Prevention (NIED). The data of all stations are synchronized to a GPS clock, which guarantees remote synchronization to the source signal of ACROSS.
Figure 3 shows the spectra for the representative three observation stations with 2.5 months' stacking from 13 June 2012. Station KORH (Hi-net Koriyama station) is located off Sakurajima Island, KURN (Kurokami station) is located on the other side of the summit on the island, and HAR (Harutayama station) is located in a deep borehole close to the source site. For each station, the spectra of three directional components of the seismometer are shown for both radial and transverse linear sources that are synthesized by combinations of clockwise and anticlockwise rotations. The notations attached after the station code indicate the combinations of the components at the sources and the receivers. The first letters, which are in capitals, indicate the component at the receiver. U, R, and T indicate the vertical, radial, and transverse components, respectively. The second letters, which are in lower case, indicate the component at the source, where r and t indicate the radial and transverse components, respectively. Hereafter, we shall use this convention in this paper.o
The unit of spectral amplitude is adjusted to meters per second (m/s), which represents the amplitude of a single sinusoidal wave. Note that the amplitude unit in this spectrum plot is not (m/s)/Hz1/2 which is generally used in conventional spectrum plots. We adopt the unit m/s, because the signal emitted from ACROSS is composed of a finite number of sinusoids with constant amplitudes. In this unit, the amplitude of ACROSS signals in the frequency domain stays constant, independent of the stacking length. If we adopt (m/s)/Hz1/2, the spectral peaks of the ACROSS signal increase with the length of the stacking period, while the noise level stays constant. We prefer m/s for the amplitude unit to make the ACROSS signal invariable while noise levels decrease proportionally to the square root of the stacking time period.
The signal from the source that is operated in a single frequency (10.01 Hz) is clearly seen even at the stations off Sakurajima Island. The station KORH (Figure 3a), which is located 19.5 km from the source site, shows a clear spectral peak at 10.01 Hz, with a signal-to-noise ratio (SNR) of about 100 in all the components, with a stacking length of 88 days. The signal of the source in FM operation is also seen in the spectrum from 10 to 15 Hz. The station KURN (Figure 3b), which is located 7.5 km from the source site on the other side of the summit, shows clear spectral peaks. The SNRs are larger than those at KORH, even though the stacking period is 71 days. The signal at HAR (Figure 3c), which is located 1.1 km from the source site, has a very good SNR not only because of the distance, but also because of the low-noise environment in the deep borehole. The SNR for a single sinusoid at 10.01 Hz is about 1,000 for most of the components, and the SNR for the FM signal is about 100. Other spectral peaks, which are typically seen at KORH, are caused by the data telemetry system at the station. Small noise that is synchronized to the GPS clock is generated in the data telemetry system, resulting in spectral peaks with multiples of exactly 1.0 Hz.
Based on the spectral data for all the stations in Figure 3, we plot the amplitudes of the ACROSS signal to show the amplitude decay as a function of the distance from the source in and around Sakurajima Island (Figure 4). We plot the amplitude of the signals for the vibrations with both the single frequency operation at 10.01 Hz and the FM operation from 10 to 15 Hz. Amplitudes are also plotted for the radial and transverse linear vibrations to show the difference between the two vibration directions. The amplitude of FM operation at each station is calculated using the root mean square (RMS) of spectral peaks between 10.0 and 15.0 Hz, in which 251 peaks are included at 0.02-Hz intervals. The amplitude decays are roughly inversely proportional to the square of the distance from the source for both single frequency and FM signals. The mean amplitude of the FM signal is an order of magnitude smaller than that of the single frequency signal for most of the stations. This ratio is reasonable considering that the generated force of ACROSS is proportional to the square of the rotational frequency, and the seismic energy of the single sinusoid is distributed among 250 sinusoids in FM operation.
As one of the ACROSS sources is operated with a frequency modulation, we can convert the signal into a transfer function in the time domain. In this process, the signal at each station is deconvoluted with the source signal in the frequency domain. Figure 5 shows the transfer functions for the seismic station we used, which are aligned as a function of distance. The two representative components of transfer functions are plotted. The top panel shows the Ur component in which P waves are emphasized, whereas the bottom panel shows the Tt component in which S waves are emphasized. In both panels, the signal is more scattered into later parts for the distant stations than for nearby stations. There seems to be no predominant later phases in the signals, probably because of the complex structure in the volcano region. The onsets of P and S, as well as the detail of the later phases, are not clear because of narrow bandwidth between 10 and 15 Hz. We will use a wider frequency range and make more detailed discussions, such as temporal changes of the travel time, in future experiments.
Comparison to Toyohashi site
The same models of ACROSS vibrators were already in operation at the Toyohashi site in the Tokai region, Japan, when ACROSS was deployed in Sakurajima. We have been operating the ACROSS at Toyohashi site to monitor the temporal change of seismic propagation property associated with the subduction process of the Philippine Sea plate. In addition, we deployed an ACROSS at Sakurajima Volcano for monitoring its activities. Before the deployment, we investigated the detectability of signal from the ACROSS source that was to be deployed in Sakurajima using the observation data in the Tokai region with ACROSS at Toyohashi. In this section, we show the results of the investigation comparing them with the actual observational result. To assess the feasibility of signal detection at Sakurajima, we used the amplitude decay relation as a function of distance obtained in the monitoring experiment of ACROSS in the Tokai region and an explosion experiment in Sakurajima. The process shown here is useful to assess the feasibility of using ACROSS at any other locations.
