At Aso volcano, Kyushu, Japan, several different types of volcanic tremor have been observed for many years. One of them is the continuous tremor, the ground vibration which has dominant frequency between 3 and 10 Hz and has approximately constant amplitudes without any clear beginning and ending. We observed the continuous tremor at Aso using short period seismometer arrays for 3 days in 1999. We locate the source of the continuous tremor by seismic array data processing. We use the semblance coefficients in order to estimate the arrival azimuth and apparent slowness by grid search. The epicenters of the continuous tremor are located around the currently active crater, and the source depths are likely to be shallower than 600 m. We find that the estimated epicenter clearly migrates synchronized with the change in the tremor amplitude. The migration often occurs periodically with a period of about 80 s, but aperiodic occurrence of the migration is also often seen. In both cases, the epicenter is located southeastward (northwestward) when the amplitude is larger (smaller). We propose that there are two or more independent tremor sources with fixed locations, and that their amplitudes modulate either aperiodically or periodically with periods nearly 80 s. The tremor signals from those sources are mixed at the arrays, and the estimated epicenter parameters vary according to which signal dominates the seismograms. The simplest model is that there are two point sources, one at west and the other at south of the crater (we call the two sources as "NW source" and "SE source", respectively), and the amplitude of the SE source changes with time. Consequently, the SE source dominates the seismogram when the observed amplitude is larger, whereas the NW source dominates when the amplitude is smaller. We generate synthetic seismograms, and apply the location technique to them to verify the validity of the two point source model and to search the locations of two sources which can explain the observed synchronization between the amplitude and the apparent epicenter location. We find that the distance between the two sources needs to be more than 400 m to agree with the observation. We also analyze the seismic array data observed in 2001, and infer that the NW source in 1999 may be identical to the tremor source of the 2001 data. (c) 2006 Elsevier B.V. All rights reserved.

At Aso volcano, Kyusyu, Japan, three types of volcanic tremor are observed: 'long period tremor', 'isolated tremor', and 'continuous tremor'. The long period tremors have the period of 15 seconds, and the spatial variation of their amplitudes are explained by a combination of isotropic expansion (contraction) and inflation (deflation) of a tensile crack beneath the volcano (Yamamoto et al., 1999). The dominant frequency of the isolated tremors is 2Hz, and their sources are located about 600m beneath the first crater (Mori et al., 2004, Yamamoto, 2004). The continuous tremor has dominant frequencies between 3 and 10 Hz and has approximately constant amplitudes without any clear beginning and ending. The model of generation of long period tremors and isolated tremors is as follows (Yamamoto, 2004): Volcanic gas ascends through the crack-like conduit, and long period tremors are generated. Next, at the top part of the crack, cylindrical conduit is deformed and isolated tremors are generated. On the other hand, we don't have the clear model about a generation of continuous tremors in the conduit system beneath the Aso volcano. In this study, we discuss about continuous tremor.

Teleseismic waves from the 1999 Chi-Chi, Taiwan earthquake dynamically triggered isolated tremor and micro-earthquakes at Aso volcano, Japan, which is at a distance of 1400 km. Immediately after the arrival of the P-waves, short-period tremor (SPT) began to occur more frequently for about 2000 s, and during and after the arrival of surface waves, 2 micro-earthquakes were observed around the crater. Observations of frequent SPT are an indication of triggered isolated tremor which consists of both SPT and long-period tremor (LPT) components. For the cases of arrivals of seismic waves from other large teleseisms from March 1995 to June 2002, triggering of isolated tremor was observed during 1998–1999. During this period increased thermal supply to the crater was observed while the background activity was relatively low, and the volcanic system beneath the crater was more responsive to external stress perturbations.

The subsurface structure of Aso Volcano, central Kyushu, Japan, has been imaged down to 10 km below sea level and notable features of subsurface structure in the central cones were revealed by a 3-D seismic reflection analysis. The 3-D reflection analysis included reflection enhancements and 3-D migration. The reflection enhancements and the NMO correction were applied on waveform data from the seismic experiment ASO98. The 3-D migration with a Kirchhoff integral was processed for 1397 NMO corrected traces to calculate the reflectivity at about 60 000 image points in an approximately 7 km by 7.5 km square region. The results of the 3-D migration show that the subsurface structure beneath the central cones can be divided into three parts, the western part, the eastern part, and the northwestern part. Bright reflectors appear in most of them and a significant discontinuity is inferred. The reflector horizon at 2 km below sea level spreads beneath the western and the eastern part and is probably the top surface of the Pre-Aso volcanic rocks or the basement rocks. Reflection voids appear in the western part and just beneath the active crater. The reflector void in the western part implies a hot region and is most probably the known seismic anomalies, such as low velocity region or attenuation region. Another reflector void beneath the active crater can also be associated with a hot region, apparently surrounding the conduit system of the current volcanic craters. The major discontinuity is located beneath the northwest dank of the central cones. This discontinuity divides the northwest region with weak reflectors from other parts. The discontinuity may be the trace of the Oita-Kumamoto Tectonic Line, which is the major tectonic line in central Kyushu, crossing the Aso caldera. (C) 2003 Elsevier B.V. All rights reserved.

A seismic tomographic study was performed for Nevado del Ruiz Volcano (NRV) using more than 1500 high-quality local and regional. Three low P wave velocity (low-VP) and low S wave velocity (low-VS) zones were found; one low-VS zone at depths 2–4 km located beneath the volcano; a second low-VP and low-VS zone at depths 5–10 km located beneath the crater, elongated and dipping to the E–SE; and a third low-VP and low-VS zone at 10 to ∼12 km farther to the east. These three low-VP and low-VS zones are believed to be the location of heat sources. A high-velocity zone for both P and S waves was found at shallow depths (0∼5 km) around the active crater. The upper part (0∼2 km depth) of the high-VP and high-VS zone was characterized by low-VP/VS ratios (<1.68), while the deeper part (2∼10 km) had high-VP/VS ratios (>1.80). The low-VP/VS zone is correlated with a steam-dominated geothermal system. The high-VP/VS is interpreted as an intrusive body of magmatic origin which includes partial melting zones associated with low-VS anomalies. A small low-VP zone in which long-period (LP) earthquakes were clustered was found to the southwest of the volcano. Based on the data obtained with the tomography in combination with seismicity, geochemistry, geology and gravimetry, we suggest a model for the seismic activity of NRV. Volcano-tectonic (VT) earthquakes that occur very often in swarm-like patterns located in several clusters around the volcano seem to be due to changes in stress produced by the passing of fluids and/or gas through many small cracks. A fault and a caldera-like structure separate the VT swarms located to the west of the volcano from the source of LP earthquakes.

A phreatic eruption occurred at Kuju Volcano in October 1995. We deployed an EDM network around the active craters of the volcano just after the eruption. Slope distances of the survey lines in the northern network have tended to contract, whereas those in the southern one extended. The maximum contraction observed in the northern network was 70 cm over 6 years. A spherical volume decrease just beneath a fumarolic area called Iwoyama 700 m north of the new craters is a plausible model for these changes in slope distances. A noteworthy feature is that over 6 years after the phreatic eruption ended, the deflation rate is still approximately linear. We also estimated the thermal energy discharge by fumaroles in the new craters, which proved to be well correlated with the observed deflation rate. It is strongly suggested that deflation of a geothermal reservoir, not any magmatic effect, causes the ground deformation around Iwoyama.

The sources of short period volcanic tremors at Aso Volcano are located using seismic array data.There are two types of short period volcanic tremor at Aso Volcano: continuous tremor and isolated tremor. Continuous tremor is ground vibration with approximately constant amplitudes without any clear beginning and end.An isolated tremor is, on the other hand, an episodic event which starts suddenly and continues about ten seconds. Short period seismometer arrays were deployed for three days near the crater of Aso in 1999 and 2001, and the short period volcanic tremors were recorded. In order to locate these volcanic tremors we developed two semblance-based seismic array analysis methods: one method which stacks semblance coefficients for many individual time windows, and another which stacks seismogram-s of the individual seismic stations over different time windows and computes semblance coefficients for the stacked seismograms. A technique is also developed to utilize three-dimensional particle motions and to extract episodic events with the same pattern of ground motions. Based on the result from the array analyses we conclude that the continuous tremor in 1999 consists of surface waves which are emitted from the vicinity of the western rim of the active crater at very shallow depth. On the other hand, the isolated tremor is S wave (body wave) and its source can not be restricted satisfactorily,but the semblance coefficient takes maximum at the direction of the south of the crater. Its focal depth is difficult to determine, but it is almost certainly be deeper than that of the continuous tremor