Explosion experiment at Sakurajima
An explosion experiment for which we analyzed the amplitude decay as a function of distance was carried out in 2008 in and around Sakurajima Island (Iguchi et al.2009). We compared the amplitude decays between the explosion source and the ACROSS source in the Tokai region.
We first estimated amplitude decay as a function of the source distance in and around Sakurajima. We used waveform data obtained in the seismic exploration experiment with densely deployed seismometers that was conducted in and around Sakurajima Volcano in November 2008. In the experiment, 15 dynamite sources (hereafter referred to as shot sources), 426 temporary-deployed one-component seismic stations on land, and 32 ocean-bottom seismometers were used. Velocity type sensors with the natural period of 2.2 Hz were used on land, and 4.5 Hz on the ocean bottom. These seismometers were deployed with 200-m intervals on Sakurajima Island and at 400- to 500-m intervals around the island. Each station recorded seismic waves from every shot for 60 s with a 250-Hz sampling frequency. The purpose of this experiment was to delineate seismic structure over an area of Aira Caldera (Miyamachi et al.2013). In this study, we used the data from seismic stations on land that are shown in Figure 6.
From the experiment dataset, we used the record of shot 2 (S02 in Figure 6) for the estimation of amplitude decay with distance. Shot 2 was located close to the ACROSS source site and is suitable for comparison. Although there were several other shots that were closer to the ACROSS site than shot 2, we do not use them because their dynamite weight was only 20 kg, while the dynamite for shot 2 weighed as much as 200 kg.
Figure 7 shows the signal traces of shot 2. The traces are aligned according to the distance from the shot point. The first 10 s of the data after the first arrival is used and indicated in bright red color. The first arrivals are identified using a variance ratio between 1-s sections before and after a point. We look for the point that has the largest variance ratio in a trace, which is regarded as the first arrival. The first arrivals are satisfactorily identified to select the 10-s-long data to evaluate the amplitude. The traces for the stations on Sakurajima Island, which are indicated by blue lines before first arrivals, have no marked later phases. Some of the traces for the stations with source distances between 16 and 19 km, which are located off Sakurajima Island, have clear later phases. The stations are located on the plain to the north of Sakurajima Island; therefore, the waves are regarded as having traveled through sediments. The waveforms of the stations that are close to the shot point are clipped. We do not use the clipped or noisy data, which are indicated by gray lines in the figure.
The waveforms are converted into the frequency domain by a Fourier transform. Mean values of the power spectrum are calculated in the two frequency bands, between 5 and 15 Hz and 10 and 20 Hz. The two frequency bands (5 to 15 Hz, 10 to 20 Hz) correspond to those of the two sources at Toyohashi. We plot the root mean square of the amplitude at the seismic stations against the source distance and fit the plot with the following equations:
where x is distance from the source and a is the intercept term that is regarded as the source intensity. The term 1/x indicates the amplitude decay by geometrical spreading for the body wave, and exp (-bx) indicates attenuation within the medium. This equation is the same as that used in the location of hypocenters of volcanic earthquakes by Battaglia and Aki (2003). As there are no marked later phases that correspond to surface waves (Figure 7), we used the geometrical spreading factor for body waves. The b is connected with quality factor Q with Equation 2, where β denotes wave velocity.
Figure 8 shows the mean amplitudes at the seismic stations in each frequency band for shot 2. The amplitude values cover the distance range of 1 to 20 km, which show the systematic decay of amplitude with distance. The blue dots show the mean amplitudes for the stations on Sakurajima Island, whereas black dots show those of the stations off it. We try two curve fits for each frequency band. We fit the data to minimize the sum of the fitting error in logarithmic amplitudes. The blue lines show the curve fits for the stations only on Sakurajima Island. The black lines show the curve fits for all the stations. The decay for the stations on Sakurajima Island is larger than that for all the stations. The former yields a b value of about 0.25 for 5 to 15 Hz and 0.30 for 10 to 20 Hz; the latter yields a b value of about 0.17 for 5 to 15 Hz and 0.20 for 10 to 20 Hz. Each value corresponds to Q values of about 50, 65, 80, and 100, respectively, assuming that the P wave velocity is 2.4 km s-1 based on the structure analysis of Sakurajima Island by Miyamachi et al. (2013). There are at least two reasons why the decay rates are different between the stations on and off Sakurajima Island. One is the difference in Q. Sakurajima Island is more attenuating than the surrounding region. The other is the difference in the nature of geometrical spreading. As seen in Figure 7, some of the traces off Sakurajima Island have marked later phases, which can be interpreted as a trapped wave in the soft sediments. Moreover, part of the seismic wave may propagate in the water surrounding the bay to transmit energy more effectively.
In this calculation, we used the horizontal distances between the source and the receivers. Let us examine the decay change when the distance along the ray for more realistic velocity structure is used. To examine the difference, we try to use the one-dimensional velocity structure that follows a simple linear function. We assume a velocity gradient of 0.5 km s-1 m-1 and surface velocity of 3.5 km s-1, which provides a maximum depth of 5.2 km for the ray traveling 25 km from the source. The corresponding b values for the ray-based distance are 0.16, 0.19, 0.11, and 0.13, respectively. These values provide Q
p
of 80, 100, 120, and 150, respectively. The differences between the two estimations of Q are about 50%, which is the uncertainty owing to the model difference of the velocity structure.
It is interesting to compare these Q
p
values with those obtained in other methods or other volcanoes. Iguchi (1994) estimated the Q
p
value of Sakurajima Volcano using the spatial decay of amplitude of volcanic earthquakes and obtained the Q
p
value of about 20. He used A-type earthquakes, in which short-period component of around 10 Hz predominates, with straight-line approximation of ray path between the epicenters and seismic stations in the island. As he assumed body wave for geometrical spreading factor, his Q
p
value can be directly compared with our result, which is about 50. His result is smaller than ours, which may result from the hypocenter locations of A-type earthquakes, being just beneath the summit crater with depths of 1 to 4 km. Low Q
p
value may indicate the high attenuation nature in the region beneath the summit crater. Hirata and Uchiyama (1981) estimated the attenuation in the Aira Caldera region, which neighbors Sakurajima Volcano, with spatial amplitude decay ratio, showing a Q
p
of about 80. This supports our inference that the region of low Q value estimated by Iguchi (1994) is localized near the vent.
Q values are estimated for other volcanoes. Sudo (1991) estimated the attenuation beneath Aso Volcano, Japan, concluding that the Q
p
is about 100. Bianco et al. (1999) estimated the Q
p
at Mt. Vesuvius, Italy, to be about 35 with the frequency decay method. Patane et al. (1994) estimated the attenuation using the records of seismic stations around Mt. Etna, showing that the Q
p
varies between 50 and 110 among the stations in the frequency range of 2 to 15 Hz. Giampiccolo et al. (2007) estimated the attenuation at Mt. Etna to fit the power law Q = Q
0
fn with the spectral ratio method. They found that Q
p
for the upper 5 km of the crust was estimated to be 16f0.8 with some ambiguity, that is about 100 at 10 Hz. Martinez-Arevalo et al. (2005) built the 3-D attenuation tomography in the shallow part (-2 to 2 km depth) at Mt. Etna and showed the large heterogeneity in Q
p
, ranging between 10 and 250, in the frequency range of 2 to 30 Hz. They also found a region of low Q
p
between 10 and 30 at the place of presumed dike intrusion in 2001. The Q
p
we obtained for Sakurajima Volcano is comparable to the Q
p
values in other volcanoes.
Amplitude decay in Tokai
We compare these results with the amplitude decay relation obtained for the ACROSS sources at the Toyohashi site in the Tokai region. The same model of vibrators as used in Sakurajima has been in operation at Toyohashi site since 2007. At this site, two vibrators are operated with frequency modulation of different frequency bands, from 5 to 15 Hz and 10 to 20 Hz. We used 26 seismic stations in the Tokai region for the analysis (Figure 9). Out of 26 stations, 24 are Hi-net stations that belong to NIED, and the other two are operated by Nagoya University. The data are stacked for 24 months to calculate the mean value of the spectral amplitudes that correspond to the signal range of the ACROSS sources. The mean spectral amplitudes are calculated as for the source at Sakurajima, i.e., RMS amplitudes of the spectral peaks within the operation frequency band for two components of synthesized linear vibration are calculated for each station. The spectral amplitudes at the stations are fit to Equation 1 to obtain the amplitude decay with distance.
Figure 10 shows the mean amplitude of the spectral signal of the ACROSS source at the Toyohashi site. In this plot, we used the signal of the source that is in FM operation between 10 and 20 Hz, which includes the operation range of the ACROSS at Sakurajima. The mean amplitudes for both transverse and radial vibrations are plotted, showing no systematic difference between the vibration directions at the source. The values cover the distance range of up to 50 km, though few stations exist at less than 10 km distance. They show the systematic decay of amplitude with distance. We fit the plot with Equation 1 as we did for the explosion experiment data in Sakurajima. The best-fit curve, which is shown by the blue line, yields an a value (source intensity) of about 2.2 × 10-9 m/s and the b value of about 0.049, showing that the Tokai region is less attenuating than Sakurajima.
We predict the amplitude decay relations as a function of distance for the ACROSS source at Sakurajima from that in Tokai. We assume that both relations for Sakurajima and the Tokai region share the same source intensity a in Equation 1, but that each has its own b that is characteristic of each region. The red curve in Figure 10 indicates the predicted amplitude decay with distance for the ACROSS source in Sakurajima Volcano. In this curve, we use b = 0.30 that is obtained for the stations on Sakurajima Island. Both lines run very close to each other below 1 km, and the curve for Sakurajima decays more rapidly and becomes one tenth of the curve for Tokai region at 10 km.