Tomographic results for P- and S-wave velocity structure beneath the active Aso Volcano, Kyushu, Japan, using 800 well-recorded earthquakes and ten shots recorded by an eight-station seismic network, are presented. A 68% variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure. Well-resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26% slower and 18% faster in P velocity, and up to 31% slower and 22% faster in S velocity, than the one-dimensional model. The largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones. Two low-velocity regions are imaged. The first region, a 10×15-km region encompassing the upper 3 km centered near the caldera wall at Tateno Valley, is characterized by P velocities up to 19% slower (20% for S). The second low-velocity region is associated with the central cones and active magma conduit system at 6 km depth. Velocities as low as 4.3 km/s (up to 26%) in P and 2 km/s (31% slower) in S characterize the 7-km-wide volume. The magma chamber is roughly spherical in shape, centered at 6 km depth, flattens at 10 km depth, and is located between Mt. Kishima, Mt. Eboshi, and Mt. Naka, the present focus of magmatism. A sharp velocity contrast at the depth of 3 km, with high velocities to the southwest and lower velocities to the northeast, characterizes different abutting structures associated with the Oita-Kumamoto Tectonic Line.

Spectral analyses for volcano-tectonic earthquakes were carried out at Nevado del Ruiz Volcano (NRV) for the period 1985–1996 for several earthquake swarms around the volcano, named North, East, West, South and Crater swarm zones. Important spectral peaks for each earthquake swarm zone were found by counting the number of spectra that had the same spectral peaks. Each swarm zone showed some characteristic peaks, which could help to differentiate between them; however, the most important peaks were similar for all the zones. These results suggest that the earthquake swarms at NRV were influenced directly by the source (activity of the volcano) and could also be influenced by the site effect. Some temporal changes were observed in spectral parameters such as a change in the frequency contents in almost all the swarm zones, and the frequency of the P-waves in the West earthquake swarm zone. Before the eruptions on November 13, 1985 and September 1, 1989, P-waves showed low frequencies (1–2Hz) at the West earthquake swarm. After the eruptions, the frequencies of P increased (2–4Hz). This fact showed that changes (decreasing of frequencies) in the spectra of P-waves at the West earthquake swarm could help in the monitoring of volcanic activity at NRV. This swarm zone seems to be related directly with the most important volcanic crises that have occurred. This suggests that the West swarm zone should be monitored in more detail in the future.

We have revealed that the source of long period tremors (LPTs) which has been continually emitted from Aso volcano, consists of an inflation of an inclined tensile crack and an isotropic expansion. The occurrence of LPTs are often correlated with the occurrence of another type of volcanic tremor with a shorter period. However, only few studies have been made at relating these two. For further investigation of the source of LPTs, we installed short-period seismic arrays in Nov. 1999. Two semicircular Aki-Chourt type array were set about 800m north and 500m west of the active crater. The hypocenters of the short period tremors obtained using semblance method are much the same as those of LPTs, while their depth are a little shallower than that of LPTs.

We deployed a network of broadband seismometers for one year around the Naka-dake first crater of Aso volcano in Kyushu, Japan, to reveal the mechanism of long period tremors (LPTs) emitted from the volcano. It is observed that LPTs with a dominant period of about 15s are always emitted regardless of the surface activity of the volcano. A typical LPT has a short duration less than a minute and its spectrum shows mode peaks at 15, 7.5, 5, and 3s. The particle motion in the frequency band for the lowest two modes at stations within a few kilometers from the crater is rectilinear, pointing in the direction of the crater. A waveform semblance technique to locate sources of LPTs is devised to utilize the rectilinearity of waveforms. The LPT sources are located at depths of 1–1.5km beneath the bottom of the crater. When the volcano is explosively ejecting steam and mud, on the other hand, a very long period (∽100s) displacement (VLPD) polarized outward from the crater often precedes an eruptive event by a few minutes. A typical VLPD is accompanied by a few long period pulses, first positively polarized and concurrent with the onset of VLPD, then negatively polarized just before the eruption. The source of VLPDs is inferred to coincide approximately with that of LPT. On the basis of these observations, a qualitative model is constructed for the hydrothermal system beneath the Naka-dake first crater. An explanation for the unusually long period nature of the LPT is discussed in terms of a class of slow waves, which exist in solid–liquid two-phase systems. A possibility of realtime monitoring at Aso volcano using the observed long period seismic signals is also discussed.

To constrain the source of long period tremors (LPTs), we deployed a very dense broadband seismic network consisting of totally twenty-four stations around the active crater of Aso volcano in Kyushu, Japan. The spatial variation of the observed signal amplitudes reveals that the source of LPTs consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The detected crack has a dimension of 1 km and its center is located a few hundred meters southwest of the active crater, at a depth of about 1.8 km. The extension of the crack plane meets the crater chain including the active fumarole at the surface, suggesting that the crack has played an important role in transporting gasses and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.

The May 1998 eruption sequence of Sakurajima Volcano was monitored by ten infrasonic stations, ten seismometers, and a video camera. During this seismo-acoustic experiment, we recorded hundreds of infrasonic tremor and long-period events associated with seismic signals, and observed a progression from relative quiescence to a Vulcanian eruption. The number of infrasonic events increased with escalating volcanic activity, and the dominant character of the infrasonic signals changed from impulsive to emergent. At 22:17 of May 19, Sakurajima released ash and gases to a height of 2 km above the vent, an event that was recorded continuously by one infrasonic and two seismic stations. We present the experimental setup as well as a procedure through which infrasonic signals may be incorporated into future eruption monitoring and forecasting algorithms for open-vent volcanic systems. In addition, our recordings suggest that infrasonic signals are more representative of processes occurring within the volcanic interior than are seismic signals, which are strongly altered by diffraction and scattering in the volcanic edifice.

Kuju Volcano lies near Aso Caldera in central Kyushu. After a few hundred years of dormancy, a phreatic eruption began with the ejection of about 20,000 m3 ash on 11 October 1995. A number of new vents have opened on a series of lines striking east–west on the eastern slope of Mt. Hossho, one of the domes of the Kuju complex, a few hundred meters from a pre-existing fumarolic area. After the eruption, there has been continuous steam emission from the new vents. There was the second ash eruption in December 1995. Before these eruptions, seismic events were rarely observed, either near the site of the new vents, or elsewhere under Kuju Volcano. In the nearly 2 years since the first eruption, several thousand earthquakes have been recorded. These events have been very horizontally concentrated just to the north of the new vents vertically between 800 m above sea level and 1000 m below sea level. Very few earthquakes have been located on the southern side of the new vents. There was clearly a strong high-frequency attenuation affecting the seismic waves which passed through the region beneath the new vents to the seismometers south of Mt. Hossho. This evidence possibly indicates a thermal fluid content beneath the new vents, suggesting that there is a seismic attenuating zone in the feeding area of the new vents. Nearly all the earthquake spectra were of dominantly high-frequency, but the percentage of earthquakes with predominantly low-frequency spectra increased at times of enhanced volcanic activity. Volcanic tremors were also observed around the times of peak activity. Slope distance measurements have been made since the eruption. The main results of these measurements are a contraction of more than 200 ppm in distances between Mt. Hossho and points further north. The significant distance changes occurred during seismic swarms. This indicated that the seismic activities influenced ground deformation, even though some of these swarms were 3 or 5 km from Mt. Hossho. The slope distance changes indicate that an area near the top of Mt. Hossho has been moving to the northeast.

Two holes were drilled to depths of 150 m and 70 m from the surface about 200 m from the active crater of Aso Volcano and quartz thermometers were installed in the holes at depth intervals of 30 m and 35 m, respectively. This series of observations is one of the first measurements of temperature at depth so close to an active crater. The ground temperature at a depth of 2 m had an annual variation with a range of about 10°C, as expected, clearly corresponding to the atmospheric temperature variation, but delayed by about 1 month. Temperatures measured at depths from 30 m to 70 m had very small annual temperature variations. The range of temperature at 30 m depth was about 0.04°C. The temperature at a depth of 60 m, however, was particularly stable, probably because at this depth the hole is in the middle of a massive lava flow. At depths of 70 m or more, small (less than 0.2°C) annual temperature variations were again observed. These variations are probably due to the effects of surface water descending to these levels through cracks and fissures. At 120 m depth, the average temperature is about 17.5°C, over 5°C above the surface average temperature, and the annual temperature variation has a range of about 2°C, out of phase with the atmospheric changes. This is probably due to the interaction of rainfall descending from the surface with convecting hotter fluids from below. The temperature gradient below 100 m depth is very high, with the average temperature at a depth of 150 m being about 31°C. The temperature variations at this depth are dominated by long-period variations, with a steady decline after a peak in November–December 1989, overlain by a rather irregular seasonal variation, with a range of about 0.5°C. October 1989 was the time of most active volcanic activity, with Strombolian eruptions depositing ash to a distance of 50 km from the crater, accompanied by very high amplitude volcanic tremor. So the temperature changes at 150 m seem to be mainly the result of volcanic activity. The maximum temperature at this depth occurred 1 or 2 months after the peak of observed volcanic activity. So it is likely that the temperature variations show a delayed influence of the level of volcanic activity. These results show that in the unconsolidated materials often found by active volcanic craters, the effects of seasonal atmospheric variations are carried to substantial depths by groundwater flows, so that at Aso Volcano, only at the maximum depth of 150 m are the temperature variations clearly dominated by the level of volcanic activity.

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (approximately20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano.